Engine Another Poor Idle/Running Rich Thread

B-Dawggg

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Apr 15, 2019
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Hey everyone,
I have a pretty much stock 92 LX 5.0. I've always had a slight surge in idle with this car. Never was terrible and would fix itself once the car ran for a minute or two.

A few months ago I started having a problem where the car would run fine and the idle would be fine, but when It'd idle for a while or while I was stopped at a traffic light, the car would randomly and suddenly drop the idle and start running rich. I found a little trick where keeping a little throttle in would stop the problem from happening. Kept doing that until the problem finally went away.

The cars been running pretty good for a month or two now until the other day when i was stopped at a traffic light again. This time is suddenly dropped to about 500 rpm, running terribly rich, barely any power and chugging basically the entire rest of the ride. Now it just does it constantly and is pretty much undriveable.

A while back when I had first had gotten this problem i pulled the codes. Idle out of range, Bap, and both oxygen sensors running lean. I basically ignored the problem until now.

I reset the computer while i changed the bap sensor 2 days ago. Car still ran the same, terribly rich. When i got home i tried pulling the codes again. Koeo ran perfectly. Got a 67 and 81. Tried the koer test. Got the cylinder number. The throttle blipped twice really quick as if the computer was struggling to raise the rpm. After the two quick blips of the throttle, it threw a code 12 and basically froze. Left it running for 10 to 15 minutes after that with it still stuck on 12. No 10 or 11 codes. Not sure what the problem is there. Repeated the above 3 times with no avail.

So I've been following jrichker's guide. Tested the bap sensor wiring. Under 1 ohm from the bap to egr/tps. Under 1 from bap to negative terminal. Under 1 from the self test connector to negative terminal. Also tried the act and ect to bap and that was fine. Cleaned salt and pepper shakers. The next step im going for testing to the computer pins and testing oxygen sensor harness.

I tried a while ago to see if there were any vaccum leaks. Sprayed almost every connection i could see in the engine bay. I know there is one vaccum leak somewhere regarding the AC. AC only blows on defrost. There was a break at the tree on the firewall. Tried fixing it and didn't change anything. Not sure if that would be enough to cause such a terribly rich condition though.

Anyone have anymore advice?

P.S. Smog pump went bad. (Extremely loud noise) Was bypassed with a shorter belt but the entire system is still there.
 
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jrichker

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Code 12 &412 -Idle Air Bypass motor not controlling idle properly (generally idle too low) - IAB dirty or not working. Clean the electrical contacts with non flammable brake parts cleaner at the same time.

IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC. An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.

attachments\58887


Recommended procedure for cleaning the IAC/IAB:
Conventional cleaning methods like throttle body cleaner aren’t very effective. The best method is a soak type cleaner used for carburetors. If you are into fixing motorcycles, jet skis, snowmobiles or anything else with a small carburetor, you probably have used the one gallon soak cleaners like Gunk or Berryman. One of the two should be available at your local auto parts store for $22-$29. Take the solenoid off the body and set it aside: the carb cleaner will damage some types of plastic parts. Soak the metal body in the carb cleaner overnight. There is a basket to set the parts in while they are soaking. When you finish soaking overnight, twist the stem of the IAB/IAC that sticks out while the blocker valve is seated. This removes any leftover deposits from the blocker valve seat. Rinse the part off with water and blow it dry with compressed air. The IAC/IAB should seal up nicely now. Once it has dried, try blowing through the bottom hole and it should block the air flow. Reassemble and reinstall to check it out.

Gunk Dip type carb & parts soaker:
21hb0QWbOeL._SL500_AA300_.jpg



Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it.

When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.

An engine that whose idle speed cannot be set at 600 RPM with the IAC disconnected has mechanical problems. Vacuum leaks are the #1 suspect in this case. A vacuum gauge will help pinpoint both vacuum leaks and improperly adjusted valves. A sticking valve or one adjusted too tight will cause low vacuum and a 5"-8" sweep every time the bad cylinder comes up on compression stroke. An extreme cam can make the 600 RPM set point difficult to set. Contact your cam supplier or manufacturer to get information on idle speed and quality


Code 67
Revised 18-Mar-2017 to include warning about the necessity of having a 5 speed O2 Sensor wiring harness when bypassing the wiring for test purposes

Cause of problem:
Clutch not depressed (5 speed) or car not in neutral (5 speed and auto) or not in park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code will prevent you from running the Key On Engine On tests.

External evidence from other sources claims that a code 67 can cause an idle surge condition. Do try to find and fix any issues with the switch and wiring if you get a code 67.

