Engine 1990 Mustang (331) Slight Miss After Driving A Mile Or Two

Although not a newby when comes to foxbodys, this issue has be stumped. So, I changed the clutch but it took me about a month to make time to get it done. Before changing the clutch I had no issues. Since then, after I drive for about a mile or two, it sounds like a plug just fowls out. After about 2-3 minutes it magically stops and drives fine thereafter. If I cycle the ignition, even while driving, it will do it again. I have a narrow band A/F meter that tells me it's off the charts lean but once it "corrects" itself it goes back to stoic range. It doesn't buck or backfire, doesn't matter if the car is cold, warm or atmospheric temp changes. Car idles just fine in the morning, doesn't idle search, ever. My gut tells me its O2 sensors since those are the only electrical items I touched.

Basic layout of mods include, 331 bottom end, Victor Jr. heads, Perform II intake, BBK fenderwell CAI, 24lb injectors, 255lph pump, BBK fuel rail and regulator, Autolite 3924 plugs, MSD 6AL, coil, wires, cap, rotor, distributor.
 
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The computer switches from the open loop program to the closed loop program after a short warm up. - about 2-3 minutes.

In open loop, the computer works off of a fixed set of tables for fuel delivery and ignition timing. It uses a very minimal set of sensors for input data.

In closed loop, the computer takes input from all sensors to calculate the proper values for fuel delivery and ignition timing. If one sensor is malfunctioning, it may not set the check engine light, but can affect performance and economy.


Therefore...

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

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If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

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The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

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The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10
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Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus Digital Ford Code Reader (3145) Equus - Digital Ford Code Reader 3145.
It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.
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KOEO codes: 81 = inlet air control check malfunction
82 = secondary bypass solenoid circuit malfunction
85 = canister purge solenoid circuit malfunction
84 = electrical vacuum regulator circuit malfunction
11 = system pass code, no faults

KOER codes: 94 = secondary air system inoperable
44 = Thermactor air system inoperable
33 = EGR valve, no flow detected
 
It appears most if not all of the codes pulled refer to the smog pump system. I've removed this many moons ago. I do still have the EGR but the vacuum lines have been plugged. Eventually I will get a delete plate and remove it all together. However, the codes I pulled don't seem to answer the issue I'm experiencing.
 
Time to do some thinking - it still seems that the problem is one of open loop vs. closed loop. Run for a while , engine reaches operating temperature, switches to closed lop mode, problem starts, the goes away. Turn the ignition switch off, turn I back on, computer thinks it is a restart and operates on open loop mode until it thinks it is time to go to closed loop, switches and the problem happens again.

Lets do some testing that will confirm or deny the O2 sensors as being the source of the problem.
1,) Are you running long tube headers?
2.) O2 sensor testing
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.

Backside view of the computer wiring connector:
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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 (L\RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/pink 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 (LH O2 with a Gray/Lt blue wire) and 43 (RH 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.


Testing the O2 sensors 94-95 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. 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 Os 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 Dark Green/Pink Gray/Lt blue wire on the RH Os 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 Os 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.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.
 
I'm with you though jrichker. I was driving today and it seems like the ECU just gives up, the A/F meter drops lean, then the slight miss presents itself. All of a sudden, almost like a click, it's fixed, the A/F meter is showing results again and the problem is gone. The only other issue I've never been able to solve is the idle dropping after sitting at a light or in the driveway. Been that way for years, but that's another issue all together.
 
Alright, O2 testing done. The O2 sensor test from the ECU was an almost complete fail; very sporadic readings and nowhere near .9 volts at operating tempurature. So, I moved on to the harness test. I physically pulled the harness and draped it over the fender (also to inspect the wiring). First I unplugged the harness from the O2s and at the engine by plug. I tested both LH and RH from each end of the harness. LH 2.5 Ohms, RH 2.0 Ohms.

As a quick reminder, I did purchase new O2s a couple days ago.

I reconnected the harness at the engine bay to test the entire length of the harness. From the plug where the O2 would connect to the ECU, readings are: LH 6.6 Ohms, RH 2.6 Ohms.

With my data and the fact that Ohms "should" have been > 1.5 Ohms, I am led to believe that my O2 sensor harness is crap, right? If it were this bad, shouldn't I have gotten a code when I did the dump today? Looking for some advice on if I should purchase another harness, and if so, from where.
 
Winter is well on its way here in Alaska. I'm also having knee surgery in a couple days, so I broke down and replaced the signal wire lines in the O2 harness. Ohms test was only 2 ohms on each side (much better if you ask me). I plugged everything back in and it fixed my engine miss. However, now she barely stays running upon cold starting...lol! I'm going to give the ECU a few run cycles to see if this clears up. If not, I'll check out some of the other posts on possible issues. In the end though, she runs like a different car now. Real smooth puttin' around town and strong power band at WOT. Thank you all for your inputs