1997 Blazer with 160K miles. Misfires at idle terribly. Swapped out all electrical (Coil, Cap, Rotor, Plugs, Wires) except for the Ignition Module. Distributor gear looks good (Had it out for an intake gasket change). Not much play on the rotor. When I watch the TECH2 misfire screen it is typically either cylinder 3 or 5 (Usually sets a P0300, however). Seems worse when car is hot. Misfire will lessen / quit if I rev the engine at idle to about 1500 rpm.

Trying to decide if the fuel pressure readings are OK or not. Followed the book method post. When the pump primes and the pump is running, I am at 60 psi. After the prime, it drops to around 55, then seems to rise slightly to 56, sometimes 58. When idling, the pressure bounces a bit but is at 52 -53 psi. If I stab the throttle, it'll jump to 60 psi, then return back to the 52-53 value.(Are these acceptable?) I did install a new filter, but achieved the same readings.

I also did an injector balance test and pressure went from 57/58 to 48 psi after the pulse on all injectors, so I could not see any difference. Trying to either rule out the fuel pump or not. Thinking it may be the spider assembly based on many of the posts.

 

Any thoughts on this one. I'm still stumped? Trying to prep for the weekend work.

 

If my memory is correct, and I won't swear that it always it, fuel pressure should maintain around 60 psi. Any less is not enough. I cannot imagine, however, lack of fuel pressure causing a misfire on the same cylinders repeatedly. The dealer tried to sell me $1K fuel pump swap because my pressure was 59#......ran it for two more years until it quit. I think you are on the right track with the ignition. How does the car drive? Does it miss when you're moving or just at idle?

 

Misses primarily at idle. Once I get some load on the engine it clears up significantly. I can watch the TECH2 while I am driving and if I clear the screen once I am moving 30 mph or so, and slightly accelerating, the misfire is essentially gone. Even when in Park if I rev the engine above 1500 rpm or so, the misfire cleans up significantly. The symptons are really confusing me.