Running rough at idle

[1984rangerdanger]

1984 2.8 3spd with the duraspark conversion. Engine sounds like it’s missing on a cylinder. Heavy vibration at idle in gear with the brake off. There is a puff of blue smoke on start up, and a small puff when I accelerate after idling. Definitely audible valve noise.

I have checked the spark plugs and they all look about even. No oil, gas or excessive burning etc. when I did the conversion the new distributor and ignition system added a ton of power, but didn’t fix the rough idle problem which existed with the old system as well. Once the engine revs up it seems pretty smooth and has adequate power.

All the ignition parts are new.

Could out of adjustment valves caused this problem or am I better off looking deeper, like a leak down and compression test? I definitely don’t have the time for a head gasket rebuild at the moment, and I have already done the glove test on the radiator which showed no obvious head gasket leaks from there.

Any advice is appreciated!

 

[RonD]

I would do cold engine compression test just to see if a cylinder is low, so that IS the problem, or to take compression issue off the table
Otherwise you will be chasing your tail with spark or fuel issue

2.8l runs 8.7:1 compression ratio so expected is about 140-150psi

A low compression cylinder could be rocker/lifter issue and it would ID which cylinder to check

Remove all spark plugs first, then do the compression test
Have a straw and bottle of oil handy
After testing all 6 cylinders and writing down results

Dip straw in oil bottle hold finger on the top of straw and put some oil down that low cylinder's spark plug hole
retest compression, it will ALWAYS go up
But how far it goes up tells you what the problem might be
If it does not go up past other cylinder's levels then its a valve issue
If it does then its rings

 

[1984rangerdanger]

I would do cold engine compression test just to see if a cylinder is low, so that IS the problem, or to take compression issue off the table
Otherwise you will be chasing your tail with spark or fuel issue

2.8l runs 8.7:1 compression ratio so expected is about 140-150psi

A low compression cylinder could be rocker/lifter issue and it would ID which cylinder to check

Remove all spark plugs first, then do the compression test
Have a straw and bottle of oil handy
After testing all 6 cylinders and writing down results

Dip straw in oil bottle hold finger on the top of straw and put some oil down that low cylinder's spark plug hole
retest compression, it will ALWAYS go up
But how far it goes up tells you what the problem might be
If it does not go up past other cylinder's levels then its a valve issue
If it does then its rings

This is great advice. Thanks for writing it all out! Will do this ASAP

 

[1984rangerdanger]

I just did a compression test. Bad news, all cylinders but #2 have around 125psi. #2 has less than 20.

I pulled the valve covers off and can’t see any visible issues with that particular cylinder. Springs are intact and when I spin the motor things look to function as they should with the exception of their not being much oil up in this area. Photos attached.

Failed ring or a piston with a hole in it?

 

[AndyB.]

Time for a leakdown test to tell you if you have a valve issue or a piston issue.

 

[1984rangerdanger]

Time for a leakdown test to tell you if you have a valve issue or a piston issue.

Leak down test on cylinder 2 at TDC for that cylinder there is air coming out the tail pipe; none out of the PCV valve or carburetor and no bubbles in the coolant. I guess I have my answer! Time to pull and rebuild the valves. Anyone ever done this job themselves (the rebuild)? Or would it be better to send the heads off to a shop?Might as well drill the coolant holes down there while I’m at it I guess…

 

[AndyB.]

I’d pull the head and see what you’re dealing with. You may be able to get away with lapping in a new exhaust valve or just cleaning up one that’s stuck.

I wouldn’t modify the head by drilling additional coolant holes, they do pretty well without it.