Fuel Pressures Question

[mendez]

Hi,

I dropped my engine 2 weeks ago to do some preventative maintenance and while I have it out I'm changing all my fuel lines. Before I dropped the engine I measured my fuel pressures because I was having a bad warm start problem.

Here are my fuel pressures for my 1977 911s:

System Pressure: 4.9 bar (temp 19C) - SPEC is 4.5 to 5.2 so I know this is OK.
Cold Control Pressure: 0.3 bar (temp 19C) - SPEC is 1.7-2.1 so that is BAD.
Warm Control Pressure: 2.65 bar (temp. 23.6C) - SPEC is 2.7-3.1 so I judge it OK (it's on the lower end though).
Residual Pressure: BAD; it will immediately goes to 0 once I turn off the pump.

Questions:
1. Residual pressures are linked to the fuel accumulator, right? From my research it looks like I might need to replace it. Any tests I can do to troubleshoot it?

2. Should I send the WUR with Fuel Distributor for a rebuild? My control pressure seems to be perfect and I wonder that I just need the WUR; I rather spend that extra money on other engine components because the bill is adding up!

Thanks a lot!

[PeteKz]

Add info to your profile on the mileage on your car.

First, DO NOT throw parts at the problem. That will complicate your troubleshooting. Figure out what's wrong, and only change ONE thing at a time.

The SP is fine, the CCP is way too low, but that won't give you hard warm starting problems. The WCP is a bit low. But don't do anything to your WUR yet. Also check the resistance of the internal heating element (across the two pins in the connector) and compare it to the specs for your WUR. Post that measurement.

Residual pressure is held by 3 things: Fuel pump check valve, FA, and pressure relief valve on the fuel distributor. Of the three, the most likely problem is a bad FA. If it's original, almost certainly the internal diaphragm has deteriorated and ruptured by now, due to time and increasing ethanol contamination in the fuel.

Test the FA by detaching the line at the bottom of the FA, plug that line, run the fuel pump and see if gas comes out the bottom of the FA. If it does, it's bad. Another way to check is to remove the FA and put pressure on top the connectors, or vacuum on the lower connection and see if it holds pressure or vacuum. If not, it's bad.

If bad, replace the FA first. That most likely will solve the warm/hot start problem.

If the FA is not bad, then check the fuel pump check valve by plugging the output of the fuel filter, then run the pump and shut it off. If it holds pressure, the FA and check valve are both good. That means pressure is getting past the pressure relief valve on the Fuel distributor. If it does not hold pressure, then the FP check valve is bad. Replace it.

WUR and FD: AFTER you get the car running again, recheck the pressures. There are detailed instructions on on this forum that describe how to disassemble, clean, and adjust the WUR. I believe yours has the 033 WUR, correct? Hold off doing anything to the FD until after you get through these tests and fixes.

[Schulisco]

I totally agree with Pete.
1. residual pressure is not only dependent by the fuel accumulator. It also hangs together with the check valve at the fuel pump. (Some pumps have integratet cv, original Bosch's have an external cv). With a leaking cv (or other leaking components of the CIS e.g. injectors) the fuel accumulator cannot do it'S job (holding pressure at least half an hour above ~1,3bar; when all is working fine it holds the pressure for hours!).
2. To successfully troubleshoot the CIS do one step after another - any time! You'll confuse yourself by making this and that in parallel and you loose the overview of what being done or not. A CIS can mimic many issues in several ways! This is the most confusing on a CIS!

Thomas

[boyt911sc]

Originally Posted by PeteKz

Add info to your profile on the mileage on your car……….

Residual pressure is held by 3 things: Fuel pump check valve, FA, and pressure relief valve on the fuel distributor. Of the three, the most likely problem is a bad FA. If it's original, almost certainly the internal diaphragm has deteriorated and ruptured by now, due to time and increasing ethanol contamination in the fuel.

