losing power when warm - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 08-18-2013, 01:05 AM   #1
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I have a 98 bonneville v-6 that i use on a mail route. just replaced the fuel pump. now, it is harder to start (cranks for 5-6 seconds before it will catch) and the fuel gage does not read correctly. but here is the big problem - after about an hour of start and stop driving, it acts as though it is not getting fuel, or is choking. this gets progressively worse until it finally stalls. if i let it sit for about 10 minutes, it will start again and run for about 30 minutes before the problem repeats. any idea where to start looking for the problem?
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Old 08-18-2013, 05:45 AM   #2
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Go check your coolant level.

Do you know when the last time the intake gaskets have been replaced?
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Old 08-18-2013, 08:48 AM   #3
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sounds like icm or crank sensor
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Old 08-18-2013, 02:36 PM   #4
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I'm leaning towards crank sensor too. They are sensitive to temperature when they start to go bad; no signal from the crank sensor and the ECM will shut everything down (spark and often fuel too). Since you use it for a mail route, the temps under the hood will be warmer than if you were driving at a steady 50 mph, so temperature-related failures will be much more likely and you will be more sensitive to them.

However, since you have just recently changed the fuel pump, there is a chance you got a bad pump out of the box; also, since your gas gauge is not reading right, I would double check the electrical harness at the top of the tank (where the fuel pump and sending unit plug into the harness) and also check the ground strap from tank-to-chassis. Also, and this is occasionally overlooked, but when the car loses power, what'* the fuel level? Are you over 1/4 tank? Fuel in the tank keeps the fuel pump cool; if you have lower fuel levels, the pump runs hotter, and will fail sooner. (Speaking from experience here, I've been a substitute driver for a few different newspaper routes.)

Another thing you may want to check is restricted exhaust. Because of the increased temperature of a car that doesn't have much air circulation around it (low speeds and lots of stop-and-go), plus the car'* age, you may have a bad catalytic converter.

Yet another possibility is a failing Mass Airflow Sensor, which sometimes doesn't set a trouble code. All I can offer is possible solutions without a little more info. Is the Service Engine Soon light on? The Security light? When you say it gets progressively worse, is it a sudden loss of power that then gradually gets worse and worse? Or is it pretty much a steady decline in performance from a cold start?

Don't go throwing money at the Bonnie though, until you do a little further diagnosis. I'd hate to see anyone replacing a myriad of parts in the attempt to fix a problem, and not many people have disposable income like that!

Also, the sending unit float may be rubbing against the internal "basket" it sits in. *IF* you drop the tank again, pull the sending unit out and move the float slowly back and forth in its full range of motion for several minutes. I've had great success in fixing "sticky" fuel gauges this way. But this won't cure your loss of power...
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Old 08-18-2013, 10:03 PM   #5
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thanks for all the input! i just checked, and the coolant was about two quarts low. i've only owned this car for two years (and 100,000 miles) so i don't know if the gaskets have ever been replaced. the car has 240,000 miles on it. maybe i better give you the long(er) version of this story as well - this is actually the third fuel pump in about three months. the first replacement would make a very loud whine after it got warm. i waited to long to have it replaced and it failed as well. the second replacement again made the loud whine. this time the shop used a genuine gm replacement pump. after the first replacement, the check engine light came on. now, this is really bad, but i almost always ignore that, as every vehicle i own has that light on all the time.

i'm about as mechanically inclined as an electric eel, so i will probably take this to a shop for repair. the shop that did the work (and the place i work) is sixty miles away, so i will probably have someone local look at it even though warranty may (or may not) cover part of the cost. i was hoping there may be a fix easy enough that i could do it (unlikely i know) or at least have some idea what to suggest at the shop since making the car replicate the symptoms is nearly impossible. and yes, because i know so little about it, having some idea what they may find makes me feel more confidant with their diagnosis. i trust the shop i use here, but their specialty is more towards oil changes, alignments, etc.

again, thanks for the help, and any other thoughts you may have would be greatly appreciated
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Old 08-19-2013, 07:22 AM   #6
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you get or borrow a fuel pressure gauge and rig it so you can see it while driving and if the pressure doesnt change when it does act up. if it does could be the pump connection, my inlaws had a 96 that they thought was going through pumps but actually turned out to be the pump connector was intermittent and a bit corroded, so i soldered and heat shrunk the wires together
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Old 08-20-2013, 02:54 AM   #7
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wow. just wow. got word from the mechanic that it is indeed a bad fuel pump. is supposed to run at 45 psi, this one starts at 26 and drops from there. they assure me that this is the problem. is there anything else that would cause the pump pressure to drop? i've heard suggestions (not from here) ranging from a bad gas cap to something in the tank. not looking forward to throwing another $400 at this problem, even if they do guarantee this will fix it. after all, so did the last shop. thanks for letting me vent. (get it..fuel system - vent. yup my skill as a comedian no better than my mechanical skills)
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