240 K-Jet Pressure Testing Question

I tried to do the fuel pressure test per the the green book for K-Jet, specifically a 1983 non-I/C B21FT.

First I unplugged the ignition computer and the CPR connector (thanks, flat blade screwdriver!).

I made sure to 2x check my set up with the instructions and CleanFlameTrap - ball valve side to CPR hose, non-valve side to CPR inlet fitting @ distributor. Had to pop off the shield, but it's on there.

Battery is plugged in and making a healthy 12.99 volts.

Fuel pressure was "okay" enough to drive the car into the garage a few weeks ago and i know for sure the start injector isn't leaking, intake couplers are on tight, and the distributor cap & plug wires are good. Redid all the wiring too and vacuum lines are plugged in right.


Turned the key to start the ignition as the book said and...no fuel pressures or anything. Gauge read zero.


Stupid question: was I supposed to plug the ignition computer back in or is the best practice to get power to the fuel pumps by way of connecting fuse 5 to fuse 7? The green book specifically mentions plugging the ignition computer back in as one of the last steps after you finish, but perhaps I misinterpreted the instructions or didn't print a page.


I noticed that the others who have done the test have not unplugged anything and the BMW guys claim "it's all mechanical, so you don't need that", but reality seems to tell me a different story.

Some years the ignition switch/ECU 'primes' the fuel pressure, some years don't.

Jump the fuses. Read the pressure.

TestPoint said:

Some years the ignition switch/ECU 'primes' the fuel pressure, some years don't.

Jump the fuses. Read the pressure.

Click to expand...

I jumped 5 & 7 and the pumps work as expected.

Related question: Fitting H is putting up a fight on the ball valve side. I expected to get the hex swivel fitting on hose E to spin on until hand-tight and give the 9/16ths wrench 1-2 turns to snug it on, but I've really had to give it quite a few turns beyond hand-tight just to cover the threads on H and it's leaking.

For those of you who've done this, is this normal and I just need to keep tightening E until it's good & snug or did I get a defective Fitting H?

Try using some thread sealant on the fittings. They will seal up easier without having to super tighten them and possibly break the fitting. They are usually brass so you have to be careful. Also be sure to use sealing washers where the banjo fittings are.

OK, not sure if I'm using this right. Need yet another sanity check.

I've triple checked everything and sourced a spare fitting H from the diagram above.

I set it with the valve gauge cock pointed in the #1 position. I ran fuse 5 to 7. It looks like the "leak" is fuel being pushed back out the line going into the CPR and the gauge reads 0 psi in valve open position (in line with the hoses) suggesting I have 0 psi, which doesn't seem right to me.

What am I doing wrong? I'm not running the engine like some videos.

The valve should be open so there is flow to both the gauge and through the hose. Fuel flowing is the control pressure. Close the gauge off and you read system pressure. Close the gauge off and turn off the engine and let the pressure decay is rest pressure. You shouldn't read zero pressure so there must be a leak. If the control pressure is too low it could read very low pressure.

If you have the test gauge connected to the center of the fuel distributor and the 10mm side of the control pressure regulator with the fuses correctly jumped and you are reading 0 psi then the most likely issue is your fuel pumps are not running . . . or the tank is now empty.

That is not a sarcastic comment. I chased such a situation for a half hour before realizing there was nothing in the tank to pick up.

The gauge should read the 20-50 regulated pressure or about the 70 psi of the FD regulator. In no case should it read 0.

If you have fuel pressure feeding the input to the fuel distributor and no reading between the FD and CPR I guess the FD pressure regulator could be blocked with trash. That center connector is pretty directly connected to the regulator.

TestPoint said:

If you have the test gauge connected to the center of the fuel distributor and the 10mm side of the control pressure regulator with the fuses correctly jumped and you are reading 0 psi then the most likely issue is your fuel pumps are not running . . . or the tank is now empty.

That is not a sarcastic comment. I chased such a situation for a half hour before realizing there was nothing in the tank to pick up.

The gauge should read the 20-50 regulated pressure or about the 70 psi of the FD regulator. In no case should it read 0.

If you have fuel pressure feeding the input to the fuel distributor and no reading between the FD and CPR I guess the FD pressure regulator could be blocked with trash. That center connector is pretty directly connected to the regulator.

