Most of the time, my car runs fine. Occasionally, when I start it, the fast idle "circuit" doesn't seem to engage. And, until I shut it down for a restart, it won't recover.

Specifically, the car IDLES around 900 RPM when in motion (I have a stick). When the car comes to rest, the idle drops to 700 RPM.
This appears to be controlled by a vacumn diaphram near the brake master cylinder.

When the fast idle fails to engage, it always idles at 700 RPM -- even when the car is in motion. When coming to a rest, it does not drop from there.

Thoughts?

-Gregg

 

I could think of some causes, like an intermitent vaccum leak and maybe gas is not reaching the injectors because lines are clogged or fuel pressure drops, etc.

If the car is at idle and rpms rise a little and then drops, those can be the problems, however, if you are driving around, with the car in motion, you should not see any changes in idle because the engine by itself will produce a huge vaccum.
When the fuel mixture turns out to be lean, you can sometimes feel and see at the tach the rpm increment.

Good luck

 

It sounds to me like the Idle Air Control - IAC - is getting tired...

 

Gregg: This is a normal condition for an 89 as my auto trannie car will do this too! I have the bad habit of placing the trannie in "N" while coasting to a stop The VSS is telling the ECM the car is in motion and the idle is raised. When the VSS is not showing motion, the IAC takes over and controls the idle. I do not think a vac diaphram is involved in this situation.

 

It may be as simple as: (but maybe not)

Check if the IAC pintle has buildup on it. Take it off and see. Clean it with TB cleaner, if so. Be careful not to push or pull on it (damaging it). Replace carefully. You may consider cleaning the TB too, making sure the butterfly valve closes completely.

It may not be the IAC, but a dirty pintle has caused poor idle in the past.

 

Idle isn't controlled by vacuum, but a stepper motor or the Idle Air Control Motor, which allows air to enter the plenum with the throttle plate barely cracked open. The motor is connected to a pintle which rests in a bore that is cast into the throttle body. To let more air in, the pintle is unseated and that increases idle speed. Basic test is to note idle speed and then turn it off and restart it. The idle speed should now be higher, but return to the previously noted speed within 10 or 15 seconds. If so, it's working correctly.

Idle speed is controlled by the ECM based on inputs from the coolant temp sensor. The colder the coolant temp, the higher the idle speed. Based on your symptoms, assuming the IAC checks out, I'd suspect this sensor or the ECM. Since it's intermittent, it would be best to scan it. I had similar symptoms about 2 Years ago and a scan showed that the Coolant Temp Sensor was toggling between 230 and 250 degrees, even after the engine had sat overnight, though from time to to time, it would act normally and show the correct temperature (and idle correctly when it did). Further troubleshooting pinpointed it to the ECM. Absent a scan you could put another Coolant Temp Sensor in it, but you may be throwing money at the wrong part. You might also note your engine coolant temp on dash and then see if the main fan is on. If it is on and the dash indicates less than 226 degrees, that's a good sign the ECM is getting a higher temp from the Coolant Temp Signal and with that you should troubleshoot this sensor, it's circuitry and the ECM.

 

98% of the time, this is what happens. Otherwise, the idle speed doesn't raise as the test would indicate.

My other symptom is exhaust popping during warm-up. Goes away when it hits closed loop. It's not terrible as it only does it between shifts and/or on decel. I believe that the Coolant Temp Sensor was suggested as a possible culprit.

A dealership (Superior Chevy) could not find a problem with scans.

I have to assume that the car is running rich during warm-up -- for it to pop. (For it to run rich, I'd think the coolant temp might be showing low -- if it was that sensor). I also know the air is routed (via air pump) into the manifold during warm-up. When the router switches air to the rear cat, the popping stops.

I don't see how coolant temp reading wrong would eliminate the fast idle when the car is not at rest. This fast idle is designed to keep the car from stalling during sudden braking. Right?

ECM/IAC? Is the IAC, the vacuum module above the battery?

-Gregg

 

The IAC is the small cylinder on the right side of the throttle body with a 4 wire harness. It does act as a dashpot during decel, but it's not something you'll feel - or should feel. Basic test remains as previously outlined. Otherwise, a scan will track the counts which correlates to the position of the Pintle relative to it's bore (the more counts, the more air that's getting through and the higher the idle. Most track 10 to 16 counts with it warmed up; a/c off).

