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high idle speed on cold engine startup


grahamr1
12-04-2011, 06:06 PM
vehicle=1995 chv s10extcab with 115000km with 4.3l engine
i had a wiring problem recently which led me to replace the ECM prior to my mechanic actually identifying the problem being a rusted out wire. wiring is now fixed but when vehicle is now started, idle rpm creeps from 1200 up to 2000 and can't be kicked down. ECM reset has been tried twice but no real effect. coincidentally once vehicle is warm and is restarted after driving, rpm settles around 800 quickly(about normal for me)and runs fine. i have been advised now to replace idle air controller to see if that corrects things on startup. seems like a bit of a coincidence that the idle air controller just began acting up. any suggestions on figuring this one out?thx

j cAT
12-14-2011, 07:59 AM
vehicle=1995 chv s10extcab with 115000km with 4.3l engine
i had a wiring problem recently which led me to replace the ECM prior to my mechanic actually identifying the problem being a rusted out wire. wiring is now fixed but when vehicle is now started, idle rpm creeps from 1200 up to 2000 and can't be kicked down. ECM reset has been tried twice but no real effect. coincidentally once vehicle is warm and is restarted after driving, rpm settles around 800 quickly(about normal for me)and runs fine. i have been advised now to replace idle air controller to see if that corrects things on startup. seems like a bit of a coincidence that the idle air controller just began acting up. any suggestions on figuring this one out?thx

high idle as you describe is a vacuum leak. check all hoses, pcv esp. , any hoses under the TBI is a big problem area. also the hoses under the hood for the ac/heater ..

the brake booster leaking [vac] can also cause this.

the pcm/computer adjusts the fuel mixture so it is always correct. with a leak too lean so the pcm adds more fuel to get the mixture correct the result is a high idle.

using a vac gauge is a sure way to find this. being a 1995 I would say you may have many bad vac lines.

grahamr1
12-14-2011, 07:15 PM
thx j cAT
if i do have a vaccuum leak wouldn't you expect that that the engine would consistently have idle upon startup and whether the engine is started up cold or after running a while the high idle would be consistent?
can you get that much variability with vaccuum leak?
thanks

j cAT
12-15-2011, 08:27 AM
thx j cAT
if i do have a vaccuum leak wouldn't you expect that that the engine would consistently have idle upon startup and whether the engine is started up cold or after running a while the high idle would be consistent?
can you get that much variability with vaccuum leak?
thanks

WITH VAC LEAKS you are sucking in dirty unfiltered air this damages the engine. the pcm adjusts the mixture for the added air this causes the low MPG , and wears out the automatic transmission. Idle should be 600 RPM any more that that there is a leak.

with a engine that has a carb the vacuum leak would cause low idle this is because the engine sucks gas out of the carb.

with fuel injected / tbi vehicles the high idle is the vacuum. once the vacuum leaks are fixed/checked by a vacuum gauge so you now have this corrected then other items like the IAC/passageways need inspection or cleaning with the throttle body cleaner [not carb cleaner]..........


mixture controls of engine vary with the engine temp. on cold start you should have high idle. when the computer cuts in then the idle drops to 600 RPM...the colder the engine the longer the cold start high idle.

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