What the NSS (Neutral Safety Switch) does:
5 speed transmission: It has no connection with the starter, and the engine can be cranked without it being connected.
Auto transmission: It is the safety interlock that prevents the starter from cranking the engine with the transmission in gear.
What it does for both 5 speed and auto transmission cars:
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running computer diagnostic tests. It also checks to see that the transmission is in Neutral (5 speed and auto transmission) and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

The following is for 5 speed cars only. Do not do this unless you are sure that you have a 5 speed O2 Sensor harness!!!! Smoke, sparks and expensive pain in the wallet may ensue if you don’t.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!

a9x-series-computer-connector-wire-side-view-gif.71316
 

B-Dawggg

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Apr 15, 2019
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So I got the car to start pulling codes through the engine running test. Code 67 was caused by my error leaving the car in gear during the engine off test.

Got a 12, 21 and then 13. Once it showed a 13, test seemed like it froze again. Left it running for another 10 or 15 minutes after that and was still stuck on 13. I'm guessing the ECT is probably my problem. Tried testing resistance on the sensor itself and got nothing. Just oL. Tried the same with the ACT just for :poo:sand giggles, got nothing. Double checked the meter just to make sure. Looks like both of them are bad.

Cleaned IAC and throttle body... and the MAF just for fun. No change. IAC is getting just over twelve volts. Idle is even worse now. Around 500, running so rich I can't stand in the garage with the door open. Base idle reset was done prior to this problem with the IAC and spout unplugged.
 

jrichker

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So I got the car to start pulling codes through the engine running test. Code 67 was caused by my error leaving the car in gear during the engine off test.

Got a 12, 21 and then 13. Once it showed a 13, test seemed like it froze again. Left it running for another 10 or 15 minutes after that and was still stuck on 13. I'm guessing the ECT is probably my problem. Tried testing resistance on the sensor itself and got nothing. Just oL. Tried the same with the ACT just for :poo:sand giggles, got nothing. Double checked the meter just to make sure. Looks like both of them are bad.

Cleaned IAC and throttle body... and the MAF just for fun. No change. IAC is getting just over twelve volts. Idle is even worse now. Around 500, running so rich I can't stand in the garage with the door open. Base idle reset was done prior to this problem with the IAC and spout unplugged.
If you don't have catalytic converters, the engine will smell like it is running rich, even if it is OK. The tip off is that you didn't get codes 41/91 or 42/92 which are for fuel mixture management/O2 sensor codes.

Code 13 &415 - Key on Engine off - ISC did not respond properly (extends to touch throttle then retracts for KOEO) – ISC

Key on Engine running - Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too high)

If your idle is above 725 RPM, the computer will set this code. Normal idle speed is 650-725 RPM. Higher than that means that someone has mechanically set the idle speed by use of the idle speed screw, and has effectively disabled to computer’s ability to control idle speed.

Code 21 or 116 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.

[color= blue]Revised 6-Apr-2017 to add diagrams and resistance check for ECT wiring.[/color]

Note that that if the outside air temp is below 50 degrees F that the test for the ECT can be in error. Warm the engine up until you get good hot air from the heater and then dump the codes again.

The computer Engine Coolant Temperature sensor has absolutely nothing to do with the temperature gauge. They are different animals. The ECT sensor is normally located it the passenger side front of the engine in the water feed tubes for the heater. It has two wires that connect by a weathertight plastic connector.

The water temperature sender for the temp gauge is located in the driver's side lower intake manifold. It has a single wire that connects by a push on connector on the temp sender.


If you have replaced the ECT sensor and are still having ECT like problem symptoms, check the ECT wiring .

Computer wiring harness connector, wire side
71316.gif


Computer wiring harness connector, computer side
88243.gif



See the graphic for the 10 pin connector circuit layout.
68512.jpg


Check the resistance of the green wire on the ECT connector to the green wire on pin 7 of the computer connector. You should see less that 1 Ω (ohm)

The ACT & ECT have the same thermistor, so the table values are the same

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate readings.

Pin 7 on the computer - ECT signal in. At 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. At 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer.


Voltages may be measured across the ECT/ACT by probing the connector from the rear. A pair of safety pins may be helpful in doing this. Use care in doing it so that you don't damage the wiring or connector.

