Extremely Weird Problem

[V8_Kind_Of_Guy]

So Im trying to help a buddy out with his car. Maybe someone has some better ideas than us. The car starts, but idles at 3000 rpm and the check engine light is on. But when you push in the clutch, the idle drops to a normal 700-800 rpm and the check engine light turns off.

Hot or cold, in gear or neautral, the problem is always there. It has gt40p heads, 1.7 rr and gt40 tubular intake manifold. Its actually my old motor as listed in my signature, minus a few items. The motor ran great in my car, and it ran well in his for awhile. It has been sitting for about 4 months. He also got a reman A9L computer from late model resto and gave his old one away. Tried pulling codes but KOEO tests will not start, and cant do KOER test either. Verified that we have the jumper wire in the right place. His old computer only allowed us to pull KOEO test codes and thats it.

We've changed out his MAF for mine, both programmed for 24# injectors, and still nothing. It has a brand new IAC from a few months ago as well. Adjusted TPS to .996 volts, vacuum is good, timing is good. The only other thing Im thinking is stale gas, since its been sitting for so long, about 6 months ago since last fill up. But could this really be a problem? Havent added new gas yet.

Anyone got an idea let me know, thanks.

 

[SpartaPerformance]

Not a fuel issue. If you can't pull any codes with the CEL on then you may have a bad computer.

 

[V8_Kind_Of_Guy]

Should I try swaping it with the computer from my car? Just worried if there's an electrical problem that might fry my computer. Are there any checks you know of
that I can do on the computer?

 

[apollo12]

sounds like a bad computer,I had a problem the Mustang would start and idle at 3,000 rpm and wouldn't idle down.I couldn't pull any codes and I wasn't getting a reading on the tps,I put an extra computer in that I had and it ran great

 

[V8_Kind_Of_Guy]

After doing some more checks, we found the TPS is showing 2.4 volts at idle. It goes smoothly up to WOT showing 4.6 volts. Its non adjustable style, so we swapped the throttle body for mine that is adjustable, but even adjusting it, the lowest reading we get is 2.37 volts.

I thought my buddy said he adjusted it but I misunderstood him

 

[liljoe07]

Posted via Mobile Device

Is all the wiring in this car the stock wiring? The fact that you can't pull codes could be a problem with the SIGRTN cicuit. If the wrong 02 harness is used, the problems like this happen.

What are you using to pull codes?

 

[V8_Kind_Of_Guy]

Tried pulling codes with a test light and the wiring is stock as far as we know

 

[liljoe07]

Set the test light up as normal. This time, run a jumper from the STI terminal to a ground. Then try and run the Key on Engine Off codes. See what happens.

View attachment 233375

 

[V8_Kind_Of_Guy]

Ill have to try that on Saturday cuz I work and go to school, what do you think, bad ground if that works?

 

[liljoe07]

If that works, and codes flash, then its a problem with the signal return circuit. And would need some more testing. The codes will tell if there is a signal return circuit problem. Let me know

 

[V8_Kind_Of_Guy]

I definitely let you know this weekend, thanks for the help

 

[V8_Kind_Of_Guy]

I got around to doing some more testing. I tried gounding out the still connector and it did allow to run the koeo test.
these are the codes
22, 23, 34,67

Posted via Mobile Device

 

[liljoe07]

Posted via Mobile Device

You may need to Ohm out the Signal Return circuit from the connector the codes are ran on, to PIN 46 at the ECM connector. Looks like there is an open somewhere between there. Do this with the ECM unpluged.

Check the black wire at the TPS to a ground, and list the voltage. Should be minimal if not 0. Your only showing 2 codes from sensors that use the Sigrtn circuit, so it looks like the ECM is good as far as that goes.

Try cleaning the 10 pin connectors behind the upper intake. See if that fixes the voltage at the TPS sensor, It may fix the MAP/BP sensor code also.

 

[liljoe07]

Posted via Mobile Device

Also was this an auto car that was converted to a manual setup?

 

[MyHoFoSho]

i had almost the same problem with my car when i first got it. Except it would idle at 700-800 when you didnt have the clutch pushed in but as soon as you would push the clutch in it would jump out to about 3000 rpms. Check to see if the TPS sensor has a ground. My tps was reading the same as his till i just spliced in a good ground for it. Car ran perfect after that that would be my best guess it is worth a shot.

 

[V8_Kind_Of_Guy]

The cars electrical is always in question, bought it as basically a rolling shell. No trans, engine, interior or anything like that. It did have all the wiring harnesses in it. The Guy that sold the car said it was a 5 speed but converted it to an auto cuz he was gonna race it. When my buddy bought it, we changed the pedal assembly as my buddy wanted a 5 speed.

 

[liljoe07]

Posted via Mobile Device

Unplug the O2 harness from the main ECM harness by the Maf sensor, then see how it acts.

 

[V8_Kind_Of_Guy]

Did some more testing this is what we found. Unplunging the pcm and ohming out the signal return wire pin 46, to the tps, egr and map all have about 5 ohms. So it looks like those wires are all touching each other somewhere in the harness. We unplugged the harness behind the motor along the firewall, and ruled that out, no problem there. Need to take apart the rest of the harness Ans keep checking

 

[liljoe07]

Posted via Mobile Device

They do share the SIGRTN circuit(PIN 46)

TPS,EGR,MAP/BP, ECT, and ACT sensors all share PIN 46. That's their ground source.

 

[V8_Kind_Of_Guy]

Sorry, I meant voltage refrence. Pin 26, 48 and 46 all had the same resistance no matter which one you touch on the other end. Meaning if I plug one of the test leads to pin 46 on the pcm with pcm unplugged, and the other lead to pin 26 or 48, the same resistance is there.