New Rockers Cause Rough Idle???

[mbboy]

I just replaced a couple of worn rockers on the #8 cyl. of my 81 380sl to get rid of some valve clatter, and now my car is idling rough when it's cold. However, once it warms up, it idles at specs, and runs great.

Although I didn't shim the new rockers, as I guess I should have, I can't imagine that they'd be causing the car to misfire when it's cold, but not when it's warm or cruising. So, before I start "rounding up the usual suspects (WUR, ICV, vacuum leaks, etc) " I was wondering if anybody had any thoughts on the matter.

I should mention that I recently replaced the coil, distributor cap, and Overload Protection Module, and the car was running fine until I replaced the rockers.

Thanks in advance for the usual good advice.

[meltedpanda]

well since this "happened " after the rockers, I would look to the repair as the issue , lash them a little tighter to see if that helps
Heat "expands" and closes gap, so start out tighter? Just a thought

[mbboy]

Originally Posted by meltedpanda

well since this "happened " after the rockers, I would look to the repair as the issue , lash them a little tighter to see if that helps
Heat "expands" and closes gap, so start out tighter? Just a thought

Thanks for your thoughts, Ron. I'm sincerely hoping that you're right, since the compression test I did yesterday showed that the compression in one of the cylinders that got a new rocker was way down (40psi vs 165-175 for the rest), and I also seem to have some gray smoke coming from the exhaust. (I mistakenly said it was only #8 that got a new rocker when I actually put one on #8 and one on #7.)


I never would have thought that failing to shim the rockers could cause these problems, but a search revealed that someone with a 560sl had cured the very same problems by shimming the rockers on a cylinder with low compression:
87 560Sl intermittent oily smoke.

So, I'm going to try doing that with the hope that it's not something more serious. As I did the valves just 2 years ago, and haven't put many miles on the car since then, I'd hate to have to go through that again so soon.

Any other thoughts on this would also be greatly appreciated.

Steve

[meltedpanda]

certainly worth a try!

[Genbiltstein]

You might want to perform a compression test this way.
Pull the plug to the cylinder being tested.
Pull the adjacent plug as well.
Compression from one cylinder will throw off readings to the adjacent cylinder.
Perform a dry compression test on all cylinders.
Perform a wet compression test on all cylinders.
Compare the results.

[Benz Dr.]

I'm not new to this but I'm not sure what '' shimming the rocker '' means. I know there are lash caps in there that can be of different thickness so maybe that's what you're referring to?
I put some 4.5mm lash caps in an engine I did a while back and it rattled something awful. Once I changed them for thicker ones all the noise went away. I found out later on they got mixed up with 280SL lash caps which are 4.5 standard.

Are you sure the elements are holding pressure? Sounds like the valves aren't closing properly rather than not opening enough. There's a special tool for setting the base height. I bought one ( exspensive ) and really never use it.

How does the adjacent cylinder have any affect on the one beside it while taking compression readings. That's a new one on me. I'd like the hear about that.

[mbboy]

Well, I don't want to jinx myself, but the problem seems to be solved.

As it turned out, the problem valve had a 6mm shim, technically called a "thrust washer" on it. Evidently, after I had the valves done 2 years ago, I replaced the original 5mm shim with a 6mm shim, because the Go-No-Go gauge that MB designed to measure rocker/valve height showed that the rocker was sitting too low.

So, when I recently replaced the worn rocker with a new one, the new rocker/valve height didn't allow the valve to close or open properly, thereby causing the engine to misfire and burn oil. However, replacing the 6mm shim with the OE 5mm shim did it allow to open and close properly again.

This leads me to believe that some, if not many, of the rough idling problems people have reported here may be caused by worn rockers and valves that need to be re-shimmed.

But, then again, who knows anything for sure about these cars? :-)

[meltedpanda]

glad for you, hope that was it , sure makes sense we were all talking about the valves not fully closing, and as I said, always fo back to what you did last (lol)