James Bailey

O K this has me wondering...." Pump runs jumpered with no effect on rail pressure...." so with it jumpered it makes no pressure ???, Have to ask do you have gas in the car ?? Do not rely on the gas gauge. I would (after confirming fuel) probably jumper the fuel pump and loosen a fuel line fitting near the pump and see if fuel sprays out. Be very careful wear eye protection do not do this in the garage and have a good fire extingisher close and the garden hose ready.....

Bill Ball

I had written a long diatribe blaming the LH until I re-read the thread and saw that you jumpered the fuel pump and the rail fuel pressure was still low. Excepting that, your symptoms are one of the classic LH failure modes; one which I had (car died abruptly, would restart but run very poorly and soon die). They do not fit relay or CPS failure or ignition monitoring system trip, as in the former two the car will not run at all and in the latter case the car runs but will be low on power.

Anyway, as it stands now, I think Dan and Jim are now leading you in the right direction.

928 at last

Has me curious too now.....

You are correct. With the jumper in, the pumps runs smooth and quiet as usual. Absolutely no effect on the rail pressure. When I tested it, the rail was showing 28 lbs, and that 's where it stayed. (Ususally runs about 54 lbs).

Car is pretty much full tank right now. I had just filled it up about ten miles previous. So, it's got plenty of fuel in the tank.

Right now the rail is at zero pressure, and I haven't cranked the car again yet to see if it comes back up to the 28 lbs. That's why I was wondering about the system upstream of the pump to the rail gauge. The only thing I can think of is that somehow the regulator is cutting flow/pressure.

Judging by the way the car is acting, once I get the pressure back to the rails, it will run.

Just had a thought while I was typing this......checked for "tank vacuum" and no evidence of a breather problem. One more possibility eliminated.

928 at last

It hasn't started today, but did briefly twice last night. It "wanted to" when the rail had the 28 lbs, and caught for a second, but no luck. I was worried about the LH, but the test came up with no error codes, and the car does "want to start".

Imo000

Rob,

I have my brother's S4 in my garage for a TB/WP job, if you need to borrow some parts to test your car, let me know.

928 at last

Thanks Imre!
Appreciate the offer.
Not sure where to go next, but I'm leaning towards the FPR right now. I just don't know if it 's possible that it could fail in a way that would create the issues I've got.....

James Bailey

Some terms...Cranking is when it goes whaa, whaa, whaa and never fires. Trys to run would be if while cranking it fires sputters but fails to continue to run when you let off the key from the start position. I am not sure what "Wants to start" means except that is has a good attitude Still not 100% clear on what the fuel pressure is with the pump relay jumpered... Zero or 28 Psi ? Also being a 1990 it probably has the in tank fuel pump if so perhaps that is what is running smooth and quiet and the main high pressure pump is dead ??

Bill Ball

Well, yes, in theory, it sure could be the fuel pressure regulator. The injectors won't spray below about 30 lbs.
I suppose you could PARTIALLY pinch off the return line either by the fire wall or back at the top of the fuel tank (under that large plug in the hatch area right side). Use a "soft" clamp with just enough force to almost close off the lumen. Crank the motor or just power the fuel pump briefly and see if the pressure is higher. I wouldn't run the motor this way of course because the pressure is not regulated properly and may go too high. Anyway, if the pressure rises I would say that implicates the FPR.

James Bailey

The fuel return line is at the top of the fuel tank on the fuel level sender. Under the carpet under the black plastic plug in the rear cargo area. You could carefully pinch the return hose with padded vicegrips to see if the pressure will rise when you block the fuel return (pump jumpered).

Bill Ball

Yeah, let me rewrite what I wrote above. Jumper the fuel pump relay socket (30 -> 87) so the pump is running continuously. Then soft clamp (padded ViceGrips, like Jim said) the return line at the front or rear. If the pressure rises above 28 or whatever baseline, the regulator is faulty.

928 at last

Thanks!
That's tomorrow after work then!
Will report back.
"Good Attitude" is about right. Spins up on the starter and will catch for a sec, but not enough to actually run for more than a second or two. Certainly no revs or power.
Where is the external high pressure pump located?

928 at last

Rail pressure was at 28 lbs when tested. no effect on pressure noted. I haven't tried jumpering from zero as yet, but that's on the list for tomorrow.

dprantl

The main fuel pump is under the fuel tank at the back of the car under a metal cover that is held on by two nuts just forward of the rear bumper. The fuel filter is also under that cover. There is a short rubber hose that goes from the pump into a tank fitting. Connected to that tank fitting is the in-tank pump via the infamous short rubber hose that usually fails within two or so years.

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

928 at last

I'll report back tomorrow evening after I check the external pump is running and what the jumoer does to fuel pressure from zero.

James Bailey

Not only does the car have a good attitude....so does the owner !