Clamp off the air hoses to the headers and see what happens.
Generally, air to the headers after closed loop is picked up by the O2 as a lean condition and it adds more fuel than it needs so it runs rich. A scan will show that condition and if it corrects while the hoses are clamped off, you troubleshoot the air pump circuit.

I'd still look at the Coolant Temp Sensor signal while the problem is occuring. You start with it cold and the temp the should be about the same as ambient and the signal from the Manifold Air Temp Sensor. As it warms up, it should gradually rise until it reaches the thermostat. It should also track the dash readout within a couple of degrees.

 

The idle speed is controlled by the temperature reading that is given to the ECM by the ECM temp switch. Here are the values that are programmed into the ECM on my 89 Coupe:

Idle RPM vs. Coolant Temperature

Deg F RPM

306 575.00
284 575.00
262 575.00
241 575.00
219 575.00
198 575.00
176 575.00
154 600.00
133 600.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
03 1050.00
-18 1050.00
-40 1050.00

If the temp reading is not correct (originating from the sensor), the ECM will adjust the idle to the temp it is seeing. Please note the temp reading you see on the dash comes from a sensor that provides this reading. The temp that is being seen by the ECM is from ANOTHER temp sensor. Hope this makes sense. I recommend you get a scan tool and see what is really going on with your engine.

 

I disconnected the hoses leading to the air router and plugged them. Same symptoms.

It acts like its running rich BEFORE closed loop. I was told the O2 is not monitored during open loop.

gp

 

I don't "feel" anything. I only hear that fast idle is not extending until the car comes to a complete rest. (This is intermittent and only happens during an occasional "session" of driving. If I shut off and re-start, the problem usually disappears).

I'm 99% sure I saw the vacuum diaphram above the battery acting to provide this dashpot operation. Are we talking about 2 different things?

gp

 

Rephrase:
I don't see how coolant temp reading wrong would eliminate the dashpot slowing of the idle as the car comes to a rest.

Kapish?

 

You're right, during open loop, the O2 isn't monitored, but you also need to know when it goes closed loop and make sure that the air pump has diverted air to the CAT. Of course with the hoses removed and the check valves plugged at the headers, that isn't a concern. Which brings us back to the Coolant Temp Sensor signal. Best to see what it is. If it isn't following the rise in engine temp, you'll end up rich until it goes Closed Loop. Other than that, check compression and perform a vacuum check. Weak or broken valve springs or a leaking head gasket - watch #7, could also effect combustion.

 

Mine does it when the TPS gets wet, open or closed loop. Usually if I drive it alot in this condition I end up with a Code 45 from the idle being up too high causing a persistent rich condition.

I would look at both the TPS and IAC, theres a procedure on my site.

 

Good suggestions on the IAC, TPS, and CTS. Remove the TB, then remove the IAC from the bottom. Clean both with TB cleaner as suggested making sure the tip of the IAC pintle is no further than 1 1/8" from the IAC flange. Install with a new gasket (none sold alone unless you make one from gasket paper or buy a new IAC). New TB gasket and then check the TPS. 89 vette correct? The TPS reading should be .54v and the voltage should increase without any hesitation as the throttle plates are open to WOT and should read between 4v and no more than 5v. Check Vader86's site.

The CTS located in the front of the intake manifold can be ohm'd. Do this cold and hot. (Easiest to do this from the passenger side with the air duct removed) Disconnect the connector (yellow and black wires on my 86) and touch the ohm meter lead on the CTS terminals. The resistance reading will vary with engine temp (temp up resistance down and vise versa). Take the ohm reading and engine temp and verify the normals in the FSM. Do this cold engine and hot engine.

These can be done without the use of a scan tool, but a scan is best.

 

Thanks for the tips.

 

Nobody has really answered this, so I thought I'd pipe in. The vacuum diaphram above the battery is your Cruise Control Servo. If flakey, it could be acting on the throttle. Simply disconnect this cable at the Throttle Body (Don't lose the little clip!) and see where you are. Then if you still have a problem (probably, but you'll have eliminated this as a possibility), follow the excellent troubleshooting advice given earlier.

 

Gregg,
Do not go back to the dealership for a scan, I have a laptop based scantool (EASE).
Call me next week sometime and we'll take her out for some data logging and find out what's going on.
Can't this weekend, I'll be in Nashville.

 

Heres the IAC gasket part number, Call Chris May or any delaership. This is the gasket alone, without having to buy the whole kit.
GM PART # 25530697