1988 Idling issues

[Fizz88]

I've recently acquired a black 1988 Ford Thunderbird Turbo Coupe, and it's been having a few idling issues. At first, after a while driving or very rarely right after starting it up, the RPMs would go to 2,000. Now, theres another idling issue. The RPM fluctuates a little then does a decent drop in RPM, this is new. I checked for vaccum leaks, I didn't find any. I've also cleaned the Throttle body and the IAC valve plus the MAP sensor, all to no avail. If anybody has any clues as to why this is happening please let me know, I'm going to research some more after posting this but I still am pretty stumped.

Here's a video with the RPM drop:

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[64ZCODE]

IAC or ERG valve defective or sticking? Either would result of symptoms of an air leak.

 

[Tom Alpers]

I've had little luck cleaning IAC Valves. New ones often fix the issues you're having.

 

[Fizz88]

I've had little luck cleaning IAC Valves. New ones often fix the issues you're having.

Im going to put a new gasket on since its kinda rotted, last resort ill get a new IAC valve.

 

[Fizz88]

IAC or ERG valve defective or sticking? Either would result of symptoms of an air leak.

How would you test a EGR? I already did a simple test to see if the IAC works by unplugging it and seeing if anything changed, car wouldn't start with it unplugged so I dont think its really defective, but I could be wrong.

 

[64ZCODE]

To test a vacuum actuated EGR valve (some are electrically activated but I doubt that's the case with your TBird), start the engine and let it idle, disconnect the vacuum line that connects to the EGR valve, and connect a hand operated vacuum pump to the EGR valve. When you apply vacuum, the engine should run rough or stall.

 

[Fizz88]

To test a vacuum actuated EGR valve (some are electrically activated but I doubt that's the case with your TBird), start the engine and let it idle, disconnect the vacuum line that connects to the EGR valve, and connect a hand operated vacuum pump to the EGR valve. When you apply vacuum, the engine should run rough or stall.

Yeah the EGR on this car is vaccum actuated, I also tested the EGR the way you said how to do it and it the engine did run rough, so I believe the EGR is good. Though, the idling issue has gotten a little worse ever since I replaced the IAC valve gasket. It doesnt idle as high as 2,000 RPM but it does get pretty close. Probably about 1,700 or so, but the flucuating idle is definitely more pronounced now. It goes up to about 1,700 like I said, then it goes back down and the cycle continues, but it's also usually after driving for a bit. What do you think I should do next?

I've also included a video of how it idles below.


View attachment 20251030_145047.mp4

 

[64ZCODE]

Well that's weird. Apparently a known problem for turbo coupes with many car forums and YouTube discussions. If not a vacuum leak then likely culprits are a leaking IAC valve, TPS over-voltage and/or dirty MAF sensor. If it were me, I would review the forums and YouTube info and try recommended diagnostic approaches before throwing parts at it.

 

[Fizz88]

Well that's weird. Apparently a known problem for turbo coupes with many car forums and YouTube discussions. If not a vacuum leak then likely culprits are a leaking IAC valve, TPS over-voltage and/or dirty MAF sensor. If it were me, I would review the forums and YouTube info and try recommended diagnostic approaches before throwing parts at it.

The MAF sensor is under the airbox I think, not 100% for sure though but Ill try cleaning that. Also, how would you would be able to tell if a IAC valve is leaking? If it was I just thought that the car wouldn't start if so, since with the IAC valve unplugged it won't start.

 

[Sregord]

Have you run codes on this?
Sounds to me like the TPS(Throttle Position Sensor) is sending some wonky voltages...should be around 5 volts at idle, adjusted to 700-800 rpm with the IAC unplugged.

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