scuby22

I've been working on my brother in-laws 1989 toy ex-cab 4x4 3.0 5spd for a few days now. He has this eratic idling problem that we have been unable to diagnose for about six months. We have learned much patience. When the truck is cold it idles at around 1600 rpm for about 30 seconds, then the engine starts jumping from 1000 - 3000 rpm every second. I'm not exagerating the constant rapid reving is strange. But after about 2 min. of this reving the engine warms up and it returns to a normal 900 rpm idle. (if that's normal). We looked for vacuum leaks, but that would cause a high idle speed all the time. I was looking in chilton's yesterday and ran across this air valve that apparently bypasses air across the throtle body when the engine is cold to increase idle for cold start mornings. When the engine warms up, the air valve closes. The description in the book shows a flat plate type device with a coolant hose and vacuum lines attached. I can't find that anywhere on the engine. I did find a picture on the internet that showed the valve to be on the side of the throttle body? This valve is the only thing I can find that has an effect on idle speed dependent on coolant temperature. I just can't find it.

On a side note, the truck will not stay warm while driving. It warms up just fine in the driveway, then the idle smoothes out to 900 rpm, but drive down the road for five minutes, stop, then the erratic idle is back because the engine is cold again. We have put three t-stats in, and flushed the system. I can't figure out how to get this truck to stay warm. It seems even a bad fan clutch couldn't cause this.

I have been chasing this idle problem for months and any ideas would be greatly appreciated.

thanks

Churnd

you might have a faulty EGR valve. i can't really explain how it works but sometimes it'll stick open or closed and cause an erratic idle. if Jay reads this he could tell you more.

scuby22

I looked at the egr valve. If I understand correctly, the egr allows exaust gas to be recirculated back into the intake. But this only happens after 1- the engine is warm and 2- the throttle valve is open. The egr vacuum modulator which controls the egr valve only activates when the throttle is open. also there is a temp sensor on the egr that tells the ecu that the engine is warm enough for egr operation. I unplugged the vsv that the ecu uses to activate the egr and the problem didn't change. I guess if there was a leak in the valve itself, then it would allow the exhaust air into the intake all the time. Is this right?

I guess I can take off the egr valve, draw a vacuum on the intake pipe and see if any air makes it past the valve when it should be closed??? Would this work?

Jay

The Idle Air Control (IAC) Valve is below the throttle body. You will find 2 coolant hoses routed to it. I cant recall any specifics of this device of the top of my head. I dont ever remember diagnosing any problems associated with this device. (Lucky me?)

I have posted on about this symptom before. Your concern closely resembles that of the 22R's eratic idle fluctuation. Follow this LINK for that information.

Check the Air Intake hose for cracks allowing "unmetered" air entering. ( Air not measured by the Air Flow Meter ).


Lets see where this information takes us. Keep me updated. Ill keep thinking.

Jay

JSB

I know this has no relation to the Toyota 3.0, but I had an experience with my 94 RX-7 that sounds relatively close. The only difference, was it did it all the time, unless I drove it. Everytime I would slow to a stop or let off the throttle the Idle would surge like that. I finally took it to a dealer, and they told me it was my intake manifold gasket. Just a thought. It may not help but I thought I'd share.

Good Luck

scuby22

ok, I just checked the egr valve and it seems to be operating correctly. We even removed the egr system completely and it still idles with the constant surging.

Jay, thanks for the info.

I was looking for the air valve, and I did finally find it just under the throttle body. I have taken off the intake hose and checked for cracks, but it looks to be in pretty good condition.

I don't think this is a typical vacuum leak/ intake leak/ or head gasket leak. My reasoning is because when the engine warms up, it reaches a rock steady 900 rpm idle. And I can adjust idle with the idle set screw.

JSB, thanks for the idea though.

Jay, do you know how to trouble shoot the Idle control valve on this 3.0? The Chilton's manual states the following and it makes no sense to me.

"To check the operation of the air valve on a V6, start the ingine, then turn the idle speed adjusting screw all the way in - while the engine is still cold, rpm should drop; when the engine is warmed up, rpm should not drop." p.144

Now looking at this simply, if the engine is warmed up, and I turn the idle speed screw, rpm WILL change. I mean that's what the idle speed screw does. right?

I don't understand how to check this air valve thing on this V6 so maybe someone can shed some light on it.

thanks

Jay

Best I can do is check the manual tomorrow when I am at work. I might be headed that direction today, and I can possibly stop in and see what the inspection procedure is. Ill let ya know as soon as I can. I dont recall off the top of my head what type of IAC is on this motor. Generally, Toyota doesnt have much trouble with these units.

Jay