which WUR reading to believe?

[Alan L]

I have set my WUR pressures up statically - ie without engine running.
I have several CIS manuals, some call for engine running on some cars, but no reference for Porsche to do so. Most just say hook in gauge, bypass fuel relay, ignition on.
I've been hooking mine in, key on, and just depressing airplate for steady reading - which I have taken as cold pressure. For Hot, I have either hooked in when car is hot and done same, or dummied power to WUR from cold, and watch pressure rise.
Now I have been doing some tests with engine running.
The numbers are completely different.
From cold, my static pressure will rise by 15-20 psi after I start engine - even with power to WUR disconnected, and WUR remote from engine block - within 60 sec.
Similarly, my system pressure pegs my gauge under these conditions - who knows how much, but statically it is set at 95 - which is where the book says it should be. But under running conditions it seems to be well over 110 psi.
I would imagine the running numbers are the real deal - what my engine is actually seeing - but they are way different to the static numbers.
Why the difference, and which ones to use?
Alan

[JBL930]

I'd say your battery is probably shot and that it isn't running your pumps properly until the engine is running and supplying full power. Stick with the readings when your engine is running as this is quite obviously the actual reading that is important

[AngM018]

make sure you hook your battery up to some sort of charger when you are doing the test.

[RarlyL8]

and just depressing airplate for steady reading

Don't do that with the engine off. Pressing the air metering plate while the pumps are one will allow the injectors to dump fuel into your non-working cylinders.

I check the cold pressure with the engine off and the warm pressure in real-time running the car. You can secure the gages in the engine bay long enough to get it fully warm for real world readings. I let the car engine cool fully before removing the gages.

[Alan L]

Thanks guys - will look at battery - altho I have had a charger on it most time while doing static test.
Brian - I just crack the plate open til pumps kick in - takes a few seconds to come up to pressure - the amount of fuel dispensed is minimal - but I'm aware of what you say.
At idle opening you collect about 5 ml over 30 secs.
Regards
Alan

[RarlyL8]

Instead of "don't do that" I should have said "careful doing that".
There are lots of things that we do that are potentially harmful.

[Alan L]

You have persuaded me to jump the relay anyway.
Will try my new battery.
Regards
Alan

[JFairman]

I think just "cracking the sensor plate" to turn the pumps on would still shoot some fuel out the injectors. Thats about whats happening when you are turning the car over on the starter motor to start it.
It's easy to just reach around to the back of the CIS assembly and unplug the blue plug next to the sensor plate with the ignition on and motor off. Doing that wil turn on the fuel pumps.

Your question about checking the control pressures with motor on or off is a very good one. The voltage in the entire car will be somewhere around 1 to 1.75 volts higher with the motor running and it seems that would speed up the fuel pumps raising fuel pressure.

Then again I havn't checked that, but I will one of these days...

[Alan L]

I am currently re setting my numbers - based on the new battery.
My system pressure pegged on the new battery with engine off. It also pegged on the old battery, with engine off, and I dropped it down to spec. Now with new battery, I have had to drop it again.
Given the battery has this much effect - probably more significant on a 930 - two pumps, and higher system pressure/fuel flows, it would seem better to adjust with engine running.
Regards
Alan

[boostfix]

This is great info! Archiving for my winter CIS tuning project.

[Alan L]

My old battery was not that bad. It could start it easily, and after fiddling with various settings etc, it would crank it over at extended periods for about 6-7 times before showing signs of not wanting to go, before the car fired. So, it was on the way out, but still had useful life. But threw the static CIS readings right out by reading all pressures lower than it should. So, if you want to do static CIS numbers, at least on a 930, the battery should be in good nick. Mine was fully charged - I leave it on a low rate charger all the time it is not used. But their capacity to sustain high draw off diminshes, even when fully charged - they just can't hold enough to sustain draw off. A 930 would require at least twice that of a 911 for the test.
Alan