Topic: K-jetronic pressure testing (troubleshooting)

[Jan S]

I have a 1975 USA model 450 SE with an 6.9 engine (normally, the car comes with the 4,5 liter engine).

I'm having trouble with my k-jetronic. The car won't start.

I have tested the fuel flow return to tank (0 liter/ minute), the fuel flow to the distributor (about 3 liter per minute), the system pressure and control pressure with cold engine and 15 deg C ambient temp. (both 2,4 bar).

The pressures should be 5,2-5,8 bar and 1,1-1,5 bar respectively.

My hypothesis is that the pump is not delivering the required pressure, hence I need to replace the pump. But it's quite suspicious that the two pressures are identical.

Any idea why that is? Coincidence? Or is the WUR playing its game hear?

Help much appreciated. 

[TJ 450]

Off the top of my head, 3L / min sounds ok for the pump. I’d be focusing on the FD, maybe a stuck metering piston? Blocked inlet screen?

Was the car running recently, or sitting for years?

Tim

[raueda1]

I have a 1975 USA model 450 SE with an 6.9 engine (normally, the car comes with the 4,5 liter engine).

I'm having trouble with my k-jetronic. The car won't start.

I have tested the fuel flow return to tank (0 liter/ minute), the fuel flow to the distributor (about 3 liter per minute), the system pressure and control pressure with cold engine and 15 deg C ambient temp. (both 2,4 bar).

The pressures should be 5,2-5,8 bar and 1,1-1,5 bar respectively.

My hypothesis is that the pump is not delivering the required pressure, hence I need to replace the pump. But it's quite suspicious that the two pressures are identical.

Any idea why that is? Coincidence? Or is the WUR playing its game hear?

Help much appreciated.

Let's clarify some more.  You're saying that
1.  the fuel pump is delivering 3 L/min and,
2.  There is zero fuel being returned to the tank?

Can you elaborate on #2 more specifically?  The way I'm reading it is that the fuel return line is blocked.  If that's the case then you need to fix that before anything else.  Maybe look over this thread too.  The troubleshooting process is covered in some detail.
https://forum.w116.org/mechanicals/exact-fuel-pressure-test-hookup-help-needed-for-a-california-6-9/

[Jan S]

Thanks to both of you for responding to my message - it's highly appreciated!

The car has been running and starting since I bought it (April 2019). I noticed a change in the pump over the summer 2020. With ignition on the pump started and after ca. 5-10 sec the pump's sound changed ... as if the downstream system was filled with fuel and the pump operating at a higher pressure (resistance in the system). When the sound changed I could go ahead and start the car with no problems. Without the change in sound the car wouldn't start. Over the summer I drove close to 3000 km and the change in pump sound went from 5 sec to 30 sec to 60 sec to "never". Currently I have no change in pump sound and the car won't start.

The 3 liters/ minute comes from the feeder that goes to the FD inlet. The pump delivers this amount through the accumulator (?), damper cage, fuel filter and pipes, i.e. resistance is fairly low I believe. Pressure of 2,4 bar is probably sufficient to deliver that amount.

When measuring the return-to-tank flow I measured at the FD-outlet, right after the main pressure regulator. A system pressure of 2,4 bar is probably not sufficient to activate the main pressure regulator, hence no return flow.

I believe the pump is ready for the bin?

And, the fact that system pressure and control pressure are identical makes me suspicious. Is the WUR not doing its job here?

I'm not experienced with k-jet. Learning as I go forward. It's all hypotheses at this stage.

[daantjie]

If the pump is old/original to the car I would replace it for sure.  They are pretty cheap and this then gives you a baseline from pump perspective.

[raueda1]

If the pump is old/original to the car I would replace it for sure.  They are pretty cheap and this then gives you a baseline from pump perspective.

Yeah, I agree.  But if that doesn't do the trick then get yourself a gauge and start down the path described in the other thread.  It's a sequential and systematic process.

[Jan S]

Thanks to all of you for encouraging messages and tips!

I'll start with the pump (buy a cheap used one) and start testing the whole system (I have bought a gauge). I might even have to look inside the tank. Start back and work forward. When I know the general condition of the system I will start cleaning the tank (maybe?), replacing pump, filter, accumulator, ........ all depending on test results.

Look forward to going through the link above from Raueda1.

I'll keep you posted. Cheers.

[daantjie]

With respect, buying a "cheap used" pump is false economy.  To have a true baseline you need a brand new pump.  How do you know if the used one is any good?

[Jan S]

Yes, you have a point. It's just an option on the table to sort things out (e.g. tank inspection, system testing) before the new pump is installed.

[raueda1]

Yes, you have a point. It's just an option on the table to sort things out (e.g. tank inspection, system testing) before the new pump is installed.

You're not the first person to go through all this and you won't be the last.  It seems like this fuel system stuff is always the same in the end, and you'll wind up checking/cleaning everything, start to finish.  If you haven't cleaned out the tank you might as well do it now.   Odds are remote that it won't have a lot of crap in it unless somebody already did it.  And replace all the rubber hoses back there.  The fuel return hose is often NG and can restrict backflow even if you can blow through it.  then you can start moving down the chain....    Good luck!

