M129 engine running on 4 cylinders only

[Mqueretin]

Hi There,

The engine of my 250 SL of november 1967 starts easily but only on 4 cylinders, not six. Cylinder 1 and cylinder 4 (ie the first and the last in the firing sequence of the engine) do not work. All spark plugs are brand new and all spark.  Compression of the cylinders was OK last time it was last measured (around 9 bars) and when the engine was running properly, it was punchy and fast so probably no issue from that side. I can smell fuel in the cylinder when i take the plugs out which seems to indicate that both cylinders get fuel.

Any help on what I should do/what the problem might be would be greatly appreciated as I have strictly no clue and am considering, in despair, to call the local  official MB dealer (In France) !

Cheers, Marc.

[ja17]

Check to see if those two cylinders are getting spark first. Install some dry spark plugs.  If no luck, see if they are getting fuel. Crack open those two injector lines at the injectors while the engine is running. See if you get a fuel pulse.

[ctaylor738]

How did you test for spark?  If you pulled the plug out and re-connected it, that's no guarantee that it will spark under compression.  Best to test with a timing light.

But in any case, if the plugs are dry, then it's a fuel problem.  If they are wet, it's an ignition issue.

[scoot]

Any help on what I should do/what the problem might be would be greatly appreciated as I have strictly no clue and am considering, in despair, to call the local  official MB dealer (In France) !

What is the last thing that was done to the car before this problem started happening?  I would check for condition of plug wires, correct connection of plug wires (like did 2 get switched), condition of plugs after running on 4 cylinders, ...   I agree with what Joe and Chuck said.  If you smell gas then I'm thinking no spark.

[Mqueretin]

Thanks All or your feedback.

I tested the sparks by simply removing them and putting them on the engine block whilst the engine was running, and saw the sparks. indeed, I don't know of they are sparking back in under compression. However, I noticed that the plugs are getting hot when the engine runs, so I suspect they must be sparking.

The plug wiring was changed to new last year by a reputed 113 specialist workshop together with a broader engine indepth maintenance. They tuned the engine and adjusted the ignition. I suspect the timing was checked. Anyhow, the car was driving very well as never before. Very punchy and very fast. Unfortunately, the workshop is more than a 1,000 kms away from where the car is stranded.

Since the problem has happened, the plugs  are getting very dark after only a few minutes of running, except for the plugs of 1&4 (see picture attached: four of the plugs were like the dirty ones and two remained as they were before, hence my deduction that these are the two cylinders not working). The engine would not start again and I would have to remove and clean the plugs again. I have since changed the plugs and the engine starts much better (and restart after stalling), but no improvement on the number of cylinders running.

I'll be going back to the car this week end (the car is located a couple of hours driving from home) and will test the injectors: removing them (starting with the two suspected cylinders) and checking if and how they spray in a glass. I i see that no fuel is sprayed from some injectors,  my idea would be to swap injectors to see if the issue is with the injectors or upstream. If upstream, i'll check at the connection of the pipe(s) to the injector pump. If there is anything else I could try, any advice would gratefully be received.

Many thanks again. Marc

[ctaylor738]

The two sooty plugs look to me like plugs that were run for a few minutes while the engine warmed up, and then were taken out.  They don't look to me to be fouled.  The two clean ones look like they weren't getting fuel.

On the other hand, it wouldn't take too much to persuade me that you have ignition problems on four cylinders.  An inexact science.

I believe the plugs you want to run are W9.

I think your plan is a good one, but you might consider the following since the wires were replaced.

Check their resistance, to make sure it's not more than 5K ohms.  And for sure check the resistance of the coil wire.  It is not uncommon for a shop to buy a set of wires (like from MB) that doesn't include a coil wire so they throw in a high resistance carbon core wire designed for modern high voltage ignition systems.

Cheers,

CT

[scoot]

1.   I agree with your current use of W7DC plugs. 
2.   I agree that you need to check the resistance of the wires and coil wire, although I don't think that is your problem.
3.   A compression check on the two cylinders that are not firing would be interesting, as would verification that the wires are on in the correct order (although that seems unlikely to be the problem).
4.   To me it seems odd that this is happening.  You _could_ have two suddenly clogged injectors, but it makes little sense to me.  I'm more wondering if something isn't tight - either the spark plugs or the injectors themselves of the injector lines.  If the injector lines aren't tight you would see fuel leak at one fitting or the other.  If the injector is clogged you would see nothing.  If the spark plug(s) are not in properly you would also see nothing. 

If it were my problem I would:

1)  disconnect injector lines at injectors, zip tie baggies over each injector line, remove coil wire, and crank the engine for a long time repeatedly.  Are you getting the same amount of fuel in each of the baggies?

2)  If no, you have a problem with the injection pump.

3)  If yes, pressure test (pop test) the injectors and look at spray pattern. 

