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LT-1 Engine idle



I have a 92 convertable that has an eratic idle. Sometimes it is hard to start and idles at 500 to 600 rpm. Once it warms up the idle smooths out at about 900 rpm. Then sometimes you start it and it goes into a high idle and will not knock down. When it is warmed up she races at 1800 rpm. Hard to hold at a light unless I drop the auto trans into N. The local dealer wants to flush out the plenum because he says the idle motor is sticking. There are 50,000 miles on the clock. Any ideas or sage wisdom out there.


Active member
Aug 11, 2002
Washington, PA USA
2004 Black Coupe
Engine Idle

Had a similiar problem with my 89 recently. After testing and changing numerous relays, MAF sensor, IACs, resetting idle to specs we swapped out the EMC and that was the problem. Car runs great now. I had no codes appear ever. I purchased a OBDI cable and downloaded some dignostic software. When you plug in the cable to the port, the idle will rise to 1000 rpm. Mine did not. I found a ECM at the "bone yard" for $10 and it has work great for about three month now. Hope that helps!


Motor head!!!!
Feb 13, 2001
Valencia, CA,USA
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Clean the IAC first

This assumes you air filter is new or cool
and your snorkle has no rips... cracks.

Get the following stuff in advance
Throttle plate gasket set
Carb cleaner
Tooth brush
Q tips ( Lots of em )
Hose to bypass throttle body water passage.( read further )

Ok beer in hand..sleeves rolled up....

1) Disconnect the SNORKLE going to the throttle body

2) Disconnect coolant hoses ( 2 ) going to throttle body ( Put lots of rags down around bottom of throttle body)

3) Remove throttle linkage ( careful don't lose those clips)

4) Remove vac fittings...( Draw a pix of what goes where)

5) Remove electrical connections
a) IAC motor ( 4 pin connector )
b) TPS ( throttle position sensor )

6) Remove 4 10mm bolts holding throttle body to intake plenum

The throttle body should now fall off.

Put the throttle body in vice... use wood inbetween the vice jaws, so you don't dammage the throttle body casting.

Get a BIG crescent or a 32mm wrench and remove the IAC motor.. be careful and NOT dammage the paper gasket.

Now grab holding the CONNECTOR END UP
Now take some carb cleaner and carefully spray the TIP ( pintle ) of the IAC and clean the bad boy up... be careful NOT to let cleaner run down the shaft INTO the IAC body.

After you get that clean...put it off to the side. In particular the seating area of the IAC!!!!!

Not tackle the throttle body itself... get the OLD back gasket off... clean all the crud out of the passages... and CHECK how worn the butterfly shaft is.... wiggle the shaft around... the more play... the worse of an air leak you will have.

Dont get Carb cleaner on the TPS ( Throttle position sensor)

Take some compressed air and BLOW everything out.

Put her back together....

Now if you have time...... bypass the coolant lines going to the throttle body.. get a new piece of hose... and just re route.

Ok everything back together..... start the car... and before 10 seconds shut her off... then restart.. this will allow the ECM to relearn the idle routine.

Everything OK so far... now go for a test drive... drive the car OVER 40 mph for a few minutes...( this will force the IAC shut )

Let her idle... smooth??!!

Good work.....


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