'96 S10 4.3L burning up fuel pumps

Hi,
I posted a fuel pump question here in March 2006 thanks for the help. By the way, replacing the brushes in my original fuel pump only afforded me ~4k extra miles, the replacement brushes I found at a motor repair shop where to soft. Anyhow, I purchased an AC DELCO OEM fuel pump/sender module part# MU11 ($465.00) in April 2006.
My truck ran fine (just like the old days) until March 2007 when I posted here again with a stalling problem. It would run for about 15 minutes and stall then not restart for several hours. I replaced O2 sensors as suggested and did a normal tune-up. I never checked the fuel pressure as I had just replaced the fuel pump/sender module a year ago. After a few test runs it eventually quit running completely. I checked the fuel pressure, none (I guess I was in denial after what I paid for the new module). A dead OEM replacement fuel pump after ~14K miles (no return on electrical components).
I purchased another AC DELCO OEM fuel pump/sender module part# MU11 ($515.00 – inflation I guess) in April 2007. I installed new fuel pump/sender module, turned on the ignition and it fired right up (just like the old days). On Monday 6/18/07 I was on my way home from work and the truck stalled (about a 20 minute drive). HMMMM…… got towed home, checked fuel pressure there was none. Let truck set overnight. Next morning I went out, turned the ignition over and it started right up. Checked fuel pressure to see if it was Ok and it was. Now you think I left for work right, wrong, I have had enough of being stranded on the side of the road and being towed home by wife on the end of a tow strap or should I say neck snap!!!!!! Just as before the truck stalled after 15-20 minutes and there was no fuel pressure nor could I hear the pump start when I would turn the key on. Another dead OEM replacement fuel pump after ~4K miles (remember - no return on electrical components).
I have checked for voltage drop with approximately a 5 amp load (old headlight highbeam still worked) connected to fuel pump power wires and tested with a Fluke 87 Voltmeter, voltage held strong at 13.5 volts only dropped .05 volts on inrush. All power connectors looked clean and in good condition and voltage drop test seemed to be fine. I did change the fuel filter the first time around in April 2006 and checked it again in March 2007 when I replaced the fuel pump for the second time.
Any ideas what is causing the fuel pump deaths??????? Anyone now the actual AC DELCO fuel pump they use in the MU11 module (AC Delco part# EP???) AC DELCO will give me no help. The local dealership will help me for a hefty price and I will still have the same problem I have now in a few months. I have seen replacement pumps on EBAY and other websites but none of them are 100% sure that there pump is a SPEC OEM replacement. HEEEEEEEEEEEEEEEEEEEELP Please.

Thank You,
zapit480v

4-5 things that will make a go bad.
Running low on gas.
Stopped up fuel filter
Bad wiring.
Bad ground.
Bad pumps.
Bad gas.
Over or under voltage.
What is going bad on fuel pumps?
Like quiting or low pressure?

Well, I'd start buying cheapo fuel pumps from an auto parts store as the next step.

First, MT-2500 is correct about the low fuel level significantly shortening the life of electric pumps. However, the later design "fuel tank modules" include a reservoir surrounding the pump. The reservoir gets return fuel and pump bypass fuel, and remains full even when the tank is nearly bone dry. The tank has to go completely dry before the reservoir will empty, whereupon there is no longer any pump cooling. GM figured that one out about ten years ago when the stand-alone pump was replaced by the "module" in most applications.

Second, and in my limited experience, undervoltage kills fuel pumps faster than anything. It was very astute of you to load the circuit to test I/R drop, but you can test that ONLY up to the tank. The connection at the very entrance to the tank "module" is highly suspect - So much so that the aftermarket has developed a repair kit to solve the deficiency.

If you still have the first replacement pump module, you can disconnect the pump harness at the top of the module and inspect it for heating. Don't be too astonished if you find something like this:

http://72.19.213.157/files/PumpConnector.jpg

Finally, I have personally used the ebaY replacement pumps and have not had any return for failure since using the first one several years ago. Of those I have replaced, at least two of them were "Master" brand and one was clearly marked "Carter" on the pump body itself. One (I can't recall which) had a bunch of numbers stamped in the pump body, including the matching GM OEM part number. If you go this route, make sure you follow the directions for fitting the pressure hose (convoluted or "accordian" tube) properly. I chickened out and used 100 PSI Goodyear FI hose and clamps instead of the "heat and install" tubes.

Thanks for the input.

I did test voltage drop with the module pulled from the tank all the way through the circuit to the pump connector right at the connector that attaches to the pump (a little dangerous but I wanted to be sure) with an old headlight. 13.5 volts and very little inrush drop drop for a moment and came right back to 13.5 volts when filament was burning bright. Connectors are not burned/bubbled nor is the wire insulation burned/bubbled near the connectors. I did have the transmission rebuilt last year (the screw shell gear snapped for the second time, no 2nd, OD or reverse, another chevy flaw) and verified the harness was intact and not damaged to the intake manifold.

