Porsche 911S 2.7L Running Issues

Had this little 2.7L come into the shop this week for some serious running issues. The car would fall on it’s face around 3-4k rpms, bucking like a wild horse. It had been a few other places without much luck in repair, so I got to take a stab at it. The old CIS cars are very tricky to work on if you aren’t familiar with them. I know my way around them fairly decently, but am always still learning new things every time I work on one, so they can stump me on occasion. With these 35-40 year old cars, sometimes you can have multiple issues happening at the same time which can be a bugger to figure out. When you have running issues with a CIS car, the first thing you do is to check your fuel pressures. Cold and warm control pressures, system pressure, and resting pressures. This will tell you a lot about the car.

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I didn’t get a chance to take a lot of pictures on this project, as it was pretty straight forward. The first thing you need is a CIS fuel pressure gauge. This will be connected inline at the WUR (warm up regulator), between the WUR and the center port on the fuel distributor. Just disconnect the line at the WUR and install this inline there. You need to have the shut off valve of the gauge on the WUR end. You do not use the banjo fitting, that is the return line.
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The WUR on the 2.7L is found here.
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And the line runs to the center port on the fuel distributor.
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Your basic test procedures are as follows: Each model’s pressures are different, so consult your technical manual for specs on the engine you are testing.

System Pressure:
WUR Unplugged (electrical connector)
Valve on gauge closed
Engine off, ignition on
Fuel pump relay jumpered with a toggle switch so you can have the pump running without the engine running.
Record system pressure. Turn off FP or open valve to reduce system pressure since it will be in the 65-80psi range.

Moving on.
Cold Control Pressure:
WUR unplugged
Valve open
Fuel Pump on
Record system pressure

Warm Control Pressure:
Same as cold control, just plug the WUR connector in. It will take 2-3 minutes for it to rise and stabilize. Record pressures.

Residual pressure:
After warm control pressure, turn off fuel pump and watch pressure for 20 minutes. Record findings.

In our case, when I went to check the warm control pressures, nothing happened over a 5 minute period. That told me the WUR was bad. I installed a new WUR, and then had to completely reset the idle CO% mixture as it had been adjusted to compensate for a bad WUR. With the new WUR, the car was so far out of adjustment it wouldn’t even start. So you have to guess with the CO% mixture until it will start, then adjust from there. I use a Gunson gas tester when testing CO%, the key is to make small adjustments to the idle mixture as well as the idle screw together until you reach the mixture and idle settings you need.

After that, she drove like a champ!

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