Ed Scherer

Yeah, you're right: I didn't do a good job of eliminating the possibility that the fuel pump relay wasn't being energized. I should have done the "check for power on 30" test and 30/87 jumper (I had a jumper along and forgot to try this). Brent: this is a good thing to try, as it forces the fuel pump to run regardless of the input signals to the fuel pump relay. It's a divide-and-conquer test that helps determine whether it might be something upstream of the fuel pump relay (crank position sensor, etc.) And it's a no-cost test.

Oops, acknowledged. I should have remembered that: the LH failures are usually due to the failure of some Palaeolithic era hybrid circuit independent of the map EPROM, right?

So... IIRC, it would be OK for me to temporarily try my LH in Brent's car and/or vice-versa, right? Even with the '90S4 / '91 custom map GT difference, they should be compatible enough to swap?

But, isn't the typical failure mode of the LH such that the observable symptoms are injectors going random (a "clickathon") or some such? We're not hearing that on Brent's car.

Yeah, I mentioned that to Brent, but wasn't in a good position to investigate that roadside yesterday, although I believe pulling the fuel pump relay and bridging the 30/87 terminals on the relay socket as Wally recommended would be the first step in possibly (if the 30/87 bridge allows the engine to start) working backwards to seeing why the relay coils aren't being energized. One cause of that (among others) then would be the crank position sensor signal, right?




In summary... at this point it seems like a good idea to work backwards from the fuel pump, starting with what Wally suggested. Checking for power on terminal 30 and then bridging 30/87 at the relay socket is a really easy next step to further isolate the problem.

And trying a donor LH (which I'd be happy to provide, if it's advisable to try a '90 S4's LH on Brent's SCed GT) is another easy thing we could try later.

dprantl

I don't think it would be a good idea to try another LH in Brent's car. He has 30lb injectors and all the fuel maps are very different from stock. A stock LH would probably run extremely rich, if at all. The only brain that would be guaranteed to work in his car would be the one from my car

Fuel pump jumper, you know you need one. On friday, I towed the boat out of the back yard and parked it in front of the house ready for a weekend at the lake, woohoo. Tow vehicle ('91 SAAB 9000 auto) is idling while I go outside to close the gate and as I am walking back down, car dies. WTF? First thing I do is take my jumper wire and bridge the fuel pump, don't hear anything. On the SAAB it's best to jumper the right side contact of the fuel pump fuse socket to the right side contact of the hazard light fuse socket. Since the hazard lights are powered all the time, it bypasses pretty much everything (except the wiring to the pump itself) and is putting +12 directly to the fuel pump. This is the same thing as jumpering pin 30 to 87 on the fuel pump relay socket on a 928. Good thing I had a spare walbro pump; one hour later we are on our way.

Dan
'91 928GT S/C 475hp/460lb.ft

Ed Scherer

Brent: note that John Speake has a nice, concise summary of good troubleshooting techniques for dealing with a no start condition.

See Troubleshooting Porsche 928 Starting Problems.

Thanks, John... I don't think I ever noticed that page before today.

Ed Scherer

OK, noted.

Arggghhhh! And I had one along in my "spare relays and fuses (and jumper!)" bag. I could kick myself for not remembering to try it.

fraggle

If it still won't fire when you've got the pump relay jumpered I'd vote crank sensor. You need an oscope to really test it right. I have one but I'm not all that close by.

Brent

Thanks guys. Well, I'll test for power on the 30 terminal in the relay socket, then jump from 30 to 87. If I need a known working LH for my car, sounds like a call to MidNight Auto down in Atlanta.
I did get the info on the Bosch 044 FP. I need to pull the cover and see what's on the car. I got a busy week and probably won't get to it until this weekend. I'll report later.

John Speake

Yes the hybrid is the reaso for most LH failures, but not all. Not all LH failures give the "death rattle" :-)

Trying another stock LH to help diagnose this no start would be valid, Mark Anderson used stock LH with 30# in his race car some years ago.

A good check for fuel pump issues is to measure the fuel returned to the tank (RHS of trunk, under the big rubber bung, lengthen retun pipe into a container, bridge FPR for 30 secs, you should get 1350cc in the can.

Ed Scherer

I do, too, and I am.

Ahhh... another good idea I'll try to remember for this and future fuel-pump-related issues.

Ed Scherer

Nice avatar, Brent.

Hopefully, you can replace it soon with a "car in motion" shot.

Brent

Sweet. Ed you're the man. I can see my 928 armamentarium increasing very soon.

dprantl

Cool, that's good to know.

Dan
'91 928GT S/C 475hp/460lb.ft

Brent

OK, finally got to check things out. Fuse is good, contact fine. 30 on the relay panel is hot. Jumper from 30 to 87- no FP noise, no start. Did not try Ed's LH because we both feel it has to be the FP at this point. I was going to try the reverse polarity trick but the FP is turned to where I can't get to one of the terminals without removing the lines first. What's the point in that?

Question: if I change to the Bosch 044 FP, will that change the mapping/tuning on the car. I still don't have a good handle on the SC thing. And would it be a good idea to delete the internal FP if I use the Bosch 044? Something Andrew Olson alluded to I believe.

FCSFrank

Hello - Can't help you just wanted to say Hi to another Kansan. Too bad i"m clear across the state

WallyP

Before you start replacing fuel pumps, make sure that you actually have 12 vdc at the pump...

andy-gts

I have a gts fuel pump that was running when replaced 1 month ago (preventive maintenance on 105k) its yours if you want it! call me 785-505-6162 , I will be here till 7 pm
andy