Lower RPM Miss.

[Varment]

I currently have a brand new 318 in my car and it is running rough at low rpm. I noticed the pcv valve was making a clunking sound and found that it was vibrating up and down. This immediately made me think it was a compression issue because of the lack of vacuum. While running the engine I unplugged the wires to find out which cylinder(s) was leaking. Both number 4 and 6 are not firing at idle on the passenger side. I pulled the valve cover and started it and checked for bad springs, or wipped cam lobes. Everything is working fine.

It seems to me to be a bad head gasket.

What do you guys think?

 

[RustyRatRod]

Is it blowin steam out the exhaust? Is it runnin hot? Losin coolant? If not, it ain't a blowed head gasket. Try lookin at the plugs. It's normal for the PCV valve to do that. some of them are loud as all hell. It means it's workin.

 

[Varment]

It is not running hot or losing coolant, but it is kicking out some dark colored exhaust. We've checked both of the dead cylinder's plugs and both look fine and getting spark. I also don't really know how else to explain the PCV but it is definitely not normal. I can tell you that the PCV valve was not making nearly the amount of noise it is making today. I have only been driving the car for three days now for about a total of 150 miles on this brand new engine before this has started happening.

Any other ideas?

 

[daredevil]

Do a compression test on the offending cylinders. Sounds like the rings arent seated or someone lined the gaps up.On compression its loading the crankcase when it fires. Pull the dipstick and see if it has a pulsation. If so this would confirm it. May just need to be run more to have the rings seat in.

 

[RustyRatRod]

The lined the ring gaps up thing is hooha. The rings rotate on the pistons during engine operation anyway so they all line up at some point in time. They do not lose compression because of that. Besides that, there's nothing to impede the gasses escaping when the rings are not lined up. the path is just staggered. It ain't gonna slow down gases one iota. lol That's an old wives tale. This is why all engines have some amount of blowby. It's a normal occurance. If you've noticed a big difference in the PCV valve sound, then a compression test is a good idea. also put a vacuum gauge on it. It is entirely possible you may have a head gasket breeched between two cylinders that does not affect a coolant passage. A compression test should reveal this. If you get low compression on those two cylinders, add about a tablespoon of motor oil to the cylinders one at a time and try the compression test again. If the compression comes up, the problem is the rings. If it does not, it is something else.

 

[vintagetin]

It is entirely possible you may have a head gasket breeched between two cylinders that does not affect a coolant passage. A compression test should reveal this. If you get low compression on those two cylinders, add about a tablespoon of motor oil to the cylinders one at a time and try the compression test again. If the compression comes up, the problem is the rings. If it does not, it is something else.

Agreed!

 

[Varment]

I had someone come over that knew abit more about engines than me..and by abit I mean leaps and bounds. He did a compression test and we found Cylinder #6 is reading 125 and #4 is not even registering. To verify this he hooked up an air hose to the spark plug for #4 and sure enough air was flowing all the way through the exhaust with the valves "closed".

...Wonderful...

 

[daredevil]

The lined the ring gaps up thing is hooha. The rings rotate on the pistons during engine operation anyway so they all line up at some point in time. They do not lose compression because of that. Besides that, there's nothing to impede the gasses escaping when the rings are not lined up. the path is just staggered. It ain't gonna slow down gases one iota. lol That's an old wives tale. This is why all engines have some amount of blowby. It's a normal occurance. If you've noticed a big difference in the PCV valve sound, then a compression test is a good idea. also put a vacuum gauge on it. It is entirely possible you may have a head gasket breeched between two cylinders that does not affect a coolant passage. A compression test should reveal this. If you get low compression on those two cylinders, add about a tablespoon of motor oil to the cylinders one at a time and try the compression test again. If the compression comes up, the problem is the rings. If it does not, it is something else.

Try a compression test on a new motor with the gags lined up. I do agree the crosshatch rotates the rings but you arent supposed to line the gaps up during assembly as the gaps may rotate together. O.P. I guess you pulled a valve cover to verify the valves were closed? Were the heads milled? Pull the rockers and retest if so. Pushrod may be too long holding valve open.When valves closed try spinning it with your fingers. if it wont spin it may be your problem.

 

[vintagetin]

Varment, to check for a bad valve seat on #4: Pull the valve cover off. Pull off the rocker arm shaft assembly (loosen the bolts evenly so as not to bend or tweek the assemble under the valve spring loads). It makes sence to me that you should be able to lay a straight edge across the tops of the valves and this will reveal the exhaust valve on #4 sitting higher than rest indicating a dropped seat. If this is not the case, you still could have a burn valve..and I'm not sure how that would happen in your case. One loose bolt on the intake manifold is not going to cause a lean condition.