D-Jetronic engine problems - PeachParts Mercedes-Benz Forum

Graham · #1 08-31-2009, 09:28 PM

My '72 350SL with M117 engine had been running well, but a week or so ago, I had some misfiring and rich fuel syptoms on hot starts. Thought it was just because of the hot weather we had and perhaps leakdown of the injectors.

I decided to change the injectors for a spare set that I cleaned up last year. While I was about it, I installed a new rotor and cap plus new plugs.

I got everything back together this morning. I checked the fuel pressure and it held full pressure for 10+ minutes. After about an hour, it dropped to about 20psig (from 29psig). This seemed OK. Better than before.

Started car and it ran fine. Took it for a run (mix of city and highway). At first it was good and then it started to stumble and mis-fire. Worse than before I started

Even at higher speeds it was misfiring.

Checked plug and injector leads were properly seated. Made sure Distr cap was on properly. Made sure plug leads went to proper plugs! Could not see anything wrong.

Pulled plugs. They don't look good! Remember these plugs have only been driven 20 miles. They are the proper NGK plugs for this car. The previous plugs looked the same and were equivalent Bosch plugs. They had worked fine for past year or two. Pictures of plugs are here:http://home.cogeco.ca/~mbz/newplugsafter20miles.pdf

I did a compression test while I had the chance - Amazingly all cylinders read 150psig.

I still need to check the trigger points, but can't see how it could be that. And do a vacuum check once I get engine running again.

Any other suggestions?

Tomguy · #2 09-01-2009, 07:57 AM

EFI trigger points, MAP sensor hose, standard vacuum leak suspicions. You put on injectors that were an unknown. They may flow more than the others you had. You are rich, that's for sure. 20 miles shouldnt get that black!

meltedpanda · #3 09-01-2009, 08:51 AM

wow, I hestitate in saying this since you and I just went through this on mine but
AAV stuck open??

Gurunutkins · #4 09-01-2009, 09:10 AM

Hi Graham
some of those plugs are really black for a 20 mile drive, apart from Tom and Ron's suggestions you might also want to recheck your pressure in the ring, you may have one or more of your replaced injectors leaking after the workout on the 20 mile drive. do you get any smoke when just idling?
cheers
Barri

Graham · #5 09-01-2009, 10:35 AM

Thanks for input guys.

barri - reason I changed injectors, was because of leak down. The "new" ones held full pressure for 10+ minutes and dropped to about 20psig in an hour. With dash mounted gauge, I check it after every stop. Gauge comes on with radio before fuel pump runs. No smoke at any time, but exhaust smells rich and there is some sign on ground of liquid droplets spitting out. I intend to check calibration to make sure rail pressure is not too high.

Re AAV Ron - When I was on test run, I stopped and checked the AAV. I squeezed the exit elbow and it made no difference. This means that there was no flow which is unusual because it always leaked a bit. Maybe air is leaking in somewhere else and nothing is going through AAV?

Re injectors - I did flow test them after I cleaned them - My test set up is crude, but indicated slightly low flows.

I want to do some sensor testing plus throttle switch and trigger points before I clean and replace plugs. Also check that I have good spark. There is a crack in one resistor's ceramic, but I doubt that would be a factor? Need to replace it though.

Re vacuum leaks - I tried using propane around injector seals using small tube. They should be OK. Need to check the MAP hose and other potential vacuum leaks. Also the throttle switch may be malfunctioning on idle.

I have the feeling this may take a while!

bwostosh · #6 09-01-2009, 11:36 AM

How about leaning the MAP sensor,
Pop off and catch the black cap and with a hex wrench turn CCLW 1/4 turn (per test) and road test. I was able to perfect a 450SL on a 1,500 mile road test, doing this and reading plugs. mileage went from 12 to 16-18mpg. Keeps in tune better now.

Graham · #7 09-01-2009, 04:43 PM

Quote:

Originally Posted by bwostosh

How about leaning the MAP sensor,
Pop off and catch the black cap and with a hex wrench turn CCLW 1/4 turn (per test) and road test. I was able to perfect a 450SL on a 1,500 mile road test, doing this and reading plugs. mileage went from 12 to 16-18mpg. Keeps in tune better now.

I was going to do that after installing the new plugs. But they were so bad after 20 miles, that it looks like something else might be wrong. Don't want to adjust MPS until I am sure everything else is right.

Anyone have any suggestions on where to get a used CO or two gas analyzer? Only ones I have seen are the Gunson ones for about $250.

meltedpanda · #8 09-01-2009, 08:58 PM

man I looked for a co analyzer forever, no luck ,try some old service stations in your area ,you may luck out, good luck on your "hunt" keeps us posted

Graham · #9 09-01-2009, 11:48 PM

Has anyone removed the top of the intake manifold and replaced the connecting rubber tubes? Or at least tried to look for leaks in that area?

