Inliners International Forums General Discussion Engines dead cylinders

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

Double check your firing order.

timing is about the only thing i know for sure, i have checked it a dozen times, 1,5,3,6,2,4 clockwise on the cap. timing is set at 12 deg with vac advance connected, idle is around 600 rpm. truck starts great and idles once it warms up (smooth as it can on 3 cyl.)

Have you pulled off the valve cover and watched it while it ran? If not try that. You might have a broken spring or something.

i have in fact i even checked the tension on all of the valve springs that was all within spec. all of the valves appear to be opening and closing with no problem, there is no chatter in the valves while running. i have not checked them with a dial gauge but they all appear to be moving the distance. at least no visible differences. i have checked the vacuum at the intake and at idle it fluctuates rapidly between about 12 and 20 as soon as you give it some throttle it settles out and holds at around 18 as long as you are giving it some throttle.

Wonder if you have the wrong dist cap? I think the V6 4.3 will fit, but is wired differently inside the cap.

How would it be different? Power goes in the center, there's contacts at 60 degree intervals, and power goes out.

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.

Because most V6's are not an even fire engine, and have split intervals of varying degrees of crank rotation on the crank rod journals. So a normal even firing distributor cap will not work correctly on an odd fire engine timing sequence.

Valves severely recessed into the valve seats, exceeding the hydraulic lifter travel.

strokersix: just out of curiosity what would cause that, and what is required to fix it? keep in mind this head was completely re-worked by a machine shop about 20,000 miles ago.

It is just a guess based on your curious lift figures, may not be correct. If your head is recently done then it's probably a bad guess...

High mileage and hard use can cause recessed valves. Or poor machine work.

is there a way to check that without taking the head off, would a cylinder leak down test show that?

before i pull the head im going to loosen and re-set all of my valves just to make for sure something is not off in the settings

Take your rocker arms off and lay a straightedge on the valve stem tips. I would be suspicious if there was more than 0.020" difference between valve stems. Note that the intakes may be different than the exhausts.

Could it be bad plugs or wires? You may not be getting enough spark to fire under pressure.
Will Willis

will anything is possible, i bought new plugs last night gonna give that a try today and see what it does. 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.

No, it can't have a burnt valve and still have good compression. Since you have such inconsistent valve lift, I would suspect a cam going bad or as stroker suggested, sunken valve seats.

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??

Somehow strokersix's remark reminded me of an old tired 250 where I had this happen, it would just lose cylinders at idle.

The engine ran pretty good (in fact, it is the same 250 I used for the carb/intake shootout) but was very touchy at idle. I later pulled it and pulled the head, looked pretty decent so doubt that was the problem. What I did to get it to idle:

Find the dropping cylinders. Back off those 2 valves until they just clatter - at that point if its like mine, it will pick up that cylinder again. The lifters were acting like solids - they would only idle at zero lash or while clattering. IIRC I swapped a couple lifters for new ones and that fixed it.

A little messy but it solved the problem. Probably wanted a new cam&lifters but the engine got swapped for a 292 so it is now on the wait list.

Also, try swapping 2 plug wires. If the dropping cylinder moves with it, you know you have a bad wire.

Also, don't rule out the intake gasket. Especially if it runs good at speed - could be a leaky gasket. I've even had em where they would idle good cold, but then when they warmed up, the exhaust would get hot and push the intake away from the block, causing leaks and rough idle. This came from me being cheap and trying to re-use intake gaskets too many times, but at least I knew what was going on.

DeuceCoupe,
thanks for the input I will definitely give that a try and see if backing off of those vavles makes any difference. I also ran a manifold vacuum test on the thing and at idle the needle bounces from about 8 to 20 back and forth very quickly but as soon as you give it the least bit of throttle the needle steadies out at around 18 and holds there as long as you are giving it throttle.

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?

If the head gasket was really messed up, it would have appeared on your compression test. Generally in the form of two cylinders with low vacuum next to one another. Your first post noted a steady 145 for each cylinder with little variation between the cylinders. You still have not verified whether you have the correct distributor cap. Until you do that you may be spinning your wheels a little bit. I think you really have to rule out a lack of spark before you do anything else. If you can confirm that each cylinder is getting spark when it should, then you can move on to the other things, confirming that the cylinder is getting fuel. If it is getting spark and fuel, I am with the others, may be a cam going south. Just out of curiosity, how did you measure valve lift? Try doing it from the lifter, that is pull the side covers, rotate the motor around and observe when the lifter is on the heal of the cam lobe. See if you can rig up your dial indicator to read directly off the edge of the lifter and rotate around to see what the highest reading is. See if that gives you a little more comfort level as to accuracy.

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.

Did you spin the pushrods to determine when lash taken up? That may have been your problem. They will still spin even when loaded so that is not a good way to find the lash point.

My guess was on the right track but wrong root cause...

yea strokersix thats how i determined zero lash, just tightened the rocker down till i couldn't easily spin the pushrod then added one complete turn. guess i had them way to tight.