hey all, this is my first post here, looks like a great knowledgebase! my 89 4x4 p/u has a problem my mechanic can't solve. I'm hoping someone has heard of this problem before. It's an old problem that has morphed to an seemingly unsolvable one now. I've had this truck for 19+ years now, bought new, and over its lifetime on about three occasions, have had a problem where if the engine was idleing, albeit at a light, parked or whatever, and the A/C came on the engine idle would drop drop rpms then go high rpms alternating every 2 seconds or so. This wouuld happen only when the compressor would run. My mechanic adjusted on more than one occasion over the years the vacuum knob setting on top of the engine somewhere and it woudl correct it. Another time, it did it when I would press the brakes, again same adjustment corrected. Another time, he replaced a switch of some sort under the dash, I think it had something to do with the brakes, but not sure. So I've had this issue off/on 5-6 times over last say 10 years. Two months ago, it came back but this time, much worse; very high RPM, followed by very low, almost like cutting off then recovering to go high again, about every 2 seconds. No adjusting corrected this time, and we've replaced another brake switch and now a computer, and twice he has turned it back over to me "fixed" only to have it start again within 50 miles or so. Got it back againg today after some computer "tuning" but it started doing again w/i 2 miles of the mechanics place. I keep thinking vacuum leak; he says he's been over it thoroughly looking for that, can't find one. SO, any ideas would be a wonderful Christmas present. I love this truck and don't want to part with it.

22RE engine
112k miles
minor headwork 6 years ago

THANKS!

 

If this only happens when the a/c comes on or breaks are depressed, it sounds like the TB idle bypass screw may be set too high.....crazy it may sound.

Warm the engine to operating temperature, leave the a/c off, and check the base idle setting with a tach. Check the timing and make any adjustments necessary to get the RPM's to 750. Then, check the operation of the idle up on the a/c when you turn it on again.

Report findings. If it hasn't fixed it, move on to something else.

 

TB = throttle body, btw.

 

sounds like the a/c idle up valve is having a problem.

 

Hey thanks. I should have clarified; it is doing this at all times now, not just when the a/c or brakes are engaged. Those were the triggers in years past. I'll pass it along anyway.

 

If it's cycling up and down with the A/C on, then it's likely the A/C is low on gas and causing the compressor to cycle.

 

now its doing it all the time? weird.
Is the coolant full? try purging the cooling system of air. Make sure the Idle Air Control valve is working properly. just my $0.01

 

I am not sure if the 89 has a Mass Air Flow Sensor, but it could be the problem. Try disconnecting the Mass Air Flow Sensor (it is on top of the air box behind the driver side headlight, the connection looks like a half circle), running the vehicle (it will most likely stall), then reconnect it and see if it corrects it.

 

That's a possibility^^^^. Although, on an '89 you have a vane or volume airflow meter. It's different than a mass air sensor electronically.

If you disconnect it, most likely the truck will just die altogether.....even if it has a bad circuit in somewhere. That's always been my experience, anyway.

Have your mechanic test it this way.......
http://personal.utulsa.edu/~nathan-b...33volumeai.pdf

And, if he doesn't find any problems there, try a test this way....
https://www.yotatech.com/forums/f116.../#post51007356

 

other things to check out
check for cracks in the duct from air flow meter to throttle body and PAIR system, vacuum leaks, ignition timing, valve adjustments, coolant temp sensor operation, air-flow meter, O2 sensor(s), throttle position sensor adjusted and operating properly

sorry to throw the book at it, but any of those could affect idle quality

 

x2........

And, meant to mention this before, but the present problem could unrelated to previous problem.

 

also, I'm puzzled why the mechanic would adjust the a/c idle up valve to compensate for surging when pressing the brake pedal. surging when pressing the brakes suggests a vacuum leak in the brake servo. so I'm wondering if that still isn't your problem but it's just progressed to a constant leak.

 

hmmmm i am clueless maybe its just time to rebuild????

 

The two vac lines running off the PS pump valve go to the a/c idle up valve, also.

Good point on the servo. Hadn't though of that.

And, no.......not time for a rebuild....hehe.

 

disconnect and plug the vacuum line going to the brake booster and see if the idle clears up. if it does, that's your problem- the booster (a.k.a. servo)

 

Or, a pair of vice grips on the hose. I've done that troubleshooting. Wasn't the problem, but it was easy....

Plugs are nice too, though.

 

Well, it's been a week now I've had the truck back. THANKS for all your help with suggestions and advice. I passed all of them on to my mechanic. WHich he tried/didn't I'm not sure, but here's what he told me he did.

1. Re-tuned the ECM
2. Eliminated brake booster as cause
2. Increased Idle speed higher than normal (I estimate 900rpms)
3. Cleaned injector manifold and cold-start injector manifiold (manifold may not be correct word; that's my inference)
4. Asked us to run a can of Chevron Fuel Injector cleaner through tank of 93 Octane (BP-Ultimate) fuel, and use that for a few weeks to help clean it...

It is running without surging/cutting out now, albeit at a higher idle speed than I'd like. It has done the surge once in about a week, on a very cold morning, about 28 degrees. Otherwise it has been fine. I hope we can get the idle speed dropped back down soon.

SO, does any of this make sense? I hope so. THanks again for all the notes and sugestions.

I STILL LOVE THIS TRUCK!

 

fixed my surging by taking an air hose and blowing into the small coolant line coming into the throttle body

 

The reason that it idles funny when your at a stoplight with the brakes on of just sitting there is because the 22re computer has several sensors to adjust the idle. When the brakes are pressed it doesnt inject as much gas to even out. Same when push the clutch in. Sounds like your idle scew has backed out. Get it to operating temp and adjust it to about 900rpm. Some suggest lower around 750. I have this exact problem. You can also take the easy way out and get a diagram of the computer and just jump the brake and clutch idle sensor wires out. Highly not advised though.

 

thats weird. my 3VZE does the same thing, its an 89 too wth a mass airflow sensor, and it only does it like once a month but it does do it. its frustrating