My 1994 GT - Troubleshooting?

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RufusT

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I've replaced the IAC recently. In fact, prior to replacing it, the car would not maintain a consistent idle. After replacing it, it would like around 700rpm, where it used to - but was rough and obviously had a miss. However, the computer would not enter diagnostic mode. I changed the MAF as part of the troubleshooting for the rough idle, as I had not identified the reason it would not go into diagnostics. Troubleshooting resulted in identification of an internal ground failure on the ECU. After replacing the ECU, I got the high idle. That allowed me to pull codes that indicated EGR issues and inability to adjust idle. I thought they may be related, so i replaced the EGR solenoid motor and all the related vacuum lines. No change to behavior. Sprayed some brake cleaner around to confirm no vacuum leaks. Install of old MAF reduced idle, but was inconsistent...

I have a mechanic friend who says he can connect his reader to my obd 1 and see what the sensors are sending, in order to determine what may be out of spec. My hopes are to see him Sunday of this week... I may still replace the EGR Valve, EGR Sending unit and TPS, as they are all nearly 30 years old... we will see.
 
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RufusT

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I've done a few of the other recommended checks. I reinstalled the new MAF as I am convinced the old one to be fault. the previous error codes returned. Next, I removed the EGR valve and taped over the holes, then plugged the hose that was connected to it. Starting the engine resulted in the same high idle condition. Next, I tested TPS voltage to see where it is set. The TPS shows 1.056 volts when the key is on. moving the throttle results in a smooth increase and decrease in voltage - up to 2 volts... then the multimeter shows open circuit. Should the TPS not go up to 5 volts?

I'll be reinstalling the EGR tomorrow. A friend of mine who works as a mechanic is going to come over and will be bringing his computer. He says this will let us see what the sensors are doing and help us to figure out what is going on. Hopefully, this will help us narrow it all down.

That said, I plan to also remove the IAC and confirm its functionality, just to rule that out.
 

ttocs

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there should be no open areas when moving the throttle all the way from idle to WOT. I can't remember what the voltage is supposed to go up to as that was in the video I used to figure it out.
 
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RufusT

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So my friend showed up with his computer - but without the adapter to connect to the OBDI port. We did a range of tests, most were verification steps to what I had done. Ttocs was right - major vacuum leak. Located it at the IAC... The new (and cheap) one had a major issue on the housing that was sucking lots of air in -- but could not really be heard over the noise generated from the high rpm. He listened with stethoscope and was able to find it that way. We cleaned the old one and tested resisteance on the posts. Then, we plugged it into the harness and turned the key on. He said the valve moved - which is an indicator that it was working appropriately... so we reinstalled it.

She was running a little better, but still not like she should. Idle was around 1000, but was pretty rough and wouldn't smooth out. So, we pulled the throttle body and cleaned it all up real good... While the throttle body was off, he noticed a spark plug wire had rested on something that gets hot and the rubber was missing in a spot - so we replaced all of the wires with some that I have from a donor 302 I picked up for a GT40 rebuild I plan to do... Then we reinstalled the throttle body. Still running rough... BUT... When we disconnected the MAF, the idle dropped to 800 and she was idling like I would have expected for a few minutes... then started running rough - I assume because the MAF was missing... So - we reinstalled the original MAF sensor...

She idled at 800. Still a little rough. So, I let it cool a little and pulled a spark plug... Of course, my new plugs are fouled with carbon... which may be the cause of the rough idle. Throttle response seems to be pretty well on point... I will likely replace the plugs again - as they aren't that expensive.

So now she is idling where she should, but still seems to be running rich. I still get codes:

KOEO: 558 (EGR Solenoid Circuit Fault)
KOER: 332 (Insufficient EGR Flow), 632 (Transmission Overdrive Control Switch)

I know I didn't push the O/D button, so not worried about that one. But - can anyone help me figure out the 558 and 332? I believe this causes her to run rich, which likely started most of my issues..

I've replaced the EGR Solenoid Motor, as well as the EGR Valve Position Sensor. I've tested the EGR Valve to confirm it holds vacuum. I've confirmed voltage to all of three EGR components under the passenger fender well, as well as the Valve Position Sensor... How can there be a fault in the circuit, if they are all getting voltage?
 

95opal

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Did you clear codes after vac leak repair
 
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RufusT

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The replacement MAF was a Duralast stock replacement that I got at Autozone - hopefully, I can return it..

We did clear the codes, by disconnecting the battery. The codes were pulled afterward.

Any ideas about what would make the EGR Solenoid Circuit Failure in a KOEO situation? Even though the valve tested as good and holds vacuum - could there be an issue there??
 

95opal

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The replacement MAF was a Duralast stock replacement that I got at Autozone - hopefully, I can return it..

We did clear the codes, by disconnecting the battery. The codes were pulled afterward.

Any ideas about what would make the EGR Solenoid Circuit Failure in a KOEO situation? Even though the valve tested as good and holds vacuum - could there be an issue there??
Have you verified the correct resistance on the egr circuit? Could be a bteak in the wiring. Have you verified that the valve works while the car is running? Ie. Its getting vacuum
 
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RufusT

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I will verify vacuum to the valve next time I get out there. I replaced all of the vacuum lines, but haven't confirmed that there is vacuum to the valve. The 558 code is an KOEO code - so there would not be vacuum at that time, though I can see how if there is no vacuum that could cause the insufficient EGR flow.

As for why the ECU was replaced, there was an internal ground fault on the stock/original ECU. The car was misfiring and running rich, but would not go into diagnostic mode. I did a ton of troubleshooting and eventually found a write up that detailed how to test resistance between some pins - there was open circuits. I opened the original and could see where some of the electrical components had gone bad (referenced a YouTube video that showed that).
 

dcm0123

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An EGR valve malfunction would not cause a rich running engine.
Pull the vacuum line off the fuel pressure control valve to make sure it is not full of fuel. This would be signs of a ruptured diaphragm.

I had problems with the connector on the EGR valves in the past. If you twist the flat blades on the sensor slightly, it may help if the connector on your wiring harness is starting to go. I would trace the wires if this does not help.

Glad to see you are making progress.

Found this online about clearing codes-
TThe correct way to clear just the codes is to start the test and when the codes start to display remove the jumper wire from the Diagnostic Connector. Disconnecting the battery erases the KAM which will cause the engine to run rough for a period of time while the PCM relearns the information.
 

95opal

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I will verify vacuum to the valve next time I get out there. I replaced all of the vacuum lines, but haven't confirmed that there is vacuum to the valve. The 558 code is an KOEO code - so there would not be vacuum at that time, though I can see how if there is no vacuum that could cause the insufficient EGR flow.

As for why the ECU was replaced, there was an internal ground fault on the stock/original ECU. The car was misfiring and running rich, but would not go into diagnostic mode. I did a ton of troubleshooting and eventually found a write up that detailed how to test resistance between some pins - there was open circuits. I opened the original and could see where some of the electrical components had gone bad (referenced a YouTube video that showed that).
Dont matter KOEO if a code was tripped its tripped. I just wanted to make sure you cleared the codes after the vac leak repair. Once you verify vacuum at the egr i would start inspecting the electrical circuit.
 

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