[John Moore Sr]

When is the last time the fuel filter was changed?  It could be restricting flow if partially plugged up. Another possibility (if the pump package is so equipped)  is the "Pressure Compensator" as MB terms it. This is a little inline device with a small metal line added into the filter inlet connection that connects to said "compensator",  then ties into the accumulator leak off line that connects to the inlet damper on suction side of fuel pump. It is my opinion that failure of this little device would cause a recirculation of fuel pressure back to pump intake.  There is no mention of this device, nor of it's intended function in any printed MB Service literature that I have seen.  It is possible that it may be referenced in the M100.985 engine service micro fiche.  Another possible low pressure inducing part could be the in tank strainer located in the bottom of the fuel tank. The inlet pipe to the fuel pump connects to the strainer. It has a hex head to facilitate removal with a 23??mm socket. Of course you have to unscrew the hose connection out of it, and if you are lucky the strainer will turn out with it also. Sometimes they will. Sometimes they don't. The tank has to be completely drained of fuel to perform this, and as usual use extreme caution while working with flammable fuel.  JMSr.
 

[revilla]

Wait a minute. Am i getting something wrong here?
There’s 3 l/min flow out of the pump. Why are we talking about replacing pump, filter, etc?
The car was recently running. Cleaning the tank???
If I understood well you have an engine that doesn’t start and your objective is to get it running, not to replace parts left and right for the fun of it, right?
If that’s the case, and ONLY if that’s the case simply remove, check and clean your main pressure regulator. You have 2,4 bars when it should be at 5,8. I bet you one or all of the 3 o-rings there are either broken or missing. No need to buy a full (expensive) unit. Nothing to do with the wur to answer your previous question.  Or, your return line might be clogged.
I would start there until you get fuel returning freely back to the tank and your system pressure is at 5,8.
Good luck
Ps.: sure, cleaning the tank is always a good thing. Replacing every part no doubt is a good idea too. But all depends on your own scope in your adventure with the car. You did a fuel flow test, was the fuel clean? If yes, leave the tank alone for now and put it in your ‘continuous improvement’ bag. Magic doesn’t exist with these systems, thinking you replace a part and BINGO your engine will start. There’s rather understanding of the functioning principles and TONS of fine tuning. Specially with the KJet, plug-and-play like modern semi-mechanics won’t get you there and your bill and frustration will reflect just that.
Robert

[Jan S]

Raueda1; thanks for good wishes. Yes, I guess I will go down that route - ending up cleaning and testing the whole thing.

John; the filter hasn't been changed in my ownership (since April 2019). The plan was to do it summer 2019, but no. I will change it for sure, but as long as the pump delivers 3 liter/minute at FD input, I believe the filter and the tank strainer is not the culprit (working hypothesis). Although the strainer is also on the list to be replaced. I need to read a bit more about the "pressure compensator", that's new to me. And yes, working with fuel requires certain measures, e.g. removing plugs to battery and avoiding electrical tools.

Revilla; the flow at FD input is ok (3 liter/minute), but the system pressure is too low (2,4 bar). Isn't that pressure too low for the main pressure regulator to be "activated"? As soon as the pressure reaches let's say 5,5 bar, the main pressure regulator will "move" and allow fuel to go back to the tank? That's why i believe the pump needs to be replaced.

My objective is to get the car started, then test and inspect the entire system. Then I will assess and decide which route to go.

[revilla]

Hi Jan,

Yes, you’re correct.  But that’s assuming your main pressure regulator is in working condition.  If your main pressure regulator is missing or has broken o-ring(s) it will allow more fuel flow thus giving you a reading of 2,4 instead of 5,8.  That’s why I suggest to focus on the MPR until you reach your 5,8. I have never seen a pump capable of delivering 3 l/min and not able to develop 5,8 pressure. But this could be a 1st time (?).
Removing, checking and cleaning your MPR is much easier than changing the pump that could be perfectly operational.  That’s why I always start there. You might be also missing shims in the MPR, true, but again I have never seen an MPR even missing all shims that gives a pressure drop all the way down to 2,4.  I think a MPR with no shims (usually 2-5) gives you about 5 bar of pressure. 2,4 is definitely too low.
One more thing, it’s odd that your control pressure and your system pressure are both at 2,4. Too much of a coincidence. Please make sure your gage valve is positioned on the WUR side and not the other way around. This mis-connection will make your gage always read control pressure regardless of the valve operation (open or closed).
Please let us know your findings. We’ll get there.
Regards
Robert

[revilla]

The spec calls for 1 l/30 secs, or 2 l/min minimum. The pump demonstrated flow is 3 l/min thus I believe your pump is OK. 
Again, all depends on your own personal objective. If you just want to replace parts with new that’s ok too. I just thought you wanted to start the engine.  Changing the filter doesn’t hurt either. But I doubt that’s the problem causing the difficulties to start the engine. I could be wrong of course.