Hello y'all Is there a definite link between a low compression cylinder (75 PSI) - number 1 on bank number 1 - and the spark from the plug. I'm getting no spark at all, checked by simple removing the lead from the plug while the engine is running. There is no change in the engine noise, like on any other cylinder with a higher compression (130 PSI) on the same bank. Thanks
75 sounds dangerously low, and 130 even sounds too low. Numbers like that would definitely contribute to a poor running engine.
75 psi is pretty low but it could still run. Did you remove the spark plug and look at it? It might be oil fouled.
I've check for some spark outside the engine, and the plug is sparking. That's what seems very strange. ???
There is no correlation between no spark and low compression. They are 2 seperate and distinct problems. Low compression is a sign of leaking valves, leaking or broken piston rings or a blown head gasket. Either way it requires the cylinder head to be removed and a possible engine or head rebuild. Compression of 75 Ibs on one cylinder and 130 Ibs on the others is another sign of a need for a rebuild. Normally you look for a variance of about 10 % between cylinders and remember , normal compression should be upwards of 190 Ibs across the board. Your motor is in serious need of a repair. Sorry for the bad news.
We assume that there was a compresson check to identify the 75psi cylinder. So what did the plug look like. Was it gas or oil fouled. If so it or if you installed a new plug then it can be fouled again and will not fire. 75 psi is a problem with either leaking valves or rings. Check the plug again and see what you get. You may want t run the engine until it is warm and then put in a new plug. It should run good for a while after it is installed. Buy the way does the engine smoke from that cyl bank.
most likely a valve problem unless the car is smoking bad.. could be burnt valve, or bad cam lobe/lifter. A Bent valve would produce 0 compression. If its smoking then it could be ring problem or head gasket blown or head cracked/warped (steel/aluminum). Good luck only way to fix it is to pull the head off. Have fun!!
I don't think pulling the plug lead tells you much. My 308 motor idled fairly nicely with four cylinders not firing at all when I had an ignition issue. Except for the total lack of acceleration, you wouldn't even really know a whole bank was dead. I think you need more evidence of that cyl not firing.
QFA All you need to know is in those compression readings. Big question is what are you going to do? My guess is you're up for a top end rebuild at least. You could compression test with some oil injected down the plug holes to rule the rings in or out or 'scope that worst cylinder, before committing to the expense.
there is a chance that by that cylinder not firing, the un burnt fuel could have washed the cylinder of all of its oil causing low compression, i would pour a small amount of oil in it and let it sit over night and then whith you properly working injection and ignition system start it up, let it warm up and check compression once again! if that doesn't make a difference then start getting nervous!
Thanks for all the insight Before converting my serial number into a bill, I'll go the simpliest way for now... How small an amount of oil should I be pouring in the cylinder ? Also, is it possible to check the electrical impulse on a spark lead (wire), in order to rule out the wiring. Thanks
An engine with very low compression on one cylinder like yours would "feel" like it has a bad spark plug or wire. So pulling the wire and not hearing a change in RPM/noise is not surprising. In fact, it helps verify that you have a problem with the number one cylinder (as long as you have verified the spark plug IS firing). A leakdown test should determine if its the bottom end (rings, pistons, cylinders) or the top end (valves, camshafts, rocker arms) that needs repair.
Not much...about a tablespoon, just enough to get around the rings and create a "temporary" seal. If you see no change it points to the valves. The electrical impulse can be checked with a timing light...I believe
I wouldnt just pour in the oil and screw the plug back in. After you pour in some oil, crank the motor over a few flips to blow any excess oil out. Your just trying to wet the rings, but any excess oil in the cylinder will dramatically increase compression ratio, and consequently increase your pressure readings and could give you a false positive. And while 75 psi isnt good, its not low enough not to run. If ALL cylinders on one bank are reading 75 psi I would suspect the valve timing jumped.
