Can anyone help with my misfire

[DavidBrough]

I have just completed a major overhaul of the cylinder head on my 1969 auto 280SL euro to cure an inordinate thirst for oil. Prior to the strip down everything was running smoothly with no problems evident. However, now the whole thing is back together again the story is not so rosy as I have an annoying misfire I can’t seem to cure. The engine will always start first time and seems to run fine at high rpm and is quite smooth at 60-70mph. The story is very different at low rpm when the engine is very rough and has difficulty in moving off the line and is very jerky up to 30-40mph only starting to smooth out as the engine revs rise. The head has had new valve guides with reground valves and a skim as it was a bit warped. I have hot torqued the head to 80lb and set valve clearances to 3 & 7 thou. All cylinders compression tested to exactly 160psi and I have fitted new NGK BP6ES plugs, points and condenser. Dwell has been set to 39 (this took absolutely ages, does anyone know a quick way to do it). Ignition timing is 0 deg at tickover and 30 deg at 3,000 rpm. While the head was off I had the injectors cleaned and tested and was told that they were in a very poor state. However, they cleaned up well with only one still having a slightly off par spray pattern. The tester said they would be 100% better than before. Fuel lines and injectors were torqued to spec and there are no leaks. I also replaced the cam chain and this timed up spot on the mark at TDC on number one. I always set the tickover air flow with a colour tune spark plug and have never had any problems. I do have a very slight blow on the exhaust but this shouldn't affect anything, I think. After all that and a smooth running engine before I started I am now getting very exasperated and don’t know what to do next. Does anyone have any ideas what might be causing the low speed misfire.

[ja17]

Hello David,

You will need to be certain that the linkage adjustments were not altered during the work. We can help you here if you are not familiar with the process.

Check to make sure the cam timing is correct next! The alignment marks on the cam gear and front cams stand must align or be just slightly late when the engine is at top dead center. It should never be even a little bit early.

One other thing comes to my mind with your symptoms. If the injection pump is run 180 degrees out of time it will run fine except at lower rpms where it may be a little rough and have a hesitation and be sluggish at low rpms.

It could have been possible to get things out of time 180 degrees if the distributor was off during the head work especially if the chain was replaced. You do not have to have the injection pump off to get things out of whack. If the techniciaqn was not careful he may have accidentally set up the injection timing  180 degrees off without realizing it.

Sounds like you have checked most of the standard tune-up issues.

Joe Alexander
Blacklick, Ohio

[DavidBrough]

Thanks Joe, I did all the work myself and from what you say may have got the fuel pump timing off as that seems to fit my exact symptoms. How do I check and fix this. When I took off the head I didn't touch the distributor at all and fed the new chain through before replacing the head and used the position of the distributor rotor arm to set TDC on one. I then replaced the head, connected the cam and it set bang on the mark with the new chain.

[TheEngineer]

The linkage alignment Joe is talking about, is the pushrods from the gas pedal to the throttle plate and the injection pump. The relationship between the position of the throttle plate and the arm on the injection pump is critical because it controls the mixture. The book describes how to set it up. When one does work on the head, the distance down to the injection pump is often changed. Just my two cents worth.

[DavidBrough]

Thanks Peter, I think I have the throttle linkage correct as I have set this before and it's quite straight forward. At the moment I think I may have messed up the injection pump timing when I changed the cam chain. I dont' know how it is possible to do this but does anyone know how to check and fix it.

[ja17]

Hello David,

There is no procedure for doing this without pump removal on the gasoline injected engines.  However I have a method which may save you from removing the pump if the timing is not off. Remove #4 injection line at the pump (3rd from back) remove the fitting and the check valve so that the pump piston (plunger) can be watched or felt with a probe.

The number four injection pump plunger is at the top when the engine is about 12 degrees after top dead center on the compression stroke (cam lobes up)of #1 engine cylinder.
Note, this is a "Blacklick Procedure" and you will not find it documented in any literature. Hopefully it will verify the injection pump timing situation of your engine.

The fitting on the injection pump should be torqued to 18 ft/lbs during re-assembly.

