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Discussion Starter · #1 ·
Hi all.

How smooth should the idle be on a '91?

I'm just not sure what to expect. I assume it won't ever be mass-market sedan smooth but not sure how rough is normal.

Mine is pretty lumpy, the mirrors vibrate and at times the car rocks noticeably and I can feel the engine vibration in the seat. The tach hunts around a little, maybe 50 RPM or so.

Also, when I turn off the AC the idle plummets down to about 700 RPM but recovers after a second or two (IAC goes to 0 then comes back up).

I've tried all of the usual things...
Adjusted the throttle stop to get the IAC in the high 20s at warm idle w/AC off
Reset the IAC and ECU
Checked for air leaks
Replaced the 2 manifold check valves
O2 Sensor is a few months old, as is the fuel filter, pump, regulator and spark plugs.
Compression is even across all 4 cylinders
No CEL codes before or after the ECU reset

It runs beautifully other then at idle. It sucks that there isn't another local Esprit owner here, it would be nice to compare my car with someone else's.
 

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hmmmmmm, how about the spark plug wires? Split vacuum hose? How much rpm @ idle? ---- must be around 900 rpm!
Going bad Coil ?
Check the EGR valve if functioning well --- it should not be leaking when hot, its one way valve.
 

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Discussion Starter · #3 · (Edited)
hmmmmmm, how about the spark plug wires? Split vacuum hose? How much rpm @ idle? ---- must be around 900 rpm!
Going bad Coil ?
Check the EGR valve if functioning well --- it should not be leaking when hot, its one way valve.
Thanks for the reply!

The 4 cyl cars don't have an EGR, only the V8
All the hoses and check valves are good.

I was thinking about the plug wires too, they are Magnacore wires recently replaced by the PO. Coil packs are a good idea, along with the ignition module, it there a way to check these?

Once the car is warm the ECU generally settles on a desired idle of 987, which seemed a little low to me. The Freescan manual mentioned that it may go into a no-load idle mode below 1000 where it only fires every other cylinder. That would explain the roughness but I don't know how to check if that's what's really happening...or what to do about it if it is.

I'm afraid a PO might have messed with the throttle balance screw, the cap is missing.
 

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Normal warm idle will be around 1000 give or take. When warm you will often note some slight variability and a little unevenness with some vibration.

On cold startup idle tends to be a little higher and smoother.

When resetting the computer make sure you put a load on the engine at idle.
So cold motor, pull power to reset computer, start motor and turn on AC/headlights. Then the ECU learns to handle the extra load, and keeps the rpm a little higher.
Having the idle drop low when turning off the AC, is a characteristic I've noted, but it should come back up.

I believe I've read that the computer varies the idle speed by design under different scenarios..

So a little variable or maybe "lumpy" is normal. Truly rough is not. Going a little low when hot is also normal but should be tolerable if it is as designed.

Randy
 

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Glutton For Punishment
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251 Posts
Pull your injectors and have them cleaned. Small variations are noticeable at idle, but become in detectable when you rev the car. If your car has low miles, it means it has spent far more time sitting than running. It should not shake and rattle the car at idle.
 

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Wingless Wonder
1988 Esprit Turbo
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^^^ Jon, we know this is good advice for the Bosch CIS injectors, which are no longer available....but on a GM MPFI system, why not just replace the injectors?

There was just a Topic here on the resistance of commonly-used GM injectors.
 
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