H-E-L-P ! Engine miss persists after valve job..
 

Some of you may remember my earlier posts about a '90 300E with a 3.0L SOHC engine miss. The miss was intermittant at first but then became permanent. We found that Cyl #3 was dead, no compression, Cyls no 5 and 6 were weak as well. We removed the head and #3 had a large chip out of a valve, and 5 and 6 showed signs of blow by in the intake side. So I had a valve job done by a reputable local crank and head shop. New valves, seats, guides and seals were installed.

We just got the head back on, and low and behold a miss is still there. All cyls now show 185 to 195 lbs compression - including #3. However #3 still appears to be the problem - remove the plug wire and the idle is not affected. The cyl has spark, compression, not sure about fuel - the plug seems to be wet, but only slightly and the plug has no discernable color to read. All other plugs are burning perfect light tan.

I don't even know where to start....I'm thinking that the valves may not be adjusted correctly.

This is what I know for a fact:
The head has been rebuilt - all new parts except springs
The problem cyl is still #3
#3 has spark
Plugs, wires,dist cap and rotor are all new.
There are no vac leaks - I checked every connection
The timing gear,chain,orientation were all marked and the gear was removed from the cam with the chainin place. Tension was kept on the chain/gear until reassembly - it could not have jumped a tooth
There is good oil pressure
Temps are normal
No indication of coolant leakage or burning.
When I installed the oil spray bar, the bar seated in the supply tube but only the two end bolts lined up, the center bolt did not line up with a tapped hole in the rocker assy below it, so I assume two rocker assys were switched in error. I don't think this would be an issue though, since the oiler is seated in the supply tube and the valves were all adjusted by the head shop after the valve job was done.

This is what I am not sure of:
That the head shop checked each spring against specs.
That the head shop installed the rockers in exactly the order I removed them, although I had them laid out in order. They did apparently take the time to mark them each with positional stampings, so I assume they refected how I had them laid out, but there was the issue stated above.
Cyl #3 appears to be getting fuel as the plug is slightly wet, but I may be misreading this, it could be it should be much wetter.


Any ideas what is going on here? Any experienced help would be greatly appreciated.....

Andy

I suspect you are making this more complicated than it is. Many of the things you are concerned about could not be causing your problem.

If you have compression and you have a good spark (nice blue, not yellow), but you still have a miss, well, you have a fuel problem. Your problem is isolated to this cylinder and it could very well be that what caused the cylinder to be "dead" before the valve job was not the valve, but the fuel problem you have now.

The injectors don't often go bad, but has yours? Seems like you should be looking at the number 3 injector or perhaps the fuel distributor itself.

Good luck, and please keep us posted.

Ya, what brewtoo said..............

I suspect a bad spark plug.

Tom W

Yea, the more I thought about it last night, the more I thought injector. May have been the real culprit all along, but once we saw the low compression on #3 and pulled the head and saw a chip out of the valve, it sure seemed that was the logical choice....guess not. I just hope its an injector and not the fuel distributor....

I will check out the injector today. Head needed the rebuild anyway - valve guides and seals were shot and a valve was damaged. This thing should run like a top once I get this fuel situation on #3 fixed.

Thanks,
Andy

Yes, rebuilding the head was a good move and certainly you are better off having done it. Hope I did not imply otherwise.

Not at all. Ordered the injector nozzle today from here UPS 2 day. I'll keep you posted.

A word of caution.

Do not pull plug wires with the engine running.
The high voltage of electronic ignition systems can, without plugs attached, damage the distributor, coil or more important yourself.

Rob

OK, so here's an update. Got and installed the new injector on cyl #3. The old injector was clogged for sure. Car runs great, and idles good. I can still detect just a tiny bit of roughness in the idle. I will run a few bottles of injector cleaner through the engine, and if it still exists after that, maybe start replacing the remaining injectors a couple at a time. (while I had the thing apart, I went over all the vac lines to make sure they were in good shape so I doubt it's a vac leak) Now the car feels like you have a V8 under the hood. LOTS more power than it had before.

Unfortunately, now that I have all that power back, I'm noticing a very hard shift into overdrive. Every time it shifts into high gear there is a nasty thud, unless your foot is FIRMLY in the throttle, then the shift is fine. I just had the trans checked out and serviced before the head was removed. The trans shop gave it a clean bill of health so I don't think this is a trans issue.

Any ideas here?

Flex disk(s)

That's the first thing that came to my mind as well. Thought it might be.

how about the differential mounts at the top rear.

Thanks, I'll check this out as well.

Did a few road tests lunchtime today. The result on shift is distinctly different depending on the situation:

1) Under moderate or heavy acceleration the shift is crisp but no big thunk is apparent.

2) Under very light acceleration, or if you are easing out of the throttle as the trans shifts (like in traffic) you get a BIG thunk from the rear area.

3) similar noise when trans is shifting down but thunk is not as apparent.

4) The noise does not seem to eminate from the front of the driveshaft (tail of the trans), it seems to be in the back near the differential.

If Flex disk, would you suspect rear, or both ?
If differential mounts, would there be any other characteristic to look for? Doesn't seem to be any sloppyness or anything....

Just wanted to say "Thanks" to everyone who offered help and advise during this head removal/replacement project. All the help was indispensable as this was the first time I have turned wrenches on a MB, and I didn't have any manuals yet.

The operation was a complete success and the car runs great now. I still have to remove/reset the timing chain tensioner, which now that I have the manuals to see how it is constructed, I know is no big deal to do. I will get that done this weekend (remember, I backed off the bottoming ring just a bit to reverse the tension change temporarily).

It might be of interest to some that following the head change, I decided to put my faith in the claims of the engineers at Exxon and convert the engine (with 126k miles) over to full synthetic Mobil 1. I'm happy to report that it took the change just fine, and I have no sign of oil leaks anywhere that were not there before the change, and those that were have not gotten any worse. So much for the old wives tales about converting a high milage engine over to synthetic.

Turns out that the hard shifts I experienced once the head was fixed are not the flex disks at all, and I'm convinced that a simple 1/4 turn adjustment of the modulator will straighten this out this weekend.

Next up will be a coolant system flush.