Click to expand...

It turns out this was EXACTLY the issue, I thought it was like a pneumatic gauge where it would hold the reading, but I guess not. A trip to Home Depot and I had a remote switch that let me run the pumps outside the car.

14-20 psi line pressure, 80 psi control while cold - exactly the opposite of what the greenbook told me I should see. It still seems like it's pushing fuel back out the line at the fuel distributor or something like that because there is near-zero rest pressure. I'll try some of the diagnostic measures in a bit starting with releasing any overpressure. No leaks and tank pump is still good. Maybe I'll check the pump current consumption.


Not good, but it's at least a start.

I don't think you can have higher control pressure than line pressure when the pumps are on

apachechef said:

I don't think you can have higher control pressure than line pressure when the pumps are on

Click to expand...

Hmm...I might be doing this backwards. Will be back with a picture this afternoon.

The fuel pump should be pushing gas at about 90 psi while the FD regulated pressure should be about 70 psi to the CPR. The surplus returns to the tank via the FD/fuel return line.

The CPR returns fuel above its 20-50 psi regulated pressure back to the return line via the Frequency Valve under control of the O2 sensor and computer.

If you, indeed, have no rest pressure that is not an issue with the FD or CPR but either the pump or the back flow valve at the pump.

TestPoint said:

The fuel pump should be pushing gas at about 90 psi while the FD regulated pressure should be about 70 psi to the CPR. The surplus returns to the tank via the FD/fuel return line.

The CPR returns fuel above its 20-50 psi regulated pressure back to the return line via the Frequency Valve under control of the O2 sensor and computer.

If you, indeed, have no rest pressure that is not an issue with the FD or CPR but either the pump or the back flow valve at the pump.

Click to expand...

no rest pressure can also be from bad orings on the needle thingy in the FD

Photos, as promised!

1.) General setup. Per the instructions, I had the valve-side running to the line going to the CPR.





2.) This is what it read with the valve set to the "O" position (open).


3.) This is the with the valve set to the "C" position (closed).


4.) Reading in the "C" position (closed).

That is the correct connections. The reading for the 'open' measurment is low a bit. Typically in the low 20s. The reading for the closed position is too high or at least at the very top of the range. Low to mid 70s most often.

The system pressure can be adjusted with shim changes to the FD pressure regulator.

apachechef said:

no rest pressure can also be from bad orings on the needle thingy in the FD

Click to expand...

Interesting. I noticed it?s weeping fuel/depressurizing out the WUR line @ the FD when I?ve finished cranking or running the pumps. Is there a way to identify the difference between a bad drain back valve or a bad plunger?

Oh god I hope it?s not bad needle o-rings, because that was a miserable repair. Doable, but miserable nonetheless.

snailmale said:

Interesting. I noticed it’s weeping fuel/depressurizing out the WUR line @ the FD when I’ve finished cranking or running the pumps. Is there a way to identify the difference between a bad drain back valve or a bad plunger?

Oh god I hope it’s not bad needle o-rings, because that was a miserable repair. Doable, but miserable nonetheless.

Click to expand...

the singuluar abscence of the tiny distal oring on the fp reg needle will result in unaffected cold start, running and immediate hot restart no hot restart, 10 min post shutdown.

It turns out I got my vocab mixed up - I thought the line pressure regulator and the K-Jet FD plunger were one and the same, but the internet and the Bentley manual straightened out any misunderstandings.


In any case I took that line pressure regulator out and found out the o-ring on the tip was deformed.



The whole thing was a rusty and gunked up mess, but it looks better with some applications of Berryman's B-12. Time for new O-rings!

New O-rings in and the car holds rest pressure and does warm starts alright. It still weeps gas out of the top of the fuel distributor at the CPR line. I was also able to get it to stall a few times if I blipped the throttle hard a few times.

Starting to think it's the CPR?

Two comments:

Change the copper washers on the leaking port.

If the CPR is functioning within the specs for cold to warm pressures then it is not the source of an instantaneous (blip) problem.

I believe that control pressure could be okay depending on ambient temp. Mine read around 80 system pressure as well. I used this guide from kjet.biz