idle speed increases with warm engine

[gt2993]

My '87 Turbo has an intermittent condition that I am having a hard time resolving. The car idles fine 75 percent of the time exactly at 950 RPM or so. However, occasionally, after the car is fully warmed up, the idle will surge to approximately 1,100 to 1,400 RPM and hold steady there. It does not hunt up and down, it just holds high idle. Most of the time, if I drive the car some more, the idle will drop back to its normal RPM range after 5 to 10 minutes and will behave the rest of the time without any further problems. I also noticed that when the high idle condition occurs, the RPMs drop very slowly when the engine is reved. For example, when the idle is not acting up and the car is running fine, the RPM's drop from cruising speed to idle very quickly when I depress the clutch. However, when the surging idle condition is present, the drop from cruising RPM to idle is very gradual, sometimes taking 2 to 3 seconds. Otherwise, the car is in excellent running condition. All vacuum lines are new. Any suggestions that may help me find the cause of this problem would be greatly appreciated. Thanks.

[kdjones2000]

ISV is a possible source.

My car does this sometimes as well. I would not be overly worried about it.

[kevingross]

First place to look is for a vacuum leak that increases as the engine warms. You could also have an issue within the air flow sensor.

[gt2993]

The airflow meter was recently replaced, so I am hoping that is not the cause, although it would not be totally unheard of for a ne part to fail. The ISV is one of the only parts that I have not replaced, that will probably be the next on the list. I still there a way to diagnose the ISV?

[abporsche1]

My 87 has done that for 20 years. At first it was a concern, but over time it didn't effect any normal operation so I didn't pursue a fix. Otherwise runs perfect.
Barry

[rick13]

Is your Throttle Position Sensor clicking ok?

[meeblek]

I have this exact same problem. I have tried bypassing the ICV using the diagnostic port (as per Clark's Garage) but this changed nothing.I think my problem is the TPS as the back plate of it has been blown loose from boost pressure and oil has leaked into it. Apparently this means the throttle body seals are shot and need to be replaced and the TPS needs to be cleaned out/repaired/replaced.

Check your TPS! Throttle body reseal kits are fairly cheap from Arnnworx.

[gt2993]

The performance of the car in every other way is fine, it makes full boost, power is where it should be, and has never had any problems with hunting idle. In other words, other than seeing the tach needle race up to higher RPM at idle, it does not cause any drivability issues. However, being an obsessive P-car owner where everything on the car has to be factory perfect, I want to find the cause of the problem and address it. I have a spare brand new TPS, (the one on the car clicks as it should), so I may try that just to eliminate it as a factor. The car also has a new fuel pressure regulator, fuel damper, air flow meter, O2 sensor and cycle switch. I replaced a lot of the parts on the car not because they were bad, but we occasionally show the car and I wanted everything under the hood to be clean.

I guess if the TPS does not solve the issue, I will pull the manifold off (again, the last time I only cleaned the ISV when I replaced the cycle switch and all of the vacuum lines) and replace the ISV with a brand new one. If that does not work, I may need to find a steep cliff.

[meeblek]

There is a procedure on Clark's for bypassing the ICV. See step 3 on his write up of how to adjust the idle speed. Link.

EDIT: Supposedly the only downside to bypassing the valve is that you may have to keep feeding it gas during cold weather startup until the engine warms up. Guess it acts like a choke and enriches fuel when cold.

[abporsche1]

No TPS clicking. Car sleeps indoors in a mild climate unless on long road trips usually in warm climate, so no real cold starts. It always starts immediately with no stutter.

[RRRacing1970]

ICV.. Sticking... Yup... just disconnect the Brake Vac. Boost line at the brake booster an poor in about two shot glasses of Sea Foam with the engine running and problem is fixed!
Ron...

[gt2993]

I wil try cleaning the ICV and see if it helps. I also noticed that lately the car is occasionally difficult to start cold and medium warm. The car starts normally when warm.

[DannoXYZ]

Don't just test TPS for click, electronically test it! This should be first test. Because if DME ECU doesn't see that engine is at idle, it won't control the ISV to manage idle-speed. Your ISV could be perfectly fine, but will be non-functional due to improperly adjusted TPS.

So... do not replace parts randomly just because you think it's part of the problem. Test it first and confirm it's bad. The numbers from testing can be compared to the FSM to confirm if it's bad or not. Then proceed from there. Most of the time, it's a matter of adjustments, not replacement. Because you can install a brand-new part, but if not adjusted properly, it won't be functional. How do you know adjustments are correct? Measure and come up with numbers..

Also the only way an engine can idle at 1400 is that there's enough air going into it to support that speed. Where does this air come from when throttle is closed? That's right, vacuum leaks!

[kdjones2000]

The hard start problem for me was related to the Fuel Pressure Regulator and Fuel Pressure Damper starting to die.

Check for fuel smell in the vacuum lines attached to them, and as Danno says above, getting some measurements on your fuel pressure, and leakdown, are good to do as well before replacing parts.

It could also be your check valve.