Sporadically cranks but won’t start

[BobF]

I am desperate to find an answer. Only happens sometimes but after driving for awhile and parking it turns over but won’t start. Tow it to dealer and starts after getting off tow truck. Had recent fuel pump replacement still acting up. Thanks

 

[mickeytee]

Have you had the fuel pump module looked at. It usually just fails and wont start but car will spin over..

 

[BobF]

Have you had the fuel pump module looked at. It usually just fails and wont start but car will spin over..

Thanks here’s the issue, last time it wouldn’t start I went online and found that same suggestion so switched it around with another identical module and it started. The problem is I didn’t try starting it again before switching so not sure if that was the issue and it’s been parked for the winter and has been starting every time. It has also been mentioned that it could be the engine control module so that may be my next turn after trying this module again when it won’t start agaIn.

 

[RubyRod]

First get a Fuel Pump Tester (gauge that attaches to the Pressure Relief Valve on the Fuel Rail). They're not expensive and readily available. Confirm that you don't have fuel pressure when the car won't start. If you have fuel pressure but the car isn't starting it's NOT the Fuel Pump "System". If you don't have fuel pressure then it MIGHT be the Fuel Pump "System". But that's the hard part.

The Fuel Pump "System" in the '03 - '05 cars consists of the Electric Fuel Pump, the Fuel Pump Control Module (aka "Modulator"), and the ECM. The Modulator pulses the power to the Electric Motor in the Fuel Pump to vary the speed. This is called a Pulse Width Modulated Electric Motor. The ECM sends commands to the Modulator to vary the fuel pressure according to engine demand. The Modulator also sends a signal back to the ECM. This signal is called "FPMP DIAG SIG" (Fuel Pump Diagnostic Signal I believe). I don't know what information is carried on this DIAG SIG. According to several on-line articles I've read on the XLR this system uses 3 "speeds" to vary the Fuel Pressure. Diagnosing the modulated pulses to the electric motor is ... challenging. Diagnosing the ECM is impossible, at least for DIY. I have my doubts that the dealer or an independent shop can diagnose the ECM either. I'm struggling with this system on my car at present.

I've read that most modern cars use a modulated Fuel Pump controller to vary the speed of the Electric Motor. I don't know if these systems are multi-speed like the XLR or are infinitely variable. But instead of a separate Control Module like the XLR this function is incorporated directly into the Engine Computer. If you can't isolate the problem on your own your best bet is to find a repair shop that has experience in diagnosing modern PWM Fuel Pump Systems. Maybe an independent shop, maybe a dealer. Doesn't have to be a Cadillac dealer. Any GM dealer might be helpful. It all boils down to experience in diagnosing the PWM system.

 

[BobF]

Wow thanks for all of this. It’s parked for the winter but starts at very time I try it so waiting for a non start like the last one on a trip to Idaho where i shipped it back to the us Canada border when as soon as it came off the flatbed it started but with a boost from my other car.

 

[RubyRod]

Something you might want to check is the ground point at the rear of the car on the driver's side behind the rear wheel fender liner. This is the ground point for the Fuel Pump Control Module (FPCM). I've found posts regarding problems with this ground point. Separate it and clean it, remove any corrosion you find, and re-assemble. I've also read that this is a sore point on the C5/C6 corvettes as well. So it's worth checking. A bad ground can result in intermittent electrical problems like the one you describe.

 

[BobF]

Something you might want to check is the ground point at the rear of the car on the driver's side behind the rear wheel fender liner. This is the ground point for the Fuel Pump Control Module (FPCM). I've found posts regarding problems with this ground point. Separate it and clean it, remove any corrosion you find, and re-assemble. I've also read that this is a sore point on the C5/C6 corvettes as well. So it's worth checking. A bad ground can result in intermittent electrical problems like the one you describe.

Thanks I had the dealership check that on the last visit and they said all was good but don’t know if they disassembled. I will look at that again

 

[RubyRod]

I measured the resistance on mine with my ohm meter (DMM) and it shows good but I couldn't flex the wiring harness to the ground point to determine if the wiring was OK or not when the car was in motion. I also took mine apart and found no issues or corrosion but I'm still wary of this ground. I'm trying to create some diagnostic test leads so I can monitor the ground with my 'scope when the car starts and, if I can catch it, when it won't start. It should let me know if the ground is an issue or not.

If you don't have a 'scope the other thing to try would be to wire the FPCM ground to an alternate ground point on the chassis. I may also give this a try just to rule out the ground entirely.