IceBlue91z28

My car has been backfireing lately after hard acceleration and I have absolutely no idea why. I replaced every ignition compunent, fuel filter, and even throttle position sensor and still nothing. I stopped all exhaust leaks and still nothing. I dont think my timing is off because my car runs pretty much perfect so i have no clue what is wrong. I believe vaccume leaks can cause this but where would my vaccume system leak to cause backfire and how?

Morley

Sure can. With a large vacuum leak you'll be running very lean and get a "lean backfire".

Vader

As usual, Morley is right. Any vacuum leak can contribute to backfire. Beyond the normal suspects like gaskets and hoses, you should also check the PCV system, brake booster, and EGR.

It's also possible that the mixture is lean after deceleration and the chambers are hot. Another reason to check the EGR system. You might also want to test the output voltage of the MAP sensor to make sure the ECM is getting a true picture of what's happening. A resistive connection will send a lower voltage signal from the MAP to the ECM, which will interpret that as a lower absolute pressure (higher vacuum) and trim back the fuel pulses.

IceBlue91z28

Thanks guys. Im going to check all that stuff today and maybe get a new MAP and see what happens.

BOTTLEDZ28

iTrader: (9)

Vader what would be the correct voltage for the MAP sensor? 12V? also out of the 3 wires onit which one gets read

IceBlue91z28

How would I go about finding a vaccume leak if I even have one? Theres so many hoses to check i dont know where to start. I have seriously tried everything to stop this backfire and I have no idea what is causing it. It just sort of started to happen one day and hasnt stopped since.

Morley

From your name we'll assume you have a TPI engine? Not so many hoses there. First, check all of the hard plastic vacuum lines, these are prone to cracking and breaking from heat and vibration, make sure they are all connected by their rubber couplers. Start in one place, like the vapor canister and slowly work your way around the enginebay, checking all the rubber hoses and any plastic parts they connect to especially. Also check your PCV valve and breather line on the opposite side.
Next, get a can of starting fluid (or even carb cleaner) and start the engine, carefully spray the fluid along the runners and intake manifold and EGR valve (use the straw), if the RPM changes when doing this, you found your leak.

90Formula-X-F

Be looking for a diverter ( air management ) valve problem from the smog pump. It's more than likely dumping air into your headers causing it to detonate unburnt fuel. Cause could be anything from vacume hose or hoses off the valving to bad valve, bad check valves and even engine temp being read from the computer incorectly never sending the car into closed loop.

I would recomend disconecting the hoses that lead to the check valves. see if it is dumping into the headers at idle. is exhaust coming back up through the valves ? More than likely you'll find that a check valve has burn't out and let hot exhaust into the air management valve and messed it up. If so replace all the older check valves and the air management valve.Good luck....