What Controls Engine Idle Speed

sabre1fv

New Member
Just curious about this since I'm having an idle problem and the typical fixes aren't working....

Idle speed isn't controlled by only the throttle position sensor as the idle speed is also affected by the EACV, but that I believe is controlled through the ECU, BUT the ECU must be getting data from 1 or more sensors, so I'm curious to know which ones they are. Can anyone explain the process?

Thanks.
 

TheCrimsonStar

New Member
Check all your vacuum lines. If one of those is leaking or disconnected it'll mess with your idle too.

As for sensors, MAP sensor, IACV, TPS and I believe that is it. Someone add if I missed something.

Sent from my Galaxy Nexus using Tapatalk 4 Beta
 

Nick_C78

New Member
Timing, Idle Screw, TPS, IACV, and Vacuum. A few other sensors can also make it idle higher, but those are the general ones. Anything else would be giving you a CEL.
 

Muckman

Not a M0derator
Sensors - MAP, TPS, IAT, ECT, Cam/Crank TDC, CKF (OBD2), O2
Actuators - IACV

Fast Idle Thermal Valve and the Throttle Idle bypass screw also bypass air for idle.
75% of idle problems are vac leaks
25% are dirty idle valves.
 


enoch723

New Member
There is so many things that can make it idle weird. Even a weak head gasket can. What I did to my 88 was cleaned the EACV really really well with throttle body cleaner, cleaned the throttle with the cleaner got rid of all the vacuum leaks and replaced the TPS. When I was done with it I reset the ecu to "relearn" as everyone calls it.

Thing ran fine for the year I had it before I sold it
 

sabre1fv

New Member
I definitely don't have any vacuum leaks as that was one of the things I checked, but thanks for mentioning. I'm thinking electrical since it only happens when warmed up and isn't a constant problem. Will idle fine then drop a few hundred RPM and run rough, then idle picks up to normal. Only other symptom is it sometimes stumbles when I accelerate slowly from a stop. I'm wondering about the TPS, but that is usually never mentioned as a problem - it is the original one and at 180K plus miles, who knows.

Thanks for your insight.
 
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enoch723

New Member
I had EXCATLY the same issue with mine. Mine stopped doing it about the time that I changed the main relay under the dash so I am not sure excatly what it was that solved it cause I didnt notice till a few weeks later that it had stopped. The things I had done at the time was

reorient intake cam(previous owner had it a tooth off)
cleaned the EACV with throttle body cleaner
new tps
fixed vacuum leaks
and replaced DASHPOT completely
new main relay

Keep in mind that it could be anything from a pin hole in a vacuum line to a leaky head gasket so just start trouble shooting

What I do to find out if there is a vacuum leak which is different then most people.
Most people spray starter fluid or throttle body cleaner around the vacuum lines which works good to pin point. I got my friend to come over and smoke a cigar. While he was smoking it he could blow the smoke out around the engine bay.
Obviously if the engine bogs down something is sucking in the smoke
 


sabre1fv

New Member
Yes, I used an un-lit propane torch to check for air leaks since I could direct it into specific areas and found nothing. Relay and EACV have been replaced over the years for reasons unrelated to this so they are not original - even put the original EACV back in with no change and the previous one was only a couple of years old.

I have a test harness so I'll try checking the TPS readout this weekend.

Thanks for your suggestions.
 

enoch723

New Member
If everything else test fine you might have a leaky head gasket.. I know it sounds dumb but sometimes it can cause a loop idle.

This is why I went with the 86 or 87 instead of the 88 or 89. 86 and 87 and vacuum advanced while 88 and 89 are electronic advanced. Ever single brown top I have ever had or worked on either didnt have the problem or it was an easy fix.. Black tops it usually is a simple fix but there is so many variables
 

nkerklin

New Member
I agree. Mine does that and I changed out the fuel injectors and the MAP sensor. Saw the coolant seeping and yes, head gasket.
 

enoch723

New Member
Yeah It took me 3 years to figure out what was going on with my black top and it ended up just being a leaky head gasket.

Weird thing was that it wasn't leaking coolant or even a coolant oil mix.. It was just a rain drop of oil every like 20 minutes. Weirdest thing ive ever seen
 
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