High Idle, Goofy TPS - Fixed

2001 Saturn Coupe, 1.9L engine with a manual transmission. Neighbors car, asked me to check codes, got P0131, P0300, P0326, P0507, P0621, P1599, P1620. Idles around 2300 rpm. Says idle has always been high but its been getting higher. The engine was replaced with one from the boneyard before he bought it. He cleaned the IAC pintle and passage way, checked and lubed the linkage. Looked for obvious vacuum leaks. I scanned it and looked at some of the data. IAC starts at 80 then goes to 0 count, likely a vacuum leak. The odd thing is the TPS. With the KOEO the voltage looks normal, .5 volts closed, rising smoothly to 4.5 volts when the throttle is opened. With the engine running though the TPS starts at .5 volts but snapping the throttle it rarely gets up to 1 volt. Would that mean a short to ground in the TPS circuit? Suspicious of that P0621 code as it's for the alternator and the "short" only happens with the car running. Didn't get much time with the vehicle but should be looking at it again this weekend.

Anybody have any experience like this? Any thoughts on it. It looks like a vacuum leak, just a matter of finding it but the TPS has me scratching my head.

A couple of things I would start with. Check again for a vacuum leak -- including via the brake servo take-off, the other is the main engine ground

The refresh rate on the scanner may not be catching the full range of the TPS on a quick snap.
Considering it registers the full range of voltage KOEO, I'd say a short to ground probably isn't in the cards. Although I didn't look up all those code definitions you posted.

Depending on what you're using for a scanner, you may be able to select just the TPS voltage pid and display it as a graph, then snap the throttle and see what you've got.

I like Andy's suggestions, and am going to throw my hat in the ring to check for a leaking fuel pressure regulator.

Sorry about the codes, laziness on my part. P0130 - O2 Sensor low voltage sensor 1, P0300 Random misfire, P0326 Knock sensor- pcm has retarded timing as much as possible but still have knock, P0507 - Idle speed more then 200 rpm higher then desired for more then 10 seconds, P0621 - Generator L terminal output low or circuit 25 shorted to ground, P1599 - engine stalled without shutting off ignition, P1620 - Low engine coolant. I'm paraphrasing from the GM manual. I used my Uscope to test the tps so I think I got an accurate reading. I couldn't get easy access to the alternator and had limited time with the vehicle so I didn't get to test output although in hind sight I could of checked at the battery at least. IAC checked out for impedance and like I said it shows as working on the scan tool, it's an Autel ds708. Fuel trims were normal, ect and iat looked correct as well. The coolant isn't low but it is filthy. It's one of those cars that gets nothing but fuel and mistreatment until something breaks so just trying to make it a little safer to drive. The high idle is tough to drive with since it's a manual shift. Not sure how a bad fuel regulator would cause this problem but I'll take a look at it. Thanks for the reply.

Well that blows all that out of the water!

I was thinking that the regulator could be leaking fuel into the vacuum line, causing the high idle. But the trims are good so I doubt it now...
And if you were using a scope, refresh rate of the scanner wouldn't be an issue either.
Oh well

This might be a good candidate for the water test, especially around the intake gasket area. Pour some water around the intake sealing surface and see if you can hear it suck in. It'll be obvious where it's leaking when you find it.

The TPS behavior is interesting... It never goes above 1V with the engine running? Even on a test drive? :huh: Whenever you get back to this one, it'd be cool to watch the TPS percentage, as this is the PCM's interpreted value of the TPS voltage. It should sweep to near 100% at WOT, and always return to 0% at closed throttle. If it never reads zero, or if you can tap the throttle and make it read off zero when closed, then that'd be a strike against the TPS.

I plan to use the water test if I get another look at it. Was hoping to this weekend but it didn't work out. I didn't drive the car but in the driveway the highest voltage I saw on the TPS was 1.2v with the car running. Not sure if I had it matted or just revved it high, definitely over 4000 rpm. He was talking about taking it to a shop so I might never get the answer, that would be a bummer.

Yeah, not getting to finish a diag is always a bummer. But, whatcha gonna do?

I have seen GM PCM's fail internally and cause issues like this. One recent one had a resistive short between the TPS and the MAP sensor signals inside the PCM, causing surging at highway speeds and idling problems. Proving it is tough - I had to cut the TPS wire close to the PCM while watching scan data.

Anyway, your low TPS voltage symptom got me thinking. :lol:

Getting another look at it this afternoon, Woo Hoo.

Any luck, tiride? Not trying to nag, just curious.

Not sure what happened, I replied before I went on vacation but I don't see it here so I'll try again. Second time looking at car the TPS worked as it should although it now had a P0122 tps low circuit voltage code that wasn't there before. IAC worked, at least it moved with cycling the key and resistance was in spec. Cleaned the throttle body, it was filthy. Found a broken ground strap on PCM which we fixed. No difference. Used the water test around the intake mostly, got a couple of times where engine seemed to smooth out a bit but nothing definitive. Used acetylene around the intake while watching the O2 sensor. Best I could get was about 3 seconds of smoother running and a rich O2. Figured intake gasket but with no smoke machine suggested he take it to a local shop to get it smoked and prove it before doing intake. He did and the intake is leaking. He's going to change the gasket tomorrow. I'll update after the install.

Thanks for the update! The P0122 couldn't have been accidentally set by you or the owner, right? Wondering if the TPS isn't acting up intermittently. :huh: That'd sure help to explain the readings you were seeing earlier.

Thought about the TPS code being a result of our playing around, quite possible I would think. I'm kind of hoping we can duplicate the TPS symptoms we saw before just because I'm curious, although I'm sure the owner of the car is hoping changing the intake gasket fixes everything. Between the alternator code, the broken ground strap and the TPS issues I'm sure there's more going on but one thing at a time.

Neighbor got intake gasket changed today, says the car idles properly now. Have to wait and see if codes come back but for now calling this a fix. Thanks for your help.

tiride62 wrote: Neighbor got intake gasket changed today, says the car idles properly now. Have to wait and see if codes come back but for now calling this a fix. Thanks for your help.

Fixed enough for me! Let us know if more issues come back, and we can dig further.

So, now that you know the intake was indeed leaking, what are your thoughts on the water test?

It's definitely a useful test, easy to do and cheap but of course it has limitations. This is the third time I've used it. So far a hit, found a leaky purge valve on a 5.3 L chevy truck. Limited success on this Saturn, hard to apply the water because of positioning of intake but I did get some results. Enough for me if it was my car but not good enough to call on someone elses car. No dice on a 6.2 L GMC that I could tell had a vacuum leak but couldn't find it, turned out to be slight head gasket leak at the rear of left head. I think I need to find or make a smoke machine for evap and vacuum leaks.

tiride62 wrote: It's definitely a useful test, easy to do and cheap but of course it has limitations. This is the third time I've used it. So far a hit, found a leaky purge valve on a 5.3 L chevy truck. Limited success on this Saturn, hard to apply the water because of positioning of intake but I did get some results. Enough for me if it was my car but not good enough to call on someone elses car. No dice on a 6.2 L GMC that I could tell had a vacuum leak but couldn't find it, turned out to be slight head gasket leak at the rear of left head. I think I need to find or make a smoke machine for evap and vacuum leaks.

Agreed - the challenge is getting the water to the suspected leaks. :lol: Some other members here have made their own smoke machines with awesome results. The cool part is how you can dial the pressure up higher than the usual 1 PSI most smoke machines run.

Do you know of any threads that talk about making a smoke machine? Seen a couple on Youtube so I have some ideas but any resource I can use is welcome.