My 1991 is backfiring when I let off the gas and coast down a hill. Or if I've been giving it a lot of gas and suddenly let off.
It does not backfire on accelleration.

I recently started getting a Code 23 (mat sensor) and plan to replace the sensor with the relocation kit.

I also get an EGR valve code only after driving the car for extended periods of time.

I think the code 23 could be causing my issue because it could be sending the wrong fuel/air mixture.

Any thoughts?

 

Could be an exhaust leak.

 

an air leak in front of the mufflers

 

sometimes a sensor tells the truth. it is not a politician.

 

Love that sound. Leave it alone.

 

What is a "mat" sensor?

If you mean MAF sensor, this one could cause Rumble, or slight backfire during Decell if it's dirty or faulty...

 

MAT sensor... (manifold air temperature sensor). Located under the plenum. Measures temp of air entering the engine.

 

It could also be timing too far advanced. Check your timing.

 

I checked the timing recently and it was right on the money. I will check again to be sure.

 

Check your AIR pump selenoids. If they aren't functioning properly they'll be injecting air into the exhaust when your coasting and cause the back fires.

Mine was doing that and I just unplugged it, took care of the problem.

 

I don't know about your year, but it may be similar. This is for '86 to '89 with 1227165 ECMs: The MAT sensor only determines when EGR is activated. That's ALL it does. It has no other function.

Check your EGR system.

 

I would check to see if the EGR valve moves freely when you move it with your fingers.

I believe an EGR solenoid is used and I would check it to see if the coils resistance looks good.

I had a similar problem where the coil in the solenoid was good but the valve did not operate properly. I found the problem by externally putting 12 volts to the coil (it clicked) and blowing thru the valve portion. It did not close and open properly and then pieces of debris started to fall out after examining it for a while.