I checked ebay for replacement pumps and checked the feedback for those sellers related to pump returns and was discouraged. The other ebay discouragement was the fact that GM says that there is no replacement pump for that paticular MU11 model and the ebay sellers won't deny of confirm it is the pump I need. Their ads say it is but when asked for spec no one can give a straight answer.

I then decided to purchase AC Delco complete MU11 modules from the chevy dealership to attempt to insure I was buying a quality part.

My tank is rarely below 1/4 of a tank.

No auto parts store or GM dealership have just the pump for this specific truck, only aftermarket versions of the complete AC Delco MU11 module.

But since I did find a pump on ebay that at least states that it will only fit my specific vehicle (replaces pump in MU11 Module), chevy s10 4.3L V6 and the GMC Sonoma 4.3L V6 and not all models from '95-'02 which I know is incorrect and the seller offers a 1 year warranty (wow..someone warranties an electrical component, we'll see) I guess I will give it a try $65 is a lot cheaper than $500+ each burnout.

Thanks and I will post back if this pump burns up tooooooooooooooooooo.

Thanks,

zapit480v

So, you DID test the entire harness, which is good. That doesn't leave much but the pump. The problem is that you still have to determine if the failures are electrical or mechanical. Sediment, water, astringent cleaners, and contamination in the fuel can wreak havoc on a pump impeller, bushings, and winding insulation, as well as dissolve the brushes and erode the commutator. You might want to consider using fuel from a different supplier, just in case.

One possibility is the variant fuel pumps used in the "Robust" fuel system, intended for use in Flex Fuel / E-85 use. Not only are those pumps different, but the fuel filter and several other components are different from the "regular" fuel systems.

It is possible that is why the replacement pumps are listed as "not all models" but I'm not certain if that's why.

I do purchase the majority of my fuel from the same gas station (meijer "C" station, not sure who their supplier is) up the street from my home and have for about 7 years.

I cleaned the tank when I replaced the first pump module. The first time it was somewhat dirty and had some water in it, I could see a bubble of water below the fuel (the truck was 10 yrs old at that time). The second time (1yr later) it was clean and I could not see any water or debris so I left it as it was and replaced pump module.

I will try one of the ebay replacement pumps and see what happens. Maybe it was just a fluke that I got 2 bad replacement fuel modules, not completely out of the question I guess.

Blue Bowtie, the pic you showed of the module connector with the corroded male connection prongs, is it removable for inspection with out damaging the module (I pitched the first 2 modules in the trash)? I would like to verify there is no connection/corrosion problem there that may not have shown up in my voltage drop test before I replace the pump. If it is corroded I only tested for a couple of minutes (didn't want to wear the battery down) and it may not have generated enough heat at that point to show up on my DVM. As I said, right now the vehicle will run for 15-20 minutes before the fuel pump stops pumping and the engine stalls. At that point you cannot hear the pump prime for several hours and as soon as you do hear it prime it will start right up again, but only for 15-20 minutes and then the same cycle starts. During the time that pump will not prime I can pour fuel into the throttle body and it will run until the fuel is consumed.

One last thing I just thought of that I did not before is that I have seen AC relay contacts act similarly when overheated or pitted. But then again there is a lot less mass to cool than the pump has. Also when I changed the modules previously I did not have the same symptoms until after the fuel module had been in service for awhile so maybe this is a tangent I should not spend the time to pursue. MT-2500 sent me a wiring diagram of the fuel system the first time this happened so I could go through the entire system and check for continuity wire by wire. Maybe I will at least replace the relay just in case or at minimum verify the cold contact resistance matches the other contacts on similar relays (if I remember correctly there are 3 relays side by side behind the access panel in the glovebox including the fuel pump relay).

Thanks for the input All,

zapit480v

One other thing I just realized that I forgot to check is whether I even have voltage at the pump module connector after it stalls after the intitial 15-20 minute run when the symptoms occur.......DUH. I have only checked pressure at the manifold and listened for the pump to start when the symptoms occur. Maybe I will check that today and post back. Although again when I have replaced the fuel modules in the past the symptoms do not appear until after the pump has been in sevice for awhile, 1yr the 1st time and 5mos the second time, but I will check it anyhow just to to rule it out. Sorry I have been doing electrical design for so long now that my electrical troubleshooting skills have gotten a little rusty.

Thank Again

zapit480v

Hi again,

I did check the voltage at pump connection when truck would stall, voltage was present even though pump was not running.

Needless to say I purchased a replacement pump from ebay (considerably cheaper than purchasing the complete MU).... The truck is running fine again!!!! For how long???????????? I'll just have to wait and see.

After reading some other postings here I will also be changing the Bosch Plugs, which I just replaced back in April during a tune-up, to AC DELCO..... I have to look on my receipt but if the Cap, Rotor and Plug Wires were not AC DELCO should I replace those also or are they not as touchie as the plugs???

Thanks Again,

zapit480v