I have read all the threads on high idle here and tried all the fixes and still can't get the car to idle below 1500 rpm. I mean I must have printed out a dozen threads and tried all their tests and possible fixes.

History: Car sat for 12+ years with old gas, gas tank was rusted out. That is when I got possession of it.

Work done - I put in a brand-new gas tank, sending unit, fuel pump and filter, all new rubber and metal lines from fuel tank to fuel rail. Fuel rail was taken off and cleaned, new injectors and fuel pressure regulator. All O-rings and gaskets on upper plenum were replaced with new. New TPS and IAC installed. New air filter.

I can cover the IAC port inside the intake area of the throttle body and the car idle will drop down to 800 and sometimes it will die, sometimes not.
I removed the IAC valve, and I can get the idle to drop down.

We have tried the propane hose test, carb cleaner test and Monday we did the cigar smoke test. Can't find any leaks.

Today I did the Pepsi can between the EGR tube and bottom of the upper intake thing and it is still is idling high.

It starts fine and idles rough around 1000 rpm for a few minutes. Then the idle starts fluctuating between 1000 and 1500 rpm, eventually the idle rises above 2000rpm.

I am at a loss on what to try next. I'm hesitant to dump any more money into the repair because it's not my car and the owner has spent a ton of money on all these new parts already.

Could the problem be the ECM and is there a way to test the ECM?

I have a new IAC valve installed. Is there a way to make sure it is getting power? I read in one of the posts that a guy wasn't getting power to his IAC (problem at connector), but he didn't post details about what the fix was.

[This message has been edited by ketstang (edited 07-07-2023).]

Idle Will Stay High if can't drive above 35mph after battery is discon or dead.

See vac leak page in cave.... https://web.archive.org/web...cast.net/~fierocave/

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

quote Originally posted by theogre: Idle Will Stay High if can't drive above 35mph after battery is discon or dead. See vac leak page in cave.... https://web.archive.org/web...cast.net/~fierocave/

I have disconnected the battery for a couple minutes and then drove the car about ten miles with speeds up to 50 mph. When I get back home and put it in neutral the idle speed is 2000-2500rpm.

I will look at the link you posted for more ideas.

Thanks for replying.
Kevin

you can check that the IAC is functioning by setting key to ON with diagnostic mode pins jumpered. the IAC will audibly be etiher extending or retracting(i cant remember which.) shut off, pull out the IAC after ~30 seconds of this. then if the IAC pintle is extended, push it back in. repeat, and if it is extended the second time, you'll know its moving.

quote Originally posted by ketstang: I can cover the IAC port inside the intake area of the throttle body and the car idle will drop down to 800 and sometimes it will die, sometimes not.

Covering the IAC port takes the IAC valve out of the equation... but to rule out if too much air might be getting past the TB butterfly valve (due to a messed-with idle stop screw), simply cover the entire TB opening while the engine is idling. If the engine still runs with all air being blocked from entering the TB, then there's definitely a vacuum leak somewhere. In this situation, there's no real point messing about with the IAC valve until you find where the air's getting in.

[This message has been edited by Patrick (edited 07-07-2023).]

If you remove the IAC and cover the hole, it should stall.
If you remove and cover the IAC hole, and the top of the TB it should definitely stall.

I bet Patrick is on the right path, and you have a vacuum leak.

The most often placed missed for solving high idle on 2.8 is the cold air tube hidden behind and below the throttle body. It's a hard plastic elbow with o-ring seal that connects TB to the intake via a short section of rubber hose. My hose was mushy almost to a goooo feeling and the O-ring on the TB end was not a tight fit. At this stage it will always suck air and keep the idle high.
Another cause would be poor grounds between engine to body, engine to chassis, a weak or corroded negative battery cable, IAC connector dirty, etc.
see pic below,, Borrowed pic, Disregard the arrow. See item 5.



Spoon


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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

[This message has been edited by Spoon (edited 07-13-2023).]

My car was having a very similar issue and it turned out to be the Coolant Temperature Sensor that was causing the high idle, as evidently the ECM wants the idle high till it reads a certain temp. These cars have two temp sensors, one that goes to the gauge, and one that goes to the ECM.

The one that goes to the ECM and causes the Idle issue is up right under the Thermostat housing. it is a little tricky to find and see, but it is very easy to remove. The factory plugs tend to have issues as well, and you can easily buy a plug with the sensor.

The high idle was driving me up a wall and this fixed it. Sounds crazy, but this sensor can cause a high idle.

[This message has been edited by Jeffdylanproctor (edited 07-13-2023).]

quote Originally posted by Spoon: The most often placed missed for solving high idle on 2.8 is the cold air tube hidden behind and below the throttle body. It's a hard plastic elbow with o-ring seal that connects TB to the intake via a short section of rubber hose.

