I have a 1999 Suburban LT with 5.7L V8. Just bought the thing from an auction and had a bad miss in the engine. Throwing code P0300, Mulitpule cylinder miss fire. Check wireing and plugs. Also did a compression test on all cylinders. Thats when I found Cyl. 6 & 4 have only 10psi compression. IT starts fine all the time, just has a bad miss when at idle. Under a load or going up hill it sounds like a bad rod knocking, but it doesn't do it idling. Can this just be bad valves and not rods? All the other cylinders check fine.

 

How many miles, and what do the plugs look like? It could be burnt valves which would explain the low compression, and rough idle ect.
NGK Spark Plugs USA

 

It has only 149000 miles and the plugs look clean. They are not wet or have any oil on them at all. look new so does the plug wires. I did find on inspection of the fireing order of the plug wires were wrong. 6&4 where back words. Wonder if they ran it so long like that it cuase the valve to burn and feather out. When its under a load it knocks so bad it sounds like a bad rod failure. Wonder how I can test for bad valve ver a bad rod without takeing the engine apart.

 

If you are sure the noise is from the underneath engine, you can drain oil, drop the pan, and pull rob bearing caps. That would tell you if a rod (also main) or main bearing.

 

So it is possible to drop the oil pan without having to jack up the motor?

 

I don't know as i have a 2000 model. But if you do, pop the motor mounts, and go from there.

 

but wouldn't a bad rod knock all the time not just under a load? You would hear it at idle if it was bad. That is leading me to think it is bad valves that cuase a really bad denotation sound under load conditions.

 

i thought i saw some you tube videos of people checking the rod bearings with the engine in the truck mine looks open enouph to get the pan off might want to the pull the valve cover of to see if there are any valve train problems,i had a friends sunbird do this and it was the head gasket but he had no engine noise

 

Will a bad intake gasket cause low compression on the cylinders? I know they had a problem with the Dex-Coolant and wonder if this could be my problem with loosing two cylinders on the same side next to each other. Could it be just a bad head gasket?

 

IT was just what I thought it was. I finally got around to removing the intake and couldn't find anything. So I dug deeper. Took the heads off and right in between 6 and 4 the head gasket was missing. Was causing blow by to the exhuast valves. I am glad that is all it is. So working on installing new head gaskets. Does anyone know if you have to put any kind of sealer on the new head bolts?