Warped valve? Diagnostic help please!
 

Subject Car is my 1980 450SL, 201,100 miles. Okay, so my car has new injectors, plugs, wires, cap, rotor, WUR...

STILL, cylinder ONE is missing and the new plug is BLACK (sooty and dry black). It is obviously the missing plug (all others are clean).

Compression on cylinder one is GOOD (144 psi) and in line with all other cylinders (which are mostly between 143 and 145).


So I pulled the valve cover today, and pulled apart the rockers, springs, etc. of cylinder one. I removed, and put on new valve seals, though the old ones seemed okay (isn). There seemed to be very little play in the valves. The intake valve rotated easily, and the Exhaust valve would rotate, but not as easily.

BUT here's what I also found: When compressing the EXHAUST valve spring for cylinder one, it would freely compress a few mm, then it would seem to stick for a mm or two, then compress further freely. As if it had a bump.

I am guessing, but this seems to me to indicate a warped or bent valve stem. Yes?

Could something else cause this? Someone in the shop commented that they thought my plug fouling was due to an exhaust problem with this cylinder. I suppose a bent/sticking exhaust valve could be the culprit?

Car is my 1980 450SL, 201,100 miles. It is running excellent except for the loping/missing idle.

To hear it (youtube):

https://www.youtube.com/watch?v=DMP72a45nP0

Above 2500 RPM, it's smooth as silk. Under 2000 it gets a bit rough. Unfortunately, smog on the dyno ends up being around 1500 (and I am not passing smog).


SO: If you have experience with a warped/bent valve stem, or any other cause that this seems like, PLEASE comment. I really would like to determine what is causing the misfire in this one cylinder affirmatively before I go off merrily ripping the head off...

is the valve spring able to pull back the valve? - if it can then the engine should idle proper unless the camshaft is worn down.

dry black sooty plug is usually caused by excessive fuel in the combustion chamber - try a grease pencil dots on the exhaust manifold runners on cold engine startup and see if there is any variance of its burn time.

quick work can be done with a laser thermometer. your injector to that cylinder seems as the culprit.

The spring brings the valve back up, and there is no play in the rocker. You can see this bank in action here:

https://m.youtube.com/watch?v=A3HRNERdjOQ

℗ unfortunately, you can't see the #1 exhaust valve that well (video shot last week, before I felt the binding). No tapping noise in cylinder 1.

I am now considering that the binding I felt may have only been the spring compressor possible rubbing against a fuel line. (Doh?!)


Regarding injectors:

All injectors are new, with new seals, new seats, and new O-rings.

All spark plugs, plug wires, cap, and rotor are new.

The O2 sensor is new and it has also been strapped to the chassis for a better electrical ground.

Also new are the WUR, the air filter, and the fuel filter.



It has been missing is in cylinder one only. The compression for cylinder one is good. And I am now at a loss to figure out why the plug is sooty, and the cylinder still misfires.

You have CIS correct?

I vote for an injector that is dribbling rather than making a mist. Black is a rich mixture / miss fire.

The bump you are feeling, is this when you are opening the valve ( pushing on the stem ) or compressing the spring to remove the keepers? If pushing on the stem, this is semi normal on high miles motors were carbon builds up on the stems. When hot it could be holding the valve open but there are few things to try first.

With decent compression your problem isn't likely to be valve related.

Start by swapping the wire , plug and injector to another cylinder to see of problem follows. Have a very close look at the cap for arching at cyl 1. Yes i know this is new but we need to make sure. Did you reuse any of the plug wire ends? There is a resistor in some that can fail and cause your symptoms.

If none of this helps, and you have CIS, move the fuel tube on the fuel distributor to one near by and see if problem follows. The fuel distributor isn't " timed" to engine rotation so moving a tube isn't an issue.

I take it the compresion test was done, cranking the engine.
Then the rings should seal and valves must be closing correctly.
If I remember right, the valves are supposed to turn to keep the seats cleaner.
It seems strange, but there is always an explanation for such behavior. Perhaps the compression test should last much longer for Cyl. 1? If the pressure is still correct, then there is something going on with the combustion. Are you sure the Spark Plug isn't losing pressure?
I'd just throw in another new plug on 1 and see.
Now, the pressure is highest when the piston hits the upper dead point. This doesn't mean that the there couldn't be a leak on the Head Gasket (just a thought) then again it seems far fetched, since the pressure is good.
Perhaps a weak spark?
Did it run before the new stuff?

+1 on swapping injectors.

I like to listen to the injectors with a stethoscope. You can hear them tick (or should).

Last thought, check the spark plug wire? And cap.


Sent from my iPhone using Tapatalk

I had a problem like this one time on a small block chevy. All new plugs, wires, cap, rotor, good compression on all cylinders, including the one that was not firing. Turned out to be a broken valve spring. The SBS has an inner and outer valve spring and the outer was broken in a place that couldn't easily be seen. The inner spring could close the valve at cranking speed so the compression looked good, but could not get it closed when running. I believe the 450 also has an inner and outer spring. I know you just did the valve guide seals but maybe if you just pulled the springs off, set them aside , then reinstalled you may have missed a broken spring.
Just a thought.
The way I found this problem was I removed the valve cover and was just looking and thinking. I just used my fingers and pushed down on the 2 valves for the bad cylinder. One of them actually moved down a small amount with just that little bit of pressure.

PaulM

Cylinder leak down test.
 

I have had engines that have a good compression test with a sticky valve.But will show up on the cyl leak down test. Can't hurt to compare the cylinders percentage of leakage.
If on this CIS system it turns out to be the injector leaking,buy a complete set of good Bosch injectors,Do not just replace the bad one only as they will not match opening pressure of your other old injectors,and also do not buy the cheapest ones from china they are junk! I made this mistake on my CIS injection system.:cool:

Thanks for all comments.

Just FYI: before checking this valve, I had replaced all injectors with new Bosch. Also replaced injector seals seats and o rings. Also replace all plugs, wires,,cap and rotor.

The spring isn't broken - it was removed and cleansed and in good condition.


At the moment, I believe the problem is the fuel distributor, I believe it has a damaged diaphragm and is pumping out too much fuel at low rims to cyl 1. I had originally thought it was a leaking injector, so I replaced the injectors. The only conclusion I can come to now is a fuel distributor.

Rebuild kit arrives today ($94 on ebay from CIS-jetronic).

I have reason the believe the "sticking" when compressing the. Valve was possibly related to the MB spring compressor having some interference such as against a fuel line. With the springs removed,valve travel and spin weren't that bad.

Initially I had thought that the fuel distributor was less likely to fail rich, but this was based on my reading about distributors with stainless steel diaphragms. This car has a rubberized fabric diaphragm, know to fail often with modern ethanolized fuels.

The new diaphragm (from American Precision) is claimed to be resistant to ethanol. Here's hoping.!

Followup
 

Just to followup - nothing to do with the valve. The fuel distributor had a pinhole in the diaphragm. I rebuilt it using the eBay kit from eBay vendor CIS-Jetronic.

The missing went away.

To pass smog, I did still have to replace my catylitic converter - the old one was totally clogged from missing and over-rich running.