Intermittent p0304

[quadboy75]

I have a 2000 z71 tahoe with the 5.7, old body style, been having issues with an intermittent code p0304 and misfire when pulling a hill, in town and on flat ground it runs and idles perfectly. I’ve replaced the tps, map, plugs and wires, cap and rotor, and coil all recently. The intermittent problem was there before I replaced everything and is still there now. Any help would be appreciated.
Also, compression test shows 160 on basically all cylinders, 260k miles, and I know it has a faulty speed sensor that I need to replace.

 

[quadboy75]

I have a 2000 z71 tahoe with the 5.7, old body style, been having issues with an intermittent code p0304 and misfire when pulling a hill, in town and on flat ground it runs and idles perfectly. I’ve replaced the tps, map, plugs and wires, cap and rotor, and coil all recently. The intermittent problem was there before I replaced everything and is still there now. Any help would be appreciated.
Also, compression test shows 160 on basically all cylinders, 260k miles, and I know it has a faulty speed sensor that I need to replace.

Also, the hill that I can get it to miss on every time is a 7% grade for about two miles out of a 35 mph curve, at the top when you reach about 50-55 and it starts to level out is when it starts vibrating from the miss and trips the light, shortly after shifting into high gear.

 

[Eman85]

Can you make it miss powerbraking it?

 

[quadboy75]

Can you make it miss powerbraking it?

Haven’t tried that

 

[exp500]

Does your tach "bounce" occasionally? About a 1-200 rpm swing. Have you tried in injector cleaner in the gas? Have you adjusted the valves?

 

[quadboy75]

Does your tach "bounce" occasionally? About a 1-200 rpm swing. Have you tried in injector cleaner in the gas? Have you adjusted the valves?

I run injector cleaner every oil change, Lucas, and have for years, the tack does jump up a little just before the miss and the engine revs a little like it’s almost hitting passing gear. And no I haven’t adjusted the valves. It runs like a ***** ape in town, you step on it from a stop and it takes off without any hesitation.

 

[Hoesgottaeat2]

Another one to throw at it would be to clean the MAF sensor.

 

[quadboy75]

Already did that too, sorry I didn’t mention that

 

[quadboy75]

Haven’t tried that

Update, tried powerbraking several times, both from a standstill and at around 40 and no light no miss.

 

[east302]

Do you have a scanner to see individual cylinder misfire counts and check the fuel trims?

Since it’s specific to cylinder #4, a barely-faulty injector could be a culprit since you look to have replaced the ignition components with no change. Are these the original injectors and have you checked the fuel pressure? Spec is 60-66 psi with key on, engine off.

When were the lower intake gaskets last replaced? Any coolant loss?

 

[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