Author Topic: K-jetronic pressure testing (troubleshooting)  (Read 519 times)

Jan S

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #15 on: 03 October 2020, 10:50 AM »
Revilla; thanks so much for swift reply and good advice.

Valve is on WUR-side. I agree - it's too much of a coincidence that the system pressure is equal the control pressure. Something is going on there ....

I measured the flow of 3 liter/minute when de-connecting the fuel supply at the FD inlet. That means flow is coming directly from the rear system without passing through the FD. The pump works against less resistance, hence higher fuel flow can be delivered at lower pressure (pump curve - pressure as a function of volume - high pressure and low volume vs. low pressure and high volume). I assume ...

The flow at the FD-outlet "return-to-tank" is zero. Nothing is going passed the MPR at 2,4 bar. I would assume the pump would reach its "dead-end" pressure which is probably 6-7 bar if the flow back to the tank was close to zero. But no - it's 2,4 bar.

Is taking out the MPR straightforward? Anything special to be aware of? O-rings that has to be changed etc.?
1975-mod W116 450 SE with 6.9 engine

revilla

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #16 on: 03 October 2020, 11:16 PM »
Hi Jan,

The most interesting observation is the zero return at FD outlet. I want to be optimistic, thinking your FD internal organs aren’t damaged.  But first, let’s remove the MPR. It’s quite simple actually.  The hypothesis is the following. Injectors opening pressure is between 3-3,5 bars depending on their condition. You have only 2,4 so they won’t open. Thus engine can’t start.  Later when possible can you please take a picture of your gage connections?

It’s not at all difficult to remove the MPR.   On the front/upper side of your FD you’ll see a 16mm (or 17?) nut. That’s your MPR. Reconnect the return line. Run the pump until you have your 2,4 bars. Unscrew the MPR but VERY slowly while watching the gage. What does it measure while you unscrew it? The assembly should come out after 5-7 turns. Do not unscrew it completely. Now your gage should be at zero as you have lost some fuel.  Right? If not at zero stop immediately.  Then unscrew the inlet line a few turns until you have zero in your gage.  Re-tight. Back to MPR, unscrew completely. Delicately remove it from slot. Sometimes the piston comes out and sometimes it doesn’t.  If piston (not magnetic) doesn’t come out give the pump a MICRO pulse keeping a rag close enough to the opening. You should now see the piston.
Take a picture of the MPR and share please.  Reinstall the MPR but WITHOUT the piston. Disconnect the return line. Give the pump a short pulse while keeping a rag and suitable container under the FD outlet.  Fuel coming out now? I hope so! If not, we have a major situation unfortunately. Remove the MPR again. Reconnect the outlet line. Micro pulse to the pump. Capture the exiting fuel. Observe. Any rubber or particles coming out? Do you have clean fuel flowing out of your (removed) MPR slot?

Please report back.  We’ll get into the MPR itself at the right time, but FIRST it’s key to understand the zero return flow and the 2,4 system pressure.

Regards

Robert

Robert
W116 1977 280SEL & 1979 280SE

Jan S

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #17 on: 04 October 2020, 03:05 PM »
Thanks a lot, Revilla!

It may take a few days before I can test according to your advice. I will get back to you on that.

It's interesting that you think the pump is not at fault or at least not the first thing to attack, when zero return flow and 2,4 bar system pressure. Can you elaborate on that?

Back in a few days.
1975-mod W116 450 SE with 6.9 engine

revilla

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #18 on: 04 October 2020, 11:43 PM »
Hi Jan,

The pump might be faulty, certainly. But based on your flow test it demonstrated 3 l/min which is beyond the spec value of 2 l/min. I understand your test was done so fuel didn’t go through the FD (measuring at inlet hose rather than at return line). Still 3 l/min sounds like a healthy pump. The fact you’re getting zero flow at the return line indicates there’s a problem with the FD itself.  It could be your MPR or the internals. I share with you it’s a mystery why you’re getting 2,4 bars with no flow at the return line. That’s why we need to understand how you connected the gage and also the condition of your MPR.  The test recommended before without piston will tell us a lot.  Another easy test you could perform to rule out a faulty pump is to connect one end of your gage (valve closed of course) to the inlet hose at FD and briefly run the pump to measure her ability to raise and maintain pressure.  This test will tell us whether the problem is with the pump (which I doubt) or rather with the FD. Again, this test is more practical than buying and replacing the pump. But if you want to buy/install a pump to check if your car would start that’s valid too.  It’s just a dart in the dark based on the fact it showed 3 l/min.  I prefer always to do the most practical tests first before buying/replacing parts and crossing my fingers.  A pump capable of producing of 2,4 bars of flow pressure is MORE THAN SUFFICIENT to get fuel out of the outlet port. That’s why I believe the issue is rather the FD/MPR than the pump.

