dr bob

The fuel pump relay on your car IS a standard 53B. Replace it with a known-good (read: NEW) brand-named relay and you'll have solved half the problem.

Carefully reassemble everything you've taken apart, paying particular attention to possible vacuum leaks at the MAF and throttle-body connections. Inspect the boot at the throttle for cracks/splits/blowouts, hoses disconected or plugs blown out, etc, especially if it's the original boot. They have been known to split under boost, allowing enough air to leak in to fool the airflow sensor during cranking. I have no clue what mapping Tim uses for cold-starting, particularly with the bigger injectors; guessing it's pretty light to avoid flooding before airflow is detected. Hence the concern about air leaks past the MAF, especially with the low-vacuum readings you report. Check for cracks at the oil filler, loose oil filler cap, split or otherwise leaking oil vent and air hoses under the intake, etc. Any could be a false-air source. Dwayne's guidance in intake refresh includes a plumbing-parts fitup that allows you to pressurize the intake to find leaks. If you have compressed air and a small regulator handy, make one and use it to prove that there are no intake air leaks. I haven't needed one (yet...) so don't have one to send/lend.

Jfrahm

Well the only thing the jumper might do is make it possible for me to walk around and read the fuel pressure before it drops off, the relay might be OK. I might not have any pre-run of the fuel pump however, if I am supposed to.

dr bob

I'm pretty sure there isn't any pre-run on mine ('89). The CPS sees the first few teeth pass by as the starter cranks, relay closes, pump runs, car starts.

When you mentioned idle fuel pressure same 20 PSI as cranking pressure, that tells me the relay is closed and supplying insufficient current.

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My car has actually stratnded me exactly twice. Once in the garage after an extended business project kept me away from home for a seriously extended time. Didn't know it would be that long when I left. The other time was at the bottom end of my driveway, when a weak/tired fuel pump relay affected running. I shut the car off and tried to restart it, but no luck. That second episode caused me to miss a critical lunch meeting with two 928 VIP's (Rob E and Greg B). Once I got the car dragged back up the hill (it was l o n g driveway...) it took just a few minutes to figure out the problem after fitting the fuel pressure gauge and jumpering the socket. Car fired right up with the new relay. That new relay, incidentally, was in the little carry-along spares bag in the tool compartment, as was the jumper and the relay extraction tools. Moral: Replace the critical (FP, LH, EZK, X-bus, FI) relays all at once, before you find them weak. Do it in the comfort and convenience of your garage. I still carry a few spares, in case my horns or foglights don't work or something.

Jfrahm

If there is no pre-run then I expect I have higher fuel pressure when cranking and it drops to 20 by the time I get around to the gauge to read it.

My next try I suppose is to put the supercharger pipes back on and see if for some reason it will cold start in that configuration.

I may also borrow an oscilloscope or buy a cheap pocket digital one, or a USB laptop one.

outbackgeorgia

Fuel pressure should go to 38 and stay there. Why is it dropping? It should take hours to drop below 30
Dave

Jfrahm

I do not know, probably a lazy check valve. I do no think it is related to the cold start problem as I can pin the fuel pressure high by jumpering the fuel pump relay connection.

Jfrahm

Tried another ECU, problem seems gone, starts right up when cold on the other ECU.

I am not sure what is different between cold cranking and hot cranking, maybe the Temp 2 input to the bad LH is blown and it just does not deliver enough fuel for a cold engine.

John Speake

The cranking fuel pulse has its own table in the LH software that defines the pulse at the injectors.
Injector pulse versus temperature.

At an engine temp of 20degC (70degF) the amount of fuel injected is 200% of the "hot" temp setting. At an engine water temp of 1deg C (33degF) it is 400%.

dzaprev

Fuel pressure at 20 psi ?

Ralph Newman

Joel,
20 psi sounds way low. My 88 with 8 lbs of boost, 30 lb injectors uing the stock fuel regulator will take the injectors up to about 80 % duty cycle. My pressure gauge stays right at 60 psi at idle and cruising in the garage.
Ralph