serious miss cylinder #1 1991 560sel

[acm680]

I do a lot of diagnosis on cars, and have a friend with a 1991 560 sel.
Cylinder #1 refuses to run for 3 years now. All ignition parts are new.
Engine was rebuild 3 years ago.
Injectors are new, so is the fuel distributor. Compression is OK.

I have seen issues with camshafts, and cylinder #1 is suspect for lack of lubrication.
Apparently the engine had this miss even before rebuild.
Cams were not replaced, soo.. Is there a likely hood of a cam problem?
No valve noises at all.
Spark plug on #1 show signs of running much richer, than the other 7.
Injector and fuel distributor swap result in no change.
The miss is consistent, it stays even at higher RPM's.

Any suggestions?

Henri.

[autozen]

Henri,
You've got a good one there. I read your post carefully, and it looks like you've covered everything. Now it's straw grabbing time. I can only come up with two longshots. Possibly there is a restriction in the intake runner. Also I've seen lifters on newer MBs cause this problem but never seen it on older models like the 560s. You could swap lifters with another cylinder. Let us know what you find out. This is a very interesting problem.

[ctaylor738]

Well, you could pull the rocker/cam cover off and look at the cam lobes. But if the cam lobe or rocker arm were worn enough to cause the cylinder not to fire, there would be a huge amount of noise.

My suggestion for diagnosis:

1. Do a compression test to make sure that the valves are closing
2a. Swap the plug wire with a firing cylinder's
2b. Put a timing light on #1 and see if there is spark
3. Pull the injector out and test or swap it with another cylinder. Install the injector with a new seal.
4. Blow the line from the FD to the injector out with compressed air
5. Swap the line to #1 to a different position on the FD

[autozen]

What did the answer turn out to be????

[Jim Anderson]

A vacuum leak on that one cylinder would cause it to miss.

[david s poole]

if i read your post correctly this engine was rebuilt 3 yrs ago and #1 hasn't fired since that time.you also say that there is no valve noise.perhaps it would be a good idea to do a cold leakdown test and see if the valves in #1 are leaking.it is not uncommon after a rebuild to find a lifter stuck or that the spacing shims need to be replaced with smaller ones.if lifter wide open and filled with sludge not allowing valve to close all the way or when head resurfaced old spacer shim too big.[if valves making a little noise at least you know they are closing] good luck david poole european performance dallas tx.

[t walgamuth]

Originally Posted by ctaylor738

Well, you could pull the rocker/cam cover off and look at the cam lobes. But if the cam lobe or rocker arm were worn enough to cause the cylinder not to fire, there would be a huge amount of noise.

My suggestion for diagnosis:

1. Do a compression test to make sure that the valves are closing
2a. Swap the plug wire with a firing cylinder's
2b. Put a timing light on #1 and see if there is spark
3. Pull the injector out and test or swap it with another cylinder. Install the injector with a new seal.
4. Blow the line from the FD to the injector out with compressed air
5. Swap the line to #1 to a different position on the FD

i dont see why a worn lobe would make noise.

could you kindly explain. i agree a worn rocker would rattle.

tom w

[ctaylor738]

I have never seen a cam lobe wear without damage to the rocker. And generally the wear starts with a weak ball stud, or lack of lubrication and that will certainly make noise.