dead cylinders

[66CHEV]

250 6 cyl. starts fine but dead on cylinders 1, 3, and 6. I have good fire at the plugs, compression on all 6 cylinders is around 145 psi with less than 5 psi difference between all 6. i have taken the exhaust loose verifying there is no restrictions there, plugs on the dead cylinders are not wet when you take them out, which lead me to believe it is not getting fuel to those cylinders?? any thoughts. i was chasing this same demon about a year ago, when an icy bridge put the truck on the sideline for a while. i now have most of my body work fixed and am back to chasing the engine troubles again. any ideas would be appreciated. thanks

[TJ's Chevy]

What do you mean by dead cylinders? Is it spitting and sputtering? Or does it run fine and have NO power. If the compression is ok and the plugs are firing you probably have a bad cam or something is screwed up with the valves. It could have a blown head gasket and could be leaking water into those cylinder causing the engine to spit and sputter. I have a cylinder that drops out on my 305 V6 once in awhile and I'm sure its from the blown head gasket.

[66CHEV]

by dead cylinders i mean that i can start the truck and while it is idling you can take the wires off those three cylinders and it doesnt effect the engine at all. if you take the wires off any of the other three it will almost kill the motor. and while driving it has low power not really a spit and sputter but you can hear the miss.

[TJ's Chevy]

Sounds to me you have valve train issues. Maybe take the valve cover off and make sure your valves are opening and closing properly.

[enaberif]

Very seldom are cylinders ever dead. You could have a stuck valve that isn't opening properly.

Stick some compressed air into a cylinder not a lot and tap on the top of the valves and you'll hear a distinct noise if its stuck or free.

[66CHEV]

i do know that with the valve cover off and the engine at idle, all 12 rockers are moving up and down. just by eyeballing it they appear to be moving the same distances but i havent checked that with a dial gauge or anything. but i can say they are all moving. i will give the compressed air test a go and see what i find from that thanks.

[66CHEV]

ok so last night i checked the valve lift for each cylinder with the following results:

Cylinder 1
E = .328
I + .266

Cylinder 2
I = .219
E = .265

Cylinder 3
E = .359
I = .282

Cylinder 4
I = .250
E = .234

Cylinder 5
E = .265
I = .281

Cylinder 6
I = .297
E = .297

could someone who knows valves help with this info. I know that these numbers look a little low considering the stock cam should be somewhere around .388. the thing that seems strange is the three cylinders that are not firing however have more lift than the ones that are firing.

[chevy_mike]

Definitely weird lift numbers. Are you 100% sure you are getting spark to all 6 cylinders? I found my original points setup was not firing on all. Actually it was bad plug wires. I was moving to a HEI so dizzy, wires and plugs were all replaced. Runs great since.

Going to move this to the Engine area for more help.

[66CHEV]

chevy-mike the way i checked the spark was take the plug out and hold against metal while cranking the engine and the spark looked good at each cylinder. i do have a petronix plate and flame thrower coil to replace the points. i have not installed that yet. i wasnt thinking about the issue being fire related but i will go ahead and install that and see what happens. thanks

also the lift measurements came from me measuring from the head to the top of the valve with a ruler that measures to 1/64 inch so its possible that i could be off 1/64 on any of the measurements.

[chevy_mike]

Okay, that can explain the random valve lift numbers. The head is not completely even across the surface. You need to use a dial indicator to get accurate numbers.

If you know you are getting fire at each cylinder another issue could be a vacuum leak at the mounting point with the intake and head. With it running, take some starting fluid spray and lightly spray around the mounting surface. If the engine changes in how it runs, you likely have a leak and you are pulling too much air in these cylinders, hence they won't fire.

[66CHEV]

i bought some starting fluid going to try that this afternoon. just out of curiosity, a couple days ago i tried that test with carb cleaner. would it give the same effect?

[66CHEV]

one thing to note i held a piece of paper up to the exhaust last night and it blows the paper away then sucks it back into the exhaust pipe. i know most people say thats a burnt exhaust valve problem. my question is it possible to have a burnt valve and still have good compression.

[66CHEV]

ok so I done a leak down test on the cylinders today and everything seems normal. the cylinders are slowly leaking down which I can assume is normal. the only place that air can be heard is at the small holes in the top of the head next to the pushrod holes. I assume this is normal as well??

[frankslagoon]

Very strange. you would think if your getten gas in the cly. and the plugs firing something should happen. if you do have a intake leak like chevy mike suggested maybe your not suckin anything from the carb. some of these carb cleaners we buy now aren't to flammable. try the starten fluid around the intake?

[66CHEV]

I agree man something should be happening but nothing I do seems to make even the slightest bit of difference. I tried the starting fluid last night, even undone the fan belt so it didn't blow back on the intake and I soaked that intake and exhaust both with starting fluid while the truck was running and nothing?

