QuestionQUESTION: Hi wayne, the GMC expert is on vacation...but it seems like you know your stuff no matter what kind of SUV. First of all, my gas guage is broken b/c every time I get near empty, the fuel light comes on but the needle goes haywire. It says I have 1/2 to full tank when I really don't have any. I have learned to cope w/ that. BUT...as of the last 2 weeks here it has been really cold and car wouldn't start last week. AAA came out and b/4 tow, guy said maybe frozen fuel pump so he tapped under truck driver side w/ ball pinned hammer as I started it ans WHAMMO...it started right up. I got it looked at and they wanted over $900 to replace but car was starting fime and guy said to see what happens b/4 I spend. NOW, 2 weeks later, I fill up gas b/c it got low again and left car at my church for few hrs. Temp dropped under 30 degrees and now it won't start. My real question is, A)Do I have a frozen fuel pump and B) What should I be tapping on with a hammer to start? How will I know what the fuel pump or whatever the AAA guy was tapping b/c I am gonna try it tomorrow. I had to leave car overnight and I'm hoping the sun will fix the problem and it will start tomorrow. Do I need a new fuel pump? Is it OK to tap on fuel pump to start? Do I have water in the gas tank? Do I need to flush system w/ any bottle of stuff? Thank you so much
-Brian from Philly
fuelpump
ANSWER: Hey,
A: You have a burnt out sending unit in the tank for your gauge.
B: What happens to GM fuel pumps is that they have two carbon brushes that run on a copper commutator ring to conduct voltage to the armature windings. This pump spins between 4500 and 6500 RPM depending on fuel load requirement. Sooner or later the copper windings either wear down to the insulator or the brushes on the pump do not contact properly and get stuck in their holders or get uneven ends. SEE PIC 1.
What the driver was doing ws tapping on the tank. Tapping on the tank sends a shockwave to the pump and the vibrations help the brushes settle on the commutator ring. The perfect test for this is to use a low amperage probe with a digital graphing oscilloscope. If a technician runs the pump and monitors the waveform of current going through the supply wire he can determine the condition, RPM and winding continuety of the pump on a scale of perfection. Its not water or anything else. The cold pump is just not running because the brushes are probably not contacting the commutator ring.
Wayne
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QUESTION: thanks for the info Wayne, I am going to try to start it now and then get it to pep boys for a new fuel pump. The cost there is only $600+ for everything instead of $900+ at the dealer ship. I hope I can get it started. A)Do you think I definitely need to get a new fuel pump? B)Will a new fuel pump fix the problem?-Thanks Brian in Philly
ANSWER: Hey,
A) I think what you are describing is indicating that the fuel pump has a problem
B) if you have the pump tested with a low amp probe and scope and it shows the anomalies that I drew in windows paint jpg you definitely need a pump.
Diagnostics based on descriptions is not the most accurate form of auto repair but it sounds like you are on the right track.
Wayne
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QUESTION: UPDATE: Pep boys has the car, they ran diagnostic and called to tell me, " You definitely have the burnt sending unit for guage, but the fuel pressure was good. The diagnostic went well but before you spend almost $700 more dollars, we will keep it overnight and run the diagnostic again for free. That will tell us more. It may be a fuel pump but might only be the fuel regulator, which is a lot cheaper". So, that's where I stand folks, I'll keep in touch to let you know. If you have any home improvement questions or wanna know anything about the HOME DEPOT or Roofing, I am your expert- Brian from Philly
AnswerHey
Ask them to do a waveform analysis on the pump. If they say "huh?" Or "we tested pressure and its good" they don't know how. With the technology of low amp probes and graphing meters you can test a pump without popping the hood and take the guess out of it all.
Wayne
