Help - 1980 fuel pressure drop and hot start problem - where to look next

[bluetarga]

I have a 1980 911 with a hot start problem... starts fine when cold but after its hot leave it for 20 minutes and it takes for ever to start.
Checked the pressure, After starting it the control pressure goes up to 3.4 bar. System pressure is 5.0 bar.
Turn off the ignition and on the control pressure side it drops to 2 bar in about 15 seconds then plumets to 0 in about 5 seconds. Exact same reaction on the system side.
So I know I have a pressure problem.
I have clamped the warm up regulator return - same result
I clamped the input into the fuel pump (check valve check) - same result
At the same time I clamped the return to the fuel tank (pressure regulator check) - same result
So the Bosch CIS manual says its the cold start valve that needs replacing...so just in case I clamped both tthe input and return to it - same result which makes me wary that it is the issue
I have seen lots of posts about fuel accumulator issues so I clamped off all 3 connections to it and - same result.
Really not sure what to check next..any help is greatly appreciated.

[SCWDP911]

sub'd - good luck with it

[cmonref]

Well, you've covered the waterfront.

Pull the bottom connection off of the accumulator. If fuel comes out of the accumulator, the diaphram has been holed and that is your problem. If your reported clamping was fully effective, that should have isolated the problem. But I would still perform this test anyway.

If your control plunger in the fuel distributor were sticking UP when the sensor plate returns to rest, it would bleed control pressure into the six injectors. But they should seal at something like 2.5 bar. If one of THEM did not close, then THAT cylinder would be full of fuel to the point of hydrolock.

[bluetarga]

to check the accumulator, when do you pull the line ...before you start it?
i pulled the return line once and noticed some fuel but am concerned it is coming up from the return line rather than out the accumulator...possible??
clamped that puppy tight more than once thinking that would be the best check. thx

[cmonref]

When you take the bottom fitting off of the accumulator, did the gas come from the return line or from the accumulator? If the accumulator is OK, there will be a trapped pocket of air inside the accumulator below the diaphram. It can fill with fuel only if the diaphram is torn and lets fuel from the upper chamber (which IS pressurized) into the lower chamber (unpressurized return line). If still in doubt, crack the fitting with pressure in the accumulator. If is sprays, the diaphram is ruptured. Or, with the lower fitting off, "blip" the fuel pump for a few seconds (until the injectors "sing") to test for ruptured diaphram.

DO be careful with open fuel lines in the engine compartment.

[robert walsh]

Had The Same Problem 0n 83 911sc, Installed New Accumulator And Problem Was Resolved.

[Poor-sche Lover]

Maybe this is a long shot, but I had the same problem. I found that the o-ring on the pressure regulator valve INSIDE the fuel distributor had crumbled. Luckily, I used one from another distributor I had. Good luck.

[Aurel]

Hot start problem is vapor locking of gas in the lines. It can be caused by two things: a bad check valve at the fuel pump, or a bad fuel accumulator, as stated before.

Aurel

[jrcook320]

Aurel, I think you mean to say the hot start is caused by vapor lock, which is caused by low rest pressure, which can be caused by either a leaking accumulator, check valve, pressure regulator, cold start injector, of fuel injector.

You've certainly covered the bases, but I would be wary of relying completely on clamping to isolate the problem. The most common causes are the check valve, accumulator, or pressure regulator.

The regulator O-ring is industry standard size #008 which can be had in a fuel resistant nitrile at any hardware store, you can buy viton o-rings from mcmaster.com, or you can buy the specialtauto's "hot start" o-ring kit for just a few bucks.

[Superman]

Originally Posted by Brian K. Haggard

If your control plunger in the fuel distributor were sticking UP when the sensor plate returns to rest, it would bleed control pressure into the six injectors. But they should seal at something like 2.5 bar. If one of THEM did not close, then THAT cylinder would be full of fuel to the point of hydrolock.

I doubt it. It does not take much fuel volume leaking to draw pressure down to zero. But yeah, it could be an injector that is leaking this pressure.

FP check valve
Accumulator
cold start valve
injectors

In that order. At least, that's what I've heard.

[tsuter]

Fuel accumulator.........