high control pressure (cold and warm)

Volkovich · 04-13-2003, 01:38 PM

I am a newbie at playing around with my Porsche (1979 911SC) and only know what I can read in my manuals (both Wayne's books plus the Bentley with the Bosch Fuel Injection - Engine Management on the way.)

I got my fuel pressure gauge hooked up and jumpered the fuel relay to turn the pump on with the engine off. My pressure under this condition is at 4.8bar. According to Bentley that is right in the middle of the range.

When I start the motor and leave the gauge attached I thought the pressure should have dropped down to 2-2.5bar (it was about 65-70F out today) and then rise up to 3.2-3.6. I actually get a slight increase in pressure (up to 5.0bar). If I open or close the gauge shut-off valve there is no change in the gauge reading.

After I shut the car off the pressure seems to hold pretty well. It's been about an hour now and it is still at 1.3bar. When I open the valve I don't see any additional drop in pressure.

From what I have read it sounds like my warm-up regulator could be the problem. I was wondering if anyone could give me their opinion on whether this is the case or if something else should be checked first.

john walker's workshop · 04-13-2003, 02:17 PM

system pressure should be 4.8-5.2 bar. control pressure 3.4-3.8 bar. control pressure starts low, like 1-1.2 bar and goes up to the top range in just a few minutes. if the w/up reg had power to it for very long, it would have started to warm up. if you had excessive control pressure on cold starts, the engine would be very cranky for a few minutes, until it warmed up a bit. CIS gauges won't allow the engine to run very well if the lever is in the system pressure mode. the system pressure check is done quickly, and then the lever is turned to the control pressure mode. system pressure stays the same, control pressure rises.

Volkovich · 04-13-2003, 02:41 PM

You described my symptoms perfectly. My car is very had to start when cold. It sputters, dies, over revs, dies, then eventually it will idle.

So if my control pressure (valve to WUR is open) and system pressure (valve to WUR is closed) are the same it would seem the WUR is not doing what it should be doing. In fact, I would think it isn't doing anything.

When cold it should reduce the pressure very low (spring compressed) and as the bi-metallic strip heats up it would increase the pressure (decrease pressure on the spring) but not to the point that it is wide open.

If my understanding is correct then what is wrong with the WUR and can it be fixed?

john walker's workshop · 03:12 PM

a "fix" can be attempted. notice the 1/4" wide, round, depressed spot on the top surface of the WUR. it's actually the top of a pin in a hole. if you can depress it a tad more, the cold mixture will be richened. with the gauge attached, engine cold, and WUR plug pulled off, so it doesn't warm up, use a hammer and punch to move it just a few thousandths deeper. watch the gauge as you try tapping it down kind of gently, because if you go too far, and make it too rich, where it surges, you will have to drill and tap the pin, and use a small bolt, nut, and washer to pull it back up. shoot for 1-1.2 bar. (search for "adjustable warm up regulator").
before you do that, follow the top vacuum hose to the thermo-valve, just above the left valve cover, over cylinders 2 and 3. the valve needs to be closed when cold, so the WUR can run richer cold. when it opens, vacuum reaches the WUR, and it goes leaner. the hose should not have vacuum when cold, until the t/valve warms up. check for power and ground at the plug on the valve.

Volkovich · 04-13-2003, 03:58 PM

What would happen it no gas is reaching the WUR? I just had it apart and noticed a lot of oxidation in the bolt holes that hold it together and on the fuel return line bolt. I looked inside and saw a screen in there. After that I took EVERYTHING apart and found the thin metal diaphram and plunger and noticed the tiny holes where the fuel enters and exits.

I dumped some carb cleaner down that hole with the screen and initially nothing came out on the other side Finally, I did notice it weeping through from the inside hole. I put everything back together (is there an easy way to make sure the metal pin on top of the springs gets sitted in the bottom of the plunger?) and reinstalled on the car. I did a system pressure check and it was at 4.8bar.

When I turned the car on my pressure was 3.6bar and it shot up to 5.0bar when the gauge valve closed. The car also started to run really rough. In a few hours I will see what the pressure runs with the car dead cold.

Have you heard of the WUR getting clogged?

ohecht · 08-25-2003, 09:24 PM

JW, I have been troubleshooting cold start enrichment problems that I think are related to the WUR. I have chronicled the diagnostics on a thread called "CIS idle rising over time".

I have system pressure of 5 bar, cold control pressure of .8 bar that rises quickly to 3.5 bar when I reconnect the electrical connector.

I am curious about the vacuum connection on the WUR. Mine is an 82, but the WUR has the vacuum connection, which I think is only for earlier cars. The connection is routed to the throttle body, and I have never heard of or seen the thermovalve you mention above.

The car has always run fine, and still runs great when warm (even when cold, just with an annoying low idle).

I was leaning towards the WUR until I just saw the control pressure rise so smoothly, now I am wondering again if there is a vacuum leak under the CIS airbox that is aggravated when the engine is either cold or warm. I cannot find a good procedure for removing the airbox, and the connections in the back are intimidating to think about taking apart in the car, although I am sure that would be easier than reassembly!

Olivier