I have not done a test yet, but the other day when I was checking the new injector seals for leaks, I accidentally let some propane discharge just above the opening in the intake manifold - engine almost stalled. I thought it might have been sucked into the throttle intake, but thinking back, it seems more likely it went down to area where connector pipes are. I thought the rpm should increase, but the opposite happened.

Chas H · #10 09-02-2009, 12:02 AM

Clean the grounds, clean the trigger points. Other suggested problems will affect all cylinders, not half.

Genbiltstein · #11 09-02-2009, 05:37 AM

I don't know right off hand how many cylinders a 117 engine has but I do know the D jet has a cold start injector. That injector on mine is what I refer as my ninth injector.
If your cold start injector is continuing to pour fuel into the problem then you are going to run extremely rich. There is an electrical signal that opens that injector when you are starting the car.
I am not so sure if this injector should fire off fuel until the engine warms up. I know for certain that the cold start injector had best shut off or you will suffer problems like you are stating.
There is a temp sensor that tells the computer that the cold start injector can fire.
Your spark plugs are extremely rich with unburnt fuel. I would blame the cold start injector not shutting off when needed after the engine warms up.

meltedpanda · #12 09-02-2009, 02:20 PM

Quote:

Originally Posted by Graham

Has anyone removed the top of the intake manifold and replaced the connecting rubber tubes? Or at least tried to look for leaks in that area?

I have not done a test yet, but the other day when I was checking the new injector seals for leaks, I accidentally let some propane discharge just above the opening in the intake manifold - engine almost stalled. I thought it might have been sucked into the throttle intake, but thinking back, it seems more likely it went down to area where connector pipes are. I thought the rpm should increase, but the opposite happened.

I would like to know as well, cant be too bad, and the idea on the CSI is a good one, if they leak, they are an issue, did you replace when you did the others?

Graham · #13 09-02-2009, 03:18 PM

Quote:

Originally Posted by meltedpanda

I would like to know as well, cant be too bad, and the idea on the CSI is a good one, if they leak, they are an issue, did you replace when you did the others?

It looks like a reasonably easy job to pull the top of the inlet manifold. Wish I had thought of it when I had the injectors out.

I did not change the cold start injector. I did check it for leakage and it held pressure. But what I did not check, was if the ECU was telling it stay open. I think I will clamp it off before I restart the car (that is once I get it all back together!)

Right now, I think I am on to another problem, that could be causing the rich idle on these cars. The Throttle Position Switch has to be set properly. When the throttle plate is completely closed and the external lever up against the stop, contacts 12 and 17 of the switch should read 0 ohms. After a very slight movement (0.4mm at the stop), the reading should be infinity. The closed contact signals to the ECU that the throttle is closed and presumably sets a lower fuel injection rate? This was not set right on my car.

In process of cleaning and resetting.

ctaylor738 · #14 09-02-2009, 04:37 PM

I agree with Chas H. Something is causing individual cylinders to run rich, others look a bit lean. That means either injectors, trigger points, or wires, with an outside chance the brain is frying. So my $.02 would be to look first in those areas.

Can you switch injectors from a rich to a lean cylinder and see if the problem moves?

Graham · #15 09-02-2009, 07:42 PM

Quote:

Originally Posted by ctaylor738

I agree with Chas H. Something is causing individual cylinders to run rich, others look a bit lean. That means either injectors, trigger points, or wires, with an outside chance the brain is frying. So my $.02 would be to look first in those areas.

Can you switch injectors from a rich to a lean cylinder and see if the problem moves?

The pattern on the plugs was almost exactly the same with the previous plugs and that was with a completely different set of injectors.

The plugs in real life all look rich - sooted down inside the body. Just different degrees of sootiness

The pictures are a bit misleading.

I have checked resistance of wires from distr cap contact to centre of a good plug. They all come out about the same 6.1 Kohms, I believe.

Just in process of cleaning up contacts around coil and resistors.

Trigger points could be the problem. Still have to pull them.

The other thing I probably mentioned already, is differing leakage on the tubes in the inlet manifold. I will test this when I get car going again. But so far as I can see, using propane, I will just be able to check front vs back.

Finally, today I pulled the throttle body, cleaned the TPS and set the idle position and linkages carefully. I suspect the idle switch may not have been activating. BTW, inside of inlet manifold was black and damp. Only way fuel would get there, would be from start up injector?

As of next week, I will have an exhaust analyzer available. Should be able to check CO levels.