‘69 280sl - Rough Idle/Rough Running

[cjpoulos]

Hey all. I have a 1969 Mercedes 280sl that I brought home in June of ‘24. I’ve been chasing down some demons ever since I took ownership, and I like to think that I’m mechanically able. I’m having some pretty tricky rough idle and rough running (when it feels like running) issues. I’ve search through the forums and pieced together info on many issues, but I felt like it would be worth it to make this post. This car is verified to have only 14,800 original miles, but it was stored nicely in a garage in non-running condition for 25+ years. It’s in amazing shape cosmetically, but it’s not the best mechanically at the moment. I’d love some advice on changing that last part!

Here’s a list of the engine related work I’ve done minus a few things:

Replaced Fuel Tank
Cleaned Electric Fuel Pump
Replaced All Soft Fuel Lines/Filters
Cleaned Hard Fuel Lines
Verified Functionality of CSV
Valve Adjustment
Valve Seals
Compression Test - All cylinders hold about 160psi
Cooling Channels Cleaned in Block
New Water Pump
Throttle Linkages Are to Spec
Valve Timing and FIP timing are set to correct spec

There’s lots more necessary work that’s been done to revive this car with the goal of preserving it and ensuring it’s health for many miles to come.

Now, the engine cranks well. It will typically start quickly on the first go. It idles POORLY (makes a deep rumble too). Maybe sitting at about 500rpm idle, does not like to rev up, and does not smooth out at higher revs. It tends to die after about 30 seconds. At a second attempt, it cranks but doesn’t start, although it tries. By the third attempt it only cranks. I don’t crank it for very long obviously, but I can’t figure it out at this point! The plugs are fouling with carbon very quickly, they’re gapped to .030, and black after maybe 30 seconds or running. I know these cars like to running rich, and maybe it’s just because there is excessive fuel when attempting to start, but that seems like a short amount of time to foul plugs.

Like I mentioned I’m an able mechanic and like to think that I’m not a hack, but this car is kicking my ass! There is also some fuel in the distributor vacuum line if anyone has some special knowledge on what that may cause.

I’m definitely missing some info here and I would very much appreciate some advice - and if I mentioned something that you know to be very wrong, please tell me! Thanks!

[Lori]

Sounds similar to when I woke up my car from its 12 year slumber. 

After initial start, have you tried to adjust the idle air screw?
Have you tried leaning out the fuel injection pump idle adjustment?
Have you checked the fuel injectors?  After sitting, they are likely to have an erratic spray pattern. 

Also, have you look at the linkage adjustment tour?
https://www.sl113.org/wiki/Restricted/Linkage-tour

[mdsalemi]

Check all the plugs, wires, and wire ends (plug connectors)
If rough running I bet the plugs are fouled; not necessarily the cause of your problem but it's not going to run properly with fouled plugs OR with a bad wire or connector.

[Bonnyboy]

You note you cleaned the fuel pump-have you measured the amount/volume of fuel returning to the tank to make sure its up to spec?

Ian

[cjpoulos]

Good advice!

I have tested the spark plug leads about a month ago and they all tested very well, including the coil wires.

I have not tested the fuel injectors or measured the returning fuel volume. I’ll look into that shortly. I think MercedesSource sells a bench tester for the MFI systems, so I’ll have to pick one of those up.

Before the head was pulled recently, the idle air screw was set properly, but check on it again.

I haven’t changed any adjustments on the FIP yet, but that could a cause of what I’m expecting now that I think about it. I’ve found some comprehensive explanations on the adjustments so that will be done as well.

Thanks!

[Pawel66]

Not sure what "Throttle linkages are up to specs" - not sure what it means.

I do not see in the list two activities that are absolutely basic:
- ignition timing
- linkage tour

Have you done those?

[cjpoulos]

Yes, I've gone through the linkage tour and that's essentially what I meant by "up to spec". I have also set points gap to .012 and dwell is at 36 degrees I believe - whatever the recommended settings are for each, I don't quite remember if those are the correct settings of the top of my head.

