Hi all, The instructions for TPS adjustment say to adjust with the engine off but why is this? I have the TPS set to 0.54V with the engine off but when the engine is started and idling the TPS voltage comes up to 0.70V - which is out of spec. I guess this is due to the battery voltage coming up with the engine running, and since this is when the ECM is reading the TPS I would have thought this would be the best time to set the TPS. Enlighten me please.

Since no one seems to know the answer, how do we know the way we're told to do it is the right way?

I think this info is in a FAQ. I don't think mods would allow it to be put in there if it was wrong info.

You are done ...Thats correct, should be reading 0.54 with engine off...I have adjusted mine with engine off, manual says so...No more adjustments needed....Now drive that bad boy ...lol

Are you having additional problems with the car?

But do the instructions as per the manual take into account the state of battery voltage with the engine off compared to with it running? As I said before, my battery voltage comes up with the engine sitting at idle. Presumably this upsets the calibration?

Yes, I am actually having some issues with the car. It seems to all of a sudden have a high idle, up to 1000RPM. I know the IAC has a high count because I can hear the difference in sound the engine makes (louder sound of air being sucked through the air filter), and a ALDL log confirms it also. The question is why? The car seems to drive fine otherwise.

If it were a vacuum leak then I would have thought the IAC count would be low because the engine would be getting extra air elsewhere.

Ideas?

But do the instructions as per the manual take into account the state of battery voltage with the engine off compared to with it running?

I do not believe "the state of battery voltage" has anything to do w/ your calibrating the TPS. The minor amount of voltage that is fed to the TPS can certainly be handled by your battery even if it was a little low in power, which it is not as you are not experiencing that problem. If you have followed all instructions and you still have a high idle, we can continue to help.

I believe I have followed all instructions correctly. The higher idle revving is constant, it's always about 1000RPM and prior to this behaviour I have not made any adjustments to the car at all. This symptom pretty much "just happened". I suppose the upside to this is that at least it's constant rather than sometimes showing up.

Ideas please?

you have to take into consideration that when engine is cranked that the other sensors may be idling the engine up a little. when engine is off throttle blades are completely closed. when idling the iac and other sensors and computer are making adjustments all the time. if air conditioning is on the iac or ac solenoid will speed the engine rpms thus moving the position of the throttle blades,jimmy

That might be true but doesn't explain why my base idle is now always at least 1000RPM, when it's supposed to be, and has been in the past, lower. About 700-800 from memory. The question I keep asking is why?

you didnt play with the minimum idle screw did you?

No, haven't gone near it. That's what I don't get, this symptom "just happened" without me doing anything.

hmm, have all vacuum lines been checked? brake booster connected? how about the IAC vale itself, perhaps stuck open?

just adjust the idle screw,also you can check inside the throttle body for a small burr on the blades or the body itself. if youre displeased with the voltage put a scanner on it and adjust it that way,jimmy

I agree w/ Chevy8588, check vacuum lines. That is how crap happens, it just starts one day. W/ a vacuum leak the motor tries to rev up to compensate for the low vacuum-easy to check-listen for a sucking sound, not at your wallet, yet, but under the hood-check all lines for cracking, check connections for broken plastic, hoses, etc.

dont play with the screw, that screw is set from the factory, it will not solve your problem, if you mess with that screw you will NEVER get your idle proper again.

well how do they set it at the factory??

yes it was put on there from the factory for a reason so you can adjust the idle. it was adjusted for the sensors that came on it from the factory .however these sensors were not made to last a lifetime .some sensors act different than others (different brands) tps springs can be a little stronger on one than the other causing a little faster idle,jimmy

earlier in your post, you said that ALDL log confirms a high IAC count. what else does the ALDL data tell you?

Sorry for the late reply guys, life just likes to throw some busy days in for good measure!

I have checked the vacuum lines and I can't see anything wrong there. If I did have a vacuum leak, I would have thought the IAC counts would drop off, not go up, no?

My idle screw is adjusted from factory anyway as I don't have a stock engine anymore. If I adjust it any further then it's going to throw my TPS off adjustment, which means I'm going to have to elongate the slots on the TPS to get 0.54V with the engine off. Those slots are maxed as it is.

I used to get somewhere between 20-50 IAC counts at idle in gear, but now it appears to be about 80. To make the car a bit more driveable I did a new PROM which forces the IAC count not to exceed 125 - the car jerks less when putting it into drive when stationary as the RPMs have been forced to be lower with this method.

The ALDL data log also shows when my foot is NOT on the accelerator, the TPS volts is about 0.69V. Cruising around town the IAC counts are around 80, but go up when I'm idling. Not down like they're meant to.

Thoughts are appreciated.

You may want to try to figure out your PROM data on the DIY/PROM forum of this site. I would have to do a little reading on this to be of any help.

I think I have a lead on what the problem relates to. I checked out a ALDL log I made a couple of months ago, and another one even older. These were recorded before this problem showed up. Both logs show idle voltages of 0.55V when the engine is cold and running - well within the specification.

You'll recall earlier on in this thread that I was seeing 0.70V with the engine running. This is quite a big change and is out of specification. I wonder if my TPS is bad? However, watching either a ALDL log or voltmeter hooked into the TPS, the voltage climbs steadily and uniformly to over 4 volts like it's meant to. It's just that base voltage when the engine is running is bad.

Ideas?

I managed to find the source of the problem. I did not think to mention before that I have an LED throttle position display module. Each LED represents 10% throttle and hooks into the TPS signal that feeds into the ECM. This was faulty and causing the voltage drop I was seeing. As soon as I unplugged it the car reverted back to its normal state and operates properly.

glad to hear it

you are the man!