Check engine light - Did I cause it?

[jcm-3]

I just got through doing a number of things to get rid of alot of annoying idle and throttle issues, and the car is driving great. The idle is nice, it comes down quickly, and surge is almost completely gone. I am getting a check engine light when letting off of the throttle under load. It doesn't have to be at redline, even when letting off of the gas after flooring it in third or fourth will cause a check engine light to come on, and then go off after a few seconds.

I'll try to keep this short.

I did the following:
- base idle reset, set it low and used the bypass plate to get rid of the surge
- reset the tps
- redid the computer grounds
- cleaned the 10-pin connectors

I have a number of codes, but believe that I have always had them:
KOEO
15 - EEC, probably because of the chip
67 - neutral drive switch, circuit failure
Memory
66 - VAF/MAF sensor fault, below minimum voltage - probably because of the tune?
87 - Fuel pump primary circuit failure
96 - Fuel pump secondary circuit fault/high speed fuel pump relay open

I know that Joel had a nice video about how the neutral safety switch can cause surging, need to research that further.

The MAF one I think is because of the tune, alot of MAF tuning to get the 60 lb injectors working right.

The fuel pump ones might be related to the Kenne Bell boost-a-pump, not sure.

But, here's where I think I might have messed up. I had hidden my wires last year, and recently 'fixed' my grounds. I ran another 1/O ground cable from the battery to the tranny, and extended the computer ground wire to connect to that same place to make sure that I didn't have an iffy connection using a sheetmetal location under the fender liner. But, I wonder if splicing in and extending it will cause it to have trouble.

Due to ignorance, I thought the connector on it was an inline fuse, but of course it isn't. I was trying to avoid having that connector inside of the fender liner in case I needed to replace it, so I extended the wire to get the connector out in the open. I know my splicing technique is good, because I have done tests with my voltmeter to make sure that I don't lose any current. I also noticed that this wire seemed like it was just standard, stranded wire.

Please see pic belowfor where I spliced it and soldered in an extension.

Will this cause the engine to throw a code? Even though there are a number of codes above, I believe I always had them and didn't have the check engine light. The only real change after last weekend is related to the gounds. But, another ground cable isn't likely to cause any problem. Extending the computer ground might. ???

View attachment 258740

 

[jcm-3]

A bigger question might be, could it cause the computer to have an issue adjusting the air/fuel, which could cause an expensive problem?

 

[jrichker]

Extending the computer ground should not be a problem if you used 10 gauge wire and did a quality job on the solder splice of the wire. It should be grounded as shown in in the diagram.

There is nothing in tuning the engine that will set code 66.
Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

If you have a K&N flat panel filter or other filter that requires oiling, excess oil may coat the MAF sensor element and cause problems.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

Changes in RPM causes the airflow to increase or decease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and ground. Make your measurement with the MAF disconnected from the wiring harness.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Engine Information

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position when codes
where dumped. Possible neutral safety switch or wiring problem. This code may prevent you from running the Key On
Engine On tests. You can generally ignore this code, since it has no effect on engine performance.

The computer wants to make sure the A/C is off due to the added load on the engine for the engine running tests. It also
checks to see that the transmission is in Neutral or 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 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.

IF YOU JUMPER THE WIRING TO BYPASS THE NSS, BE SURE TO REMOVE IT PRIOR TO DRIVING THE CAR!!! YOU COULD END UP CRASHING AND BURNING!!!

Code 87 – fuel pump primary circuit failure. The fuel pump lost power while the engine was running. Check fuel pump relay, check inertia switch, wiring to/from inertia switch, red wire going to inertia switch for +12volts. Check the other side of inertia switch for +12 volts.

To help troubleshoot the 87 code, follow this link for a wiring diagram for 89-93 cars http://www.autozone.com/servlet/UiB..._us/0900823d/80/19/59/5a/0900823d8019595a.jsp

Diagram of the fuel pump wiring for 86-90 cars

Diagram of the fuel pump wiring for 91-93 cars.

Code 96 – KOEO- Fuel pump monitor circuit shows no power - Fuel pump relay or battery power feed was open - Power / Fuel Pump Circuits. The fuel pump lost power at some time while the ignition switch was in the run position.

Look for a failing fuel pump relay, bad connections or broken wiring. The fuel pump relay is located under the Mass Air Meter on Fox bodied stangs built after 91. On earlier model cars is under the passenger seat. On Mass Air Conversions, the signal lead that tells the computer that the fuel pump has power may not have been wired correctly. See MAF - Mustang Mass Air Conversion - StangNet - The Mustang Network

Diagram courtesy of Tmoss & Stang&2birds

Look for power at the fuel pump - the fuel pump has a connector at the rear of the car with a pink/black wire and a black wire that goes to the fuel pump. The pink/black wire should be hot when the test connector is jumpered to the test position. . To trick the fuel pump into running, find the ECC test connector and jump the connector in the lower RH corner to ground.

86-90 Models:
Using the diagram, check the red/black wire from the fuel pump relay: you should see 12 volts or so. If not, check the inertia switch: on a hatch it is on the drivers side by the taillight. Look for a black rubber plug that pops out: if you don't find it, then loosen up the plastic trim. Check for voltage on both sides of the switch. If there is voltage on both sides, then check the Pink/black wire on the fuel pump relay: it is the power feed to the fuel pump. Good voltage there, then the fuel pump is the likely culprit since it is getting power. No voltage there, check the Orange/Lt blue wire, it is the power feed to the fuel pump relay & has a fuse link in it. Good voltage there & at the Pink/black wire, swap the relay.

 

[jcm-3]

Wow, thanks for all of the information so quickly. I usually get a few fuel pump codes, because I have a kill switch and occasionally I try to start the car and forget to hit the switch. So, the fuel pump isn't working when I turn it over.

I just resolved a slight lean issue above 5500 rpm with some fuel changes (60 lb. injectors, new GSS340, removed T-Rex, added Kenne Bell boost-a-pump), so I am pretty sure that the fuel system somponents themselves are mechanically sound. I have no issues with my air/fuel ratio at 500 rwhp. Could I still have iffy parts somewhere and it not show up on a dyno? If I had a bad or failing relay or wiring, would I still be able to get enough fuel?

I will definitely check the MAF element. I just got through with about three separate tuning sessions while trying to resolve the air/fuel ratio, and the tuner was able to read MAF voltage data. He did say I was close to maxxing it out, though. I haven't checked the element in quite a few years, so it could be dirty. If there was a problem with wiring, would it have surfaced during the tuning sessions and the reading of the MAF data?

Odd that I was not getting a code until the recent work.

Thanks again for your time.

 

[ID89GT]

Hmmmm, I have the same codes about the fuel pump, and my pump is wayyyyy loud. I can be outside the car about ten feet away and hear it clear, and it drowns out the exhaust easily.. All I have is a Walbro 255 lph pump, and haven't done anything else with the fuel pump circuit. What could cause it to be so loud? Could the codes he mentioned have anything to do with it, cause it is really starting to get annoying.

 

[jcm-3]

Minor update.

I took it out again tonight, and noticed something different about the CEL. It happens that it comes on when I am coating in 6th gear. It doesnt seem to do it in any other gear, and doesn't have to be after I got on it.

It feels as strong as ever, and the idle is perfect, right around 850. It comes down from revs quickly, and is a pleasure to drive again.

Very odd how the check engine light only comes on if I let off the gas in 6th. I didn't see it happening when I did the same with the other gears, and let it coast down pretty slow before putting the clutch in and putting it in neutral.