Stumbling, low oil pressure, predetonation
 

2003 Express 1500 4.3. Has 140K miles.

I've searched all over the forum for this stuff, hope you can help me.

Van has some erratic stumbling idling. Rides fine until about 3k rpm then acts like it is missing, and jerks. Doesn't want to go over 55/60 mph. Seems to stumble less when in Tow/Haul mode.
Under load it pings - sounds like predetonation to me, but then I don't really know what valve chatter sounds like.
Has low oil pressure - about 5psi at idle and 22psi at 60mph.
I've replaced the rod bearings, didn't change the oil pressure. There was very little wear on the rod bearings.
Doesn't have a noticeable knock I can hear.
Throws the P0335 Crank Position Sensor error, and the P0110 MAF error. If I disconnect the battery to left the codes clear, the codes won't show up until I start up and shut down and then start up again. Still misses and drives bad even when the codes aren't showing.

Appears to have a new MAF. Have replaced the Crank Sensor.

Local mechanic reckons it needs to do the crank relearn procedure but he thinks the rod bearings are bad and doesn't trust the engine to handle the high revs of the relearn. I'm not sure I trust that advice on the relearn.

Do you think I should look at:
oil pump replacement?
Fuel pressure regulator?
Main Bearings?
Plugs, wires?

Stumped.

Thanks

Additional info
 

I forgot to say that when I bought the van, it had the wrong crank sensor in it. So I replaced the crank sensor with the right part. The old sensor was worn down from rubbing on the disk thingy it reads from. Disk looks OK.

Are you reading oil pressure from the dash board guage? If so, I'd suggest verifing it with an external guage. P0335 is cps incorrect signal, did you put the correct one back? Not familar with the 4.3, but my 3.1 utilizes 2 cps, one is 7x, the other is 24x. This is so chevy can sell more parts imo. It reads the 7x up to 2k rpm, then reads the 24x.
The maf sensor code I don't have, however you think it has been replaced, these things have to be correct for the intake.

Thanks for the reply Kevin.

Yes, the oil pressure was checked with an external gauge - it is low.

Yes, replaced the crank sensor with the right part. It doesn't throw a code the first time you drive it. Just the second time. It's really weird.

I'm thinking of replacing the oil pump, and having a look to see if I can get at any of the crank bearings while I've got the oil pan open.

then put in 10-30W to give the hot oil more viscosity (I hope I've got the numbers right there), and adding Lucas oil stabilizer to give the oil some sticking power, and see how she goes.

What oil are you running, and when did this problem occur?

Running 5W30 synthetic.

Had this problem since I bought it a few thousand miles ago.

Is this what is listed on the oil cap? You might (I dunno where you live) try a 10-40. Oil pumps are gear pumps that run in oil, so wear is negligible. If you think to change the oil pump, be sure to prime the new one when you install it.

5W30 is the spec for this engine. 10w40 would be less viscous at running temps, so that would be lower oil pressure wouldn't it?

No, the higher number the more viscosity it has. All oil will drop in viscosity as it heats up. I stated I didn't know your location as the first number 5 is low viscosity for starting in cold, less resistance of the oil. The second number 40 is what it will do at operating temps.

Yeah, you're right - higher number = higher viscosity.

I just replaced the pulley tensioner wheel on this van and had a poke around. Apart from the power steering pump pulley being a little loose, there is nothing obvious wrong. I revved it up to about 4000 rpm, and there is no missing, stumbling or low end noise. Maybe a slight lifter tick at idle, but that's all.

maybe it just needs the crank/cam relearn procedure?

While the belt was off, I pushed and pulled on the crank pulley, and it moved slightly up and down, not like a bad roller bearing, but it did give a little. Not exactly a scientific check, but I wonder if that indicates bad main bearings and that looseness is what is throwing the crank sensor code? But if the bearings where that bad, surely I'd hear low end noise?

Oil is at 45 psi cold btw.

Stumped. Oh well. Perhaps the stealership can have a look at it and over charge me to tell me they can't tell what's wrong without taking it apart? :)