Cadillac XLR Forum Discussion, Default What looks like a bad fuel pump but isn't a bad fuel pump? in Past Cadillac Vehicle Discussion; A few months ago I began to have periodic starting problems on my 2006 XLR. It frequently wouldn't start although ...
- 08-21-18 11:31 AM #1Cadillac Owners Member
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- Aug 2018
Default What looks like a bad fuel pump but isn't a bad fuel pump?
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.
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- 08-21-18 06:06 PM #2
Re: Default What looks like a bad fuel pump but isn't a bad fuel pump?
If the pump (there is only one for both tanks) is good, the Fuel Pump Speed Control Module is another thing to check, in conjunction with the ECM output. This assumes the battery has been load-tested and checks out.
The FPSCM runs the pump with a variable voltage level (128 Hz Pulse-Width Modulation) based on a signal from the ECM (another circuit to check). When the ignition switch is initially turned on, the FPSCM is commanded to full power (up to 16 v). The pump delivers fuel to the rails and injectors. The fuel pressure regulator in each tank controls the amount of fuel fed from their respective tank, and allows excess fuel to remain in the tank. The regulators are part of the tank sender assy. The ECM changes the pump speed by varying the PWM duty cycle. At idle, the pump voltage is around 9.5v. When higher volume is needed, the voltage changes to 1100% of the battery voltage.
The fuel tanks and their operation are identical to the Corvette from that period. As long as the fuel filter is clear, if the pump is good (easy to determine with a Tech 2 or your dealer's diagnostic tools) then the electrical path between the ECM and FPSCM should be verified (by monitoring the data on a Tech 2, etc.) Most of the symptoms you've described are attributed to faulty FPSCMs or their electrical connections.