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Vladgmru
Vladgmru, Automotive Electrician
Category: Chevy
Satisfied Customers: 2509
Experience:  I've been working with American made cars since 1992.
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2000 grand am intermittent high idle,stock engine., should

Customer Question

2000 grand am intermittent high idle,stock engine., should be at 600 rpm's, but when i start it up some times it jumps to 2000 rpms in park/ neutral and then go to 1100+( i put the + because i can let my foot off the brake and the car will drive itself and the rpms will move like i have my foot on the gas when i do not). By turning it off and on it goes back to 600 rpms, but might start doing it again while i am driving. i took it to my Pontiac dealer and they got nothing from their diagnosis. no trouble codes, nothing. Any ideas?
Submitted: 5 years ago.
Category: Chevy
Customer: replied 5 years ago.

2000 Pontiac Grand Am Gt 3.4L V6


Already Tried:
took to dealer for diagnosis and came back with nothing, replaced mass air flow sensor, IAC
Expert:  Vladgmru replied 5 years ago.
HelloCustomer
My name is Vladimir.
You should ask your dealer to verify 2 important things.
a) They should check all vacuum lines and hoses for any leaks.
b) They should connect a TECH 2 scan tool and observe the engine data while your car is running at high idle. They should pay attention on TP SENSOR and TP ANGLE data.
I will attach the two pictures of my TECH 2 snapshot.
The pictures were taken from the same year Grand AM, but it had a 2.4L engine.
On both pictures the TP SENSOR voltage was 0.47V, but the faulty PCM understood it as 4% throttle opening while the new PCM got it correctly as 0% opening.
graphic
graphic
Customer: replied 5 years ago.
So a faulty PCM will only show up at the time of high idle?
Expert:  Vladgmru replied 5 years ago.
If a PCM has the same kind of glitch as was in my example, yes it can be discovered while idle RPM is high.
Anyway a technician has to catch and determine the nature of this faultiness( I mean high idle problem) before any parts replacement.
Vladgmru, Automotive Electrician
Category: Chevy
Satisfied Customers: 2509
Experience: I've been working with American made cars since 1992.
Vladgmru and 3 other Chevy Specialists are ready to help you
Customer: replied 5 years ago.
I took it back to the dealer yesterday when it was high idling and they said it was a PCM problem. It was not communicating with the IAC. Thank you very much for helping me out! i hope when i get my car back it runs like normal.
Expert:  Vladgmru replied 5 years ago.
Thank you for the accept.
I am glad to be helpful.
It would be great to hear any news after PCM replacement.
If you have a time, please reply in this topic.
Regards,
Vladimir.
Customer: replied 5 years ago.
I tested it out when i got it out. It worked great, but when i drove 50 miles to school it started doing it again, but its not as bad. i contacted someone who had the same problem and he told me his was a throttle body gasket. This is so frustrating.
Expert:  Vladgmru replied 5 years ago.
Hi!
So, did they replace the PCM ?
Did you show them my snap shots?
Customer: replied 5 years ago.
Yes, they put a brand new PCM in it. What hoses/ vacuum lines specifically would make the engine high idle?
Expert:  Vladgmru replied 5 years ago.
I can't say exactly, any vacuum leak( include maifold gasket leakage) would make the engine high idle.
Do you have to pay for the new PCM if it doesn't cure the problem?- I am just curious how it is in the USA.
Once I had a Chevrolet Caprice with intermittent high idle. It was a hard to catch problem , but I succeeded in finding it. It was a broken wire, one of 4 wires going to IAC valve. The rupture was near the PCM connector, so a contact on this circuit was intermittent. When this circuit was open, idle RPM was high.
When this circuit was OK, idle RPM was OK as well.
Ask your technician to check IAC wiring( 4 wires) from PCM to IAC valve for continuity, they have also to wiggle a wiring harness during the check.
Customer: replied 5 years ago.
that sounds like it could be the culprit, because they said when it was high idling that the PCM was not communicating with the IAC. i will let you know what happens.
Customer: replied 5 years ago.
o yea, LIM(lower intake manifold gasket) has been a huge problem in these cars. if they are not functioning properly or are leaking could that cause this?
Expert:  Vladgmru replied 5 years ago.
I suppose that a lower intake manifold gasket leakage would cause engine hesitation rather than RPM rising.
The PCM doesn't communicate with the IAC valve, the PCM controls it ( moves its pintle back and forth).
Thanks for your replies.
Customer: replied 5 years ago.
ok, so i just dropped it off at the dealer and when they put the tech 2 scanner it read at the 4% tp angle. however, when i saw it and said something to them and showed them the pics they said it was normal and alot of cars do not read at 0%. They are going to look at the new pcm they put in and see if it is faulty. I also forgot to mention i got in a wreck 3 years ago. i rear ended someone. They did not replace the engine, but did replace some other stuff. So the dealer is looking at the grounding of the car as well to make sure thats not what the problem is.
Expert:  Vladgmru replied 5 years ago.
I especially provided the 2 pictures with the BAD and the GOOD PCM's. A TPS voltage is important data too.
