need help diagnosing an engine problem...

[jekbrown]

hey gang. I have a problem with my 1991 Ex and need some help trying to figure it out. Basically, when you start it up, the RPMs quickly move up to about 3000... neither letting it warm up and/or gassing it a little bit seems to get it to drop back down to a normal idle level.. it just does it on its own at some (seemingly) random point. In any case, when I put it in gear (its an automatic) the RPMs drop to normal, but sometimes while driving they will head for 3000 rpm again... with NO driver input. Pretty scary... its like someone is pushing the gas pedal for you even if you want to slow down or stop... ya have to fight it with the brakes and/or put it in neutral until it goes away. The problem is intermitant but after driving for 30 mins or so it seems to happen less and less. The check engine light came on today on the way home from work for about a minute and then turned off.

I have only owned this rig for 6 months or so... so far it hasn't had any other major issues... just standard stuff you'd expect from a 15 year old rig. Anyway, anyone have any tips? things I should look for? As far as the check engine light, can you get a guy at NAPA or Autozone to plug in a gizmo to tell you what the computer says or do you have to take it to a mechanic and pay some ca$h? I like to do auto work myself when I can (yeah, Im poor) so hopefully it won't cost tooo much.

any help would be greatly appreciated, thanks in advance!

j

 

[aldive]

Have you checked the IAC?

Is the CEL illuminated?

 

[jekbrown]

what IS the "IAC"? Im a carb'd Chevy guy stuck with an Ex... lol. Need all acronyms spelled out if possible.

j

 

[aldive]

what IS the "IAC"? Im a carb'd Chevy guy stuck with an Ex... lol. Need all acronyms spelled out if possible.

j

IAC - Idle Air Controller.
CEL - Check Engine Light

 

[exploded99]

If the truck is trying to stay at 3000 rpm then I would check your throttle position sensor. I think if you search this website you will find a fair amount of info on them. You can cheack this urself with a voltmeter and it only works out a couple of braincells.

Also, go thru those vac lines and check carefully for vacuum leaks.

 

[jekbrown]

cool beans, thanks for the tips guys... I'll bust out the Chilton and see what I can find out tonight. Thanks again!

j

 

[jekbrown]

woo-hoo! Fixed it. I wasn't 100% sure what the problem was, but I knew that the previous owners weren't big on maintenance so I went with the "shotgun" approach. I swapped out the TPS, and cleaned the IAC and MAS (did a forum search to learn how). Anyway, I did a test drive and the throttle stayed exactly where it was supposed to, both during start up and normal driving conditions. yay! Total bill, around 35 bones. Not bad. Now that its fixed my wife can drive it again (the throttle issue scared her a bit I think). thanks again guys, you rule!

j

 

[exploded99]

Careful, don't lose it and go overboard here or you might actually improve your gas mileage!

 

[jekbrown]

damn it all to hell. lol! i was pretty happy with myself earlier today... fixing the Ex and all. Well, we hopped in the beast to drive out to dinner tonight and whaddya know, the rpms zipped up to 3000+ and stayed there. Grrrrrr!!! anyone have any ideas? I did a search and read something about checking the volts going to the TPS.... whats that all about? which wire(s) do I put the meter on? what should it read? if the voltage is out of spec, how do I change it? any help/advice would be appreciated.

j

 

[jekbrown]

bump bump. anyone?

j

 

[jekbrown]

help! rpms still racing... any ideas at all?

j

 

[Creager]

yr gas petal or throttle has to be getting stuck... i dont see this being computer/sensor related at all...

i really dont know how to lube the throttle cable, but if you can, try that. It just sounds like its getting stuck...

