granprixweiss928

So sometimes my 928 is hard to start when hot.

It cranks longer, and when it catches, it kind of stumbles a bit and then gets going ok. If I depress the throttle a little when cranking, it sometimes help.

Its done this for some time. and Ive always got it to start- except last weekend.

I drove home from a cars and coffee, about an hour continuous at 80 mph on the freeway. Temps ran just a tick above the first mark on coolant. Pulled in the driveway, parked. Came back out about 15-20 min, and cranked and cranked. like 5 times for each about 10-15 sec . no start for the first time ever.

I went to work in my truck- had no time to trouble shoot at that time.

When I came home several hours later, it fired right up.
When cold, it starts instantly. every time.

History/Info

1981 928 4.5l 5spd 61K mile
Engine rebuilt last summer
fuel injectors cleaned/tested about 2 year ago witchhunter < 2K miles
all new hoses and gaskets seals etc on fuel injection
thermotime and coolant sensors replaced within 2 years- 2000 miles
All fuel regulators and dampers replaced <2K miles
tested last year for residual fuel pressure - OK within spec (but cold?)

Ive read lots of threads on this topic already. Suggestions like leaking fuel injectors or the one way valve by the fuel pump failing, but I dont think so on this one...

one note- the fuel tank was less than 1/4 full- closer to 1/8 tank

Id think that if the fuel residual pressure had dropped that it would build by cranking (pump runs) and then eventually start?

Could it be fuel vapor lock? Or too much pressure in tank? Or maybe the one way valve failing only when hot? Or ignition related? (coil new also in last year or so)


thanks

Mark

GregBBRD

Fuel pump check valve. Pressure goes down in the lines and the fuel actually vaporizes (boils) in the lines....vapor lock.

Or, getting more and more common:

The charcoal canister evacuation valves fail and will allow the intake to pull a vacuum on the fuel tank.....all the time. When the valves have small leaks, this can cause the fuel to vaporize (boil) at much lower temperatures and stop the car from starting until the fuel cools off. In severe cases, this will actually suck the fuel tank inward and crack the tank.

granprixweiss928

Thanks greg.

Ill test my residual pressure tomorrow.

Regarding the tank suction- I recall this. perhaps you mentioned to me at one point, or here on RL. the valve is part 92811042906 ?

granprixweiss928

Residual pressure test- At Idle- 2 bar. At rest engine/FP off 2.4 bar.

After 25 min- 1.9 Bar.

Charcoal canister vacuum valve (by windsheild washer tank filler neck) tested OK with mity vac- holds pressure. At Idle Im seeing about 20 in Hg at idle on the vacuum feed line from the throttle body.

The Forgotten On

In the L jet cars we still have the special fuel pump relay (unless you changed yours for a 53) that primes the engine when we turn the key to the on position.

So a leaky check valve wouldn't be serious unless it had completely failed and yours seems okay.

GregBBRD

"So sometimes my 928 is hard to start when hot."

Step 1: Replace the check valve.

If it still does it, then you can start hurting your head.

9two8

Had the exact same symptom last year, easy thing to replace the Fuel pump check valve.., that fixed it !

Ken
80 928 S
5 Speed

granprixweiss928

Parts are on order!

STRIKEMASTER

I just checked my '83 owner's manual...On page 22 it says "When the engine is hot, FULLY depress the accelerator pedal while starting."

9two8

Yes your correct it does, however with a faulty Fuel pump check valve, it makes no difference.

granprixweiss928

fuel pump check valve replaced, seems to have solved the issue

9two8

...,

Ken
80 928 S
5 Speed

UK

Jørn

Hi

I seem to be experiencing similar problems with my K-Jetronic 1978mod. It needs a lot of cranking when it has been parked and it is semi-hot (the accumulator is quite new so doubt it is that).

My question is; is the check valve a separate component you can buy or is it an integral part of the fuel-pump?

Also is there a way to check for it without having equipment for measuring residual fuel pressure?

Chalkboss

Although I am basing this on a S4, the check valve is available separate. Be sure to get the proper copper crush washers. You can remove it and examine the valve, I think someone here did this recently and noticed a broken spring. Other than that you need to measure the residual holding pressure in the line to determine if it's operating properly.

traxtar928

That's correct, I posted those pictures of the faulty check valve. The spring in the check valve eventually fails, so the ball that prevents backflow can't do it's job anymore and fuel returns to the tank instead of keeping the lines pressurized. Also, since fuel pressure is not maintained, the boiling point of the fuel lowers and you get vapor lock that must be overcome by additional turns of the engine.

I bought a Bosch check valve online for a great price, but it did not include the 3 copper washers that need to be replaced, so I recommend just going to Roger for the whole valve/washer assembly.