As you and your technician can see on both pictures TPS voltage is 0.47V, but the BAD PCM interprets it as 4% of TP angle, while the GOOD PCM interprets it as it MUST be 0% of TP angle.
Of course they should check all the grounds.
I whish I could diagnose your car, I like such a problem :-)).
Hope they will find the problem.
Customer: replied 5 years ago.
Ya, when i took it to them they told me it was 50/50 it was the PCM. They are going to tap on it and several other parts to see if its something like wiring problems or something else. Because when they hooked up that Tech 2 and tried to up the rpms it was not registering with their device they put on the car. they put an instrument that was round with a clear face and had 2 lights. when they were testing the car by revving it with a black box the lights would turn solid red and green when it was revved up and then when they brought it back down they would both blink really fast. Do u what that device is?
Expert:  Vladgmru replied 5 years ago.
Yes, I have known about this device. That is an IAC signal tester.
Ask your technician to load test the four IAC control circuits between the IAC connector and the PCM connectors.
I use a 51Watt #9006 bulb to load test any suspicious wires.
Such a bulb draws about 4 Ampere current and if a circuit has any problem, a bulb will not glow or will glow dimly.
Customer: replied 5 years ago.
well, they did not find anything yet so my car will be there over the weekend, ill message when i get an update.
Customer: replied 5 years ago.
ok, so i called them and they told me they are putting in auxiliary grounds. They found three grounds that were not grounded properly. What is your take, can the grounding of the car really cause this problem?
Expert:  Vladgmru replied 5 years ago.
Hi.
Thanks for keeping me informed.
Yes, bad grounds can create a lot of weird problems and not only in PCM controlled systems.
A bad ground can also skew PCM input data.
Hope this will finally cure your Pontiac.
We'll see it In the near future.Smile
Regards,
Vladimir.
Customer: replied 5 years ago.
It is still acting up, i am taking it back again. They told me they are not charging me (they did not charge me for the grounding). I will let you know if they can find anything.
Expert:  Vladgmru replied 5 years ago.
Ok.
May I ask , are they a dealer shop or GM specialized service station?
Maybe you should print out all this topic to them, hope it will help somehow.
Wish you good luck.
Customer: replied 5 years ago.
its a dealer service shop with GM certified technicians. The problem is not as bad as it was. The engines in cars these days last and perform way better, but I know though the technology is making cars worse to work on. The intermittent problems must really be frustrating for anyone who attempts to work on them because the only way to know whats wrong, is for the car to be doing it. I think automakers need to find a way for the computers to record everything so technician's can better pinpoint a problem.
Expert:  Vladgmru replied 5 years ago.
Yes, I agree with you.
If we (technicians) could contact automakers engineers, it would be great for customers and the automotive repair industry as well.
By the way, a TECH 2 tool has got an ability to record data.
You can drive your vehicle while a tech is recording it on the passenger seat.
Customer: replied 5 years ago.
So they put a new PCM in it again and it had NO effect whatsoever on the car. it has been the exact same from the 1st pcm to this one. i remember them saying at one point the tps voltage was .63V when it was high idling. should that be a concern? also, are there any relays/fuses that have anything to do with my problem that i can check? The dealer is so stumped. They said if it was a vacuum issue or something mechanical it should do it all the time. i took off the electrical component that goes on the IAC when the car was acting right and it had no effect at all on the car, i even turned it off and started it again and still no effect. Is that normal?
Expert:  Vladgmru replied 5 years ago.
Hi!
1. "the tps voltage was .63V" - seems to me a bit high. What is the TPS voltage when it is normal idling? Is it possible that your throttle cable or the throttle sticks? It must be verified.
2. I think there is nothing to do with relays and fuses. If there was a problem with the PCM related relays or fuses, the car would not start.
3. What did you mean by saying :" i took off the electrical component that goes on the IAC" ? Did you imply the IAC valve's 4pins connector?
4.Has your dealer tried installing a new TPS sensor and a new IAC valve? I have met such intermittent glitches with those parts, but on other GM's vehicles.
5.Could you ask your dealer to take two Tech2 screen shoots, one is for normal idle and another is for high idle?
Customer: replied 5 years ago.
yes i took off the 4 pin connector to see what it would do.I think the normal is .47V. My big question is that if you say the TP angle MUST be 0%, what could make a PCM see it as 4%. because all three of them have done that, so i was thinking maybe something is messing with the pcm and throwing it off. When they did the "noid" test with the IAC tester and their tech 2. they could not get the IAC to respond with their tech 2. Another reason i think something is messing with the PCM's communication is because like i said it has been doing this for 3 years. Only recently has it been this bad though. if it was the TPS or IAC(it could be because i replaced with an autoparts store part not a factory one, i also did not put oil on the O ring so maybe their could be a leak?) wouldn't that throw a trouble code? I am taking it to them Tuesday because i work 6am to 6pm and they are closed before i get off.
