Alright, I've got a '91 z28, 305 TPI. At highway speeds, if I'm at a relatively constant speed for 10 miles or so, and I let off the gas completely, my car, I assume at least, backfires rather loudly.

Any ideas? I know it's pretty vague, and could be dozens of things, but I don't even know where to start, except maybe my cat isn't flowing correctly or my car is running too rich, which it shouldn't be, since it just got tuned.

 

Check for vacuum leaks. This would be a good start.

 

Do you have aftermarket exhaust? If it is sort of a "Crackling" sound it may be exhaust "Cavitation".

 

Already done, sorry, I should have mentioned that. I made a long trip today, so I gave my engine a good once-over.

 

Stock exhaust, it's not a crackling, I know exactly what you're talking about and my '78 does it quite well

It's a pretty loud popping noise.

 

LOL, so does my RS.. How is your timing? Was the ecm plug disconnected the last time it was timed?

 

Make sure the EGR is functioning properly. The chambers may be getting very hot after lean cruise.

 

Sounds like what Vader is talking about. EGR cools the cylinders down to cut down on the NOx emissions. Also, timing could be an issue. Let us know what you find after you check these items.

 

Yea, I meant to check if there were any codes today, but I got caught up claybarring, spray waxing, waxing and sealing the whole car.

Oops haha, I'll check first thing tomorrow, and let you guys know.

 

Alright, update time.

Pulled the engine codes tonight. Got 2, EGR code and O2 sensor code. However, I also noticed that my speedometer is off. Up to about 25-30, it's accurate, but past that it starts gaining speed, up to about 15mph off around 55.

So, I'm gonna replace my VSS, since I know the VSS relays idle/AF information to the EGR.

 

Which O2 code did you get? Rich or lean?

 

Rich.