1988 Idling issues | Ford Thunderbird club group 1955-2005 T-Bird models

1988 Idling issues

Fizz88

Fizz88

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1988
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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IAC or ERG valve defective or sticking? Either would result of symptoms of an air leak.
 
I've had little luck cleaning IAC Valves. New ones often fix the issues you're having.
 
64ZCODE said:
IAC or ERG valve defective or sticking? Either would result of symptoms of an air leak.
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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.
 
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.
 
64ZCODE said:
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.
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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
 
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.
 
64ZCODE said:
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.
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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.
 
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.
 
Sregord said:
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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How would I run codes, and how would I adjust it with it running with the IAC unplugged? The car won't start or continue running If I unplug it.
 
Fizz88 said:
How would I run codes, and how would I adjust it with it running with the IAC unplugged? The car won't start or continue running If I unplug it.
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1988 Fords use OBD-I (On-Board Diagnostics, First Generation), which stores engine fault codes, and you can read them by blinking the Check Engine Light (CEL) with a paperclip or jumper wire in the diagnostic port (like the one under the dash), or by using a dedicated Ford OBD-I scanner, which displays codes like '12', '18', or '88' for system checks and specific issues without needing a modern OBD-II tool.

Method 1: Paperclip Method (No Scanner)
  1. Locate the Port: Find the diagnostic connector, usually under the driver's side dash (a square, multi-pin port).
  2. Jumper Wires: Use a paperclip or jumper wire to connect the two specified pins (often the top right two, but check your vehicle for the correct ones).
  3. Key On, Engine Off (KOEO): Turn the ignition key to the "On" position (don't start the engine).
  4. Watch the Light: The Check Engine Light will blink in patterns. It usually flashes a "12" (one flash, pause, two flashes) three times to show it's ready, then flashes the actual trouble codes (e.g., three flashes for '3', then a pause, then one flash for '1' equals code 31).
  5. Record Codes: Write down the code numbers as they flash and look them up online or in a repair manual.
 
I'm sorry Tom... if the TurboCoupe has a diagnostic connector under the dash, I'd be real surprised. I've had 4 FoxBody Mustang interiors completely removed.
The connectorScreenshot from 2025-12-12 16-43-08.pngis under the hood on the driver’s side, usually between the brake master cylinder and the battery...on the Mustangs just in front of the shock tower. There are 2 connectors the trapezoid and a single.
 
Good evening,
Fizz88
Sorry, I reread your original post, and see you’re maybe brand new with the 2.3 Lima Turbo engine.

My past 25 year history is with this motor, in 2 - 85.5 SVO Mustangs. And I would love to pass on what I have learned about this great little power plant…. and the Fox Body chassis.
This port injected turbo motor was a massive improvement over the suck thru carburetor before ‘84.
The EEC-IV Powertrain Control Module (PCM)… Engine Control Module(ECM) originally mandated by our federal government makes things much easier to troubleshoot engine ..and later automatic transmission issues.
Tools are important… so get one of these:
www.autozone.com

Innova Ford OBD1 Code Reader

Shop for Innova Ford OBD1 Code Reader with confidence at AutoZone.com. Parts are just part of what we do. Get yours online today and pick up in store.
www.autozone.com www.autozone.com
...I guarantee you will use it more than a few times.
The connector for it is under the hood on the driver’s side, usually between the brake master cylinder and the battery...on the Mustangs just in front of the shock tower. There are 2 connectors the trapezoid and a single. Tom is correct, The ECM will deliver codes without the engine running, because ECM has a memory.

There is a booklet that will walk you thru the Key On Engine Off
...and there is a list of 2 or 3 digit codes that will tell basic needs for the ECM to work correctly.

Now for your current “idling issues”.
Watching your video: are you concerned about the little hiccup at the end? Seeing the volt or amp gauge would help. If there is a load A/C? that turns on, or the EGR maybe is kinda normal.

It would be best to get the trouble codes, as they will tell you if you need to buy a part/sensor. But just to answer your “The car won't start or continue running If I unplug the Idle Air Control(IAC).” The link(https://www.stinger-performance.com/tech.html) in my earlier post walks you thru that. Shannon at Stinger Performance is very knowledgeable.

Also: sorry I had the TPS voltage backwards, should be .94ish at idle.
To explain how the 3 components (IAC, TPS, & Idle Speed/Throttle adjustment screw) work together.

#1. The TPS at .94 to 1.0vdc tells the ECM the throttle plate is nearly closed, that voltage gets higher(up to 5vdc) as you open the throttle and the ECM then knows to increase fuel volume/time open length to the injectors. If that is wonky the ECM is either flooding or leaning the engine of fuel. It is the first tuning step for these to get right. The code reader will give you a 22(voltage too low), 23(voltage too high) or 24(voltage out of range) fault code if its out of adjustment or wornout. If you can get a voltmeter positive probe into the TPS connector to the harness(green or black/green wire) and the other to a ground you can measure the voltage with the key/ignition on...engine does not need to be running.

#2. The IAC is how the ECM controls a properly set idle, based on added or lessening load, as in while you’re releasing the clutch or the air conditioning compressor engages.

Start the engine, with a screw driver twist the throttle adjustment screw(likely clockwise) so the engine increases in rpm a little. Unplug the the IAC. If it dies adjust the throttle adjustment screw more until it runs. It should idle without the IAC at 7-800 rpm. Plug the IAC in, let the ECM figure out the change(20 to 30 sec), it might go faster, (I’ve never had one go slower)but it should settle back to the 8-900rpm. If then the engine idle changes(much higher) then the IAC has an issue. The code reader should have given a 12(circuit failure), 13(position sensor), or 15(stuck open or closed).

There is also the coordination of the Vane Air Meter(VAM)...there is no MAF ... out under the air cleaner, and the O2 sensor determining/regulating the amount of fuel. The TPS is more telling the ECM if you have the throttle wide open(WOT).

Again this is a GREAT little engine when the ECM is happy. Its very rewarding with just a little understanding about how all the sensors work.
There are alot of website resources. Just google “Turbo Ford”. and remember this is a 2.3l Lima/Pinto not an ecoboost.
 
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