Mechanics

ENGINE REVS UP AND DOWN AT IDEL

1993 Toyota Tercel

Rebuilt engine, 26k miles. Stumped on this one. Engine pulses acceleration after warm up. Won't idel. This all started when I tried to clean throttle body and had removed; but reinstalled the linkage for throttle and stop sets for idle up. Tried to replace MAP sensor; but no luck. I think it has someting to do with the idle up sensor and/or MAP sensor. No " check engine" light comes on. Tried to adjust throttle positioner and idle set RPM's but still won't idle. Pulses up rpm's and then down. When I unplug the MAP sensor it will idle rough but will stop pulsing. Can someone help with this problem. I'm stumped.
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Daveandpatty
April 27, 2007.



Sounds like there might be a vacuum leak. Did you replace the throttle body gasket? If you get the base idle adjustment too far off, the computer will fight it for control of idle.

Tiny
Jack42
May 13, 2007.
Thank you so much for an answer. Yes, I did replace all the gaskets; but that base idle set just may be the problem. I didn't think about reinstalling it to the original setting; because that mark got removed. I just screwed the linkage back to the body and hooked up the throttle cable. I'm going to try and reset it per the factory manual instructions. I'll post the results here. Thanks again for the help.

Tiny
Daveandpatty
May 13, 2007.
I reset all the adjustments of the throttle positioner and the idle stop set to the factory specs. Engine still wants to rev up and down at idle. I tested the vacuum sensors and the throttle positioner and they test good. The engine will hold steady at around 2500 rpm's; but when you let back on the throttle to idle, that's when it starts to rev up and down by itself. I'm without a clue now. Can someone please help?

Tiny
Daveandpatty
May 16, 2007.
Try the idle air control valve clean it-than fire it up now adjust the idle screw hidden under a rubber plug.

Rasmataz
May 17, 2007.
I cleaned the air valve, the throttle body, the EGR valve body which had a little carbon build up, installed new gaskets; but engine still won't idle smooth. I checked the vacuum sensors and the vacuum switches, they all check out OK. I checked all the vacuum hoses for leaks and all were good. Engine still revs up and down all on it's own. 800rpm to 2700rpm then back to 800rpm. When I disconnect the vacuum switch for the fuel rail pressure reg the dash pot for the throttle positioner will start to pulse the throttle up and down. When I disconnect the MAP sensor the engine will idle rough and then die. It also will register a trouble code for the MAP sensor; but will clear out when I reset the ECU. So I think the MAP sensor is good. I actually replaced it a few days ago. I'm really stumped on this one. Can someone please help. Thank you.

Tiny
Daveandpatty
May 22, 2007.
I've tried all the recommendations and no go. Engines still revs up and down all by itself. Could the problem be in the setting of the throttle positioner? When I unplug it, engine will rev up to about 2800rpms and hold steady. When I plug it back in, engine starts to rev up and down. Still stumped and I have donated. Thanks for any help.

Tiny
Daveandpatty
Jun 2, 2007.
Make a donation and your question will be answered. I donated and still no help with my question. I guess this problem has even the so called experts stumped. Thanks for nothing!

Tiny
Daveandpatty
Jun 10, 2007.
You mentioned the TPS have you tried testing it and initial adjustment

Rasmataz
Jun 10, 2007.
Yes, I set the TPS per the factory specs and it also tested good. I even swapped out the throttle body and TPS from another running motor of the same and didn't change any settings. It did the same thing. Someone mentioned a possible vacuum leak. Can a vacuum leak actually cause the engine to rev up and then die down, over and over? I've had vacuum leak problems in the past with other repairs and they seem to only increase the rpms and not make the engine die down and then rev up again like this problem. Thanks for the reply; but I'm still stumped.

Tiny
Daveandpatty
Jun 11, 2007.
Get the engine up to norm temp. Now use WD-40 and spray around the throttle base, intake manifold/plenum. Inspect all vacuum hoses, check the IACV, TPS, MAF sensor connector for a tight fit

Check the EGR and its vacuum solenoid operation-if its ajarred it will create a massive leak

Accelerator cable could be binding or stretch

Need more help comeback

Rasmataz
Jun 12, 2007.
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