Testing the K-Jetronic system

[Max-NL]

So the 6.9 was finally delivered to the guy who's gonna restore it (in 2 steps. First step is getting it roadlegal and 2nd step is complete disassembly after the 280S and maybe the 350sl are done).

Prologue:

When we were buying it the seller send us a vid with the engine running. So when it arrived we decided to start the car, which it sort of does. It runs for a few seconds and then it dies. We tried it several times and it did the same thing again and again. We called the seller and asked him how he got the engine running. He said that he used starter fluid to keep it running.

So we started the car, let it run for the first few seconds and then we sprayed starting fluid with an interval. after doing this for about 1 to 1.5 minutes we stopped spraying and the car ran (crappy). We let the car run and after 1 or 2 minutes it started to rise the idle speed and began to run beautiful. About 1 min after that it ran nice, we even drove the car around the block (it was a bouncy ride, all the spheres are broken). We stored the car, shut off the engine and tried starting it again. It didn't want to start even with starting spray. So we decided to leave it and started disassembly.

Diagnosis:

Given the fact that the engine ran for a few seconds and died, we are thinking that it runs via the cold start injector(which sprays for max 8 a 9 seconds), after which it doesn't receive enough fuel to keep running. So spraying starting fluid gives the engine enough fuel to run(rough). When the engine starts to warm up it stays running. So it either get more fuel, or the same amount (cold starts are very rich, after this they start to lean). When it warm up enough it even runs nice. So my money is on the fuel system. And more precisely the WUR/CPR, this being the device that actually changes the fuel from rich to good.

In order to diagnose the system we need to test the pressure. So we bought a testing kit and hooked it up. We tried to start the engine, but the engine decided to be a p.i.a. and stall constantly. We tried it several times and after that we decided to stop diagnosing for that day and remove some more body panels.

So my question is: Is it possible to test the pressure without the engine running? We have the values from the workshop manual, so we just need to check those to the actual pressure of the car.

[daantjie]

Before you worry about pressures, which are important for sure, make sure you are getting clean fuel from the tank to the fuel distributor. If your tank is full of rusty crud you can forget about k jet running right. Best to start at the tank and work back. Check all the little screens at the connections at the fuel distributor and WUR. If you see crud there the system will run crap guaranteed.

[polymathman]

For my 280SE, main pressure is checked with the engine off, fuel pump actuated by jumper.

The second reading is from a tee - between the top of the fuel distributor and the warm up regulator. If you disconnect the wire to the WUR you can check the cold pressures.

You don't have to have the engine running for these tests.

[Max-NL]

Daantjie:
We already removed the fuel line towards the distributor and ran the fuelpump. The fuel that came out of the hose was clean, so the tank screen and fuel filter are doing their work (we're going to replace those anyway, along with all the rubber hoses). Once the car warmed up a bit (after 5 a 6 min) it runs great.

Polymathman:
Thanks for the confirmation.

I assume one could jump the fuel pump by removing the relay and insert a wire with a switch. Or does something else need to be done to get the pump running?

[revilla]

Hi Max,

It's much easier than that.  There are 2 options to run the pump without starting the engine.  Either you pass the ignition to position 2 and remove the safety plug at fuel distributor, or you can also put the gearbox in R or D and move ignition to start position.  Both will make the pump run, but not the engine while you take pressure readings (system, control and static pressure).

There's a complete kjectronic manual available that explains the full procedure.  I can email it to you.

Hope it helps.

[Max-NL]

Robert,

Thanks! That indeed makes it easier to test it. I'm interested in the manual, I'll pm you my email address.

[revilla]

Max,

Manuals PDF's sent.  The video below explains the basic operating principles of the system.  Doesn't hurt to learn how it works first.

I wouldn't touch the fuel distributor for now, but there a few rubber o-rings in pressure regulator that can deteriorarte causing bad cold/hot starts.  After I fixed WUR for control pressure I changed those with great results.  Check your accumulator too as they're notorious to fail (easy to test, fuel shouldn't pass the rubber diaphragm, if it does, replace).  And as Dan suggested, clean up the filters/mesh in fuel line and WUR connector.  Later you can also clean up the rubber/metal "valve" inside WUR but leave as is for now as it's pretty delicate and likely not your issue of rough cold starts.  Starting with setting up the correct pressures is the way to go. 

