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Pontiac Repair/1990 Pontiac grand prix 3.1L surging


QUESTION: I have this 1990 Pontiac grand prix 3.1L that I have asked you questions before about. Every sensor we can think of has been changed. It sounds good but when it gets warm it surges. It is driving me and my husband nuts. Could it be something as obvious as the fuel filter, plugs or oil change? We are at our wits end. Of course there is noone with scanner around here that can do this model car. Please help. It just runs too good and due to elderly woman having it and selling it before the kids could fight over it it only has 107000 miles on it. Last thing changed was map and it did not help. we did the idle learned procedure.

ANSWER: refresh my memory on this I answer a lot of questions. You replaced the engine control module (ecm) computer and did the idle learn procedure. Yes something like a fuel filter can make it run bad especially if it plugged solid. They are over looked all the time. another thing to look at is the PCV valve only use a AC Delco valve not a "fits all" get the correct one for the engine and year. check for vacuum leak with it running spray around any of the vacuum lines with carb cleaner. if the engine idle picks up or the idle smoothes out you found a vacuum leak. one other thing on these depending on the emission system and when it was built it could have a vacuum controlled EGR valve or a digital egr valve with three solenoids on it. if it has the vacuum style valve when the engine is hot snap the throttle off idle and look at the diaphragm on the valve it should not move up and down. if it does the valve is bad. last thing to check is the fuel injectors. those old one the insulation breaks down inside the injectors and then they short out and cause all kinds of issues. you need to remove the top of the engine "intake plenium" and check each injector separately. unplug each injector and with a digital ohm meter check each injector the resistance should be between 12-13 ohms. when the engine is cold. if you have an injector that is higher or lower then what I said that is the issue. good luck :-)

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QUESTION: We finally got a diagnostic scanner and it says that MAT sensor is one code and the oxygen sensor is the other code showing up. Where is the MAT sensor located that would be the problem? We think on the intake manifold but could you give us specifics? I am so sorry to keep bothering you but I have to get to work. This is the only problem with the car. Frustrating. I keep pulling up online and it is giving me MAP sensor which is nothing like the MAT in appearance and it is not what we are trying to locate. Thank you again.

ANSWER: there is nothing called a MAT sensor. what is the code # you are getting?

A MAP sensor for sure can cause a erratic idle. The MAP sensor is the main load sensing sensor for the engine computer. it may not be a bad sensor  it nneds adiquite vacuum to give the correct reading and the wiring between the ecm and the sensor has to be ok. does the service engine soon lamp come on all the time?

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QUESTION: We are getting 13 and 23. That is what it says in the book that came with the scanner. my husband thinks it screws into the intake manifold and there is one online for sale at Advance Auto Parts. Advance says it is the Mass Air Intake Sensor. It just surges when it gets warm and dies when start and put in drive or reverse. We cannot find where it is located.

9 out of 10 times a code 13 is a bad oxygen sensor. as for the air intake sensor it's in the air cleaner housing. it just has 2 wires on it. you may want to do a google search to see if you can find a trouble cahrt to follor to see if indeed it's a senor or wiring issue. my onl;ine service info only goes back to 1998 cars/trucks.

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Todd Puccio


General auto repair,electrical issues,driveability issues,anti-lock brakes,SIR systems,HVAC,engine performance,Gm specific. I have worked at my current dealership since the 1990 model year anything much older than that I have limited exposure to. I do not have very much transmission experience so those question you may want to ask of another expert. When you send me a question let me know the exact make/model/engine and year for the car or suv you have it makes it easier to answer the questions that way. If your happy with the answer I gave you please feel free to Donate. This is a free service to you and I am not compensated at all by this web site. I have 22 years of experience to share with you and my knowledge wasn't free to me. I had to pay for my education.


Experience in the area: I have been a technician in GM dealerships for over 22 years and I am a ASE master technician. I work at a Buick and GMC truck store Previously I worked at a Pontiac GMC store for 18 years. I Served a a lead technician when dealership was on a team system. I just recently moved to a Buick,Hummer,Saab Store when my franchise was purchased by this dealership. Organizations: ASE master technician


Education/Credentials:High school diploma,2 year vocational school degree 2 year college degree. Associates degree in applied sciences (auto technology)Awards and Honors: GMC master technician

Awards and Honors
Master GMC truck technician Master ASE technician in 9 categories

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