I think suggestion was to look for paths for water to leak into gas tank. I saw a post once where someone said water was leaking somehow to top of tank under bed and getting into tank thru what I don't know - seal or rust pin hole.

 

The ECM has been all cleaned up already, perhaps the chassis and engine grounds just need replacement at this point though.

 

i missed a lot of your post apparently. but has anyone looked at the o2 sensor?

 

Yes, it was not replaced, but tested fine. Although when I unplug it, nothing changes at all. I thought it should go into open loop if you unplug the O2, or does that not happen with a one wire O2?

 

ECM tries to go into closed loop after some start up criteria are met - like some time period & temperature, not sure if anything else. When it goes into closed loop it expects to be able to make the O2 sensor go up and down when it commands the engine to go rich and lean. Something like above 700mv on rich side and below 400mv on lean side. If it can’t do that it drops into open loop. So, yes if you unplug it you stay in open loop. I’m not sure in what conditions or whether it sets a code in these years. New trucks will set code, I believe if it doesn’t see what it expects from O2 sensor. I have a 93 and had a lazy O2 which would not stay in closed loop because it didn’t vary enough but that did not set a code.

How do you know if you are in closed loop or not? Are you scanning data?
I didn’t see problems like your describing from a bad O2 sensor- mine ran fine, I was just failing emissions. I don’t think you’ll get missing, backfiring as a result of a bad O2.

Did you rule rule out possibility of getting water into gas tank after rain storm like Kevinkpk wondered?

 

No, I'm just unplugging stuff to see if it acts different or kicks into open loop. Seems like the surge doesn't change no matter what sensor I unplug. Yes, I sat under the truck for a bit the other day during a heavy rain storm and there just isn't any water getting onto the top of the tank by the fuel pump. Plus the entire truck has no visible rust anywhere. It was a western truck all of its life.

 

does the "check engine light" come on with the key "on" engine off?

Troubleshooting sensors and how to test

typical Symptoms of a Defective 02
Poor fuel economy
Higher emissions
Stalling-Hesitation

how do the injectors spray look? i know its kind of hard to compare them to good injectors as far as too much fuel but do they at least have a cone shape?

 

I haven't looked into the injectors. Yes, the check engine light does come on when key is in "on" position actually.

 

ok, just wanted to make sure the bulb is in working order

 

I was reading through this fourm because I have a 1995 k1500 with 190k miles with the EXACT same problem as the 1990. Rough idle after rain/humidity then when it dries out it's fine. I replaced/checked everything that CChevy did with his truck, with the exception that I had my TBI unit rebuilt. I just bought a ALDL bluetooth scanner for obd1 and I'm going to do a comparison of the readings when the truck is surging vs. when its dry and runs fine. Any updates/advice would be appreciated.