Intermittent p0304

[quadboy75]

No coolant loss and no vacuum leak and no I don’t have a scanner like that, just one that reads codes. Fuel pressure I’m checking next, the pump is only a few years old and I didn’t go cheap there so I’m hoping that’s not it, I am suspicious of the injectors since I think they are original, it’s hard though because it runs so good even at full throttle on flat ground. The thing that’s strange is right before it starts missing it’s like it shifts out of overdrive and into third, the rpm’s climb about 300-400 then it starts missing.

 

[east302]

GM did publish a TSB in response to sticking injectors for emission vehicles. Here’s a link to it, but the gist is that the dealer would suggest replacing the poppet style with the MPFI version if a cleaning exercise didn’t work. It’s a common swap today, but those with California emissions could, I believe, have it covered as part of some extended warranty that GM ended up offering.

https://usermanual.wiki/m/f1c8171761936903280641339b85c3e01e3f256c4a93f3be971353d85cc6f8ce.pdf

At the end, note that the TSB calls for adding a GM injector cleaner in the gas tank…part number 12345104.

It might be worth dumping a bottle in the tank.

Since you’re only seeing one cylinder misfire code, a fuel pump issue would seem less likely as that should impact all cylinders. To me, the downshifting and stumbling sounds like a normal result of a misfire.

There was also a TSB for valves causing cylinder-specific misfire codes. Now that I’m looking for it, I can’t seem to find it but seems like it was for gummed up/sticking valves or weak springs.

 

[exp500]

"Bouncing" up to a few hundred RPMs is usually a failing CPS or harness.
Valve adjustment helps sticky lifters,especially hi mileage ones. I prefer engine running adjustment.
And the distributor cap/wires are the first stop for a misfire, 5-10000 miles on a new cap is enough for a failure to happen....parts problem. Try a different wire on #4 and a new cap. That at least leaves you with emergeny spare cap.

 

[repairman54]

I had under load misses in my old '99 Yuk with the 5.7. After replacing cap, rotor, plugs, etc. it was a bad distributor. It had the OEM crappy plastic housing one.

 

[quadboy75]

Update
I noticed that the miss started getting a little worse and idle became a little rough, so I changed the ignition control module and boom, no more miss and I’ve tried several times and several hills. Purrs like a kitten now. hope that’s it