My 1994 GT - Troubleshooting?

RufusT

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I just replaced the ECU in my 1994 Mustang GT. I'll save the details for another time. When I started it up, after installing the new ECU and the machine would not idle down. It runs between 2000-2500 rpm, never dropping below.

After it was up to operating temp, I pulled codes, which produced 412, 411 - both codes basically say that the ECU can't adjust the idle.

So I unplugged the Mass Airflow Sensor and the rpm dropped to around 800, but the motor was not running smooth. When I plugged it back it, the rpm went back up.

So I did some research to see if I could test the MAF. It sends a near constant 2.8 volts to the ECU.

Thoughts? Should I just replace the MAF and see if that changes the situation?
 
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RufusT

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Replaced the MAF sensor and air filter today. When I first started it, she idled in the 700-800 range. I pressed the throttle a few times and it hesitated a little before responding to the pedal push. I go out and looked at the engine bay and noticed that I had some white smoke coming from behind the intake. I don't know what possessed me, but I removed the IAC sensor to see what would happen and the rpm jumped back up to the 2200-2500 range... again. When I reconnected the sensor, no change. Turned off the machine, counted to 30, started back up - high rpm again...

Ran codes again. KOEO showed a code for EGR circuit failure. KOER threw codes for Insufficient EGR Flow, as well as the 411 and 412 which indicate the computer cannot control the idle.

Will the EGR circuit failure (I assume a failed solenoid) cause the valve to stay open, resulting in unmetered air running into the system - like a vaccum leak would cause - thereby making the rpm at idle high?
 

dcm0123

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Check behind the intake to see if the PCV hose is disconnected, broke or if the valve has come out.
Could be related to EGR as you suggested.
I would unplug the IAC valve power and let us know if it changes the idle. This is supposed to open and close at the computers command to change the idle.
 
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RufusT

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I'll check the PCV valve sometime tomorrow when I get a chance to get out there. I replaced the IAC as a step in my troubleshooting of why it wouldn't maintain idle.. I still have the old IAC - may clean it up and reinstall it to see if that makes a difference... parts can come defective out of the box.

I check the constant power coming to the IAC, with the key on, but engine off - had nearly 12 volts running to it. I'll check to see if disconnecting it makes a difference, but I think I did that earlier today with no change to the engine behavior.. Best to confirm that tomorrow, though.
 

dcm0123

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I have had problems with aftermarkets IAC's from Autozone. Rock Auto sells Motorcraft.
They can be cleaned with carb cleaner as well.
Attached is an article which states how they operate. It is a 2 wire system. 12V positive should be constant when key is on. The computer pulses the other terminal (negative) to open the valve.

My recollection is as soon as the key is turned on prior to cranking, the valve should open fully. You can check this with the valve removed. Turn key off and it should close.

The valve should not be open if power is not applied. If it is open, it is sticking or defective.

If it is idling high with the power removed, remove the valve and put a piece of duct tape on the intakw manifold to block the holes. If the idle drops, the problem is the valve.

 
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dcm0123

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Been a long time since I worked on the EGR valves.

First pull the hose off the valve and pull a vacuum on the EGR valve to see if it opens. If it opens (you should see the shaft move on the valve while on the car), hold the vacuum and see if it stays open. If it does not, the diaghram in the valve is ripped.

If it opens then check the sensor on the valve. Recollection is resistance between two contacts should go "smoothly" low to high when the valve moves in one direction. When you test using the third contact, it should go from low to high when the valve moves in the opposite direction. If it does not change or the resistance opens and closes intermittently as the valve is moving, you have a bad sensor.
You will have to figure out which is the common pin (signal) on the sensor which will be mated with the other 2 pins to check the resistance with the valve moving in both directions. See link for pin identification.
Ford has been using the same sensor design since early 80's and the problem is typically the sensor.


To check the solenoid valve, you will need to put a tee in the line going to the EGR and attach a vacuum gauge. It may only open if the engine is under load. It will not open until the engine is fully warmed up.

1641009530240.png
 
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RufusT

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Went out to do some playing around... While investigating which vacuum lines run to what parts, I broke a small, green line that connects to the EGR valve - that appears to route under the intake manifold...

Looks like I am going to be removing the upper intake, so I can be sure to reconnect this line. While I am at it, I am probably likely going to replace all of the lines that I have access to.. may as well..

The upside is this will make it easier to get the EGR and PCV replaced.
 

dcm0123

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My recollection on out 94 5.0 is the EGR line feeds to the solenoid valves which are under the passenger side fender need the shock tower.
The line you broke maybe the vacuum source feeding these valves.
 
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RufusT

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I removed the upper intake, fenders and bumper (i have some replacements to mount anyway). This gave me easy access to all of the vacuum lines - which I replaced. I also replaced the solenoid motor, as my original error code indicated a solenoid failure during KOEO diagnostics.

