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Peter Bagley
Peter Bagley, Toyota mastertech
Category: Pontiac
Satisfied Customers: 669
Experience:  master tech at Toyota dealer
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Poor acceleration, low mpg. Replaced everything under the

Customer Question

Poor acceleration, low mpg. Replaced everything under the sun you can think of. Slight vibration at idle. No codes. Had this happen to 3 other vehicles I've owned. Talked to veteran GM mechanics. No solutions. Said everything is fine. But I know something isn't quite right. Use to run smooth, good acceleration. Good mpg. Heard a click under hood and conditions of engine/performance started. Absolutely puzzled. Maybe a bad relay/solenoid?. But not bad enough to trigger a code?. I'm stumped.
JA: How many miles are on the car? Has the car been starting normally?
Customer: 115,000. 2000 Grand Prix GT (non sc)
JA: Are you fixing your Grand Prix yourself? What have you tried so far?
Customer: New plugs, wires, fuel filter, trans fluid and filter, cleaned maf, new oil and filter. Regulator is good. New pcv valve. Cleaned throttle body. No avail. Problem still exists. Gas pedal use to be easy. Now stiff. I think the pcm adjusted something.
JA: Anything else you want the mechanic to know before I connect you?
Customer: That should do it. Thx
Submitted: 9 months ago.
Category: Pontiac
Expert:  Peter Bagley replied 9 months ago.

Hello and welcome to just answer and allowing me to assist you sometimes another eye can spot thigs others can't so lets give it a this a cable throttle or by wire?

Customer: replied 9 months ago.
Expert:  Peter Bagley replied 9 months ago.

Have you disconnected the cable from the throttle body to see if that stiffness is in the cable or throttle body?

Expert:  Peter Bagley replied 9 months ago.

Have you ever had the injectors power flushed because if they don't have a proper spray pattern to animize the fuel to burn properly it will do those symptoms. at this mileage i would illuminate that and put a good fuel system cleaner such as Lucas in the tank to follow up. Without ant codes I would start there. Also do you or can get a vacuum gauge to check manifold vacuum/?

Customer: replied 9 months ago.
Ok thx. I'll try that. I'm not positive but I'm almost certain it's something electrical telling the engine to adjust. I've also experienced coil packs going bad that will not trigger a code. The engine doesn't "chug" or stumble. Tells me coils are good. It's only averaging 21 mpg on highway. New plugs, wires, fuel filter, regulator.
Expert:  Peter Bagley replied 9 months ago.

check you EFI and engine fuses

Customer: replied 9 months ago.
Will do. Will it still run even tho fuses may be blown?..that would make sense if it runs poorly and not receiving proper signal. Thx again.
Expert:  Peter Bagley replied 9 months ago.

depends which one is gone. but still wouldn't hurt to do that injector flush at that mileage.

Expert:  Peter Bagley replied 9 months ago.

The other thing is and I know this is why people come on here looking for a quick fix but because of the complexity of engine managements on these cars today, I strongly recomend to do a full scan and active test , this shows a good tech how all the perimeters are operating and gives one a better idea where to start.

Customer: replied 9 months ago.
Thank you for much needed info. I'll try and do all those things. Injector flush, scan, fuse check. See what we come up with. But what if all those check out good and problem still exists. My solution is trade off car. Lol
Expert:  Peter Bagley replied 9 months ago.

LOL lets not shoot the goose yet until we have all those findings.

Customer: replied 9 months ago.
Just all fuses. All good. Started car. Let run a couple mins. Put my hand on the intake and feel some vibration. Acts like its needing something. Just don't know what. I tell by listening to it that it's not smooth. Maybe a thermostat? New fuel pump relay? New coils perhaps? Can even begin to tell ya. I just feel that's it's something electrical controlling smoothness, power, fuel economy, slight vibration, etc. no codes. Absolutely mind boggling.
Expert:  Peter Bagley replied 9 months ago.

Your thinking too hard lets do this in logical steps as I mentioned or you'll go bananas. illuminate one at at time that may cause this.

Customer: replied 9 months ago.
Ok. Lol. Think I'm already bananas.
Expert:  Peter Bagley replied 9 months ago.

remember one step at a time.

Customer: replied 9 months ago.
Good morning. Wanted to give you a lil update. Cleaned throttle body and IAC sensor. Now harder to start and idle is everywhere and then calms down after I "peddle the gas" to keep it running. No change in mpg. Also put in FI cleaner. Wanted to let you know.
Expert:  Peter Bagley replied 9 months ago.

Morning, ok going over our notes.Have you disconnected the throttle cable to see if that stiffness was in the cable or other wise? have you power flushed the injectors? This is what I would recomend now, power flush the injecters ,replace the throttle body with tps sensor.

Customer: replied 9 months ago.
Ok thank you again. Will do. Will let you know as soon as I do those things. Thx
Expert:  Peter Bagley replied 9 months ago.

very well. the difference with doing a proper injector flush and just putting cleaners in the fuel tank is the stuff you put into the tank is very deluted and works over time ,but power flushing goes right to the injecters with a strong cleaner to properly clean the nozzles for maximum spray pattern .