Backfire through exhaust under load...

cougar_68 · Aug 1, 2010

Test Drive #2 (Fail):

Took it out again, IAC unplugged from harness from the very beginning. Still drove like POS. Next?

cougar_68 · Aug 1, 2010

Re-did the TPS as mentioned above (TPS voltage was 4.99V and got the TPS sensor set to .9XX. Reset computer. Will take for a drive tomorrow night. If that doesn't fix it (I have no faith that it will).

I will try to get at the computer and jump 30 like mentioned and do the Key on engine running test. This is so frustrating!

cougar_68 · Aug 2, 2010

I really did it now.

My computer is under the dash of my car. I had to move the computer so I could get to the terminals (to jump terminal 30 tricking it to think it is in neutral, even though it is in neutral, stupid POS). When doing so, a wire under the dash sparked. Don't ask me what, I still haven't been able to figure it out.

Now when I turn the ignition on the fuel pump doesn't come on. Don't know if it's just the fuel pump relay, a fuse somewhere, or if the computer is toast.

I checked terminal 1 on the harness and it is getting 11.99 volts when the key is on. I checked terminal 37, no power. Is there a fuse in the computer or in the harness somewhere that could be blown?

cougar_68 · Aug 2, 2010

Ok, got to thinking. What if the #1 terminal has power all the time? Apparently it does (does now anyway...). So with key off still have 12V.

This means that there has to be a key on terminal. Don't know which terminal that is? Anyway, I think the short (sparks) I had earlier hurt something in the key on curcuit because there are other things that should be working with the key on, that don't work. I have no idea what. Checked the fuses in the fuse block and they are ok.

cougar_68 · Aug 5, 2010

Ok, let's just say, you know hypothetically speaking, that my computer isn't getting a sufficient amount of power. Could that cause the problem I am having?

Also, is the power for the Fuel pump supposed to come from the computer or is it a signal that turns on a relay?

jrichker · Aug 6, 2010

Pin #1 on the computer always has 12 volts: this is the keep alive voltage for the memory that holds the settings. Pins #37 & 57 are the switched power controlled by the ignition switch. Pins #40 & 60 are the computer power ground. Be sure that the computer power ground (heavy gauge black/white wire) is grounded to the same place as the battery body ground. On Mustangs this is over near the windshield washer filler and the starter solenoid.

The computer will operate reliably anywhere from 8-18 volts as long as it has a good ground. This is because there is a voltage regulator inside the computer that drops the voltage down to 5 volts. The circuitry inside the computer uses the 5 volts to operate the electronics. Some of the items that share the same computer power feed require more minimum voltage than the computer to operate reliably.

The fuel pump powers up for 1-3 seconds when the ignition switch turns on. Then it shuts off until the engine speed gets up past a certain point. This helps to prevents the engine from flooding.

Diagrams courtesy of Tmoss & Stang&2birds

cougar_68 · Aug 8, 2010

Think I got the under-dash wiring issue solved. Was able to do the Key On Engine Running test with the tester.

First problem: The cylinder ID code came up as 6.

The codes I got were:

31: EGR valve control system below minimum voltage (I don't have EGR)
91: HEGO sensor voltage low/system lean

What would cause it to think it's a 6 cyl? Bad plugs? When I checked them the other day two of them had way more resistance.

Mustang5L5 · Aug 8, 2010

cougar_68 said:
Think I got the under-dash wiring issue solved. Was able to do the Key On Engine Running test with the tester.

First problem: The cylinder ID code came up as 6.

The codes I got were:

31: EGR valve control system below minimum voltage (I don't have EGR)
91: HEGO sensor voltage low/system lean

What would cause it to think it's a 6 cyl? Bad plugs? When I checked them the other day two of them had way more resistance.

Cyl I'd coming back as a v6 is pretty common actually. Ignore it.

Code 91 suggest an o2 sensor may be faulty....which could be the cause of your prob. Ypu could try swapping o2 sensors and see if the code 91 turns into a code 41... Then you know the o2 needs to be replaced.

Code 31 should not affect way car runs.

I suggest at the end of the koer codes, blipmthe throttle half way and let the car run a cylinder balance test. If it shows a 9 all cylinders are good. Any digit 1-8 shows that cylinder is not contributing like the others.

cougar_68 · Aug 8, 2010

I'm going to have to try the balance test tomorrow. Will it work properly even though it said 6cyl?

I'm also considering bypassing the factory harness and running brand new wires for the O2 sensors. The wires for the driver side have been cut (not sure where). The passenger side is the only side hooked up. Thought I read somewhere that if you only had one hooked up it should be the driver side.

Mustang5L5 · Aug 8, 2010

cougar_68 said:
I'm going to have to try the balance test tomorrow. Will it work properly even though it said 6cyl?

