Smoke after throttle release

[sens]

Hello Pagode friends!

My 250SL has an overhauled 280 engine with less than 1000km on it.
The engine runs very well at idle and at also at cruising speed.
I have mounted a new exhaust.

When I drive at 80 miles/hour (100km/hour) on the high way and I release the throttle, I can hear some light puffing, and I noticed some smoke   from the exhaust. The engine is not smoking when driving or at idle.
It looks like the engine is running rich when I release the throttle.
Attached a picture of one of the spark plugs: a little oily and rich? What do you think?

I could measure CO% at 3000 RPM and try to adjust the barometric compensator to make the mixture a little leaner. Or do the split linkage test to see if it is running rich at higher RPM.
But before starting  to adjust, I would like to ask your advice where to start.

Thanks a lot!!!

Halvor

[Sead]

it sucks oil too , most probably through valve guides cos vacccum is build up after throttle is closed. typical symptoms.
spark plug is oily. could be also through valve adjuster seats in head if not sealed properly while rebuild.

[mdsalemi]

Hopefully you'll get some more "pinpoint" advice from our expert members like Stickandrudderman and Dan Caron/Dr. Benz...

...but it sure looks like some spark plug fouling to me. As I understand it, white smoke indicates a cooling system issue leaking internally, blue smoke tends to be oil burning, and black smoke is unburned or partially burned fuel in the exhaust (rich running) - none of which are good. The popping sound is probably unburned fuel exploding in the exhaust system.

When you are running at high speed, and suddenly release the acceleration pedal, you create a high vacuum condition.

As will always be recommended here, do the split linkage test as a first test.

[sens]

This is a picture I took before the winter stop. Maybe not the same cilinder.
Looks a little rich to me. Agree?

[sens]

I think the smoke is coming from oil out of cyl No 4, as you say, Sead and others.

It is the only one that looks oily, as you can see on the picture with all the 6 spark plugs.
The rest looks OK to me, maybe a little rich.

Next step will be to inspect the valve guide seal of cylinder No. 4.
It might have come loose. Maybe I can glu it back in place... we'll see.

The "popping" might come from a small leak in the new exhaust. I chose to clamp it in stead of welding.
I will do a "smoke" test to find any leaks and solve that.

I'll keep you posted and thanks again for guiding me in the right direction.

Much appreciated!

Halvor

[Pawel66]

Sorry, I am not an expert here but...

Before getting to serious things in the head, maybe it is worth checking the injector from that cylinder for spray and pattern?

[sens]

Thanks for your comment,  Pawel!

The injectors are relatively new and tested.
You can inspect and eventually change the seal(s) without removing the head. Just lift the valve cover.

Regards, Halvor

[ja17]

Sometimes newly replaced valve seals can lift off their valve guides. I would take the valve cover off first and make sure they are all in place. Pay special attention to #4.

[sens]

Perfect plan, Joe!
That is what I will do!!

Have a good day!
Halvor

[clunker]

I assume you have a manual transmission. One thought you may already have considered, is that on early 280SLs - and hence maybe on your 250SL/280 setup ¯\_(ツ)_/¯ - there is a 2nd shutoff solenoid on the FIP. This is triggered by a series set of switches 1. 3rd or 4th gear selected (gearbox switch) 2. Clutch engaged (clutch pedal switch) 3. Throttle closed (firewall throttle bar switch). Without this, the over-rich mixture on throttle lift off at speed will cause unburnt fuel to go through to the exhaust, with popping and possibly smoke on the overrun. Perhaps check if this setup applies to your 250SL with 280 engine. My early 280 SL has the series circuit disconnected - I live with the popping.

[sens]

Thanks a lot for your suggestion!
I will have a look!!

Halvor

[sens]

I took the valve cover off.
First impression is that the seal are in place, however it is difficult to see through the spring.
I guess I will have to remove the springs for a good inspection. Did that before.
Or is there an easier way to check?

How could I inspect the valve adjuster seats? Maybe oil is coming from these adjusters.

Thanks again!

[ja17]

Just take a good light and inspect all twelve. You might have to turn the engine to un-compress some of the springs compressed by the camshaft. It should be fairly obvious if that is the problem. If they are OK you can move on to the next possibility.

[MartinK]

I had this Issue on a new engine 230SL, too. My spark plugs looked like yours. Five light grey, one dark and sticky. The valve guide seal from the intake valve was slipped off the valve guide. You have to check with a very small screwdriver if the seal is moving. Good luck.

[sens]

I inspected all 12 stem seals with a strong light and a little screw driver, as you guys suggested.
They are all at the bottom and I cannot move them; it looks OK to me.

However...
I also inspected the resistance of the plug wires and the spark plugs.
The resistance of all spark plugs (1150 Ohm) + wires (0 Ohm) is approximately 1150 Ohm.

Except.... for the one that has the different color. Here the resistance of the plug is 2200 Ohm!

I suppose that I had a bad combustion in that cylinder, causing the different color. The "oil" might have been not evaporated fuel, because one day later it looked less "wet". (see picture)

Anyway, one spark plug had a different color and that happened to be the one with a different resistance.

I changed the plug for a good one and I will let you know if that solves the problem!

Thanks again for your support!

Halvor.