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sprinkles08, ASE Certified Technician
Category: Pontiac
Satisfied Customers: 22809
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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98 Grand Prix GTP 3.8L Supercharged. When the engine temp

Customer Question

98 Grand Prix GTP 3.8L Supercharged. When the engine temp hits around 130 degrees or so the idle starts to search and the engine chugs and just generally runs crummy. It was dying and barely running at all until I changed the MAF sensor. I checked to see if the fuel pressure regulator was leaking and it is not. Watching the data stream on the scanner, it runs just fine until it goes into closed loop, then it starts to act up. The warmer the engine gets the worse it runs. The data stream reads as follows: Fuel system 1 : Closed (when chugging), Load value: 3.1, Coolant temp: 150, Short term fuel trim: Bank 1 -14%, Bank 3 +64%, Long term fuel trim: Bank 1 -21%, Bank 3 +100%, Intake absolute pressure 14.1Hg, Timing advance 20 degrees, Intake air temp 82 degrees F, Air flow from MAF 9 g/s, Absolute throttle pos .4%, O2 sensor output B1S1 .875v, O2 sensor output B1S2 .785v, Short term FT B1S1 -2.3%, Short term FT B1S2 +99.2%
I don't have the first clue what's going on here. All I know is when it warms up above 130 degrees and it goes into closed loop the idle searches and it runs very poorly. I could use some help.
Submitted: 1 year ago.
Category: Pontiac
Expert:  Juan Crespo replied 1 year ago.
Hi there.Some of the PIDs you post are way out of the norm. The OBD II parameters for fuel trimming are generally 25% either positive or negative before setting P0171, P0172, P0174 or P0175 codes. You post STFT B1S2 at +99.2%, yet mention no codes. So I'm thinking either the scan tool is not very good or the PCM is really screwed up.When adding the long term trim to the short term trim, you obtain a basic “total trim number”. This number should be very close to zero, never exceeding 10% positive or negative. For example, a LTFT of 3% and a STFT of 1% would give a total trim of 4% which is within the normal range.Other data that would help with the diagnosis would be, first of all any codes current and pending; then actual fuel pressure and engine vacuum (both at idle and under load) as read by a gauge, not through that scanner.You might also want to conduct a thorough visual inspection of the Oxygen sensors, including not only their connectors, but the rest of their harnesses as well.Also, engine temp should not have to reach 130 degrees for the system to go into Closed Loop - you might also want to get a manual reading through a gauge.Please let me know what you find and we'l take it from there.
Customer: replied 1 year ago.

A. You're talking way over my head

B. I have a test light, a read only scan tool, and a DVOM.

C. There are no codes, but the O2 monitor and the EGR monitor are not set.

Expert:  Juan Crespo replied 1 year ago.
I’m sorry, but I don’t have any other information that could help you at this time. I’ll opt out so others may try. Please do not reply until another expert responds or the site will cycle this request back to me.Best Regards.
Expert:  sprinkles08 replied 1 year ago.
Hello and welcome to JustAnswer!Do you still need help?
Customer: replied 1 year ago.

Thank you, no. I sent the car to the GM dealer.

Expert:  sprinkles08 replied 1 year ago.
Let me know how it goes!