380SL Engine Freezes While Cranking

[ctaylor738]

I am working on a 1985 380SL with 201K. It has several flat cam lobes and a variety of other problems. It was running very badly but had oil pressure. I changed the oil and spark plugs, and got the control pressure more or less correct, verified spark, etc.

I was attempting to start it by cranking, richening the mixture a bit, cranking, etc. Each cranking burst was less than ten seconds. On the third or fourth crank, the engine stopped turning over. It would not budge with the starter. The starter has a good clunk when you hit it.

I put a socket/breaker bar on the crank hub. It will not budge a millimeter in either direction.

I pulled the cam covers. All of the the rockers are in place. No obvious problems with timing chain or slide rails. Pulled the plugs out. Removed the tensioner and pulled the sprockets off the cams. None of this made the slightest difference. This thing is stuck as tight as [supply your own phrase].

Any ideas about what could go wrong at cranking speed that would lock up an engine?

Thanks,

[Tomguy]

Timing chain jumps usually occur at startup or shut off, when oil pressure to the tensioner is the lowest.

Combining the worst of both worlds (no pressure to the tensioner AND the engine ceasing to move), your chain probably jumped. You can't move it backwards because the cam is trying to open the valve in that direction (it's closing when you move it forward), but can't move it forward because the piston is kissing the valve.

This is just the "Most likely" scenario. Your engine moved backwards a teeny bit due to compression and the chain moved on the cam. Try moving the cam on the right side of the engine first. A bit counterclockwise (AGAINST its normal direction of travel); MAKE SURE to mark its spot first. If it doesn't budge then move it 1 link clockwise. If it does, then try the other one.

Note: It could be the other way around but I would think it's less likely (that your valve is opening, because you can't seem to budge the engine backwards)

[MunichTaxi]

Because this is an Interference engine, somehow you are 'out of time' between the cams and the and the pistons.
I had the a similar probem when I tried to install a new timing chain. Learning curve

It doesn't seem wise to turn the engine by hand, with the tensioner removed.
How do the gear teeth look? Pointy?

Additionally, It has been written here that the crank hub can shear a key, and really confuse the issue.

[CamelotShadow]

I hate to hear of these things
Our engines in distress
Our hearts trying to heal them
get our wheels spinning once again on lifes highway

[ctaylor738]

I will try moving the R cam a bit and see if I can turn the engine a small amount with the wrench. Since the cams are worn and need to come out anyway, maybe I'll just remove them.

Personally, I don't think the chain jumped because there are no marks from chain slap and the tensioner was holding it tight when I took it out.

But I don't have any better ideas.

Has anyone ever heard of the starter gear jamming the flywheel?

I bought a 1952 Ford 8n tractor that supposedly had a locked up motor, the first thing I did was to remove the starter. When I had it loose, I heard a pop and the motor was free. The po tried to pull start it, and the wheels would lock up when he released the clutch.

I know the starter is a pain on a R107, but I would remove it to make sure the bendix isn't bound up on the flywheel.