Expert:  GMTech77 replied 5 years ago.
Hello, I know you are already working with another expert and dont want to step on toes or take anyones accepted answer. Your expert you are working with knows what he is talking about. The problem you are having I have seen in a few like vehicles. Your TPS voltage is too high and is tricking the pcm into thinking the throttle blade is open and the pcm is kicking up the idle. You most likely have a bad TPS sensor or the wires at the connector for the sensor are at fault. Have fixed the ones i have seen with a new TPS sensor and connector. If it were me thats what i would do next. Hope this helps you out, I know this can be a frustrating problem.
Expert:  Vladgmru replied 5 years ago.
Hi!
I completely agree with GMTech77 expert.
I would: install an original TPS, load test 3 TPS wires between the PCM and the TPS, make sure that connectors and pins are OK.
Now about this weird percentage.
This percentage represents the PCM's opinion on the TPS voltage it receives.
If the PCM sees 0.47V and understand it as 4% of throttle opening, this PCM is faulty.
I suppose it is an internal PCM problem. I don't see any other reasons for it.
You can also try to disconnect the A/C refrigerant pressure sensor.
I have seen various problems on older GM's vehicles, when that A/C sensor was skewing other sensors readings.
Customer: replied 5 years ago.
when the tech had hooked up the tech 2 and i looked at the screen it read TPS voltage .63 and TP angle 4% at the time of high idle. i have not seen what the tech 2 reads when the car is not high idling. I am going to ask that when i take the car there.
Expert:  Vladgmru replied 5 years ago.
Ok.
We will see it.
Customer: replied 5 years ago.
So when i dropped it off today i asked about the tech2 read when the car was not at high idle and of course its the same exact reading as when it is in high idle. This gets weirder everyday. What do you make of that? I am waiting now to see if they can find anything. I also found out they have called the corporate garage and are communicating with them.
Expert:  Vladgmru replied 5 years ago.
Hi!
Look, in automotive repair works, we have to be more specific. Every trifle is important, especially in intermittent and mysterious cases as yours.
Quote : "and of course its the same exact reading as when it is in high idle"
The same reading of TPS voltage?
The same reading of TPS percentage?
To analyse a problem, we have to compare all engine data that whould be taken in both cases ( normal idling and high idling).
Another thing I want you know, a NEW part, does not mean a GOOD part. It is only supposed to be this way, but sometimes a NEW part is only Never Ever Worked part.
Customer: replied 5 years ago.
the tps voltage are .63v and 4% tp angle AND from what the tech told me, any other data on the tech 2 is the same when the car is high idling and when it is not high idling. Sorry about not being more specific.
Expert:  Vladgmru replied 5 years ago.
As I stated before, 0.63V seems a bit high for me. Are they ( dealer staff) able to take readings from another Grand AM and compare the TPS voltage?
Customer: replied 5 years ago.
ok so i got the car back. A different tech guy was able to take a look at the car and what he found was that there a was terminal messed up. He explained to me there are 80 terminals that connect to the PCM and the wire for terminal 38 (which is one he said that controls the IAC) was in terminal 39 and vice versa. He said after he did that he put the tech 2 back on and it read 0% TP angle. He also said he did the "noid" test and everything was flashing perfect. So i will let you know how it runs.
Expert:  Vladgmru replied 5 years ago.
Sounds magically!
Well, some questions arise, who has inserted the terminal #38 into the empty( not used) #39 hole ? Why did the car operate OK in past?
I cross my fingers, hope it will be a final step in this story.
Customer: replied 5 years ago.
It is still high idling, BUT it is not nearly as bad as before. I will be taking it back to them on Tuesday again and let you know what they find. I am not sure he checked all 80 because he told me he only got an hour to look at the car. So with this issue out of the way I think this might be the last time i take it their because he knows the car should be at 0% Tp angle and should pass the noid test on the IAC when not high idling. I let you know what they find.
Expert:  Vladgmru replied 5 years ago.
Quotation: "It is still high idling, BUT it is not nearly as bad as before."
I think, that this kind of fault could not be Better or Worse.
It is or it isn't.
How long have you experienced this problem?
Customer: replied 5 years ago.
The high idling has been going on and off for three years.
Customer: replied 5 years ago.
ok, so on December the 12th they put in a new TPS and after driving to work for the past three days it is working flawless! So i am going to call it fixed. I really appreciate your help. Thanks Vlad!
Expert:  Vladgmru replied 5 years ago.
Hi!
I am so happy to hear that good news.
It was a quite long story.
Thank you for your reply and you are always welcome!

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