Nick33483

Help! After happily driving my 1996 993 on Sunday, it would crank but not start on Monday morning. Have replaced DME relay and fuel pump fuse, checked immobilizer, checked battery voltage. I suspect fuel pump (though it has a Bosch unit and fuel filter replaced March 2022) as the best I have managed is a thump when testing with jumper wire between 87, 87b and 30 with DME relay removed. I never hear the fuel pump prime though I do smell fuel after cranking. I was planning to pull the fuel pump but then noticed that when cranking, the needle on my tach does not move at all, not even a bounce. Wouldn’t that indicate flywheel sensor instead? Thinking I need to eliminate that first. Or does the tach needle not move on a 993 when cranking? Any advice will be much appreciated.

Nick33483

Update: Got fuel pump to prime with a jump wire between 30 and 87b -- so I believe that eliminates fuel pump. Started checking around DME with a multimeter and got the following results with ignition key off: 12.6v at battery, 12.6 volts at #30, 12.4v at #86 (also got same 12.4v at #86 with ignition key on), 4~20mV at #85 (not sure if this indicates a proper ground or not). With DME reinstalled, and a wire from #87, I get 12.4v with ignition key on. My next steps will be to remove blower and check ground connection at number one intake runner. Then I will move on to examining the flywheel (crank reference) sensor. Any other suggestions are welcome.

Nick33483

Further update: over the past few days, many attempts have been made to start the car after trying different things. Result was always the same, crank but no start. Having earlier yesterday managed to prime the fuel pump, I decided to try to start the car again, which it did, immediately! I let it idle for a few minutes, shut it off and started it again. Everything seems normal now. So I have absolutely no idea what was (or perhaps still is) happening, Now just wondering if/when the problem will resurface.

pp000830

If the problem presents itself again, and before digging too deeply check to see if there is a spark when cranking?
If not investigate the coils and the circuit that supplies them.

IainM

You don’t say how many miles but this is kinda how fuel pumps fail, they go intermittent first.

dbf73

Try more than one replacement DME relay? They’ve been known to be bad out of the box

pp000830

Hi Iain,
If Nick can get access to Durametric & an MS Windows Laptop or some other OBD Reader that supports actuations the pump for his 1996 OBDII 993 can be turned on and off with it. If he encounters a non-start in the future and the pump won't manually actuate with the reader it most likely identifies the problem as either the DME relay or the pump. The Durametric's "enthusiast cable" version at $287 will pay for itself many times over in short order.
Even if Nick doesn't plan to service his 993 on his own in the future, running the codes and looking them up on Google when he has a problem and what others have done to resolve them will arm him with information. Information to understand if what a servicing technician is telling him makes sense. This can avoid costs by avoiding unnecessary labor, parts, and upselling.
Andy

jay@EZimmoblock.com

The tach needle doesn't move when cranking, but you can check it at the OBD port with a DVM. See here for where to probe and what to look for.

Steve Weiner-Rennsport Systems

Doug is spot-on here. These have had dubious quality for a very long time. I used to re-solder the CB inside them on each & every one before I sold or installed them to ANYONE. No exceptions.

The number of cold solder joints and cracked traces was staggering.

IainM

Great to see you back Steve, long time no read

Gbos1

Wow! Steve is back!

Nick33483

Thanks for the replies. As a further update, the car continues to start and run perfectly. If & when this issue resurfaces, I will post again. I suspect it was the fuel pump even though it was a new Bosch unit replaced March 2022 with only 600 miles since.

NineMeister

The problem will be the fuel pump. We've had a spate of failures of brand new "Bosch" fuel pumps and after investigation and discussions with Bosch directly, it transpires that the market has literally become swamped with so many poor quality counterfeit pumps that Bosch have suspended selling them through their network until the problem has been resolved. The only supply exception for original Bosch pumps are Porsche, hence if you want to guarantee that you have a genuine Bosch pump you have to buy a new one from Porsche.

I suspect that you have inadvertently been supplied with a counterfeit fuel pump.