So I come home about a month ago, trucks running great. Get in it the next day, runs like crap. Very rough idle.

I've pulled the codes. 1, 2 and 5. 2 is because I'm running a different AFM, the 5 has me stumped. I replaced the 02 sensor and now it will idle smooth on the high circuit, but you kick it off that and it runs like crap again.

I've tested the wires and plugs. They're good. I've tested the injector harness, tester says I'm getting signal. And I've swapped the ECU. No change. So I know the mechanical side should be good. I'm looking at management.

I did notice that there's a small break in the insulation for the 02 sensor wire. Looks like MAYBE the wire could have gotten kinked. I know that's a shielded wire and that you can't just splice it, so how would I go about fixing that to at least eliminate that from the list of things it could possibly be?

 

Even if your 02 sensor was completely out of the loop, the ECU would run in open loop fuel control and it wouldn't run bad, maybe just poor fuel economy.

Please explain your "AFM" set-up. More issues come up with altered vane-air/flow set-ups.

 

It's a lid off an 88. Has that extra fitting on it that isn't used on 85's. It's tossed the code for the meter ever since I put it on, but it's never affected how the truck ran.

And on a newer EFI setup, I'd go with you, but the fact that I've tried two different ECU's (Which have both been run in the truck btw) and no change on either, I have to believe there's an issue there, I'm just not smart enough to see it.

 

Update: went thru entire fuel and electrical system, replaced no 2 injector, no change. Ran compression and leak down. 200 on 1, 3 and 4. 145 on 2. Comp ring gone. Not sure why it went from running fine to dropping a ring, but I may have found my issue.

 

you sure it's a ring and not a valve not seating?

 

Yes. Blow by thru the dipstick tube.

 

FYI the top of the cyl head is open to the oil pan, maybe it's a valve seal? also looking back on your compression 200 is way high. I think the spec is like 153-170.

 

I agree with xxxetrem. I'd do a dry/wet compression test before tearing into the lower end after a ring.

 

Leak down test and adjust the valves to spec.

 

Well I FINALLY got time to yank the motor and tear it down. Timing guides and a large part of the cover in the pan. Internals all look ok. Have another motor built already with steel guides, lol. This one will get gone through and put into an 82 or maybe an 87.