kwela_man

What cel code will flash when my 1989 pickup (v6) has a loose or defective gas cap? I've tried searching old threads and can't find a clear answer.

A month ago, someone cut my fuel filler neck and stole about three gallons of gas out of my truck. I replaced the filler neck and cap (which they also stole), but now I'm getting an intermittent CEL Code 21. Before I tear into the filler and vent hoses again to look for a leak, I'd like to know if the CEL even lights up for cap/neck issues.

Exhaust, catalytic converter, and O2 sensor are new. Several fresh tanks of fuel have been through the system. CEL comes on erratically, but usually between 30-45 mph and 2700-3500 rpm.

Sometimes it's a quick flash, sometimes several seconds, sometimes almost a minute, but upon diagnostic testing, I only get code 21 (for O2 circuit). I've cleared the code several times, but it keeps coming back.

This never happened before the fuel theft.

Topteke

If your O2 sensor is new and you're getting code 21 for O2 sensor, check out the wiring coming from the new sensor. Sounds like a bare spot making contact somewhere to me.

kwela_man

Thanks, Topteke.

I've checked the wiring before, but didn't find anything.

Although - just to be sure, on your suggestion I checked again just now and found a worn spot on the O2 sensor's protective sleeve from where it passes the edge of the heat shield.

I can't tell if any wiring is bare, but I will tape things up and see how it goes.

Where there's wear, there's gotta be wear there somewhere. Right??



I'll let you know.

Gamefreakgc

I lost my cap once and ran fine with no issues. There's no way for the ECU to know if your cap is off or vapors are escaping the tank, so no that would not be it.

Seeing how it turns on and off, yeah I'd double check wiring. If it really were a bad O2, it would turn on and never turn back off since it would not read the right voltage.

Terrys87

iTrader: (1)

^^^Agree, these trucks computers are not so up to date to realize a gas cap issue. The aftermarket catalytic converters are not that great in my opinion. Seen you said it was new, was this just recent?

kwela_man

Well, despite my previous examination of the O2 wiring, checking it again and reinforcing the insulation seems to have cured my intermittent CEL. It was the simplest solution. Topteke, thanks for reminding me to check again. I wish I hadn't spent so much time searching for other ghosts . . . Silly me.

Topteke

That's awesome, nice job. I could tell you all the times I've come up with an over-thought solution and racked my brain after being wrong, only to find it was something simple that I had already "eliminated" from my list. But that would create a really long post. Congrats again!

stanprophet

Like others have said, EVAP monitors were not mandated until OBDII, same with catalyst monitors. Most OBD1 systems are merely fault checks, and basic malfunction checks. Although these older trucks do have a crude Evap emission system with a charcoal canister, the system is not monitored for leaks. So the gas cap will not cause the MIL to illuminate. Just like a bad converter will not throw a code on these trucks since there is no downstream 02 sensor to monitor the catalyst efficiency.