Comments please:

I'm not sure the cause of my rough running engine and hope one or more of you may lead me in the right direction.  Your comments are appreciated.  The symptoms are as follows:

• When engine cold, starts up with a smooth idle and steady 17 in vacuum
• After driving for a half hour - idle becomes rough, and vacuum bounces between 12 and 15 in.
• Let it cool off overnight.  The next day, it is back to a smooth idle and 17 in vacuum.
• I also notice slight white smoke out left exhaust, none out right.
• Compression and leak down test show all 8 cylinders A-OK.

 Engine is mildly modified - Edelbrock 600 carb, cam, headers, alum. intake, msd, holley "blue" fuel pump.

 Is this the symptoms of a cracked head or something else?  The engine has 40K and runs great ( for 1/2 an hour)

Comments please.

Bret

I suspect the valves are adjusted to tight, hydraulic or solid can do the same thing.  As the engine warms up, things expand and become too tight, slightly holding the valve open.  This would explain the rough idle and erratic vacuum.  Yes, I have seen it before.  Several times.   Unburned fuel can create some white smoke, so I suspect the right side has the tight valve(s).   Misfire or rich conditions do create black smoke, but completely unburned will create white.kstyer38480.81875

Thanks Ken.

The lifters are hydraulic.  Do you think 1/4 turn loose would do it?

Thanks,

Bret

I would try it.  It's possible you just have one lifter hanging up.  You won't need to replace it, just loosen it a bit.

Do a complete readjust.  Just keep track of where they are now as you loosen them, and then readjust them making sure not to come within a 1/4 turn.

Or even better, get the car fully warmed up so it's acting up.  The back them off one at a time just a bit.  That way you can tell which one is the culprit.

If the engine is too hot to pull the covers, or it takes too long and the engine cools, go back to plan A.

Thanks Ken.

I'll give it a try.

Bret

 

I would warm the car up and do a dynamic readjust..

 

I had an old dodge that use to run great when cold and would run and die half them time once warmed up.  Turned out to be the intake manifold gasket.  Once it warmed up it expanded enough to seperate the gasket at the valley on the intake and cause the vacuum leak.

Thanks Ben & aftershark,

dynamic adj. is the plan.  As for intake gasket leak, I have checked for leaks, none found.

Your ideas/ suggestions are appreciated.

Bret