Warning:  video quality is not optimal, probably uploaded from Beta or VHS format of 70's/80's  but still informative

https://www.youtube.com/watch?v=a4fJAfXYxWk

You know where to come if you have questions.  Good luck

[Max-NL]

Robert:
Thanks for the video and manuals. I'll pass them along to my father.

We'll check the pressure and accumulator first, and then find out what to do next. At least we have something to do this Saturday .

[Max-NL]

Tested the pressure today by hooking up the test gauge between the distributor and WUR. Disconnected the connector at the distributor to run the pump, and disconnected the connector at the WUR. Control pressure steadily rose to somewhere between 3 and 3.4 bar in less than 1 minute (maybe even 30 seconds). Measured the system pressure and this was also 3-3.4 bar. Which is odd because it's supposed to be somewhere between 5.0-5.6 bar. After this we hooked the gauge to the supply hose of the distributor and guess what, it was 3-3.4 bar.

So it seems there are two issues (correct me if I'm wrong).

1. The control pressure is too high. Ambient temp was around 5-15C, so looking up the chart it should be between 0.5 and 1.3 bar (cold). Don't think the lines are clogged because when the engine is getting warmer it'll run good. So it looks like the WUR is faulty.

2. Supply pressure is way to low. This should be 5.0-5.6 bar (cold and hot engine). Did not hook the gauge up to the pump, we'll first change the filter and measure right after it. If it's good we'll work our way forward. If it's bad we'll work backwards.

I'll keep you guys updated.

[daantjie]

If the pump is still original I would replace it out of principle. They are not too dear. Also the accumulator is often a culprit for hard starting though it does not do much once the car is running.

[Max-NL]

I actually had a look at the fuel assembly. Filter and pump looks to be replaced some time in the past. We're going to change the filter anyway though. Isn't the accumulator more important for hot starts?

[daantjie]

More for cold starts really. It keeps pressure in the system to ease first start. Like a fuel buffer I guess is the best way to describe it. It has an internal diaphragm which can rupture and leak off pressure when it sits. Check the leak line. If you get fuel coming out when you disconnect the leak line it's buggered.

[Max-NL]

I'll check it the next time.

[revilla]

Hi Max,

The pressure regulator on the head of fuel dist is the responsible for system pressure -together with pump of course. But that's strange you are only getting 3.4 when you close the gauge valve. That's barely enough to open your injectors and your engine would run bad cold and hot (which is not what you described at the beginning).  I would check your test setup first. Make sure the valve is located between wur and manometer and NOT between distributor and manometer (mistake I did once). 3.4 CP is right on spot for hot op temp, so no need to touch the WUR for now. Once you check your test setup, test again; if problem persists, take the pressure regulator carefully out. Check the big spring and the washers at the bottom. Also the bigger o-ring. There is also a plunger in there. For system pressure to be that low the plunger is letting tons of fuel go back to tank, meaning o-ring at plunger is broken, spring is deformed (unlikely) or you are missing a few of those washers. But again, that's not consistent with your car running smooth when hot. Agree with Dan, testing the accumulator is easy and explains rough cold starts as you described.  Let us know how it goes. Good luck

[Max-NL]

So today we worked on the 6.9 again. We jacked up the car and put it on axle stands. We were happily greeted by a rusted out hoseclamp which need to be loosened via a Phillips screwdriver (hate those things, I always replace them with ones that you could loosen via a flexible clamp screwdriver). Of course the head got stripped so we had to saw it off with a little hand saw. Once removed, we disconnected the hose and let the pump run. The Niagara Falls would be jealous of the flow that came out of the accumulator.

After this we decided to quit. Some cars needed to be pushed around and we installed a radiator into the 280s (where we found out it leaks  )

So it seems that we're going to need to replace the accumulator.

And for the fun a pic of the 6.9 with its brothers. 6.9 is the one with the hood open, behind that is the 280S and on the right under the plastic cover is the 350SL.