Yes, you're right Brett, to maintain pressure, there must be a closing at both ends, end of fuel rail and
pump.
I think that our differences are partly due to semantics... I use the term "check valve" only to designate a valve that prevents backward flow in the system, so the check valve in the fuel system is at the fuel pump (or anywhere before the fuel rail), and it prevents backward flow of gas from the fuel rail back through the pump and eventually back into the fuel tank.
The fuel pressure regulator must come after the fuel rail and maintains pressure between itself and the check valve (thus generating the required pressure in the fuel rail). If leak down occurs through the fuel pressure regulator, the fuel leakage will be forward to the return line to the fuel tank (not backward through the fuel pump).
Quote:
Originally Posted by syljua
In that last article, it's even a description of leaking FPR, that will cause
fuel to enter the manifold, and cause rich running;
br,
syljua
This is a different leaking, a leak from a failed diaphragm that is operated by a vacuum line from the intake. Failure of the diaphragm allows fuel to be sucked directly into the intake via the vacuum line (a path that fuel isn't supposed to take).
To sum it up, we're discussing two different failure modes of the fuel pressure regulator: 1. One in which it fails to maintain the pressure in the fuel rail (either only when the car is turned off or also when the car is running). 2. Failure of the vacuum diaphragm in the regulator which allows fuel to be sucked into the intake via the vacuum line causing a constantly overly rich mixture.
Yes, syljua, both of us should be right (in principle) about the resting pressure loss and either the fuel pressure regulator or check valve being the cause of gold benz' problem.
Brett
Last edited by Brett San Diego : 07-05-2008 at 03:53 PM.
Yes, syljua, both of us should be right (in principle) about the resting pressure loss and either the fuel pressure regulator or check valve being the cause of gold benz' problem.
Here is a diagram of the fuel pump from my S600, which I think will be the same thing for the S500. Autohausaz.com lists the same part number pump for the two.
Part number 3 inside the pump is the check valve. I don't think this is the same as the check valve that you have shown for the S500. I don't know where this check valve is located. But, there is no reason that you cannot remedy a failure of the internal check valve by installing an external check valve as long as it is properly installed.
LOL, great analysis! Which should I replace first? My auto skills are about a 4 on the 10 point scale. If I can find the FPR or check valve location I can probably change it though. I'll look over the diagram.
LOL, great analysis! Which should I replace first? My auto skills are about a 4 on the 10 point scale. If I can find the FPR or check valve location I can probably change it though. I'll look over the diagram.
Neither. If you suspect a loss of resting pressure, what you should do is hook up a fuel pressure gauge, pressurize the system (probably by starting and running the car), then turn the car off and then watch the pressure gauge. It should maintain the fuel pressure above a certain level for a certain period of time. I'm sorry, I can't tell you what factory specs are for resting fuel pressure leakdown. More reading necessary.
But, it may not be a fuel pressure issue at all. I'm not knowledgeable enough to provide all possible causes, but another I can think of is the computer getting an improper engine temperature reading and thus giving an improper mixture for starting.
The fuel pressure regulator and check valve are probably the most obvious possibilities to suspect, and maybe the easiest to check via a fuel pressure gauge. There is a valve on the fuel rail which is for the purpose of hooking up a fuel pressure gauge. But, I can't give details since I've never personally hooked up a gauge to a W140. I have checked fuel pressures on my 1978 911, however.
Brett
Last edited by Brett San Diego : 07-05-2008 at 05:00 PM.
I'm not set up for testing the pressure. I'll have the mech do it next week though. We'll have this problem solved soon enough. The car is running really near perfect.
I'm really pleased with everything now. I took a few friends out last week, they we're honestly raving about the car. Everyone seems to drive a teeny car these days. It's nice to ride around in a limo sports car.
I'm not set up for testing the pressure. I'll have the mech do it next week though. We'll have this problem solved soon enough. The car is running really near perfect.
You know, though, this seems such a minor thing, I'd probably not do anything about it. It's not like it's having a problem not hot-starting at all. I just noticed your signature saying 37,000 miles. Because of the low miles, the fuel filter probably has not ever been replaced, but it's been on the car 11-12 years. May as well change it now. It can only help for the long term.
You know, though, this seems such a minor thing, I'd probably not do anything about it. It's not like it's having a problem not hot-starting at all. I just noticed your signature saying 37,000 miles. Because of the low miles, the fuel filter probably has not ever been replaced, but it's been on the car 11-12 years. May as well change it now. It can only help for the long term.
Brett
That's kind of what I was thinking when I bought the fuel filter. Just about everything was original with the exception of the tires and the fan regulator. I'll have the fuel pump and fuel filter changed next week then. I might even get a tune up and change the plugs. I have 43,000 miles now.
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