testing residual fuel pressure...what should it be?

Kemo · 05-26-2009, 12:12 PM

I have a suspect fuel check valve issue im sorting thru and I hooked up a fuel pressure gauge between the fuel accumulator and the fuel pump. the fuel pressure goes to just over 5 bar when running and when I turn the engine off it drops to about 2.4 bar. while sitting, the pressures slowly go to about 1.2 bar after 30 min.

does any of this indicate a bad check valve? also, do I have the gauge in the right place for this test?

Kemo · 05-27-2009, 04:11 PM

..

Joe Bob · Joe Bob's Garage

That's not good. The check valve is cheap....replace it. It MAY be the culprit.....most likely.

Kemo · 05-27-2009, 04:23 PM

mikez,
thanks for the input. I have read over and over to check the pressures but i havent seen any threads that tell me what the correct pressures should be or where to put the pressure gauge.

Also, I forgot to mention that I have a new fuel filter and accumulator already installed...

ossiblue · 04:59 PM

Most testing of CIS pressures is done by connecting the gauge between the fuel distributor and the WUR. To quote the Bosch "Troubleshooting Guide K-jetronic" (p.66):

Cars fitted with vacuum controlled warm-up regulator:
Engine off 2.9+/- 0.2 bar
Leak test
minimum pressure after 20 min: 1.0bar

Except for where you placed your pressure gauge, it sounds like your residual pressure is within specs. Since you wanted to measure only residual pressure, it wouldn't matter where in the closed loop you put your gauge. Your running pressure, however, is inaccurate because it is being measured before the fuel pressure is regulated by the WUR. Your reading of 5 bars is actually the full pressure of the pump.

Kemo · 05-27-2009, 05:00 PM

ossiblue,

thanks for the numbers. i will have to figure out a way to get the gauge between the wur and the fuel distributor. this was a hand-me-down harbor freight gauge

ossiblue · 05-27-2009, 05:19 PM

Kemo,

You will need a set up that looks something like this:

The long hose goes between the fuel distributor and the WUR and has a "T" connector to attach the gauge. There is a shut off valve in the long hose that isolates the WUR from the distributor, when needed. Also, you need the correct size fittings to screw into the distributor and WUR. Someone will know the size.

Joe Bob · Joe Bob's Garage

What's a check valve go for now? $30?

Kemo · 05-27-2009, 07:10 PM

Quote:

Originally Posted by mikez

What's a check valve go for now? $30?

ive been investigating this and my 78 pump has an internal check valve...rumor has it, i can add a check valve.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by Kemo

I have a suspect fuel check valve issue im sorting thru and I hooked up a fuel pressure gauge between the fuel accumulator and the fuel pump. the fuel pressure goes to just over 5 bar when running and when I turn the engine off it drops to about 2.4 bar. while sitting, the pressures slowly go to about 1.2 bar after 30 min.

does any of this indicate a bad check valve? also, do I have the gauge in the right place for this test?

Kemo,

Normally the pressure gauge is set between the WUR and FD to measure the residual, control, and system pressures. So I'm curious to know if the residual presure between WUR and FD would be the same or not compared to your measurement.

This is the residual pressure for 3.0 CIS:
After 10 minutes.............1.3 bar (19 psi) minimum
After 30 minutes.............1.1 bar (16 psi) minimum

BTW, I think your check valve is OK at 1.2 bar after 30 mins. But do the test using the conventional method and keep us posted. Thanks.

Tony