Joe Alexander
Blacklick, Ohio

[Raymond]

David,
After a total engine overhaul, I had the exact same symptoms.  I did everything you described.  I know you checked the linkage, but check it again.  I went through everything, I thought, three times and was really getting depressed.  Finally I disconnected the rod that goes to the throttle butterfly valve. The rod was just two turns too short.  It was holding the valve open just a gnat's hair.  It was enough to cause a two inch drop in manifold vacuum.  

Search the site for the idle/mixture procedure that I think Joe wrote up.  It is the bible for this procedure.  Set your mixture and idle with a vacuum gauge and RPM meter.  My car is running well with idle set at 15 in. vacuum at about 750 RPM.

Ray
'68 280SL 4-spd Coupe

[Cees Klumper]

David - sorry to hear about your troubles. I once forgot to properly bolt down one of the brackets that holds the gas linkages (the one on the intake manifold with two 13-size bolts) and the engine ran very poorly. Just tightening them down made all the difference in the world, so before you rule it out positively, I would try tweaking the linkages one more time.
If you do a search on this forum using search terms like "timing fuel injection pump" you will for sure come up with all the relevant information.
FYI taking off and re-installing the FI pump took me under four hours when I did it for the second time.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[Bearcat]

as a non mechanical head but i wonder is it ignition? i had all types of problems upto when i fitted the crane xr700 module. i know its taking away the originality but i didnt feel guilty when the same module was fitted to a 600 grosser that came back after a 60k overhaul in te states to ireland.


1970 280SL

[DavidBrough]

Thanks guys, thats given me a few things to double check. As you say, sometimes you can just go round in circles and keep missing something simple. The symptoms Joe described for the injection pump being 180 deg out sound spot on but I will check the linkage first as thats much easier (or at least I thought it was). I'm still not sure how the pump can get out of sync though.

As the engine seems to run OK at high rpm I had pretty much excluded anymore electrical issues, like ignition leads and distributor cap, would this be correct?. I changed the coil and balast resistor last year for the more powerful red versions and all had been working well. I also held and touched all wires and distrubutor/coil with the engine running and as my eyes didn't light up assumed there were no shorts!

I'll keep you posted when I can get my spanners out next and think I'll invest in a vacuum guage if the pump and linkages seem OK.

[DavidBrough]

Joe help - I have just tried your procedure for checking the injection pump timing and successfully located and removed number four injection line and its union connection to the pump. However, nothing seems to move at all, well according to my wife at least. I have tried to attach a photo but can't seem to do that either. So I'll try and describe it. Removed injection line and the union connection between the line and pump to reveal the pressure valve. This has what seems to be a sealing washer round the outside and a hole in the middle into which I inserted a small allen key. I then positioned "she who must be obeyed" with her finger on said allen key with instructions to apply slight downward pressure and to shout very loudly when movement was detected. Needless to say, she remaind unusually quiet despite my protestations that something must be moving after turning the engine three times. Am I checking the wrong bit, missing something or need to replace the wife.

[ja17]

David,
You must remove the pressure valve. Find a brass fitting which will screw onto it and pop it out! the threads do not need to be exact. In fact these threads on the outside of pressure valve are used only for removal and have no other function.

Joe Alexander
Blacklick, Ohio

[Ben]

One thing I havent seen mentioned..............have you identified which cylinder is misfiring, if possible ?

You know what I mean, see if the misfire is distinctly one cylinder ! I get the impression that it isn't and that it is more of a stumble than a definite miss but if you can decide this then you pretty much rule out any igintion issue !

All plug leads loose, use an insulated pliers to pull each end off one by one 'till you identify (or not) which one is causing the issue. I know you probably knbow all that and have done it but since it wasn't mentioned I thought I'd state the obvious since the simple things can get overlooked !

If you do identify a misfiring cylinder, swop the plug, then  the lead etc. and even the injector to see if the fault follows !

Other than  that I'd say the linkages are the best place to start !



Regards,
Ben in Ireland.
'64 230SL 4sp.

