Why Does High Fuel Pressure Cause Rich Running?

[Gauche1968]

Can someone explain why higher than normal fuel pressure from a failed regulator would cause you to run rich? Does higher pressure in the rails cause the injectors to spray more fuel than the computer wants them to? Does the higher pressure cause the injectors to leak or does the excess pressure cause a leak back through the vacuum line into the intake? [/code]
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1984 Vanagon GL
1984 Vanagon Westy

[Jake de Villiers]

Yes and yes. The good news is that a new regulator will cure the problem promptly.
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'84 Vanagon GL 1.9 WBX
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[jberger]

A properly functioning O2 sensor will compensate for increased fueling. It's surprising how much it can deal with. I would imagine huge amounts being sucked through the regulator from the vacuum hose would end up in your oil sump. Does the oil smell like gas?

J

[Wildthings]

The regulator is there to give a specified fuel pressure so that the injectors will spray the required amount of fuel during the time they are open. Too much or too little pressure means too much or too little fuel. Yes the O2 sensor can make up for a fair change in pressure, but not at full throttle when the system goes open loop.

If the FPR isn't doing its job then buy a new one. Probably not a bad thing to just do every 10 years or so anyway. Bluebus's idea to use fuel rated hose for the vacuum line and to use clamps on the hose isn't a bad idea either, as you don't want a failed regulator diaphragm to end up spraying fuel all over the top of the engine including over top of the distributor. IMO VW engineering on locating the FPR was a fail, I didn't like it the first time I saw it.

[sjbartnik]

[Gauche1968]

Thanks for the responses everyone. I am trying to understand this, as I have two vans where the fuel pressure is not behaving the same in each van. On one (the one with the hot start problem from my other thread) the fuel pressure seems normal while idling, 32psi with the hose on, 36psi off. When the van is shut down, fuel pressure stays at about 32 for a few minutes but gradually tapers off. After ten minutes, it is 27-28 psi or so. It takes approximately 30 to 45 minutes for the fuel pressure to bleed off completely.

On the other van, fuel pressure at idle also seems normal 32 psi while running with the hose off, 36 off, but on shut down, fuel pressure jumps back to 36 psi and stays there for hours. After 24 hours, there is still some level of residual fuel pressure in the rails. This van has no hot start problem, but I suspect it also "runs rich," and on start up--before the OXS kicks it--it has a low, sickly idle, but runs fairly well when up to speed.

So, which van is behaving abnormally? Part of the reason, for having two is to compare them when there is problem, but that's not going to work if they are both broken!
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1984 Vanagon GL
1984 Vanagon Westy

[sjbartnik]

There is a check valve built into the fuel pump that is meant to keep fuel from bleeding back into the tank after shutdown. The idea is to keep the fuel system pressurized as long as possible so that next time you go to start it, it will start quickly. Whereas, if pressure has bled off completely, it may take a few extra cranks of the starter before it fires.

So in that sense, Van 1, where the fuel pressure bleeds down completely after 45 min, would seem to have a leaky check valve.

Van 2 sounds like it is behaving properly but there are lots of other things that can cause low sickly idle besides rich mix.
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1965 Volkswagen 1500 Variant S
2000 Kawasaki W650

[Wildthings]

[jberger]

The O2 system controls fueling over all. Even at idle, once warmed up and several moments after a warm restart the O2 takes over everything. It really does not care about fuel pressure, as long as it is adequate. If you spray more fuel the feedback loop will adjust for it in the blink of an eye.

[240Gordy]

These are dinosaur FI systems with little sensitivity and even smaller range of adjustment. These are not wide-band O2 sensors
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[jberger]

[Ahwahnee]

FWIW - I had a regulator that would not hold pressure at all once the engine was shut down. I only discovered it by accident when I was checking pressure as part of an unrelated diagnostic -- I never noticed any starting or running issue.

I did replace it and I carry the 'failed' one as a spare as it would certainly work well enough to get me home.

[Gauche1968]

[Wildthings]