Strange Idle Problem

[The Slider]

I noticed a couple of times when I start the engine cold, the Idle will rev up and down to almost die then back up again, etc.. when I put it in drive it stops.. I put it back in park and it revs up and down again and again. but stops when Its warmed up. I Sprayed around looking for Vacuum leaks but haven't found any, can someone maybe shed some light?


Thanks!
Chaz.

 

[RonD]

Could be a dirty IAC(idle air control) valve, they can be cleaned.
An IAC is a controlled vacuum leak on the upper intake, this is how fuel injected engines control the idle level.

When engine is cold the computer(PCM) runs the engine rich and advances the timing.
It also increases the idle, if IAC is dirty PCM opens it but air flow is intermittent causing a ranging idle, in gear creates a load on the engine so ranging stops at the low idle point of the range.

There is also the ECT(engine coolant temp) sensor, this is the Choke on a fuel injected engine, it tells the PCM when engine is cold and when it is warm.
If ECT sensor is telling the PCM engine is warm or almost warm, then PCM might be running normal warm engine fuel/air mix, this would usually cause stalling and general poor driving until engine warmed up.
Good read on the ECT here:
http://oldfuelinjection.com/?p=28

This is not the same sensor that runs the dash board temp gauge, thats a 1 wire sensor, ECT is a two wire.

 

[The Slider]

Ill pull my IAC of and try cleaning it and if that don't work Ill just order a new sensor, they aren't that much.

Thanks!
Chaz

 

[The Slider]

Well I had another IAC on a good running 2.9l and changed it, made no difference. After it warmed up I did a scan of the codes and everything passed! Do computers like the ones on the 86 need time to LEARN again like the new ones do? Im going to try this again tomorrow and see if the idle is still erratic..

Chaz

 

[LIMA BEAN]

Clean the MAF sensor too, if you have not done one before here's how.

http://www.therangerstation.com/tech_library/CleanMAF.html

 

[The Slider]

Clean the MAF sensor too, if you have not done one before here's how.

http://www.therangerstation.com/tech_library/CleanMAF.html

I don't have a MAF sensor. Its a 86 2.9l.

Chaz

 

[RonD]

Yes, they do learn, when changing sensors or other parts it is good practice to unhook the neg. battery cable when you start the work then hook it back up when done, that will reset the CPU(PCM).

With no other symptom other than up and down idle, cold or warm, I would run some Seafoam injector cleaner in the tank, and wait and see.
You checked for a vacuum leak, with a spray, you might try a smoke test.
Don't forget the MAP sensor vacuum line on the firewall, it can be hidden.

Good cigar, glass of scotch, blow smoke thru the vacuum lines, nice test if your still sober when you find the leak.
I try to do this test once a week, I love DIY

 

[LIMA BEAN]

Yes, they do learn, when changing sensors or other parts it is good practice to unhook the neg. battery cable when you start the work then hook it back up when done, that will reset the CPU(PCM).

With no other symptom other than up and down idle, cold or warm, I would run some Seafoam injector cleaner in the tank, and wait and see.
You checked for a vacuum leak, with a spray, you might try a smoke test.
Don't forget the MAP sensor vacuum line on the firewall, it can be hidden.

Good cigar, glass of scotch, blow smoke thru the vacuum lines, nice test if your still sober when you find the leak.
I try to do this test once a week, I love DIY

Aw yes Highlander tech, reminds of when I was a young lad in Fort William and earning me potatoes as a blacksmithy. Then along came this firey redhead Dutchess....

 

[The Slider]

Well after cleaning everything, now when its cold I just have a HIGH Idle of about 1500 RMP and when its warm it drops back to normal. but it will stay high idle for about 5 mins or so....

CHaz

 

[RonD]

It should have a high idle when cold, that is the Computer's "Choke" setting, Open Loop, run rich with advanced timing(higher idle from advanced timing).
But I would think about 1,100, not 1,500rpm.

5 min. is about normal for engine to get to operating temperature, but idle should start to lower a bit as it warms up.
Make sure you use the factory thermostat temp, I think it is a 192deg or 195deg, running lower temp t-stats cause delayed warm up, lower MPG and less lubrication.

I would test the ECT, it is a two wire sensor usually near the t-stat housing.
http://oldfuelinjection.com/?p=28

This is the sensor that tells the computer the engines temp, so turns on the "choke", it is one of the main sensors on the engine.
Its resistance changes gradually as it gets warmer, if it is out of spec then computer can't run engine correctly.

 

[The Slider]

have the factory t-stat. I was wrong about the RPM its 1900 RPM!! I saw the link but how do you test the ECT? I didn't know there was a test... How do you test it?

Thanks!
Chaz

 

[kimcrwbr1]

Did you clean good around the throttle butterfly when you cleaned the IAC. Did you disconnect the battery for at least 1/2 an hour to reset the values. It can take a couple complete warm up cycles for the computer to set the new values. Have you replaced the PCV probably the most neglected part of preventive maint. Put a new PCV in every time you replace the plugs it cannot be cleaned and they are cheap. Reset the computer clean the throttle butterfly/venturi and replace the PCV and see how it does.

 

[kimcrwbr1]

I believe you have a ACT also air charge temp sensor clean it with rubbing alcohol or elect cleaner. Every time you clean or change a sensor you need to reset the computer by disconnecting it for 1/2 an hour. Even if you run it for a short time with a sensor disconnected reset the computer.

 

[kimcrwbr1]

http://www.therangerstation.com/forums/showthread.php?t=1226

 

[RonD]

You can test it with a Volt Meter or OHM meter, the How-To is in the link.
ECT sensors rarely fail, not never but rarely.


1,900rpm is a lot above 1,100rpm.

I would try what kimcrwbr1 suggests, especially reset of the computer, although the other systems should effect operation cold or warm.

In open loop(cold engine) the computer will open the IAC all the way when first started, so RPM should go high, then it should drop(IAC closes a bit) after a few seconds to about 1,100, but this depends on the temp the ECT is at(or THINKS it is at), if it is very cold idle will stay high.
After a few minutes of warming the idle should drop to under 1,000rpm then to about 700rpm when engine is at normal operating temp, you said that does happen.