4)   If testing the injectors is not an option, then switching them around might be an interesting test too.

5)   If you had a couple of valves adjusted way wrong I could see this causing you to not have compression on some cylinders (or if valves are stuck).   I suppose that would show up on a compression test.

Other thoughts?

[Mqueretin]

Thanks a lot for the tips and suggestions.

Re valves, they were tuned last year and the car was driven less than a 1000 miles before the problem occurred (which was after the car was left without running the entire winter last year). I'll do the tests and hope to be reporting good news next week - if not that all cylinders are running, at least that the issue is identified !

Cheers,

Marc.

[ja17]

Yes, pay close attention to your ignition.  The ignition points may have closed up or could be burnt. A fuel pressure check is very important for a diagnosis.  You must have a good clean supply of fuel with adequate pressure.

[waltklatt]

As everyone is stating.

Also the injection pump could be stuck.
Check the rack movement for free play.
Crack the injection lines open a bit with motor running.  Just a little to see if gas is seeping out.
If there is fuel then the injectors are clogged.
You could check to see if all cylinders are firing if you spray a little starting fluid into the air intake.
That would rule out any electrical issues.
Pull cam cover off and check the valves and adjust if necessary.  Maybe valves are not opening-out of adjustment.

Walter

[scoot]

I'm not sure fuel pump pressure is relevant -- if at least some of the injectors (consistently the same ones) are firing then the fuel pump is doing well enough.
It seems like if you are getting fuel to the cylinders but no spark (or spark at wrong time) that you would smell gasoline at the tailpipe (and possibly get backfire).

Something isn't adding up here.

Are plug wires 1 and 4 switched?

[Mqueretin]

Thanks again.

the plug wiring and distributor are new from last year (see picture) so I doubt there is any issue with the wiring per se. The valve adjustment and timing was checked at the same time, ie hardly a thousand miles ago. Having asked the workshop that took care of the car last year, they're adamant that it is unlikely to be an ignition problem. As per earlier feedback received, I 'm going to test first for any fuel related issue, with the hope that there is either no fuel coming in the cylinders, or also possibly that it does not spray as it should in which case the problem is likely to be solved by taking care or replacing the injectors (which should be within my skills). i'll also test the compression in the cylinders. I should know by Saturday.

Marc.

[ja17]

Check the ignition points. Newly replaced ignition points may run-in after some use and tend to close-up.

[twistedtree]

Since the two affected cylinders are adjacent in the firing sequence, it might be worth a close inspection of the distributor cap for cracks.  And as someone else mentioned, be sure the two plug wires aren't reversed.

[Mqueretin]

Hello again,

some update after I spent some time over the week end on the car.

The issue is definitely a fuel related one: strictly no fuel is coming out of the injectors of cylinders 1 and 4. After removing the injectors from the injector lines, still no fuel coming out. The injectors of 1 & 4 are working fine on other cylinders. I have undone the fuel line of cylinders 1 and 4 at the injection pump, and here still no fuel getting out of the pump, so this is (unfortunately) where the problem lies. So I suspect that there is no other choice than removing the injection pump and get it checked.

I have tried to remove it - a real challenge in view of my limited skills in mechanics - by following the process described in my Haynes workshop manual. I could do everything up to the last two steps as i)  I was not sure about the identification of the oil lines including the line to the oil filter - if anyone had a picture showing these lines, I would be grateful), and ii) I was not sure  where to find the mounting bracket bolt which was shown in the workshop manual to be somewhere behind the solenoid at the rear side of the injection pump.

what puzzles me slightly is that the injection pump delivers fuel on four cylinder, and not on two: it cant' be blocked as if so I would have fuel nowhere.

cheers, Marc.

[Mqueretin]

more on the same:

there is apparently no need to remove the injection pump: removing the pressure valves on the injector lines and pushing the pistons while cranking the engine, possibly with the help of a good old WD40 spray, could do the job. The problem seems to be removing the pressure valves as a special tool is needed - the thread inside the pressure valves is not standard.

cheers,

Marc.

[Naj ✝︎]

Do a search here on the inj pump valves.
I'm sure a member used a bolt and 2 nuts to pull the valves.
I believe the thread on the valves is 12mm x 1.0 or 1.25 mm pitch.

naj

[Mqueretin]

Hi there,

just a quick update to report that after taking the pressure valves out of the injection pump and tapping the pistons gently, the car is now working on all 6 cylinders again with a very nice noise and very smooth functioning. I have unblocked piston 1 and 4 of the injection pump (i have also taken out the pressure valve of a working cylinder for comparison before mounting everything back). The only real difficulty was to extract the pressure valves, and I used a long screw driver as a lever with my extractor to pull them up without damaging anything.

Thanks to all those on the Forum who have pointed to a fuel issue and to Naj for referring me to posts on home made pressure valve extractor.

Marc.