Here's the table :

50 degrees F = 3.52 v
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

Diagram courtesy of Tmoss & Stang&2birds

88-91_5.0_EEC_Wiring_Diagram.gif


94-95_5.0_EEC_Wiring_Diagram.gif
Wiring_Diagram.gif[/

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Vacuum diagram 89-93 Mustangs
 

B-Dawggg

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Apr 15, 2019
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Thank you jrichker. I do not have cats, haven't for a few years. Its still for some reason running terribly rich. To the point of barely running. The odd part about this is it'll run fine, with a normal idle, no shaking or stumbling, no power loss until a minute or two after running. Then it suddenly drops the idle, starts gurgling and getting very stinky. At the same time it looses all of its drivability. This used to be a once in a while problem, usually when stopped at traffic lights for any amount of time, but has now basically become how the car runs all the time. I'm guessing some sensor or wiring problem is causing the computer to switch to a rich condition. Months ago when i originally had this problem, I was getting 41/91 lean codes. One o2 sensor is newer than the other, only a couple years old. Could they both possibly have gone bad?

The idle speed when it was showing codes 12 and 13 was at around 450 to 500 rpm, shaking and barely keeping itself running.

Tested the resistance between the two pins on the ect itself. Just getting oL. Done with the engine hot. The ground wire has a good resistence (under 1ohm) with all of the other sensors and to battery ground. Haven't had the chance to test to the computer for voltage.

Just to add as much information as possible... Plugs, wires, fuel filter and cap and rotor were done 6 months ago.

Thanks again.
 
Last edited:

B-Dawggg

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So I bought an ECT sensor. Reset the codes. It ran like crap originally, drove it around the block a few times and the idle improved to about 750 after a while. Still shakey and way more stinky than usual. Pulled the codes again. 81 and 12 again. Bogs consistantly while driving.

Checked new ECT sensor and thats showing nothing too. Possible dud?
 
Last edited:

jrichker

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Thank you jrichker. I do not have cats, haven't for a few years. Its still for some reason running terribly rich. To the point of barely running. The odd part about this is it'll run fine, with a normal idle, no shaking or stumbling, no power loss until a minute or two after running. Then it suddenly drops the idle, starts gurgling and getting very stinky. At the same time it looses all of its drivability. This used to be a once in a while problem, usually when stopped at traffic lights for any amount of time, but has now basically become how the car runs all the time. I'm guessing some sensor or wiring problem is causing the computer to switch to a rich condition. Months ago when i originally had this problem, I was getting 41/91 lean codes. One o2 sensor is newer than the other, only a couple years old. Could they both possibly have gone bad?

The idle speed when it was showing codes 12 and 13 was at around 450 to 500 rpm, shaking and barely keeping itself running.

Tested the resistance between the two pins on the ect itself. Just getting oL. Done with the engine hot. The ground wire has a good resistence (under 1ohm) with all of the other sensors and to battery ground. Haven't had the chance to test to the computer for voltage.

Just to add as much information as possible... Plugs, wires, fuel filter and cap and rotor were done 6 months ago.

Thanks again.
No cats will make the exhaust smell like unburned fuel. The only way to get rid of the stink is to put a catted H pipe back on and make sure the smog pump works properly.

Did you do ALL of the base idle procedure or just parts of it? Cleaning the IAC with spray cleaner won't do a through enough job to get the results you are looking for. You need to soak the IAC in the can or bucket of GUNK or Berryman carb cleaner.

No code 41 or 91 means the O2 sensors pass the minimum test.
Here's the Code 41/91 test path for reference....

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 24 Aug 2018
1.) To correct the RH & LH mismatch on 91-93 5.0 Mustangs
2.) To add Tmoss’ wiring diagrams for 88-95 Mustangs


Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:

"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor, you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Computer wiring harness connector, computer side.
88243.gif


Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.gif



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor

The computer pins are 29 (RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/lt green wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor

The computer pins are 29 (RH O2 with a Gray/Lt blue wire) and 43 (LH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


94-95 5.0 Mustangs; note that the 94-95 uses a 4 wire O2 sensor.
The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. . The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH O2 sensor harness and the Dark Green/Pink wire on the computer pin 29


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 27


There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged, and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness. The exception is that the 94-95 uses a 4 wire O2 sensor.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Complete computer, actuator & sensor wiring diagram for 94-95 Mustangs
94-95_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
91-93_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs
88-91_5.0_EEC_Wiring_Diagram.gif
 

B-Dawggg

Member
Apr 15, 2019
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1
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Daytona Beach
So it turned out to be a combination of bad fuel pressure regulator and bad vaccum leak in an area I could barely see. Runs like a champ now. Thanks again!