Stumped... Can't get #7 to fire.

[sreams]

I have a '71 300SEL 3.5. The engine runs great for the most part, but for some reason, cylinder #7 won't fire (it used to). I can pull any other spark plug wire and there is a noticeable change, but no change at all with #7. Spark seems fine. I've pulled the plug, connected it to the plug wire with the end of the plug touching the body, and a bright spark is visible. I've checked fuel. I've pulled the injector and with the engine running, I can feel it clicking away in my hand (another injector was installed in #7s injector hole while doing this. I've also manually blown gas through it and even tried another remanufactured injector. No luck. Compression tests good... about 155lbs on #7. Any ideas what I could be overlooking?

-S

[Carson357]

Originally Posted by sreams

I have a '71 300SEL 3.5. The engine runs great for the most part, but for some reason, cylinder #7 won't fire (it used to). I can pull any other spark plug wire and there is a noticeable change, but no change at all with #7. Spark seems fine. I've pulled the plug, connected it to the plug wire with the end of the plug touching the body, and a bright spark is visible. I've checked fuel. I've pulled the injector and with the engine running, I can feel it clicking away in my hand (another injector was installed in #7s injector hole while doing this. I've also manually blown gas through it and even tried another remanufactured injector. No luck. Compression tests good... about 155lbs on #7. Any ideas what I could be overlooking?

-S

are the cam lobes on that cylinder ok?

[GaryF]

Have you tried swapping the injector with another cylinder to see if the problem moves with the injector?
If you've got fuel and spark, then as already said it may be a valve issue with the supply of air/fuel or exhaust to/from the cylinder.

[Mustang_man298]

OK, youve pretty much eliminated everything outside the engine it sounds like. You ran a compression test and it was good. So, you know external parts are not the issue nor is compression. Now, as mentioned, you need to check the cam and valve train. If the intake valve is still working, you will get a good compression reading, if it was not opening it would read low, also would be low if either valve was leaking. If the exhaust valve was not opening (from a flat lobe or munched rocker) and still holding a good seal you would still read good compression, as the intake will let air in during the test when the piston is drawing down, but the cylinder would not be able to breathe enough fresh combustion air in and fire while running due to trapped exhaust suffocating it. Sometimes you can hear the trapped exhaust huffing back into the intake as the intake valve opens while running.

[sreams]

Well... I've pulled off the valve cover on that side and everything looks fine. I double checked and adjusted the valve clearances for #7 (they were almost exactly right). The cam lobes look fine. No luck yet.

-S

[Chas H]

If you have compression and spark at the correct moment, the missing part is fuel. Is the plug wet? You haven't proven that fuel is actually getting into the cylinder, from what I read.

[t walgamuth]

try a different spark plug.

sometimes they spark fine in open air and won't under compression.

tom w

[barry123400]

Take a can of wd40 and spray around the local intake manifold area to see if the cylinder picks up. You may have an air leak that is thinning the mixture out too thin to fire in that cylinder as a result. Another source is any vaccum fitting nearby could be air leaking as well.