 

[jekbrown]

yr gas petal or throttle has to be getting stuck... i dont see this being computer/sensor related at all...

i really dont know how to lube the throttle cable, but if you can, try that. It just sounds like its getting stuck...

i doubt it... mainly because it will run up the rpms when you first start the rig... without ever touching the gas... and this is after it has "calmed down" the night before by the time I got home. If the linkage was just getting stuck it wouldn't work that way. Its got to be something else... hopefully my computer isn't going haywire...

j

 

[Creager]

i doubt it... mainly because it will run up the rpms when you first start the rig... without ever touching the gas... and this is after it has "calmed down" the night before by the time I got home. If the linkage was just getting stuck it wouldn't work that way. Its got to be something else... hopefully my computer isn't going haywire...

j

Well i dont know what to tell ya dude, the computer doesnt control anything on the 'throttle;' other then the TPS which just tells the computer where its set, which you replaced. The TPS wont control both air/fuel coming into the chambers.

I'm pretty sure its something mechanical, does it have cruise control? maybe thats going bogus.

shoot the throttle itself could be getitng stuck

 

[85Dave]

Sounds like a TPS to me. I know you just replaced it, but sometimes a new one will be bad. Especially if you got it from Autozone. Did you disconnect the battery to let the computer reset?

 

[MalcolmV8]

Well i dont know what to tell ya dude, the computer doesnt control anything on the 'throttle;' other then the TPS which just tells the computer where its set, which you replaced.

Yes the computer does indeed control the throttle. It powers the IAC (idle air control) which is a solenoid on the throttle body that when opened allows air to bypass the butterfly flap in throttle body there by increasing idle or decreasing it as it closes it.
Since you already cleaned the IAC and the problem returned I'd replace it. Cleaning it doesn't always work. It may still be sticking because it's old and worn.

Malcolm

 

[Brian_B]

Anyone though of cleaning the back side of the throtle body? My old ranger used to "stick" and that solved the problem. The butterfly was all gummy on the back (clean on the front).

 

[410Fortune]

Check for vacuum leaks, all vacuum lines and around the upper intake manifold

Use a mutlimeter to check voltage at idle of the TPS and also IAC valve.


I hope you reset the battery (computer) after you replaced/cleaned sensors.

Putting the truck into D when its racing at 3000 RPM is not the best idea.

I personally would check the gas pedal for any obstructions, make sure the cables are seated properly, etc.
I would remove the air intake at the throttle body and watch the butterfly open and close, if filthy it can stick but I doubt that is your problem since it is intermittant and the rpm's climb.

the cruise control also controls the throttle, you might try pulling the cable off the tb from the cruise control and see if it returns.

Most of this stuff is covered in some detail on this site.

First thought with HIGH RPM = vacuum leak, however your RPM's are a little higher then the average and because it is intermittant, we have to check everything.

It could very likely be the trucks computer.
Also sometimes even without a CEL, there can be codes stored in memory.
check your Chiltons/Haynes book for the do it yourself code check method (paper clip method) A cheap OBD-I scanner ($30) can be a great tool to have in your toolbox.

 

[Creager]

The IAC isnt going to make the truck shoot up to 3000 rpms.

So where does the computer have direct control over throttle position, other then then the crusie control?

It monitors it, and adjusts accordinly to have the correct fuel amount. Now it is possible for this to be computer/sensor related, but if that was the case then it would be obvious you would have BIGGER problems.

It would have be a coincodence of other problems to make this one issue computer related. (IE Fuel pump sending enough fuel for 3k rpm and the computer getting enough MAF for 3k also,) which i cant make any sense of if the butterfly isnt bascially next to WOT. Next to WOT, not exactly WOT, but close.

I'm not trying to be argumentive here, i am trying to steer you in the right direction. I am 99.99% certin your problem has little to do with the trucks computer or sensors.

Have you tried popping the hood, and having a buddy start the truck for you while you watched the position of the throttle?

Try disconnecting the Cruise control, so we can elimnate that.

This might be off the wall, but is just the RPM gauge going up to 3k or is the actual engine running at 3k? There is an issue with the tachs having a problem where they randomly read upward 3k while the truck is sitting idle... Usually its a sodder joint on the circut board on the back of tach which as come loose or broke. Its a cheap 10 minute fix with a soddering iron. But im sure this isnt the issue since your said you have to fight it with your brakes...

Any progress?

 

[410Fortune]

I agree. 3000 rpm's is a bit more then the air bypass valve can hit when open with a closed butterfly.