Now I am getting the following:

KOEO 558 (EGR Circuit Fault)
KOER 411, 412 (unable to adjust idle up/down)
KOER 332 (Insufficient EGR Flow)
KOER 173 HEGO (HO2S) Sensor Fault/rich
KOER 137 HEGO (HO2S) Sensor Fault, always rich

I feel confident that I have replaced any hoses that may have had a leak. I suspect all of these codes to be related to the same problem - but have not figured out exactly what it is. I'm leaning toward it being electrical - as the Key On Engine Off code is a circuit failure...

I am inches from replacing the EGR Valve and the EGR Valve Position Sensor - just to eliminate them from possibly being an issue.

Is there any diagnosis I should do before this replacement?
 

dcm0123

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Smoke check as mentioned above- problem could be masked by a massive vacuum leak.

Disconnect all vacuum lines leading from the engine which you did not replace and plug the ends to make sure something such as the brake booster, auto speed control diaphragm under the drivers fender or your heater controls in the car are not leading vacuum.

Pull off the EGR valve and apply duct tape over the 2 ports on the intake manifold the start the engine. The valves sometimes get hung up and leak vacuum.

Has anyone done work to the intake manifold or fuel system on the engine? Are there aftermarket parts such as the intake or fuel distribution manifolds?

Look for wire shorts on the EGO sensors. I had this happen before on a GM vehicle resulting in a severe rich running engine.
 
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dcm0123

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Found this in another post. May help you chase vac lines down.
1642305532483.png
 
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RufusT

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Thanks for the responses! I hate to admit it, but I don't know how to do a smoke test -- I've never done one. I've sprayed water around the lines to see if it would cause the motor to bog down, but no change in engine behavior... a mechanic friend said to try that with brake cleaner or carb cleaner, but I don't see how that would be any different.

I'm leaning toward the problem being in my "new" rebuilt ECM that I got through Autozone. The previous ECM was causing an engine miss, but was not idling high. Immediately after installing the new motor, it ran appropriately and sounded great... then, after reaching operating temp, idle raised. Now, idle is high immediately after starting... So my planned steps are to check resisitance on the three electrical parts of the EGR system. If they all have apppropriate resistance (15-45 ohms?), then I will pull the ecm out again and test pin 33.

I will likely add in the steps you guys have referenced above before pulling the ECM, as I usually assume the ECM to be the least likely suspect...
 

dcm0123

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You may have multiple issues.

I believe the ho2s is on the passenger side. Unplug the connector and try re-seating it. Check for damaged wires.

An engine which idled properly but now over revs likely has a vacuum leak. Verify the throttle is physicaly closing all the way. The lever should bottom out on the screw on the throttle body. If it is, check for leaks.

Reason I asked about aftermarket parts is I have seen upper intake manifold interfere with fuel rails and other parts resulting in a leak between the upper and lower manifolds. Fact it ran good to start with leads me to believe this is not an issue.

I would first suggest you run the experiment removing the EGR valve and taping ports to verify the valve is not stuck open causing your leak. If this does not help. I would disconnect all vacuum hoses from the engine and plug the engine side hoses then try starting it to see if it idles better since you do not have equipment for a smoke test.

I have used carb cleaner to test for leaks on the engine itself but you need to have a fire extinguisher ready.

When it idles fast, what RPM is it at?
 
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ttocs

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smoke check = get a cigar, light it up, disconnect the vac line to the brake booster and blow the smoke into the hose. Do this a few times and then look for smoke to appear where there is a leak
 
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RufusT

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Idle is usually around 2200rpm. I'll have to find a cigar smoking friend to come by for a smoke test to happen.

I'll see when I can get out there and do a little more work on her..
 
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RufusT

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So after work, I went out to the shop to see what I could figure out... checked resistance on the three components on passenger side fender area... 15 ohms, 41 ohms, 45 ohms.. my understanding is 15-45ohms is where we should be so I feel like those electrical parts are ok.

I started it up and while it was running sprayed some brake cleaner on the intake tube - no behavior change. Sprayed all over the hoses in the engine bay - no behavior change. Sprayed around the intake - no behavior change.

Then I realized that I had also changed the MAF sensor before realizing my misfire was due to ECU failure... So I reinstalled the old MAF... She runs really rough, but not at 2000 rpm. The motor was surging between 500-2000rpm. up, down, up, down...

KOEO: 558
KOER: 998, 157

So the EGR circuit failure is still showing up, but the insufficient EGR flow is not. 157 is a MAF sensor voltage too low and the 998 is a hard failure. Disconnection of the MAF causes the car to die...

Next, I started it and while it was surging, I disconnected the TPS. The surging stopped and she was idling around 1000rpm - though still a little rough.

So - I'm thinking there may be a few things at play: Dirty throttle body? defective TPS? TPS out of adjustment???

Thoughts?
 

Makoto

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so your car should idle (with all accessories off) around 6-700rpm without the IAC plugged in. if it can't do that you've got some vacuum leak or some other issue somewhere else. If you get that straight and plugging the IAC in makes things get screwy that could be your problem.
 

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