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Schrader valve fuel pressure testing

vwblue1967

Member
Joined
Apr 3, 2010
Location
NC
1993 244

Im having a poorly running, stalling, no start, runs perfect for 500 miles then won't start issue.

I'm not sure what all can be that intermittent with the fuel system but I believe it's time to test the fuel pressure. The Bentley manual doesn't mention the Schrader valve and the pressure gauge with the 90degree angle from IPD shows it doesn't fit my car (maybe not accurate). My car has the Schrader valve on the fuel rail and one in-between the tank and main pump.

I've never used one of these valves before and can't find too much info on them. My question is, how do you test the pressure? Is that a cap on a valve you can see? Do you twist the cap off and screw a fitting on it?

I've swapped relays, fuel filter, main pump, (in tank pump in the mail), power stage, CPS, coolant temp sensor.. and lots of other parts in the last two years. I tested the IAC valve and cleaned it. Checked grounds and connections. It will run as it always has then randomly freak out and stall or not start. Off and on randomly. My last 4 tanks of gas we're 25-26 mpg...not great but not bad for a 240.
 
Borrow or run down to Harbor Freight to get a fuel pressure test kit. When you have this kit in front of you you'll see how the test fitting connects to the schrader valve.
 
Don't know about the Harbor Freight kit, but you need a special 90 degree fitting that is not part of most other kits. They sure put it in a tough place.

-L
 
Im planning on going to HF today. The one they have (Pittsburgh something) says on the box fits most FI except Bosch. It also looks like it has flexible hoses but I'm not sure if it will fit.
 
Harbor Freight's kit doesn't have the 90? fitting.

The reason iPd says the kit won't fit your car is because 240's didn't come with a Schrader valve on the fuel rail. Your car must have a later fuel rail so iPd's kit should work.
 
Now it's not wanting to start... well sputter and run for a second then shut off.

Can you still test the fuel pressure from just cranking it? Or should it be running?
 
I bought the harbor freight kit and with it's flexible hose, I didn't have a problem hooking it up.

I tested with the car not running and got 41psi.

Car running with the regulator plugged about 44psi

And with it running and the regulator hooked up around 39psi.

And letting it sit it's holding the pressure pretty good. I'm guessing the fuel pressure is not the problem then. I wouldn't think the O2 sensor would make the car run this bad but I guess that's next.
 
It started fine today but hardly idled. The plugs looked black so I'm guessing it's running too rich.
 
When it's running, unplugging the AMM makes it shut off. I already replaced the coolant temp sensor too.
 
Shotgunning a coolant sensor is not a valid test, it only verifies both sensors are in the same state.

You should check the resistance of the sensor and that is in spec for the temp.
Then verify that same resistance is present at the ECU and there is no continuity to ground.

Shotgunning a AFM is the only way as there is no simple way to test one, but you can check continuity to the ECU.
It supplies the ECU with 2 signals air flow and air temp.

You can test O2 function by grounding the O2 signal and alternately supplying a signal by grabbing the O2 signal wire with one hand and the + battery post with the other and watch for a corresponding change in fueling.

Check fuel pressure regulator for leakage to the vacuum side.
 
What is shotgunning? I tested the coolant Temp sensor after putting it in the freezer and at room temp. I believe the one I had was fine but went ahead and replaced it since I had a new... tested both outside of the car.

I guess I need to check things at the ecu. I believe the Bentley manual has specs to check.
 
I want to say I checked the spark plugs when I installed them but that was a probably a year ago. Wouldn't hurt to make sure.
 
If they are copper plugs and they are a year old. Then they are worn out. The alcohol seems to wear plugs faster.

There are three temp specs to check in the Bentley. You check at the ecu connector. Check cold. a warm engine, and then fully up to temp. I've had sensors that fail when hot but are good when cold and warm. So be sure to check at all three temps. The ignition computer also uses the same sensor with the same spec for testing.
 
Here's a translation of the resistance at the ECT to voltage seen at the ECU.

ectCurve.jpg
 
You said fuel pressure didn't change when the FPR was unplugged vs not? Sounds like you have a bad FPR. You listed a bunch of stuff but not the FPR. Probably flooding at idle. Also if you run it like that long enough you'll have a melted/plugged cat...
 
I didn't know IPD had that fuel guage kit with 90 degree fitting until I read this thread. I have one coming in the mail now.

My guess is you have a crank sensor failing. Go wiggle it with the engine running and see what happens.
 
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