I'm also considering bypassing the factory harness and running brand new wires for the O2 sensors. The wires for the driver side have been cut (not sure where). The passenger side is the only side hooked up. Thought I read somewhere that if you only had one hooked up it should be the driver side.

That might be the issue........code 91 is the drivers side.

You need both sides working. You can remove and replace the entire harness or resplice the drivers side.

cougar_68 · Aug 8, 2010

Mustang5L5 said:
That might be the issue........code 91 is the drivers side.

You need both sides working. You can remove and replace the entire harness or resplice the drivers side.

The reason I'm thinking about replacing the wiring entirely is the wires I have coming from my right O2 sensor are gray/blue, gray/pink and black/white. Diagram says it should be dark green/pink.

At the computer I have gray/blue at pin #29 and red/black at pin #43. Should be dark green/pink (29) and dark blue/light green (43).

I just found the red/black wire in the harness. There are wires missing on the oxygen harness side. Should I spend the 85 bux for the new harness or just splice the wires in?

Mustang5L5 · Aug 8, 2010

Sound like you might have a mismatched harness.

Thing is, there are 4 harnesses and which to get is important. I believe the year spread is 87-89 and 90-93 with one each for aod or t5.

With that said, what year is the main engine harness and what eec are you using?

cougar_68 · Aug 8, 2010

The computer is an A9P. Part # starts E9ZF...so that would indicate '89.

The harness, I believe, is a complete EFI (MAF) conversion harness from ford. Could be why the colors are inconsistent with the diagram. The connectors are all marked. I'm not sure if I ever had the installation instructions or not? Would probably have more info on the wiring.

cougar_68 · Aug 9, 2010

Did cylinder balance test. I got code 80 - "cylinder 8 has a problem".

Once it cools down I will try to get the spark plug out and look at it. I have JBA shorty headers and that plug is a major PITA to get too. Oddly enough that was not one of the wires that had more resistance.

cougar_68 · Aug 9, 2010

Still haven't ordered the new O2 sensor harness. Is there anywhere to get one of those used? $100 shipped seems like a lot...

I didn't see any on ebay or in the classifieds.

cougar_68 · Aug 9, 2010

Funny story, when I have pin "30" grounded to trick the computer into thinking the car is in neutral, my A/F ratio guage doesn't work??? WTF?

Jrichker, any thoughts on this one?

cougar_68 · Aug 9, 2010

More info and this just seems wrong to me, the sending wire for my A/F guage runs and splices together with the green/yellow (pin #30) wire and red/black (pin #43) wire, right before it gets to the computer.

Isn't that way off? Shouldn't it be hooked up to only the red/black wire (pin #43)?

Considering the red/black wire is going to the left side (and there isn't an O2 sensor there) that should make it not even work properly. WTF*2

Mustang5L5 · Aug 9, 2010

Run the cbt three times. Each successive time the computer widens the specs. Three fails and you have a dead cylinder. I just did this test myself and found my #4 injector dead to the world.

As for the harness....junkyard?

I can't help on the a/f, but I think you are on the right path. Fix the o2 sensor and trouble shoot the #8 cylinder and I think your problem will be fixed.

cougar_68 · Aug 9, 2010

Mustang5L5 said:
Run the cbt three times. Each successive time the computer widens the specs. Three fails and you have a dead cylinder. I just did this test myself and found my #4 injector dead to the world.

As for the harness....junkyard?

I can't help on the a/f, but I think you are on the right path. Fix the o2 sensor and trouble shoot the #8 cylinder and I think your problem will be fixed.

Thanks! I'm going to have to try that one tomorrow.

cougar_68 · Aug 28, 2010

After much deliberation, I decided to purchase a universal EFI harness and re-wire the fuel injection. The original harness had so many wires that were cut/spliced, just plain cut, and missing. I decided the best plan would be to remove the old harness (cancer) and rule that out.

I have been working on this install whenever I have had chance for a couple weeks now. The new harness is completely installed with exception of the driver O2 sensor. The bung has a plug in it that I just absolutely could not get out. So I will need to have a new bung welded or see if I can find another way to get it out.

I ran the KOEO codes and got 81, 82, 84, & 85. These are all to do with EGR and Emissions stuff that I don’t have. After running the KOER test I had two codes: 98 & 66.

98 – hard fault is present FMEM
66 – MAF sensor fault; below minimum voltage

I don’t really know what code 98 is for. I will do a search in a minute. Also, I’m not sure why the MAF would be below minimum voltage. Is there a way to check the voltage?

The idle RPM was really low and erratic (was surging). I need to recheck the TPS, as it was removed. Not sure if that can effect idle or not though? Should I try adjusting the idle screw?

I decided to take it for a drive to see how it drove. So far it runs pretty darn good (as long as you have a little pressure on the gas pedal). Will have to try again tomorrow. In the past whenever I thought it was fixed it has run like crap the next day. We will see though.

So, does anyone have any insight on the codes and the low RPM/surging idle.

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