Rich fuel mix, Chevy G20, 1989

[cmac2012]

More fun and games with my old Chev. It’s been running pretty well since the intake manifold gasket install last summer.Then I started getting the check engine light, usually after about 20 miles on the freeway - it would pop up with the trouble code always “rich fuel mix detected at O2 sensor.”

One of the main suggestions on the web was to get a new temp gauge sensor. I installed a new one last summer during the other work, along with new jacks which were both in bad shape.

I had the fear of God put into me after about a week of doing a lot of driving with the engine complaining the whole time, really faltering off the line, I would floor it hard, and it would come to life. Off and on during this I would hear a funny knocking noise which would be gone the next day and then come back after extended driving.

At one point after a 40 mile drive back to my place it was really acting badly, as if some of the pistons were freezing up. I pulled over, got a Lyft home, came back the next day, and it ran much better. Then I noticed a new thing in my searching, which was that a rich mix can be a problem because the excess gas washes away some of the oil and you suffer poor lubrication. Only makes sense.

I had rebuilt the throttlebody injector unit a few years back, but didn’t renewed the injectors. So I bought some injectors from Napa and put those in.

I also installed a new rubber mounted fuel pressure valve, which is a trick to put in. When I pulled the upper part out the area around the fuel pressure device, there was an odd amount of liquid seemed too soupy, a lot of unusual liquid down in there. The old injectors had a few possible visual issues, long story. It seemed like there was less visible dripping instead of spray from the injectors during idle than with the old ones. The air filter is almost new.

I discovered the heat tube taking heat up to the underside of the long oval pipe going into the air filter box, the automatic choke valve device, was no longer in place, which beat all to hell and never stayed in place anyway, so I stuck the shut off vent open full-time. I figured a bit of jerkiness warming up was much better than what I was getting with rich fuel mix the rest of the time.

Things improved after this, but after a few days, the damn light started coming on again, plus I am still getting missing/ lurching off the line after warm-up. It runs great when I first start it which is an indication of too rich a fuel mix perhaps. And that damned knocking noise comes back after a while. It’s freaking me out, man.

I found something on the web about fuel canister purge. They were saying that if there’s too much fuel vapor, it’s going to be a rich fuel mix, with the check engine light coming on, etc. But the piece I found that is suggested to be renewed puzzles me, because I don’t remember seeing this on the intake manifold, which is where it’s supposed to be.

This canister is under the hood, near the passenger side, headlight housing. I’ve never really known what it does:



Here is the link for the part I am finding online; “fuel purge valve.”

http://tinyurl.com/2cpnvtb4

[87tdwagen]

That canister in the pic looks like an AC filter/drier.

As to your fuel/running issue. Have you checked your MAS, or air flow sensors for operation. Pondering if the ECU is reading too much air and is enrichening your mix to compensate.

[cmac2012]

Come to think of it, I think you are correct about that canister. I’ll look at it again, I think I even noticed once that it has lines leading from the AC compressor which I’m not even sure works anymore.

The MAS idea sounds good. I’ll look into it.

[INSIDIOUS]

You should be able to find your fuel vapor storage canister if it is there. And prolly no MAS on that unit.

[87tdwagen]

I read your post over again and noticed your year G20. Prob no MAS on that. Your O2 sensor is most likely the main fuel trim input. They can be tested, but if old, they are usually inexpensive to change. I assume in this year range you prob only have 1 single or possibly a double wire sensor. Simple EFI.

The carbon canister is usually located in the lower fender wells. Could be up front or even in the rear overhangs by the fuel filler tube.

[cmac2012]

I had never read about a MAS but I was going to look into it. My excellent mechanic buddy in Redwood City suggested a new O2 sensor and I put one in. The other one was only four or five years old but why not - no change. Oh well.

The web article I found about vapor capture said that when that system messed up you can get a lot of check engine soon light action, presumably linked to rich fuel mix.

[INSIDIOUS]

Did you ever reset the computer? Ever change the IAC valve?

[87tdwagen]

Some of those canisters can be opened and serviced. Excessive fuel in the canister can cause problems adding to a rich scenario so worth checking.

One other thing that came to mind was the state of the air filter, just to rule out any air starvation issues.

[INSIDIOUS]

maybe by pass it to see if there is improvement?