[dh81k30]

66CHEV,

Considering your posted valve lifts, I think you are the victim of a bad case of valve seat recession due to unleaded fuel and non-hardened valve seats. I had to replace the head on my old 292 due to recession. A couple of my exhaust valves were so bad that they were effectively not opening past the floor of the combustion chamber ( and yes, I had normal compression pressures). The machine shop wouldn't try to cut the old head for inserts because the recession was too close to the water jacket, in their opinion.

The interesting thing was I readjusted the lash on the hydraulic lifters a couple of times. It helped - but not for very long.

I don't remember how the valve heights are under the valve cover. Maybe it is possible to use a long straightedge and compare the height of the valve retainers on the dead cylinders to the good cylinders.

OR alternatively, you might have an issue with lifters (adjustment and/or hydraulic lash action) that doesn't allow the full closure of the exhaust valves on the affected cylinders. Not as likely since you have full compression pressure on all of your cylinders.

HTH and good luck,

[dirtyjim]

I've started using propane to check for vacuum leaks, works better than spaying stuff all over and leaves no residue. just use a standard hand held propane torch, since this is the internet I must also state the obvious that you DO NOT LIGHT the torch.

[jjzepplin]

Great idea on the propane! I have found a few engines with this problem from age I guess. Starts good then dies out due to heat expansion keeping the valves from seating. I adjusted the valves and fixed it. Not a long term fix but it worked.

[66CHEV]

ok tell me if this makes any sense did a leakdown test with these results. all cylinders leakage at about 35% except number 2 and it was at around 20%

test on cyl 1 can hear air coming out of cyl. 3,6
test on cyl 2 no air coming out of other cylinders
test on cyl 3 air coming out of cyl. 4
test on cyl 4 air coming out of all other cylinders
test on cyl 5 air coming out of 3,4,6
test on cyl 6 air coming out of 2,3,4

head gasket or just totally messed up head?

[mechanicalman]

Originally Posted by 66CHEV

ok tell me if this makes any sense did a leakdown test with these results. all cylinders leakage at about 35% except number 2 and it was at around 20%

test on cyl 1 can hear air coming out of cyl. 3,6
test on cyl 2 no air coming out of other cylinders
test on cyl 3 air coming out of cyl. 4
test on cyl 4 air coming out of all other cylinders
test on cyl 5 air coming out of 3,4,6
test on cyl 6 air coming out of 2,3,4

head gasket or just totally messed up head?

I don't think you did it correct. Make sure when you do each particular cylinder that both valves on that cylinder are closed, or just back off every rocker arm all the way and do it that way then re-adjust them. If you have valve seat recession, adjusting might put a "patch" on it temporarily anyway.

That or you got a very messed-up something!

[66CHEV]

the more i stared at the results i began to think the same thing the exhaust valves must be to tight because (correct me if i am wrong) that exhaust manifold is the only passage way that connects all of those cylinders together. except for the intake, but there is no leakage out of the top of the carb. going to back all exhaust valves off and re run the test this evening.

[mechanicalman]

Originally Posted by 66CHEV

the more i stared at the results i began to think the same thing the exhaust valves must be to tight because (correct me if i am wrong) that exhaust manifold is the only passage way that connects all of those cylinders together. except for the intake, but there is no leakage out of the top of the carb. going to back all exhaust valves off and re run the test this evening.

Might as well back them all off, plus then you know after you are forced to re-set the valves that they are all right for now.

If after re-adjusting the valves it improves, it's almost a certainty that the valve seats have recessed. All forms of valve train wear and tear loosen the valve train, the only other thing it could be is lifter pump-up and/or sticky lifters.

With it being the exhaust, it's all pointing to seat recession. If the rocker adjusting nuts end up higher after adjustment then it's almost a certainty. There is a small chance the problems are the result of catastrophic failure of multiple valve springs, but like I said small chance.

Probably time to re-build the head and install new valves/seats and likely guides. Costly, especially if you let the machine shop supply the parts.

These guys seem to know their way round these heads, what core to start with, how to un-shroud the valves and flow the ports. They have the right "kits" to rebuild these heads with valves, springs, guides, seats, locks, spring retainers/keepers, and valve seals.

I'd get the screw in rocker studs as well, the old 6 I've seen can do some damage on the rocker pedestals when the stud pulls out.
http://12bolt.com/250292_products/cy...nd_rocker_arms

These ARE the 6 cyl head guys, I expect they know what works.

[TJ's Chevy]

That a hydraulic cam so loosen the rockers until they clack like crazy and tighten until right when the clacking stops...do this at operating temp. If your valves are hanging open you'll have no power and burn the valves.

[66CHEV]

update on the 250 it is running much better, not 100% yet but about 90%. apparently the valves were set way to tight. i had set them by an old chiltons manual which agreed with every other book i read but for some reason didnt work.

i did a leak down test on every cylinder and found air coming out of the exhaust and the carb. so i loosened the valves till the air stopped coming out and started it up running much better, valves not clattering.

[jjzepplin]

You will probably need to loosen them more than that. Sounds like you are headed in the right direction though. Not sure how to adjust 6cyl valves but on a v8 I would tighten til the pushrod wouldn't spin and then one half turn more on the nut.