mat

I just bought a 1995 T100 4x4, manual transmission. It idles around 1900 RPM at start up and 1800 RPM at normal operating temperature. When applying the brakes the RPM drops to around 700 then surges between 700 and 1400 RPMs until vehicle speed is close to 0, then it resumes steady idle at 1800 RPM. If Idling at a stop and the brake is applied, the idle remains steady at 1800rpm.
Other than this, the truck drives well. It accelerates well and drove at highway speeds and in stop-and-go traffic for many hours (I purchased it out-of-state) with out heating up or giving any other sign of complaint.
I am not receiving any codes on my reader.
The truck has air-conditioning, but it does not cool the air at all. (May not be related, but it is the only other known problem the truck has)

I've read many other "high idle" threads, but was hoping that giving my exact situation may help narrow down the possible culprit. thanks

mat

Update.
I cleaned and tested the MAF sensor. It's good
I tested for vacuum leaks with starter fluid, but couldn't find any.
Pulled the throttle body. It looked pretty clean, but the IAC Valve was dirty. Cleaned everything. IAC passed continuity test, but did NOT open or close when battery voltage was applied per factory service manual instructions. Pulled off motor/magnet and manually turned the valve and cleaned more. Reassembled and retested with battery voltage. It now opens and closes.
Reinstalled throttle body and IAC on vehicle with new gaskets. We have improvement but still idling high, so I tried to find vacuum leaks with the starter fluid again. still can't find any, so went ahead and replaced any questionable vacuum lines, but to no effect.

Current condition:
Idles around 1000 rpm at start up and 1450-1500 at normal operating temperature.

I think the TPS looks good from what I'm seeing on the OD2: 9.4% at idle and slight opening and closing as expected when revving the engine parked in neutral.

The factory service manual says to "apply vacuum" to the throttle opener to test continuity of the TPS. How do you "apply vacuum" when the engine is off?

mat

Update:
I tested the IAC circuits to the ECU and it passed on both terminals.
I pulled the motor off of the IAC while the vehicle was idling at operating temperature. nothing happened. I manually turned the valve on the IAC and was able to make the RPM drop way down and the engine sounded like it was going to stall if I hadn't opened the valve back up. I turned the valve slowly and made it idle at 750rpm. I turned the vehicle off, reinstalled the IAC motor and started the truck again. It idled at 950. I throttled it up and let it drop back down. It always returned to 950. Drove it around the block a couple times and it started idling back up at 1350. Parked it and put it in neutral. Revved it up and let it drop back to idle several times. It always returned to 1350rpm.

So what does it mean????
The IAC and IAC-ECU Circuit both check out, but we also know the the IAC isn't closing all the way and that if it did it would have the ability to correct the high idle.

Would an incorrect input from some other circuit (O2 sensor, Air Conditioning Idle Up) cause the engine to keep the IAC Valve open?

mat

Update:
I read on some forums that some people did work on their truck, then had a high idle, but that after driving it around for a while the ECU "relearned" and corrected the issue. So, I started driving it yesterday.

1st drive was 1/2 hour and included highway, it idled at 1250-1450rpm and drove fine otherwise. Let it sit for a couple of hours.
2nd drive was also 1/2 hour and included highway, idled at 1250 at start and 850-950rpm during drive. Let it sit for a couple of hours.
3rd drive was 10 minutes city, idled at 1250-1350 rpm. let sit for a couple minutes.
4th drive, 10 minutes city, idled at 650rpm at start up and 850-900rpm after a couple of accelerations.
5th drive 10 minutes city, 1350rpm at start up, 1250rpm after a couple of accelerations.

I've read that people have had the ECU take up to 4 days to "relearn" but not sure how many drive cycles that person did in that amount of time.
Anyone know how many drive cycles it could take for the ECU to "relearn"?

mat

Drove it again today. a couple of 1/2 hour trips on the highway with no improvement. It continues to idle differently every time I turn it off and restart it.
so...
I pulled the ECU out of my other T100( It had the same part number as the ECU in the truck I've been working on) and put it in the this truck.
With the new ECU the idle rpms ranged from 550-1450 at start up and after 15-20 minutes of stop and go driving they would be 750-950 at idle. But when I turned the truck off and restarted it they could be any where from 550-1450rpm again.
Am I just not "resetting" the ECU correctly?
I've never done anything special to reset an ECU before. It always just seemed to work.

old87yota

It sounds to me that both ECUs are probably working correctly, and your problem lies elsewhere. The ECUs typically reset when removed from power for a few minutes and may take a little bit to relearn things such as fuel trims, but should be fully learned within a trip or two.

The high idle and surging while braking may or may not be related. We will find this out by tackling each problem separately.

Have you checked your throttle cable and throttle body plate to make sure they move freely and do not stick or bind?

I am not as familiar with the 5VZ-FE as I would like to be. Time to start looking in the Factory Repair Manual...

swampedout

The surge when braking sounds like a vacuum issue..

mat

Ok, I figured it out.

It was the IAC Valve.

At this point I was petty confident it had to be the IAC, the ECU, both, or the wiring between the two. Since I continued getting the same results with a different ECU and all my testing of the wiring seemed to indicate good connections, I decided to replace the IAC. My thinking is that there must have been something wrong with the spring in the IAC since it had passed all the electrical tests and the ECU was clearly able to control it while the vehicle was running, but it would open or close the valve too much when the vehicle was tuned off, making the ECU try to figure it out all over again.

-Something I may have failed to mention earlier was that the bake surging stopped happening once the ECU bought the idle down below 1000rpm, so I think that was just caused by the ECU trying to lower rpm during breaking like it is supposed to, but having a hard time because the idle was so much higher than it was supposed to be.

Hope this helps someone else with they diagnostics