Hey all,

I added a new full exhaust (headers, y-pipe, cat, cat-back) on my 1986 LG4. I did install a new O2 sensor directly below the shorty headers. Now my problem is that she's running real rich. Babies crying, mommies gagging rich.

What I know about a Q-Jet could fill a thimble, so I'm looking for some suggestions as to what to check in order to figure out WTF is going on. Suggestions? And thanks in advance from a long time lurker.

 

Any Service Engine Soon light?

 

No SES either. It had been tripping one with the stock exhaust (bad O2 sensor), but I've put about 75miles on it since the swap and no light, just very rich. I was afraid I'd be too lean with the bigger exhaust.

Could it be the carb is just worn out?

 

Doubt the carb is worn out.
If the ECM is dead, the carb runs full rich. Do you get a Check Engine light when the key is first turned On? Lift the hood and turn On the key but don't start the motor, you should hear a rapid clicking noise coming from the carb if the ECM is working correctly.

Might just be the new O2 sensor is bad. Did you use any Silcone Seal under the hood? That will kill the sensor. Antifreeze on the outside of the sensor is bad too.

Make sure there are no exhaust leaks. Leaks allow extra O2 (air) to be pulled into the exhaust which makes the O2 sensor/computer think too lean, so it richens the carb to compensate. The last thing to try is blocking off the A.I.R. tubes to make sure no air is being inject during closed loop.

 

Yes I get all the usual lights when starting (even the nice green "OK" light). No silicone seal, and the headers don't have AIR tubes.

I'll check on the exhaust leaks. I haven't noticed any, but maybe I missed something. Thanks again for the help!

 

What code was it setting when the light did come on?

Interesting. Mine was an eye burner before the mods, it's cleaner now after.

 

It was a code 13. The TPS seemed ok, and since it hasn't tripped again, I assumed it was the O2 sensor.

I did find that the driver's side collector flange was a little looser than it should have been, but no signs of leaks. I checked the spark, and it looks ok, but maybe a little weak. I did a plugs/wires/cap/rotor swap about a month ago. Could a weak spark (like from the old coil) cause this much of a rich condition?

 

Sure could.

Code 13 is indicative of a bad O2 sensor. But, you won't set a code with a bad coil or module. Or cap, or rotor, or wires, or spark plug.

 

Thanks, I'll try testing the coil next.

One other thing I noticed, with the engine off, the carb primary AV sits open by about 1/8" or so. Also, there is a lever on the passenger side that is pushed down (forward). On a friend's truck's Q-Jet (1986 also I believe), that lever is positioned straight up. What is that lever and would it possibly be contributing?

 

The primary AV is actually the Choke flap. If the choke is not opening all the way when warm, it will run way too rich!

 

Alright. So how do I fix the choke then? It sounds like we're on to something here. Thanks!

 

With the engine off and cool, the choke should be closed. It will only open after the engine has been running for a few minutes.

The green lever on the side is the fast idle cam. It should be up when the choke is on, and down when the engine is warm. If you shut the engine off warm and the engine cools down without the throttle being touched, the lever will be down, held there by the fast idle set screw, until the throttle is opened slightly. That will allow the cam to move and the choke butterfly to fully close.

If the choke butterfly never opens fully with the engine running, most likely the choke thermostat, the round black plastic cap on the passenger side of the carb, either isn't getting voltage or is shot. This will cause a rich condition and my not set any computer trouble codes.

 

The choke is getting voltage. So I assume the next check would be to swap the choke unit?

Thanks again, you all are awesome. :hail:

 

Just a follow-up. I replaced the choke and she seems to be running better now. Still seems a bit rich, but it is better. Thanks to everyone for the assist!