Warm start problem on 911 SC (1978)

[merlin62]

Dear 911 enthusiasts and experts,

based on the great support from many of you during former posts I would like to ask for your support also for the following problem on my 911 SC 3.0 (built June 1978). The car is from Orange County (CA) with a new home in Germany. The car is in close-to perfect shape, both mechanically as well as with respect to driving performance. Power, fuel consumption, acceleration and transition behaviour is all working perfect.

In spring this year, I experienced trouble during cold (!) starting the engine (backfired, high idle speed, etc.). I have successfully resolved this issue by replacing the WUR by a used 2nd hand component. Since then, both cold as well as warm start were working perfectly.

However, since about 2 weeks (and about 2000km after replacing the WUR), I experience problems during warm (!) starting the engine. 5 to 30 minutes after engine shut down, the engine is very hard to re-start. It takes about 2-3 trials of the auto starter before (few) cylinders start to run and another 10 sec. at very lean mixture before the engine runs smoothly again.
My conclusion at this point in time: Rapid drop in fuel pressure after engine shutdown, resulting in fuel vapour generation.

Potential root causes for pressure drop and warm start problems I can imagine:
1) Check valve of fuel pump malfunctions. For me, hard to believe since the (original Bosch) fuel pump (with integrated check valve) is only about 1 year old.
2) Fuel pressure accumulator. This is my personal suspicion. However, there are no outer indicators for malfunctions: no moisture around accumulator, no smell of fuel (which holds true for the overall car).

A) Does anybody have an idea on how to systematically resolve this issue?
B) Can somebody explain to me why my fuel accumulator has 3 (!) fuel lines, 2 at the top, 1 at the bottom of the accumulator (same on my 3,3 Turbo)? What is the bottom line good for?

PS: I currently do not have fuel pressure gauge capabilities at hand.

Any support welcome!

Best regards
Merlin62 (Germany)

[ossiblue]

I think you are on the right track as to the cause but since you do not have fuel pressure gauges, you can't be certain. Your suspicion about the accumulator is valid, given the newness of the fuel pump. The third connection on the accumulator is for gasoline that may be leaking from the internals--that's why you don't see leaking fuel around it. If your accumulator is bad, the fuel leaking past the diaphragm will exit the bottom port and return to the tank via the return line from the wur.

Without a pressure gauge set it will be difficult to know exactly from where your residual pressure is escaping. If you remove the bottom line from the accumulator, gas will leak out but that doesn't necessarily mean the accumulator is bad. Accumulators are expensive and you may wish to see if you can swap out a known good one just to see if the problem goes away. It may even pay to take it to a 911 expert to simply run a test of your residual pressure to track down which component is the culprit. I really can't offer a systematic way to diagnose your problem without using a pressure gauge.

[Bob Kontak]

I don't think the fuel gauges tell you where a fuel leak is. They can tell you you are losing residual pressure. It has been a while since I hooked my gauges up so I am all ears if wrong.

Given the WUR is "new" I would focus on that. Check the fittings for any leaks. However, if they were tight for several months / 1500 miles they shouldn't just loosen up.

Search on "residual pressure". Use the advanced search option. There is plenty of data.

[boyt911sc]

Merlin,

Like what LJ mentioned, troubleshooting a fuel injection system requires the use of a fuel pressure gauge kit. Residual and control pressures readings will tell you outright if the car would be easy to start or not.

But there is a very simple way to test the fuel accumulator so that you could eliminate it as a possible culprit in your problem. The three (3) port type fuel has the 2 ports on top for fuel delivery (In/Out) and the bottom port serves as drain line in case the diaphragm breaks and allows the in coming fuel from the upper chamber to go back to the tank via the main return line.

Disconnect the FA from the car and blow dry the fuel. Once you're certain that the FA is completely dry, test the diaphragm for leak/pin holes. Blow air by mouth to one of the top port to create some air pressure while the other one is plugged (use your thumb). Feel if air is coming out from the bottom port. If the diaphragm is ruptured or broken, you could feel the air coming out from the bottom port.

Or simply fill the upper chamber with gasoline. Hold it steady and observe if any fuel drips out from the bottom. Another test I usually do is very similar. With everything still on the car, disconnect the bottom fitting of FA and plugged the disconnected line not the the FA's drain port. Then run the FP for 1 or 2 seconds. A good FA will not have gasoline dripping out, a bad one will do. A suitable container is placed under the FA during this test.

Aside from a vacuum leak that could hinder an engine to start, check your control fuel pressures, cold start valves operation, and injector spray pattern. It is mandatory to have a fuel pressure gauge when doing this kind of work. Keep us posted.

Tony

[merlin62]

Dear All, thanks for your instant and great advice, in particular for the "simple" test routines from Tony. As physicist by education, have decided to go for the systematic way and get finally equipped with pressure gauges. Assume this will pay off for the SC and the turbo on the longer run. Will use the Bentley as reference. Will keep you posted on the root cause. Very best, Merlin

[longhornchris04]

In the interim, if you ever do get stuck, it is possible to manually pressurize the system.

1: Take the car out of gear, then set the key to run, but don't engage the starter. If you already tried to start it, you will need to cycle the car completely off first.

2: Pop the hood, remove the air filter, and make sure the straps are well clear of the fan

3: Gently lift the airflow sensor plate. You should hear the fuel pump kick in and the pressure on the sensor plate get 'heavy' As soon as you hear the injectors fire let the plate go. It takes almost no movement to trigger the system.

4: Go back and crank the engine... it should fire right away.

5: Replace your air filter, close the hood, and drive off.

Word of caution, don't do this if you don't have a blow-off valve on your airbox... you could get a backfire, especially if you let the injectors fire for too long. Given that you've been having CIS trouble, I'm assuming you have one as adjusting CIS without it is gambling.

Unless your accumulator is completely shot this will ensure plenty of pressure at the injectors for startup, and once the engine turns over the fuel pump will kick in and maintain system pressure.

This procedure is annoying, but it beats being stuck somewhere waiting for the engine to get cold enough for the cold-start kicks in to provide enough fuel.

As to chasing down the problem... good luck.

[TibetanT]

Hello Merlin:

Your warm start issue described here is exactly what happened to me in my 1983 911SC 3.0L this past summer.

I purchased a new fuel accumulator and the problem is no more!

Hope this helps and good luck.

[hbf]

Symtoms indicate that the check valve in the fuel line at or in your fuel pump is not holding the fuel system pressure after stop.

Before spending $$$ on accumulators and fuel pumps, check this $5 part.

Also cheaper than buying the pressure measuring tool, and worth a try.

[scarceller]

+1 on the check valve. I remember another test for the check valve. Pressurize the system by running the pump for few seconds then watch pressure if it drops within 3 minutes you have issue. Then repeat the test but as soon as pressure is established pinch off the fuel feed line from the pump, use a hose pinch tool or pair of vise grips on the rubber hose. The reason for this test is that the check valve is usually located right after the FP output. You are simply pinching off the hose to remove the check valve and the pump from the test. If after this test the pressure holds you most likely (95% certain) have a bad valve.

I recall this test from my 75 911

[merlin62]

Dear All,

I finally resolved the warm start issue: It was the pressure accumulator.
After replacing it, everything is perfect again. Yeaaah, a great feeling.

Again, thanks for all your advice!

Best, Merlin