98 Tahoe Random Misfiring

Hi everybody, I'm glad this site exists and I hope you all can lend me some insight into my problem.

I rebuilt the head on my 98 tahoe after a blown head gasket and replaced the fuel pump and fuel injectors (with the new MFI setup) on it. The car runs but has a rough idle, poor fuel economy, and throws a p0300 DTC (random misfire). Looking up the fuel trims, bank 2 is pretty much maxed out (25% LTFT 14%STFT) and bank 1 is at 10%LTFT. I've tested the fuel pressure, which seems to be fine, as well as the manifold pressure, which pulls a healthy 19 in Hg vacuum. The rough idle exsists both immediatly after startup when the fuel system is in open-loop mode and after warm up when it is closed-loop, so I don't think its an O2 sensor issue. The egr valve seems OK.

Even though its pulling 19inHg vacuum, is it still possible a vacuum leak could be causing this? I've tried checking for leaks with wd-40, propane, etc. but no change in the idle.

Could an exhaust leak do this? How large would it have to be? I replaced a busted check valve on the air system but that seemed to have no effect. There aren't any obvious sounds of an exhaust leak.

Is it likely the new fuel injectors I installed are bad? Maybe they're plugged with debris?

I'm just running out of ideas. All I know is that everything I check seems to be OK. I will be eternally grateful if anyone can help me figure this out.

you rebuilt one head or both ....? I would not think of only rebuilding one..especially on an old engine.

I rebuilt both heads.

I rebuilt both heads.

can you check replace the injectors ? if dirty, leaking , defective the fuel pressure will drop rapidly on engine shut down...so check fuel pressure and see if it holds to within a few psi with engine off for 15 min..

I'll check to see if they're leaking. Its running lean so my first guess was that they were somehow not opening.

New development: I'm pretty sure the passenger side cat is clogged. The car now has a real hard time turning over, and after I got it started for a few seconds the driver's side cat was warm but the passenger side's cold. I don't know if this is the cause or an effect of the problem, but it's goin into the shop to have this one fixed.

a bad head gasket CAN damage a cat converter and the O2 sensors...your test on the passengerside converter being cold is an indication its damaged ...


replacement of the converter and both upstream O2 sensors would be best ..then this may be the answer to your issues..

When you put in the new MFI unit, are you certain that you have the right injectors going to the correct cylinders ?? There's a specific order for them.

I was told a bad crank position sensor would go with the P0300 code, but it sounds like you have other issues, too. Still, I'd check it just in case. A bad injector should give a cylinder with the code (p0301,2,3,etc.), and bad O2 sensor or cat should throw more codes.