Fuel accumulator and warm start issues--why?

[medinfoto]

Maybe a dumb question, but I am curious as to the answer. Been experiencing occasional warm start trouble (when the car's been sitting for at least 30 mins or so), and reading up on it. Most of the stuff makes good sense, but one I cannot quite figure out is the role of a slightly leaking fuel accumulator.

On any start, the fuel pump starts running as soon as you start cranking the engine and remains running until engine is stopped, no?

Then why is a leaky accumulator a problem during warm starts? Would the system not pressurize virtually immediately like it does on a cold start, regardless of a slight leak in the accumulator?

Just curious about the mechanics behind this one... thanks in advance!

[Mrmerlin]

the accumulator will hold pressure in the lines for the next start,
if its leaking then the fuel will vaporize in the lines from engine heat thus effectively causing a no fuel start attempt.
This one part will usually fix most hot starting conditions.
Its kinda like the green wire put in a new part and keep driving

[19psi]

True, but why does it start fine with no pressure when cold?

Cold start injector must have something to do with it but it shouldn't have any pressure behind it either.

[19psi]

Check valve stuck open on mine causing the hot start issue. At least 10 seconds of embarrassing cranking before start up when hot.
Fixed valve and instant hot starts now.

Obviously fuel to main injectors flows back to tank and leaves the system with air that needs to be purged.
Could the cold start injector be positioned where a small supply of fuel is still maintained within a fuel line?

[Mrmerlin]

the other part that should also be replaced is the fuel pump check valve always use new sealing washers for this operation,
you need three of them.

Once the engine is hot the fuel will vaporiz if there isnt enough pressure in the lines.

Also if the check valve is bad then fuel will return to the tank from pressure in the lines,
thus creating and air pocket in the line so it takes a bit of cranking for the fuel to refill the lines.

When the engine cools the vaporized fuel re condenses and the pressure in the tank forces the fuel back into the line so to effectively refill portions of the line so a cold start happens quickly if the WUR is functioning properly

[medinfoto]

Thanks, that makes sense; I was trying to figure out how a warm start would differ from a cold start fuel supply wise.

So basically the cycle is like this in a "slow leak" scenario:

1) Car cold. Fuel fully condensed. No problem.
2) Car shut off for a shorter period. Not enough has leaked out to make an impact.
3) Car shut off for a longer period. Fuel has leaked, pressure has dropped, vapor in lines making it hard to start.
4) Car shut off even longer. Back to step 1.

I'll go ahead and check out the accumulator and check valve as part of my fuel system refresh this winter when it's downtime here in Sweden.

[Mrmerlin]

I would say if either of these parts are older than 10 years simply replace them both.

in the grand scheme of things not that expensive and you will probably find they are worn

[medinfoto]

Originally Posted by Mrmerlin

I would say if either of these parts are older than 10 years simply replace them both.

in the grand scheme of things not that expensive and you will probably find they are worn

Yeah, will go ahead with it. Smelled around the area today after running it briefly and there's definitely a bit of a gas smell.

I can get the check valve parts for around 30 bucks locally, or get a whole new fuel pump with check valve for 85 or so... think it'd be worth just replacing the whole thing and keeping the old one as a spare?

[19psi]

Don't know about the accumulator but my failed check valve resulted in zero pressure instantly at shut down. Disconnect a fuel line in the engine bay and there is no remaining fuel to be found, it all drains back into the tank.

There is no way a vented (or partially vented) fuel tank is going to build up enough pressure to refill the fuel lines.
Even if it was 100% sealed it would take a massive cold to hot temperature change to build up pressure.
I could shut down when it's 90º and the car would start instantly the next morning when it was 60º out. Hot to cold should make things far worse by drawing fuel back, like putting a cap on a plastic gas can when it's hot and the next morning it's buckled in due to the temperature drop.

Regardless, the starting sequence appears to be designed by Rube Goldberg. It's the only fuel injected vehicle I've ever owned that doesn't simply prime the system for several seconds when the ignition key is turned on. And the "technology back then" excuse doesn't work either...I have a 1982 Honda motorcycle that's fuel injected and its ECU primes the circuit just like every vehicle made today. Seems that a timer circuit on the fuel pump relay could have made things bit more reliable and far more simple.

[Mrmerlin]

George what happens after the fuel/engine cools down is that the fuel in the lines that was vapor re condenses,
the fuel in the tank will flow past the check valve if its bad,
back towards the engine till it reaches its level with the rest of the system,
note the lines run along the bottom of the frame,
so the fuel could run to engine compartment depending on how much fuel is in the tank
The pump check valve can work both ways but if its bad then fuel can leak past it both ways.

If you have an accumulator its wise to atleast inspect it,
there is a small screw on the top of the unit,

if you remove this screw you can smell for fuel,
if you smell fuel then the diaphragm is leaking and the unit should be replaced.

[Danglerb]

Remember the old days of carbs? Vapor lock, fuel boils in the lines so the engine is being fed vapor instead of fluid and that is much much less fuel so the engine won't run. Fuel injection initially cured this due to higher fuel pressure which has a higher boiling point, then smog stuff raised engine temps about 30 degrees F and vapor lock became an issue again. By the middle 80's it was pretty well "not fixed, but patched up", and generally fixed fixed ten years later.

The 928 is "fine" when everything works, but not a lot of everything works 928's around, more like enough works to run is more common. Fix it some more and just move on to other issues.