I was given a 1993 caprice and when I got the car it wasn't running. I changed the fuel pump and filter and it would start and idle. But give it gas it backfires through the intake. Changed the cap, rotor, icm, coil, wires, plugs, injectors, rebuilt TB, changed coolant temp sensor, o2 sensor, air filter, thermostat checked timing and it still does it. Doesn't idle high indicating a vacuum leak either but I won't rule it out. I'm trying to give this car to my brother who fell on hard times. Car only has 91k on it and I'm at a loss as to what else it could be. It pops less once it is warmed up.

 

A smoke test to help find something if a vacuum leak is there.

Firing order is right and not off on 2 plugs??

Compression check could show something too.

Does the engine run a MAF sensor or just a MAP?

 

Check the vacuum lines for cracks.

Sounds like a lean pop.

 

It only pops at part throttle under load. Ill.check the firing order again and see.

 

The only weird thing I have noticed is that the coolant temp gauge is saying it's right in the middle but on live data it shows the ecm is reading -40 degrees

 

Trace the wire and make sure the ECM is getting a good signal.

Some of those older engines had 2 sensors. 1 for the gauge and 1 for the ECM.

 

Found the issue. The owner before had changed the cap in an effort to make it run. I followed his wires 1 by 1 and kept 5 and 7 swapped. Lesson learned. Always double check the firing order.

 

Seen it happen before, figured I'd mention it atleast lol.

I'd still look into that coolant temp dealio though, the computer shouldn't be seeing -40 coolant temp.

 

If that's the case you coolant temp sensor is bad as well.... or disconnected, broken wire etc. On the trucks and I think the cars too as it didn't matter the temp probe on the intake manifold is the coolant temp for the ecm and the one in the drivers side head is for the guage on the dash. I know you said you fixed it but if the ECM isn't seeing temperature it still won't be right.

 

Changed the sensor and got that to work. Car is running really smooth. Just a few more minor things to fix and I can get this car back on the road.