Rough idle, vibration at low RPM

[nachoman]

See my previous thread about pushrods. As it turns out, the pushrods looked fine, and the valve stems and springs looked fine. Driveability was terrible, but increasing timing and idle speed was enough for me to be able to go the short distance to work. The problem started when taking a short 2-mile freeway trip with a badly leaking water pump. Temp never maxed out, but clearly I lost about 3/4 gallon of coolant.

Yesterday, I put a can of STP motor treatment 1/2 in the gas tank and 1/2 in the crank case to hopefully clean things up inside. My trip home from work things seemed smoother. This morning, I warmed it up, sprayed the remainder of a can of carb cleaner into the throttle bore as I was revving the engine, then did the old seafoam treatment. Things seem much better now, but idle is still rough, and low speed is still a bit sluggish.

Thoughts here? Drive for a few days and see? Perhaps a carb rebuild? My thoughts are that I somehow gunked up or had a valve stick partially open or a lifter get clogged up, and the seafoam and the other solvents have partially solved that.

 

[nachoman]

Today's drive to work was much better. Idle was still rough, but not as bad. I wasn't worried about stalling or holding up traffic. Engine is pretty smooth at higher RPM. I feel like I am okay to use the car gently for a few days until I can spend some time on it. Still seems sluggish at lower RPMs, but not nearly as bad as it was a few days ago.

 

[bubba22349]

I would do a compression test to see the condition of the engine first and rebuild the carb too.

 

[nachoman]

Did a few things - terminal to coil was frayed, looked like a few broken stands of wire. So, I replaced the terminal, soldered and heat-shrinked. Also did the "water to clean out carbon" trick using a small squirt bottle through the carb with the engine running. Not much blew out of the tailpipe.

Both my multimeters were dead, so I got a new one and battery said 13+ volts and voltage to coil about 6.75. Haven't tested voltage to pertronix yet, but I bypassed the resistor wire so I assume it's 12-13 volts. I also haven't done the compression test - I'll try to get that done this weekend. It seems like every time I drive the car it is a little better. Engine is actually pretty smooth at higher RPMs. I pulled the PCV from the valve cover at idle and see no blowby from the crankcase. Also no coolant in oil or oil or oil in coolant.

 

[nachoman]

OK, just tested the secondary resistance of the coil over lunch break. Speck on a stock coil says 11,000 ohms, I am getting about 7,750 ohms.

 

[nachoman]

I finally got a compression tester and vacuum gauge on it - vacuum fluctuates quickly and irregularly at idle between about 10-17 inches. Crack the throttle, and it steadies above 20. Compression using the crappy autozone loaner gauge is pretty even in cylinders 1-2 and 5-6. 3 and 4 are almost completely dead.

I think I have a blown head gasket between cylinders 3-4. Thoughts?

 

[bubba22349]

I would say you are right on in your assumption, it's time to pull the head. Good luck

 

[nachoman]

Two adjacent dead cylinders. I don't know what else it could be.

 

[chad]

the siamesed & the H2O (pump)?
hummm...

was that loss detectable in white tail pipe emissions?
/or/
a sudden disappearance of fluid?

 

[nachoman]

Coolant loss was due to leaks - first radiator, then water pump, then upper radiator hose. No smoke or steam out the tailpipe, no water in the oil, or oil in the water. I think the head gasket is blown between the two cylinders without affecting the water jacket or oil galleries.

 

[chad]

"...head gasket is blown between the two cylinders without affecting the water jacket or oil galleries...."
Ahhh.

Thanks 4 the post-back.
Good, Catch er quick enuff, less damage!

Let us know when you tear-down.

Which h. gasket do U think you'll use?
Do you think you will/can put in a 'port divider'?

I guess all will tell when U take off the head...

 

[nachoman]

I was right. Head gasket blown between 3 & 4. Got it all apart, time to put it back together. I'm going to take care of a few other things while I have it apart. Looks like I have two leaking freeze plugs, and having a difficult time removing them from the block. Also missing an exhaust manifold bolt.

 

[Gene Fiore]

Might be a good time to upgrade to head studs if you don't have already.

 

[bubba22349]

Put a straight edge on the head deck to check if it's in spec for being flat. Good luck

 

[chad]

Put a straight edge on the head deck to check if it's in spec for being flat. Good luck

how much air could ya C w/o a mill cut?
how forgiving are the gaskets?

~ any guidance appreciated ~

 

[nachoman]

This is my daily driver (actually, the only car I have ever owned - for 19 years!) so I am most concerned about just getting back on the road. Don't have time or money to do upgrades now.

I have to admit, I'm quite proud of myself. I was able to use tools and knowledge to correctly identify the problem. Lesson learned, though. Vacuum or compression tests are pretty simple on these engines. On an old engine that isn't running well, look for the obvious first. Then do the compression test/vacuum test before tearing into carburetors and such.