Hi everyone, I'm having a problem with my L-79. My #7 cylinder is missing/not firing 100% of the time. A bit of history and background... the engine up until maybe 9 months ago ran perfectly. The engine was strong, reliable, smooth all the way up the RPM range, etc. Then I noticed when it was cold and first running, it would miss a bit, then it would run fine as the car warmed up, maybe 5-10 minutes of running it, the missing cylinder would "come to life" and run fine. This went one for the past 6-9 months and it would gradually take longer for the missing cylinder to fire, but would eventually fire. Now, the #7 cylinder will not fire no matter how much driving, or how warm the engine is. It's running on 7 cylinders only. I have changed the plugs, I run R45S plugs always, and 7 plugs look good, the #7 looks fouled, not wet at all, but sooty/fouled. I put in brand new plugs, same thing, no change. I install brand new wires, thinking maybe the #7 wire was going bad, no change, same thing. I checked the distributor cap and pulled the #7 wire out with the car running, and I get a spark at the #7 terminal. The cap and rotor are fairly new, and there is spark coming from all the terminals on top of the cap. The timing on the car is fine (38 degrees full), as is the dwell(30). I pulled the valve cover off to make sure that rockers/valves were moving as they are supposed to, and all seems well on that front. I can't imagine what it might be at this point, I really need some help. I thought it might be ignition related, but I believe I've ruled that out, hard to believe it's a carb issue with just one cylinder being effected, and all others running perfectly. Thanks for your help in advance.

Adam

 

Compression test is the next logical step.

 

Caps and rotors are cheap enough that I would buy a new one of each and install them. You have eliminated the plugs and wires but not the cap and rotor. The fact that they are "fairly new" could be a smoke screen. It wouldn't be the first time that a distributor developed a hairline crack.

Next, I would pull the #7 plug, pull the coil wire and ground it, and then have someone crank the engine with your finger over the #7 spark plug hole to ensure that you have compression. Better yet, put a compression gauge on it and see what your compression is on that cylinder.

Gary

 

Adam,

Your troubleshooting pretty well rules out an ignition problem. Carburetion doesn't seem to be the culprit because there are no problems with the other seven cylinders. Your next step might be to do a compression check on #7. You may have a cracked valve, head, piston ring or a leaking head gasket.

Dave

As 67L36Driver stated:
Compression test is the next logical step.

 

by the way you got an automatic transmission?

 

Ok, I'm about to do a compression check shortly (waiting on tester to arrive). I'm wondering, since the situation with the #7 cylinder has been "deteriorating" over time, from running perfectly to the point of now being a completely dead cylinder over the past 6 months or so, and having essentially ruled out anything ignition or carb related, would that lead one to believe it is gasked related as opposed to a piston ring or valve issue, or, not necessarily ? Put another way, would a piston ring or valve problem be "immediate" as opposed to a "deterioration" in the functioning of a particular cylinder? Thanks

Adam

 

I had a very similar situation with a 928 Porsche (poocher) that I owned a few years back (read - before getting married). Did a compression check, confirmed the bad cyliner, then while removing the cam towers I noticed a broken valve spring on the intake valve that was allowing exhaust to bleed back into the intake. Replaced the spring without removing the head and I was back in business.

BTW - the Porche dealer wanted "about $7,000" to fix the problem. They wanted to remove the motor and do a valve job.

Forgot to mention - I'm located in Fairfield, so if you need an extra set of hands or eyes, let me know.

Marty

 

I thought I was the only one burnt by a broken valve spring!!

On my 350/350 at cranking speed the inner valve spring/damper moved the valve quickly enough to get a great compression reading. Once the engine was running the valve wouldn't seat quickly enough to get to a firing compression!!!

 

Ok, here's the latest, after ruling out ingition and carb, I did compression check this afternoon. Low and behold...the #7 cylinder DOESN'T even register on the gauge. All the other cylinders are in the 165ish PSI range. Now what?

Thanks,

Adam

 

Pull the valve cover and try and press down on the two valve springs at the retainer. Compare them to other cylinders. If you have a bad or weak spring you will be able to push the valve down with little effort.
You could also have a burnt valve. If the valve adjustment was too tight it would hold the valve off the seat and over time cause the seat to burn.
You could back off the two rocker arms and put low pressure air in the cylinder and listen at the carb., the tail pipe or the crank case to see where leak is coming from. Basicly a cylinder balance test without using gauges.

 

Check the valve springs, then if they are OK and not causing the valve to partially close,

Pull the head.

You will either find:

A) a compression ring broke and has been gouging the cylinder wall,and you lost all the compression,

B) you have a hole in the piston

c) something is keeping a valve from seating/sealing, which could be buildup on the valve stem caused by a broken valve spring.

Doug

 

 

Ok everyone...here's the latest...I put compressed air in the #7 cylinder with the piston @ TDC and the valves completely closed (Backed rocker arms off to make sure). The air is not coming back through the carb or the oil filler neck, BUT, it is definitely coming out of the exhaust. So I guess we've narrowed it down to the exhaust valve or seat. I would say, probably the best news given the other possible outcomes. What next, head off ? Do I need to do both heads since I have one off anyway ? Thanks

Adam

 

I would pull the head and check it out for the problem in #7 and make the decision whether or not to pull the other head based on the condition of the head you pulled. If you need to have the whole head reworked, you might want to have them both done since you are that far into it.

Dave