Help! What looks like a bad fuel pump but isn't a bad fuel pump?

[TQP]

A few months ago I began to have periodic starting problems on my 2006 XLR. It frequently wouldn't start although the starter would crank. Once it started, it would run fine until the next time I stopped. I had a few mechanics look at it and they couldn't figure out what was wrong. I finally broke down and took it to a dealer who had a mechanic who has experience with these cars. He ultimately diagnosed it as a bad fuel pump. For a couple thousand bucks, he replaced the pumps and sensors in both tanks (all with OEM parts).

When I got the car back it seemed to run fine for a couple of days, then it began doing the same thing... cranking but not starting. However, EVERY time it would not start, I could connect a jump box to the battery and it would start first time and run rock solid until the next time I stopped. Then about half the time it wouldn't start again until I put a jump box on it when it would start right up again.

I recently got frustrated and took it to a different Cadillac Dealership. They are diagnosing it as a bad fuel pump again, but I don't believe it. The probability that they are serially installing one bad fuel pump after another seems slim. Plus it doesn't explain why it will always start with a bit more power from a jump box or how it will fun perfectly and indefinitely once you do get it started.

One more thing... I've put a couple of different batteries in it thinking maybe a dead cell or bad battery. It currently has a nice Odyssey battery that tests perfectly. so that doesn't seem to be the problem.

So, my question is "What looks like a bad fuel pump, but isn't a bad fuel pump? Hopefully somebody here has seen this before.

 

[Fireduck69]

I haven’t experienced this but sounds like a corroded connection some where. This is the same fuel system as the C5 so maybe do some search on the vette forums for similar problems.

 

[mickeytee]

yep. check out those 2 ground wires on the frame at the rear behind the tire. Almoste everyone I have looked at is corroded . take them off clean the m and install a serated electrical lock washer.

 

[nerkpowers]

You should search the site, but I believe there is a similar post and indicated the ground directly from the starter to the chassis was the culprit. Please post your results so we can all learn from this one.


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[David_B]

Behind the tire? or between the door and the tire? There was another thread here about one there I thought..

yep. check out those 2 ground wires on the frame at the rear behind the tire. Almoste everyone I have looked at is corroded . take them off clean the m and install a serated electrical lock washer.

 

[mickeytee]

ground wire

the ones I see are behind the rear tire

 

[BobF]

I am having the same issue and have also replaced the fuel pump. I am having the same issue after new fuel pump so on the last tow to dealer I had them check the corrosion issue but nothing. I have had a suggestion to replace the fuel pump module so will try that once it’s back on the road. My other thought is the ECM but not sure if that’s the way to go.

 

[RubyRod]

According to my budget Service Manual for the XLRs your '06 doesn't have the Fuel Pump Control Module. The Fuel Pump on the '06 & up cars is energized directly from the Battery through a relay. The relay is in the Fuse Box under the hood. This is the same method for powering the Fuel Pumps in most older cars. It's MUCH easier to diagnose than the Modulated system used on the '03 - '05 cars. Not sure why GM changed from the Modulated Fuel Pump Controller back to the direct power method but they did.

You should get a Fuel Pump tester (a gauge that attaches to the Pressure Relief Valve on the Fuel Rail) to verify that you don't have fuel pressure when the car won't start. You can get one at any auto parts store for around $50.

I have seen claims that using a boost (jump from another battery) will allow a failing Fuel Pump that is energized directly from Battery power to start & run. The booster Battery raises the voltage (12V) at the Fuel Pump enough to allow a failing electric motor to operate. I've also read that this is a temporary solution.

If you are seeing improvements in starting & running with a boost AND the Fuel Pump has already been replaced recently I would suspect the Fuel Pump Relay in the Fuse Box. Try swapping the Fuel Pump Relay with one from the Fuse Box that matches the part number of the Fuel Pump Relay but is used on a less critical circuit. Like maybe the Rear Window Defogger.