[Pawel66]

Ok, sorry, you said that "before head was removed the idle screw was set properly, but will check on it again" - usually you correct it doing linkage tour, so I thought you have not done it as linkage tour is about setting correct idle and linkages are just part of it.

I think you have some good advise here, I am not a mechanic, I can tell only from my experience, two points come to mind:
1. Already advised fuel flow check - a very simple test that will tell you if the fuel system is not clogged (I assume you have fuel level above reserve)
2. A shot from the hip: if you have new Bosch points in the distributor - lots of those that were on the market a couple of years ago were faulty

[cjpoulos]

No worries, any advice is greatly appreciated and obviously I'm overlooking some seemingly pretty simple here haha. I'll update after I've completed some of the work here!

[ctaylor738]

Suggest checking the free movement of the injection pump rack.  It may be sticking in the rich position.  Lots of posts on this. 

Cheers,

CT

[Cees Klumper]

X2 on the injection pump rack; some of the plungers may be stuck.

[ja17]

Try checking the WRD. It is an easy test to perform. I see you are in Ohio. If all else fails, bring it down to Blacklick, Ohio and we can sort it out together in a short time!

[stickandrudderman]

What Joe said.
https://y outu.be/TrsMgUlEUaU

[mdsalemi]

Colton,

If Joe Alexander has invited you to bring the car to Blacklick—a mere 140 miles, 2:30 drive—you really ought to make the plan post haste. I can guarantee that it will be time well spent, the problem(s) will be solved (or diagnosed accurately at a minimum) and you’ll be better off for it.

[cjpoulos]

Joe, I'll definitely have to take you up on that! I'll send a message over to you. Luckily we have a car hauler I can load the SL on.

The WRD thermostat seems to function properly, and everything seems to move/flow freely so hopefully we can check that off the list.

I verified the valve train timing again last night and it was spot on, and it's holding about 160psi per cylinder when I tested compression. I'm going to test the ignition systems even further tonight to make sure we're getting strong spark.

Thank you to everyone for the help so far!

[yves]

Hi CJ, i used with succes the Injectors cleaning kit. I have pressed the pump a few times for some injectors and a lot for some other.... but finally they showed a nice spray pattern and kept the fuel without any leak.
Be aware to have the right tool for unscrewing the injectors....!

[Pinder]

I would take a guess that your Fuel inection pump rack may not be completely free and maybe binding as well as teh pistion / plungers in the rack may be stuck on some of them. This may free up over time but likely the injection pump needs a good flush out to free it up.

[twistedtree]

Colton,

If Joe Alexander has invited you to bring the car to Blacklick—a mere 140 miles, 2:30 drive—you really ought to make the plan post haste. I can guarantee that it will be time well spent, the problem(s) will be solved (or diagnosed accurately at a minimum) and you’ll be better off for it.

Plus you'll meet Joe.

[twistedtree]

The fouled plugs plus progressive difficulty restarting makes me think it's running really rich.

The fuel rack check is a good one.

After the first start, you could try disconnecting the control wire on the CSV so that is doesn't squirt more fuel on subsequent restarts.  If the restarts get easier, that would further indicate it's over fueling.

Your idle speed is low, so try opening the air valve.  If the engine speeds up and smooths out, it's further confirmation that you are running too rich.  You could also disconnect the linkage from the throttle flap and crack it open by hand.  Again, if the engine speeds up and smooths out, it's confirmation you are too rich.

It may only be loosely related, but what spark plugs are you using?  I think the original specs call for a BP6ES, but I had a lot of similar issues and at the suggestion of someone here switched to BP5ES plugs and it made a huge difference.

Check the little air filter on the FIP like in Colin's video.  When cold the FIP will be running rich, and the extra air admitted by the WRD is essential.  If that's restricted it will run rich.  Then leave the filter off so you can later confirm that the air flow has stopped once the WRD has warmed up.

Verifying fuel flow is important to do, but I don't see how it would lead to rich running - just the opposite.

What you know for certain is that you have missed something, so don't assume everything you have checked or done previously is actually correct.   Being open to questioning what you have already done is mentally one of the hardest parts of diagnostics, at least for me.

[antonio martinez]

Fuel return?