Unusual idle
Thread Starter
Joined: Nov 1999
Posts: 1,790
Likes: 22
From: Monroe,NC
Car: 90 Formula
Engine: 305
Transmission: 700R4
Axle/Gears: 9 bolt/3.27
Unusual idle
First off,my car is a 1990 TBI Firebird Formula with 700 R4. I recently had to replace the ignition module. While I was at it I went ahead and did a tune up with fresh plugs,wires,cap and rotor,pcv,breather filter and cleaning the K&N. I also changed the oil(Mobil1 synthetic) and filter too. Shortly after this I noticed a somewhat lower idle than before. I also noticed that the car felt like it was missing while idling in gear when stopped at a light but felt fine when driving. I went back and checked the IAC and reset the timing and things seem a lot better now but the idle is still low. According to my scan tool the engine is turning 600 rpm when in gear at idle but the tach is showing 400 rpm. Which should I believe and what could be causing this?
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Joined: Jan 2000
Posts: 19,659
Likes: 310
Mongoose,
Don't place a large wager on your in-dash tachometer. These instruments are not that accurate, and are never to be used for tuning and diagnosis. They're good as a relative indication only.
Since your scan tool is getting digitized information from the ECM, the ECM believes the idle speed is 600 RPM. That's probably closer to accurate, since it is counting the distributor reference pulses for the speed reference. 600-650 RPM is the programmed target idle speed in gear for a fully warmed engine, so ths system is functioning correctly.
If you suspect a problem with this, go back to the HEI module and check all the connections and grounding. If the reference pulses are not quite correct, the ECM could be over counting. Since the reference pulses are derived from the HEI module, any problems with it can cause an incorrect idle RPM.
Did you use a factory replacement module or aftermarket? Did you apply silicone grease to the underside for good heat transfer? Were the ground (mounting) screws clean and tight, or corroded? Did you test the buffer capacitor in the distributor base for damage? Are all the electrcal connectors clean and tight?
Don't place a large wager on your in-dash tachometer. These instruments are not that accurate, and are never to be used for tuning and diagnosis. They're good as a relative indication only.
Since your scan tool is getting digitized information from the ECM, the ECM believes the idle speed is 600 RPM. That's probably closer to accurate, since it is counting the distributor reference pulses for the speed reference. 600-650 RPM is the programmed target idle speed in gear for a fully warmed engine, so ths system is functioning correctly.
If you suspect a problem with this, go back to the HEI module and check all the connections and grounding. If the reference pulses are not quite correct, the ECM could be over counting. Since the reference pulses are derived from the HEI module, any problems with it can cause an incorrect idle RPM.
Did you use a factory replacement module or aftermarket? Did you apply silicone grease to the underside for good heat transfer? Were the ground (mounting) screws clean and tight, or corroded? Did you test the buffer capacitor in the distributor base for damage? Are all the electrcal connectors clean and tight?
Thread Starter
Joined: Nov 1999
Posts: 1,790
Likes: 22
From: Monroe,NC
Car: 90 Formula
Engine: 305
Transmission: 700R4
Axle/Gears: 9 bolt/3.27
Well,it was an aftermarket piece. I did use the grease and all the connections were good and tight. The screws were corroded but I cleaned them and I am unaware of any buffer and how to test it. How do you test the buffer and where is it?
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