Question: 1987 C4 Idle problem

[short1rock]

I have a 1987 C4 with 60k on it at idle normal 700 fuel range on the dash will read normal and avg will read normal the range will go to zero avg will go to 12.5 and idle will raise to 1100 problem will come and go just idling on jacks
Probably an input signal problem, Bad ground? Anyone have any input?

 

[i10fwy]

I am interested in the response on this too. I have a 87 Callaway and the idle is all over the place too!

i10fwy

 

[short1rock]

I am interested in the response on this too. I have a 87 Callaway and the idle is all over the place too!

i10fwy

sometimes when I'm driving it it gets up to 1400

The only correlation I can find is the fuel distance on the dash going to zero when that happens

Scan tool couldn't find the problem, no check engine light

 

[TedC]

This just start acting up? Do you have the GM Service Manual? I would start by checking TPS voltages using a DVOM. Should be .54V +/- .08V . Those are pretty wild idle swings. Possible vacuum leak?? Defective IAC?? Min air speed not set correctly?? Worn TB shaft?? Bad injectors?? I've found removing and cleaning the throttle body has always helped. If water enters the TPS, idle quality can get messed up as well (ex. washing engine bay). Search this site as well. Plenty of other posts on idle speed issues.

 

[TedC]

sometimes when I'm driving it it gets up to 1400

The only correlation I can find is the fuel distance on the dash going to zero when that happens

Scan tool couldn't find the problem, no check engine light

Does the MPG go wacko? Is this an auto, and does the upshift light come on?

 

[short1rock]

Does the MPG go wacko? Is this an auto, and does the upshift light come on?

tps checks out ok

egr ok

can't find any vacuum leaks

still think its electronic, as it comes and goes just letting it sit and run without touching the car

yes auto, no shift light

 

[TedC]

Scan tool readings?? Did you happen to catch the IAC counts. The ECM pulses the IAC, and these counts indicate how the ECM is trying to adjust idle speed. With a fully warmed up engine, it should read around 18-22. If the counts are 0 or over 25, then the ECM is going to have a hard time maintaining idle speed. A scanner should tell you both desired speed and actual.

 

[boomdriver]

I have a 1987 C4 with 60k on it at idle normal 700 fuel range on the dash will read normal and avg will read normal the range will go to zero avg will go to 12.5 and idle will raise to 1100 problem will come and go just idling on jacks
Probably an input signal problem, Bad ground? Anyone have any input?

Every time.

I've written about the C4 wire harness & grounds/connections so many times that I cant bring myself to go thru the details again..
but,

I can assure you that herein lies the problem.

 

[TedC]

YEP... I missed that one on my list...

 

[short1rock]

Every time.

I've written about the C4 wire harness & grounds/connections so many times that I cant bring myself to go thru the details again..
but,

I can assure you that herein lies the problem.

just getting started with all that, cleaned the one on the left side under the dash, starting to take the dash apart to clean all connections

ground on the pcm?

 

[short1rock]

update

just getting started with all that, cleaned the one on the left side under the dash, starting to take the dash apart to clean all connections

ground on the pcm?

pinpointed the problem at the plug on the ECM contacts dont really look dirty or corroded ,going to clean them first, hopefully not broken or cold solder joints, dealing with that on the climate control but that is a whole nother can of worms, what a mess!


problem with the connection on the large plug on the ecm

Maybe I'll have time tonight to go wire by wire and pinpoint the problem