…………

Peter,

Add a 4th possible culprit. And that is a leaking warm-up regulator for pre-SC CIS.

Tony

[mendez]

Originally Posted by PeteKz

Add info to your profile on the mileage on your car.

First, DO NOT throw parts at the problem. That will complicate your troubleshooting. Figure out what's wrong, and only change ONE thing at a time.

The SP is fine, the CCP is way too low, but that won't give you hard warm starting problems. The WCP is a bit low. But don't do anything to your WUR yet. Also check the resistance of the internal heating element (across the two pins in the connector) and compare it to the specs for your WUR. Post that measurement.

Residual pressure is held by 3 things: Fuel pump check valve, FA, and pressure relief valve on the fuel distributor. Of the three, the most likely problem is a bad FA. If it's original, almost certainly the internal diaphragm has deteriorated and ruptured by now, due to time and increasing ethanol contamination in the fuel.

Test the FA by detaching the line at the bottom of the FA, plug that line, run the fuel pump and see if gas comes out the bottom of the FA. If it does, it's bad. Another way to check is to remove the FA and put pressure on top the connectors, or vacuum on the lower connection and see if it holds pressure or vacuum. If not, it's bad.

If bad, replace the FA first. That most likely will solve the warm/hot start problem.

If the FA is not bad, then check the fuel pump check valve by plugging the output of the fuel filter, then run the pump and shut it off. If it holds pressure, the FA and check valve are both good. That means pressure is getting past the pressure relief valve on the Fuel distributor. If it does not hold pressure, then the FP check valve is bad. Replace it.

WUR and FD: AFTER you get the car running again, recheck the pressures. There are detailed instructions on on this forum that describe how to disassemble, clean, and adjust the WUR. I believe yours has the 033 WUR, correct? Hold off doing anything to the FD until after you get through these tests and fixes.

Thanks everyone for the help! I got my self some Porsche service manuals to learn more about it...and I'm amazed how all this CIS system worked.

The Heating Element resistance is 23.3ohms; I think that is OK, right? I read somewhere that between 20-30ohms is OK

[PeteKz]

Yes. 23.3 is good. And yes, the WUR is one more place residual pressure can leak down. However, yours is leaking down so fast that I’m pretty confident it’s the FA.

[mendez]

What about CCP? If the WUR is the problem, should I rebuild the WUR with the Fuel Distributor?

[boyt911sc]

Originally Posted by mendez

What about CCP? If the WUR is the problem, should I rebuild the WUR with the Fuel Distributor?

No. They are two (2) separate and independent components. Unless both of them are defective having them rebuilt is a prudent course of action. What problem/s are you having with the FD? Test the FD and your fuel injectors too.

Tony

[Schulisco]

As written, both the FD and WUR work indenpendently from each other, but the WUR manages the CP of the FD.
Nevertheless I personally would let check both if state and age are unknown. Both are quite important to get the engine running properly. My experience: On every CIS equipped engine not being checked/maintained at these components for the last couple of years always has an issue. But the truth is also - the engine still runs obviously good...

Especially the FD is either out of balance at one or more outputs to the fuel injectors or/and the WUR is out of specification. Both have to be checked by measuring pressures and amount of fuel delivered.

So if an FD is out of balance/specification on at least one output - e.g. one output puts out more fuel, another gives out much less fuel, a lambda meter will then show no problems. When this happens, the leaner cylinder runs hotter than it should. In my case it was running so hot that the exhaust valve was ticking much louder than the other valves! In extreme cases, this can also lead to damage to the catalytic converter. The problem will only be revealed when unburned HC is checked.
Another problem could be the injectors itself. They wear out mechnically because technically they're pressure valves opening at a particluar pressure. Sometimes the spray pattern is not good any more (drops or jets instead of cone-shaped mist) or they differ in the opening pressure. This could be also a culprit for a to quick loosing residual pressure after shutting down the engine which causes hot start issues.

Thomas