Please let us know your findings. We’ll get your engine started. I’m confident.

Regards
Robert
W116 1977 280SEL & 1979 280SE

Jan S

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #19 on: 10 October 2020, 08:09 AM »
Finally I managed to squeeze in some garage time today.

I had to rebuild my fuel pressure tester in order to test the fuel pressure (system pressure) at the feeder (input to the fuel distributor).

The result: as I expected the pressure was 2,4 bar - identical to the system pressure I measured between FD and WUR when valve closed on WUR side of gauge.

The system pressure should be 5,2-5,8 bar.

My hypothesis is that the fuel pump must be replaced. What do you think?

An observation: the fuel pressure dropped from 2,4 bar to 0 bar immediately after ignition and pump off. Accumulator shot as well?
1975-mod W116 450 SE with 6.9 engine

revilla

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #20 on: 10 October 2020, 08:44 AM »
Hi Jan,

Well done. Confirmed, pump must be replaced. Nothing like real life test/data to drive smart decisions.

Regarding the accumulator question. As your pump is kaput you MIGHT be loosing pressure back to the tank through the bad pump itself. It acts as check valve to prevent fuel return to the tank. It doesn’t necessarily mean your accumulator is bad. It rather means it can’t do its job if the pump doesn’t hold the back pressure. But it could be leaking internally in which case you would have to replace it too.
My suggestion is that as you’re disassembling the fuel pack to remove the pump, you should take advantage and test the accumulator to confirm/rule out its condition before buying a new one. Unless you want to replace it together with the pump as a preventive measure because you might not know when was it last changed. That’s totally acceptable and up to you, again depending heavily on your personal scope with the car. BTW, change the filter too as they really inexpensive these days. That wouldn’t hurt.  One area to be VERY CAREFUL is the tubes/banjo connections linking pump/filter/accumulator assembly. They are notorious to bend under tool pressure  which will certainly lead to a MUCH BIGGER leaking problem.  Washers as cheap/simple as they look are an important piece if the puzzle. Replace all with new please.

Good luck.

Robert

Robert
W116 1977 280SEL & 1979 280SE

Jan S

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #21 on: 10 October 2020, 09:47 AM »
Thanks for your advice, Robert

Yes, I will disassemble the entire fuel pack and replace pump and filter and all rubber hoses (also in engine bay) , clamps, washers and maybe accumulator. I'm not sure about the inner life of the damper cage ... need to read a bit before I replace that as well.

How about installing a small fuel filter before pump? A filter with a glass/plastic housing to see what is coming out of the tank.

I'm even considering emptying the fuel tank, replacing strainer and inspecting tank. If tank is bad I probably need a new one. One step at a time.

 



 
1975-mod W116 450 SE with 6.9 engine

daantjie

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #22 on: 10 October 2020, 12:39 PM »
I would say draining the tank and checking it out is a very good idea, if your fuel supply is compromised then your system will never run right.  A pre - filter of sorts I would say is not really neccessary if your tank is nice and clean, those things are usually of plastic and that would make me a bit nervous to have it exposed to the elements.
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #23 on: 10 October 2020, 03:16 PM »
A valid point, Daniel, regarding "the elements".

I have never emptied a tank before, looking forward to the mess, but even more so a clean fuel supply.
1975-mod W116 450 SE with 6.9 engine

Jan S

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #24 on: 12 October 2020, 10:41 AM »
Since the whole back-end of the fuel system (pump, filter, etc.) is being replaced, I can add a fuel hose to the feeder in the engine bay and let the old pump do its last job. That should empty the tank, I assume  .
1975-mod W116 450 SE with 6.9 engine

Big Tiny

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Re: K-jetronic pressure testing (troubleshooting)
« Reply #25 on: 17 October 2020, 07:17 PM »
I have had a quick read through this thread.

Pump sounds fine, 3l a min is right on a new K jet pump. Also rules out tank blackage, filter, one way valve, supply line.

Something else is up, and that is likely the fuel distributor. FPR and WUR are ruled out with pressure test.

Check inlet fitting the FD ?