Hi, Im new to the forum and I need to pick your collective brains.
I have a '98 blazer vortec 6 cyl,120,00 miles, running real rough, Did a comp test and the back 2 cyl left and right are really low at abt 35 LBs each. the others are fine. What ya think? Gasket? Cam? Is this a known issue?
TIA
Bill

 

You can check for broken or worn rings by squirting some oil in the spark plug holes and seeing if the compression returns. If it doesn't, then it's likely a head or head gasket problem.

 

i did the oil thing..no change, just seems weird that the 2 cyl that are low are opposite each other, could both head gaskets be blown?

 

valves not closing all the way for some reason?
sticking, carbon deposits, bent?

 

Which codes do you have? Just to confirm which cylinders your thinking?

If you did the oil test in the plugs without a change it doesn't sound like it's your piston rings. You can further confirm this with a leak down test. I would say it's uncommon for both gaskets to cut loose at the same time, but not unheard of. Once the engine starts misfiring and shaking this can cause all sorts of stress on things.

It's also possible as kevlar mentioned that your have a couple of sticky valves. Sometimes these things don't really become noticeable until you have a couple of them acting up on you.

In any case at 120k miles, if you are hoping to keep the truck for awhile, it's in good shape otherwise, and you can do the work yourself you might want to just consider going for those gaskets and have the heads reconditioned while you are at it. This will give the truck a new lease on life and give you an opportunity to do a full tune up plugs, wires, cap, rotor, and switch out a couple of other things that are common issues or just a pain to get to.

Upper and lower manifold gaskets
Oil cooler lines (if applicable and leaking)
Water pump
Radiator/Heater Hoses
Serpentine Belt
Power Steering Fluid change
Oil Pressure Sender (good preventative, because it's usually a pain to get to, but easy beezy with it torn down.)
Exhaust manifold gaskets (they come with a gasket kit, might as well throw them on)
Thoroughly clean the throttle body and manifolds from deposits
Coolant flush
Thermostat and gasket replace
Clean and lube (penetrating lube works good) the EGR valve
Fuel Spider (expensive, but not a bad idea, or try to clean up the old one).
Replace, or Grease pulleys

I'm just recommending these as preventative maintenance. Make sure to troubleshoot first for the real cause. You don't want to just start throwing parts at it and you also don't want to spend all the money on the above parts if the car still isn't going to run afterwards.

 

thanks for the reply.
this was my daughters "learner car" when she got her license 5 years ago, small enough that she felt comfortable, big enough that i felt comfortable. kinda. when it started acting up 3 years ago i put it in the garage. now my son is getting his license, so im thinking of getting it running again. i already changed all the ignition components. then i checked the compression. thing starts right up on the first crank tho, so maybe its worth pulling the heads.