I'd say it's number two after a cracked EGR tube, but yeah, it's hidden away and easy to miss.

quote Originally posted by Jeffdylanproctor: My car was having a very similar issue and it turned out to be the Coolant Temperature Sensor that was causing the high idle...

You bring up a very good point, although a scanner or WinALDL would point out that issue pretty quick.

However, also keep in mind that a bad CTS would not enable an engine to idle at a high RPM with all air blocked from entering the TB (unless there was also a vacuum leak).

[This message has been edited by Patrick (edited 07-14-2023).]

quote Originally posted by A_Lonely_Potato: you can check that the IAC is functioning by setting key to ON with diagnostic mode pins jumpered. the IAC will audibly be etiher extending or retracting(i cant remember which.) shut off, pull out the IAC after ~30 seconds of this. then if the IAC pintle is extended, push it back in. repeat, and if it is extended the second time, you'll know its moving.

I followed your instructions and the pintle did move. So my IAC is good and working then?

quote Originally posted by ketstang: I did check the EGR flex tube yesterday when I took off the upper intake and it looked good.

Were you able to check the entire length of the tube? Seems to me the insulating jacket does a good job of hiding most of it.

quote Originally posted by ketstang: I removed the IAC and covered the hole and it didn't stall. I then covered the TB intake and it did stall.

I don't quite understand the point of removing the IAC valve.

With the IAC valve installed, and the IAC port in the TB covered, the engine should stall.

quote Originally posted by ketstang: I can cover the IAC port inside the intake area of the throttle body and the car idle will drop down to 800 and sometimes it will die, sometimes not.

It's possible that the idle stop screw has been messed with and the butterfly valve is open slightly too far. Has the plug that normally covers the head of the screw been removed?

[This message has been edited by Patrick (edited 07-26-2023).]

Just throwing this out there but are you sure the tach is correct ? I have an issue that the tach says it idling at 2-3k when it’s not. I did install Rodney’s tach filter but still have the same prob…..I’ll have to dive into the tach itself.

Any chance the throttle cables stuck? One time messing w my intake, I had a high idle I couldnt figure out till I looked at the fuel rail. One injector didnt seat 100% and there was my yuge vac leak.

shem

quote Originally posted by PhatMax: Just throwing this out there but are you sure the tach is correct ? I have an issue that the tach says it idling at 2-3k when it’s not. I did install Rodney’s tach filter but still have the same prob…..I’ll have to dive into the tach itself.

mine is horribly erratic. at idle it shows about 1400, but idling in 1st itll show almost 3k. 6k is more like 4.5k, and redline is kinda where the "X1000" is

i checked mine's accuracy by driving in 5th at 55MPH, and doing the math for what engine RPM i should be, given the tire diameter, gear ratio, and speed. i used this calculator

quote I followed your instructions and the pintle did move. So my IAC is good and working then?

the IAC itself is working then. covering the port in the TB intake will eliminate the IAC from the equation if it doesn't stall like patrick and others have said.

Yes Tach goes wrong, speedo too. Short reason...
The Tach and Speedo have a "chip" to run the display.
Is "programed" by resistors and capacitors for numbers printed on the back to match 0-max of numbers and ark they are on.
Example: 85 miles covering nearly 300° arc on a speedo

Caps &/or resistors can "die" for several reasons and screw up the display... "chip" is still working "right" but has wrong programing now.
Many "fix" the Resistor(s) but often bad cap and gets worse because fixing/adjusting wrong part.

So don't trust either when have problems.
Use a "Shop Tach" of many types
Anything w/ GPS can check Speedo but GPS "chips" calc Speed locally and most chips lag actual speed. So only works well on roads that allow you to drive @ X speed for many minutes.
or use ECM scanner to check Tach and Speedo accuracy.

Could this Holley be the OEM throttle body? Still trying to track down my high RPM. The cap is still over the throttle adjustment screw, so it looks like it hasn't been tampered with.

Anyone see anything that would be of concern in these pictures?

quote Originally posted by ketstang: Is there anything special about this vacuum connection block? It's pretty dried up. Can I remove it and just put hoses between to connect the two lines?

I don't think it's "anything special", but it performs double duty by connecting hoses and keeping the two lines tethered together.

quote Originally posted by ketstang: While I am this far into the hoses and have the upper intake removed, is there a problem with removing the two hard lines going from the radiator filler neck to the throttle body? The car is in the midwest and not driven in the winter.

It's a common mod. I've done it to a couple of my Fieros with no issues.

