malorty

Hello all, this is my first post! Just recently bought my 91 3.0 and have been pretty happy so far except for a few issues. I have been getting about 16mpg and the engine has been stuttering/jumpy when cruising and under light acceleration so yesterday I adjusted the timing (which was around 6-7 BTDC) to the spec of 10 BTDC. Seemed to drive well after and got rid of some stuttering so I thought all was good but about 5 miles into my drive the CEL came on. Diagnosed today and it threw code 21. Would adjusting the timing have ruined my O2 sensor?

Side note, my exhaust is pretty shot. Super rusted and there is a hole in the muffler that leaks exhaust. Would this cause an issue maybe with back pressure??

Thanks for any help in the matter!

jb451

I think the timing may have had something to do with it, but did not ruin your O2 sensor. Did you reset the computer after you did that? It's possible the 02 is going off learned values from timing 6-7* BTDC, and when you changed that, the ECM detected a "fault" with the 02 sensor. Also, if you have not replaced your 02 sensor in a while, now would be a good time to do so, Rock Auto sells the for around 60-70 bucks IIRC. Yes, exhaust leaks can do weird things to O2's as well, so you should probably buy new piping to replace your rusted out stuff.

Gamefreakgc

Exhaust leaks affect the O2 system if they are upstream. Anything beyond the cat don't matter (unless you have two O2 sensors) They are two ways to check for exhaust leaks. 1st is to grab 3-4 feet of hosing, place one end in your ear and "scan" the exhaust pipe for leaks using the other end. Listen for high pitched whines or rushing wind noise. Don't burn yourself. The other would be to use some Seafoam as directed through the brake booster line. When you go to start it up after it's been sitting, if there's ANY exhaust leaks you're going to have white smoke POURING out of it. You have do this quick, or orient your truck in a way that the exhaust doesn't blow back near the engine. A flashlight helps see better.

If you do have a leak, just go to a muffler shop and have a whole new system put on. Go up to 2 1/4" piping from the headers all the way to the tip and put on a Magnaflow muffler. You won't regret it and should only cost $275 to $350. See my sig.

BTW, advancing your timing is going to most likely lower your MPG's

scope103

Code 21 is a fault in the HEATER circuit; it doesn't have anything to do with what signal the ECU is getting from the sensor. So it is very unlikely adjusting the timing had anything to do with it.

The most likely cause is an O2 sensor (heater part) that finally gave up the ghost; a new sensor is the solution. Other causes include wiring issues; the wire to the sensor is in a very tough environment. If you have holes in the exhaust (which, of course, should be fixed if you plan on staying alive much longer), I'm guessing it will be tough to remove your old sensor. In which case it might be worth testing; http://farm6.staticflickr.com/5493/9...a5c6f67b_o.jpg you CAN test the heater, unlike the sensor part.

malorty

Awesome, thanks for all the great info everyone. I did not reset the computer after adjusting timing. How do I do this? Just disconnect battery for a few minutes? I'll try this and see if it turns the CEL off. Either way I know I am replacing the exhaust since it is so rusted so I'll do the O2 sensor as well. When I do the exhaust should I just do the cat back or from the manifold back? If I do the whole thing should I replace the CC as well? I assume it is original so has over 250K on it.

Gamefreakgc, you mentioned advancing my timing would decrease MPG's. Would you not recommend I set it at 10 BTDC as spec calls for?

Thanks again!

jb451

I think gamefreak is referring to anything past 10* (numerically before)... Advancing it too much can cause the engine to run hotter, ping more, and diesel after shutoff. If i were you, i would do the whole thing. Hell, you can buy everything off of Rock auto for a smidge under 400 i think (then go and find a 5% off coupon! )

Yes, disconnecting the battery can reset the ecu, just disconnect the negative cable and wait 30 minutes to make sure it resets! It will turn the CEL off for a bit, but if the problem persists, it will come right back after a few drive cycles. My advice to you is replace the 02 and the pipe it rests on to make sure you have no exhaust leaks before the 02, see if that fixes your issue and CEL, then fix the rest!

Gamefreakgc

Retarding the timing is supposed to increase MPG's at the expense of HP. Advancing the timing increases HP at the expense of MPG's. Something like that anyway.

malorty

So I reset the battery and have been driving around today. So far no more CEL! Thanks everyone! Now to find some new exhaust.

Another side note and maybe a different topic but maybe the problems are related. My truck does fine on freeways, around town, etc. but when I was driving up a long steep back road it started to overheat. Let it cool off and turned around and temp was fine downhill. I've been reading some threads and I checked the resistance of the fan by turning by hand when cold and hot and they seem to provide equal resistance. Should I replace the fan clutch or is the next step to get the radiator flushed/inspected for blockage? Can only a radiator shop do this? Could overheating have anything to do with the rusted out exhaust??

Thanks again!

Gamefreakgc

No, exhaust has no impact on overheating. Your fan might be bad, but better to do a coolant flush and put some good, clean new antifreeze in there. Do a full flush, that means putting water in the engine while it's hot and running to flush everything out. Search the forums or Youtube for some videos on how to do it. Might want to replace thermostat too while you are at it.

You are supposed to replace the coolant every two years, per the factory maintenance schedule.