[DavidBrough]

Ah progress, managed to check the injection pump timing, thanks Joe. Unfortunately, it's spot on so I still have a problem. I have re checked the throttle linkage adjustment and set the pump rod with my trusty small piece of tube, throttle butterfly just catching and both sides opening smoothly to full throttle, all mountings tight and secure. I have ordered a vacuum gauge and will confirm tickover and check for manifold leaks. One thing I have noticed is that number two plug seems quite black whilst the others are OK. I swapped the injector between one and two and the black plug seems to have followed it. I have now ordered a new injector and will try that. Also thought I may as well get a new set of leads, cap and rotor arm at the same time. I think SLS are on for a good month. Will report back when the parts arrive and I have had chance to test drive the results. Anymore thoughts sill gratefully received. Thanks for the help so far guys.

[Ricardo]

David
If it's of any help I had a similar problem last year that turned out to be the one new injector I had installed being defective. Of course I assumed it couldn't be that and it took my son's prompting to check all the injectors and sure enough the new one had the 4 layer fine screens in the bottom of the injector turned sideways and a thread of wire from one of the screens worked down and under the seat, causing the injector to dribble constantly....I guess the lesson is that you can't overlook anything, even if you have changed out a part, it could still be defective.
Ricardo

[Ben]

Looks like that one that was slightly off was enough !

I have seen a similar misfire on a car here, it just stumbles just as you gun it !

It was the injector, but thats the only other time I heard of real injector trouble with these cars !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[DavidBrough]

Well, after checking and re checking everything with much grateful help from everyone the car seems to have cured itself. I'm pretty certain that the linkage adjustements were OK and that the problem lay with one of the injectors which has now cleared itself. I did notice one of the plugs was quite black and traced this to the injector so ordered a new one but before I could fit it the car started to run better and better until it now runs very sweetly indeed. A bit of dirt must have got in when I was refitting the injectors or lines. Still alls well that ends well. Thanks everyone

[J. Huber]

Another happy ending! And you get to keep the wife!  

James
63 230SL

[lgr]

HELP!!!

I have a 1970 280 SL that always this problem: Misfires!
I run really smooth until I feel the accelerator pedal feel more “stiff”, and then, if I don’t pull it down, it starts misfiring!

But the funny thing is that, if I push the pedal completely down, there is no misfire at all.

First of all I would like to know if this “stiffness” of the accelerator pedal on the half-way of is course, is a normal thing (because is there that that the problem occurs).

I went to a Bosh centre and to a Mercedes specialized centre but no one solve me the problem, so you are my last hope, because it’s uncomfortable to drive the car like this.

Thanks in advance for your help,

Luis  

[DavidBrough]

Hi Luis,

Sorry to hear about your problem. As ever with these things there are many possible causes and the thing to do is eliminate them one at a time.

As you state your throttle linkage is stiff that’s the best place to start as it should be smooth with no sticking points. Have a look at this post first of all and see if proper adjustment helps http://index.php?topic=5003+tour

Remember to check all the connections including the pedal itself and those under the inlet manifold for smooth operation. My car is RHD and I’m not sure how the connections transfer on a LHD but just follow the activating rods. The only real thing to be wary of is the connecting arm on the injection pump itself, make sure you do NOT move it in any way as the position is factory set.

The next check would be electrical and I would suggest fitting new spark plugs even if the ones you have are not very old. The only plugs that do not misfire in my engine are NGK BP6ES gapped at 0.032". I have tried many others including Bosch but these are THE BEST. You should then set the ignition timing with a strobe to 30 deg BTDC at 3,000 revs or more. If this doesn’t cure the problem move onto new points and re set via the dwell angle (38deg +3 or -1), do not rely on feeler gauges as these are not accurate enough.

Confirming valve clearances and compression is also a good idea.

Gives these points a try and report back progress and we can try something else is you still have no joy.

Good luck


David Brough
1969 280SL Auto with A/C

[lgr]

Hi David
Tanks for your prompt reply!
I'll do, step-by-step, as you say but, first of all, I must sort out the problem with the rev.counter (I'm waiting the flex I ordered to SLS-Germany).
I'll come back to you with the progress of the "operation".
Thanks once again!
Luis