need help fixing a truck
 

my uncle just bought a 98 chevy 1500 4x4 the motor runs like crap the number 5 and 7 cylinders are dead we did a compression check all 8 cyl are good now when you start the truck it smells like gass and pops alot i just ran 2 bottles of seafoam in it to try to clear it it still pops and dies out and still no power what do you think the prob is

5 and 7 dead as in not fireing? no spark?

hes there is spark and fuel, the timeing and fireing order are rite, and the cap, rotor, plugs and wires are all new too.

everything is new the timeing is right

Sound oddly familiar to my motor that had two blown pistons...compression checked out but it ran piss poor. Pop them heads off one more gan' maybe check the cam too(wiped lobes)?

i had the valve covers off the everything works

is it getting fuel

how did you check for fuel? maybe you can move injectors around and see if the misfire moves with injectors

98, should be just a regular vortec. A few common failures on that. The injector assembly(poppets go bad or regulator leaks). Intake gasket failure causing a vacuum leak into the lifter valley. Pull off the oil cap while it's running and see if it changes RPM, if it does there is most likely a lower intake gasket leak. The simplest one is a bad distributor cap/rotor, they tend to crossfire badly on a vortec when they fail.

its getting fuel

i checked for fuel on the fuel rail with a fuel pressure gauge

im going to check that tomorrow thanks alot

fuel pressure gauge won't tell you if a poppet has gone bad. What do the plugs look like from those two cylinders? That may be your best indication. Also as stated an internal vac leak will cause it to misfire or run bad.

Yea, just because it's got fuel pressure at the rail doesn't mean that the two cylinders you mentioned are getting fuel.

- Justin

what do the plugs look like in the dead cylinders? if theyre wet after running, spark problem. If theyre dry after running, fuel problem.

Yup. Happened on both out 98 and 96... Got a whole new "spider" injector assy.