My '89 22re 4x4 single cab 5 speed had a funny surge when I got it. When I idle it high at around 1700 rpm it would surge from 1700 to about 1000 and back about every two seconds. It made it tough to drive at low RPM's. It got worse when I installed the AC kit on it and got to where it would start missing also, probably because of heat/too much fuel. The guy who installed the AC scanned it and said the computer was reading a constant 62 degrees which caused it to send more fuel bringing idle up then EGR would send message of too rich causing it to idle down and so on. The truck had an aftermarket temp sending unit on it when I got it. It has no connection for the computer, just one wire going to gauge. Does anyone know if the truck has a plug/connection under the hood for the temp sensor connection to ECM? If so an OEM temp sensor seems like the fix. Thanks for any help.

 

There is a temp sender for the ECU, it is this green one in the front of your intake underneath the throttle body facing forward. Can be tested using an ohmmeter. An air pocket in the IACV can also cause the idle surge you are experiencing.

 

Thank you very much. I get more GOOD help on this sight than just about any forum I'm a member of. Old Toyota owners are the best! BTW do you know where the sensor for the ECU connects on the other end?

 

I only have the FSM for an 87, not sure if your 89 is the same. Schematic in the FSM shows the green wire from the thermo sensor goes directly to the ECU, Brown wire with black stripe goes to the Throttle Position Sensor (which is attached to the Throttle Body)

 

Engine surging is generally caused by a vacuum leak. The a/c making it worse makes sense since the a/c idle up intentionally creates a vacuum leak to raise the idle.

 

Hmmm. It does not have an idle up sensor on it. Kit didn't come with one. I'm sure I can find one. Do you know how it wires in?

 

Another possibility for the high idle surging is that Toyota engineers put a little bit of programming in the ECU that cuts off the injectors when the idle is above about 1,000 or 1100, RPM and the brakes are applied. Their logic is that if the brakes are applied, you're slowing down, and you want the engine RPM to be at idle when you get stopped. Best, quickest, way they figured to do that was to cut the injectors off until the RPM is down to something real close to the correct idle speed. Once it's there, the injectors are enabled again, to keep the engine running (duh).
I believe, but may well be wrong, that the TPS idle switch needs to be engaged to enable this little ECU program, as well.
If the idle is set too much too high, when the brakes are applied, the engine will surge. The ECU cuts the injectors, it slows, they're re-enabled, it climbs back up, the ECU cuts the injectors, etc, etc.
If the idle RPM are set to FSM recommended setting, around 850 or so, it won't happen. I believe the ECU can read that the AC is on, and adjusts accordingly, but I'm not sure exactly how that works. It may be the AC idle up doesn't go high enough to trigger the ECU's program. I'm just not sure. I only use 2-55 AC, so I don't need to worry about all that. You know, 2 windows wide open, 55 MPH, type air conditioning I'm cheap, errrr...FRUGAL, yeah, frugal, and don't want to pay for the fuel the AC uses up. Even when I lived in Yuma, Az, I never used the AC on my trucks, and it get's a little hot, there. I even took the belts off, so that the engine wasn't driving the ac compressor's pulley.

I'm not saying that this IS you problem, but it's a possibility.
Pat☺

 

I guess that's possible but it only happens during acceleration and not when brakes are applied.

 

Idle up for the AC works off vacuum via a diaphragm, this thread might help you. Check out post 36 in the thread, it worked for me when I had to replace my diaphragm since the original one is no longer available from Toyota.

https://www.yotatech.com/forums/f116...70/index2.html