I would invest in buying a low cost smoke machine that runs on compressed air and baby oil. If you already have a small air compressor you can buy the economy model for about $30.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

I found out and had the same experience as Jeff. I had a bad CTS. Remember, these cars are getting old, and we’re old heck, 15 years ago. This happened to me in 2009, so yes, GM at the time had quality components or sensors on GM products, but will not last forever. Plus, it’s common an engine is rebuilt , and the owner just swaps the sensors to the rebuilt engine. I would recommend replacing all sensors for peace of mind. The only sensor is Fiero owners really can’t get anymore is the cold start sensor that is originally made by Bosch. Another tuffy is the EGR vacuum solenoid.
All other sensors are AC Delco parts available today.
So, that’s my experience I have to share if that helps anyone.

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fierogt28

88 GT, Loaded, 5-speed.
88 GT, 5-speed. Beechwood interior, All original.

I am going to replace the coolant temp switch since it is inexpensive, and I am running out of options. Knowing there are two coolant temp sensors, when searching how do I tell which part number is the one that interacts with the ECM. Any help would be appreciated.

quote Originally posted by fierogt28: I found out and had the same experience as Jeff. I had a bad CTS. Remember, these cars are getting old, and we’re old heck, 15 years ago. This happened to me in 2009, so yes, GM at the time had quality components or sensors on GM products, but will not last forever. Plus, it’s common an engine is rebuilt , and the owner just swaps the sensors to the rebuilt engine. I would recommend replacing all sensors for peace of mind. The only sensor is Fiero owners really can’t get anymore is the cold start sensor that is originally made by Bosch. Another tuffy is the EGR vacuum solenoid. All other sensors are AC Delco parts available today. So, that’s my experience I have to share if that helps anyone.

quote Originally posted by ketstang: I am going to replace the coolant temp switch since it is inexpensive...

It's not a "switch".

The coolant temperature sender is for the temperature gauge.

The coolant temperature sensor is for the ECU.

The manifold air temperature sensor is also for the ECU.

The ECU needs correct data from both temperature sensors to help set the correct idle speed.

A scanner or a laptop with WinALDL (and a cable) will show you what information is being fed to the ECU by all sensors... or you can blindly throw components at the problem!

[This message has been edited by Patrick (edited 08-17-2023).]

I'm not sure if this would apply to the 2.8, but my '88 2.5 had this exact same problem when I first bought it; it would just randomly idle super high and stay there. In my case I had also already replaced the TPS, but with the lowest quality bargain basement option. After a ton of investigation, I discovered that the sensor would act erratically and sometimes read high, making the ECM think the throttle was open when it actually wasn't. Then, the ECM would command the IAC overly open and cause a high idle. Once I bought a higher quality TPS, my high idle completely went away.

quote Originally posted by Patrick It's not a "switch". The coolant temperature sender is for the temperature gauge.

Both of you are 'right'.....

Sensor/Sender, Coolant Temperature (GM 25036809)

Two Circuits in One:
Variable Resistor for the gauge
Switch for the coolant temperature warning light (bulb)

Edit: This unit is for the temperature gauge and warning light ONLY

IF you're SURE that you don't have any vacuum leaks, and still have 'High Idle Problems'

Replace the engine coolant temperature sensor for the ECM (GM 25036979 for V6)

quote Originally posted by Jeffdylanproctor My car was having a very similar issue and it turned out to be the Coolant Temperature Sensor that was causing the high idle, as evidently the ECM wants the idle high till it reads a certain temp. These cars have two temp sensors, one that goes to the gauge, and one that goes to the ECM. The one that goes to the ECM and causes the Idle issue is up right under the Thermostat housing. it is a little tricky to find and see, but it is very easy to remove. The factory plugs tend to have issues as well, and you can easily buy a plug with the sensor. The high idle was driving me up a wall and this fixed it. Sounds crazy, but this sensor can cause a high idle.

[This message has been edited by Vintage-Nut (edited 08-19-2023).]

quote Originally posted by Vintage-Nut: Both of you are 'right'...

Not really. The OP expressed an interest in replacing the coolant temperature sensor for the ECU, not the sender for the temperature gauge. There is no "switch" whatsoever in the coolant temperature sensor for the ECU.

I still strongly suggest that the OP find out what's going on with his engine by getting a reading of all the sensors first before randomly replacing them.

[This message has been edited by Patrick (edited 08-18-2023).]

quote Originally posted by Patrick: . . . before randomly replacing them.

Where's the fun in that? Shotgun approach is the best way to keep one busy.😁

As Fierogt28 said "I would recommend replacing all sensors for peace of mind"

I did too!

Well, wanted to get back to everyone on my high idle problem. After several suggestions to replace the coolant temperature sensor for the ECU, I did and stated that it didn't seem to work. I have now driven it 100 miles and it is now idling at about 1500rpm at startup but slowly idles down to 800rpm. I am very satisfied with this and want to thank all of the people who have helped me along the way!

I have a newly found problem, but that's for a new thread. It's unbelievable (to me anyway).