Forerunner

To all you techies out there, here is a problem that has me puzzled. I would appreciate any thoughts.

When in PARK or Neutral, if I smoothly apply accelerator from Idle, the RPM rises smoothly to about 1400 and I can hold it there, no problem. If, however, I continue to apply the gas pedal, the RPM actually DROPS to about 1300 and lingers there, with noticeable, but not huge, roughening of the engine. If I continue applying the pedal, the RPM kind of stays at 1300 for a little bit of additional pedal travel. Then, all of a sudden, at some point during additional pedal pressure, the RPM just goes smoothly but quickly from 1300 to about 2500.

It is like there is a sudden inflow of fuel, air, or boost. From that point on, above 2500, the rpm varies in direct proportion to pedal pressure, no more funny stuff. If I let it drop below 2500 rpm, it again goes straight to 1300. In effect, there is no way I can hold anywhere between 1400 and 2500 in any pedal position. All of this is in Park, of course, there is nothing of sorts going on in Drive. The truck runs smooth, accelerates fast, gets to 85 MPH+ easily and with no hesitation. In drive it acts perfect.

I am not suprised I have not noticed this, because how often do you rev up RPM in Park for any reason? I just did it after Seafoam application to clear up the engine. That is when I noticed it. So I have no idea how long it has been that way.

I will try to draw some sort of a linear graph, with application of gas pedal corresponding to the RPM.:


RPM in Park
--------------------------------------------------------------------------
700 smooth rise to 1400 drop to 1300 1300 1300 instant rise to 2500


PEDAL APPLICATION - continuous rising pressure from idle
--------------------------------------------------------------------------
........................................|no RPM change zone|fast change|............


It may be related to the fact that when I put the truck in drive, I have a 300-400 RPM drop from Idle speed. But I don't know. Any ideas? Thanks in advance!

Forerunner

Come on, I can't belive noone has any ideas! You guys build entire cars from scratch.. help this amateur mechanic (me) to figure it out

george

sounds too weired. no eng. codes? i am thinking of TPS?

AgRunner06

That was my guess but the hesitation usually happens in Drive. I think you can unplug the TPS and run without it but you will get a code. You can try that and see if the vehicle still acts weird. Just pull the EFI fuse afterwards for about 10 mins to reset the ECU. Will it still act weird if you just rev it really fast in Park? It couldn't hurt to give the TB a cleaning too.

Forerunner

Thanks for the replies! It was the TPS - it was out of adjustment in the way it was mounted to the throttle body. You know how the top of the TPS screw hole is shaped like a groove so you can rotate the TPS a few degrees this way or that. After searching through factory manuals (more like being lost in factory manuals) I decided to give the TPS another go. There were no codes for TPS in the ECU. I figured I would exhaustively test it in every way possible anyway.

That task was made more difficult by the fact that the factory test procedures assume that, at least for some tests, you have the throttle body removed with the TPS mounted to it. Took me a long time to figure it out, by looking at the pictures in the manual and not being able to match them to the actual truck, that the pictures were of the throttle body but upside down!!!

I got the TPS off which was not easy, considering the stupid way the screws that mount it are positioned. Tested it and it was fine. Mounted it back on the throttle body, tested it as procedure calls, by using thickness gauge.. then adjusted it according to the same procedure. Behold, that took care of the problem. Now revs up smoothly and the idle is also smoother than ever.

All the more weird, considering that the TPS was mounted maybe a degree or two differently before. I find it hard to believe that 1 degree of TPS rotation would make this much difference. If you have tried adjusting TPS on 3VZE you know what I mean, it is not a fine instrument. I have replaced TPS on many a vehicle before, and I never seen one that was so picky about how TPS is positioned. Heck, some of the TPS did not even have any adjustments in the way they were mounted.

The moral of this, at least to me, it is not enough to have a good TPS. It is not enough to mount it correctly in the right holes. Once has to adjust it precisely, and I mean precisely (it took a hair of rotation to make TPS readings show within spec vs out of spec) and you need thickness gauge just in the right place to do it. Dang I would much rather work on 22 RE engine

Another thing it seems that ECU diagnostic codes for the TPS are really useless, or at least incomlete and unable to determine out of spec parameters. I mean, I was driving with a TPS that was giving out of spec readings and made the engine behave so weird..yet there was no code for anything in the computer. Just shows ya one cannot assume anything with these older motors. Thats about the only thing our 1996 Doodge has on these older Toyotas, better onboard diagnostics (OBDII)

I have not yet measured my gas mileage since the repair. I reset the ECU after adjusting the TPS so we will see.. just eyeballing the gas gauge though, it seems that gas mileage has marginally improved.

22re_celica

Most likely a loose or unplugged fuel injector.