Bought a new boat - 69 Electra Sedan. While changing plugs, thought I'd give it a compression test. Removed all plugs, throttle wide open, no spark and cranked the motor 4-5 times per cylinder. NOT at operating temperature (oops!), I got 135-155 psi in all cylinders EXCEPT 4 & 6 - which read zero. Double and triple checked and re-ran the test - same reading. Waiting on a 2nd person to help with further testing, this time at operating temp. I'm guessing ( ) that if I had a problem in the bottom end of the motor serious enough to cause this, I'd be hearing some nasty noises. Seems more likely in the top end - could it be a bad head gasket? Or am I more likely looking for valves? What else could cause these two cylinders to both be dead? BTW, once I put it all back together, it starts and runs reasonably smooth - doesn't sound like it's missing. No oil in coolant or vice versa, but some gray smoke out the exhaust after it warms up. Any advice appreciated. EDIT: I did try running the test after re-installing a spark plug in the adjacent cylinder (test 4 while 6 was plugged and vice versa). Same result. Doug
I would check for 4/6 rocker arm motion while cranking and/or running, I assume you poured a little oil into 4 & 6 during re-testing.
Do a leak down test on the suspect cylinders. Listen for where the air is escaping. Most likely a blown gasket. Hopefully not a crack in the chambers, between seats or between exh. seat and heat crossover ports. The needle doesn't move at all and then bleed off? Quick visual of the those rockers as suggested earlier would be good too.
i dont think he can see the needle if hes doing a one man compression test, but ive been wrong before. wouldnt a blown head gasket make it run rough?
The motor that blew a head gasket on me ran well just made a strange noise. It did have like 30 pounds in a stactic compression test though. (in #4 & #6)
He could see the needle using a remote starter button under the hood and anything that causes zero compression readings will make an engine run rough. Bob H.
Last time I saw "zero" compression, it was a hung open valve. Normally a blown headgasket between two cylinder will produce an abnormally low number, but not zero (20 psi isn't unheard of). Most compression gauges are "latch" type, which is to say they show the highest reading until the release button is pressed. Most commonly, you see hung open exhaust on the inside cylinders, since those exhaust ports run the hottest.
Put a plug in one of the cylinders and rerun test. If you have a little compression in the other it is blown head gasket between cylinders. AS for running rough, I know for a fact mine didn't run rough at all with it blown between #3 and #5 cylinders. Just lost power. Car went from running a 12.30 to running a 12.85. Didn't miss at all.
If I'm not mistaken a stock 430 should idle a little smoother than a 455 capable of low 12s and if you couldn't detect a miss at idle with zero compression on 2 cylinders on a stock engine, it might be time to take up a new hobby. Bob H.
If you'll look at the edit in the original post, he specifically says he tried it with a plug in the other cylinder and got the same result.
Thanks for all your responses guys. Lost my garage access and it's been damned cold and wet so I haven't had a chance to get back to t'shooting the problem(s). I do however, have a set of redone heads, resurfaced & refinished manifolds and valve covers all ready to go. Figure I'll start tearing down and if the problem ends up in the bottom end, the rest will have to come out anyway. Now if I could just get a few dry days above 45 degrees... BTW, you guys were right - this thing IS running pretty rough on 6 cylinders! ou: :laugh:
Finally got around to taking this apart. No surprises here: http://i47.photobucket.com/albums/f180/pyro_225/BuickPartsMarch07013.jpg But this REALLY sucks! :af: I kept the :rant: :blast: :rant: to a minimum due to the hired help: And I was just gonna slap on some 67 big ports and go... Guess now I have to do it right with a new block.
Are those cracks or blowtorch marks? In the block photo, it looks to me like it'd be gone if you decked the block. And you'd want to zero-deck the block anyway to keep the quench distance good.
Cracks. I wire-brushed it a bit to get a clearer pic. Can a 69 40 block be shaved THAT much and still use stock pistons/rods? It'd be nice to keep the original #'d block for this car (not that anyone would care except me...) but I don't know if I'd want to dump a bunch of $$$ into a weak(ened) block even if this could be repaired or machined away. What usually causes this kind of damage? Overheating? Could the blown gasket have created a void that over time the block and head expanded into, thus causing a crack?
if anything i think it would be caused by getting really hot (from hot gasses passing back and forth) and then really cold. orpossibly getting hot enough and then being torqued enough to let it slip slightly. I vouched for the #'s matching block on my car, deck surfaces were perfect, and i get to have 700hp with the numbers to show its original. I almost killed my friend who took apart a 59 apache and just brought the original engine and trans to the local junk yard... I would think if its cracked like that now it will keep cracking, if its not already deep enough.
