needs help with backfiring
 

Hi, I'm new to this site and I just bought a 5 speed 89 Mustang GT. The car seems to run fine and strong doing normal driving but everytime when I try to step on it under hard acceleration (above 4000 rpm), the exhaust starts to make some sputter noise, the engine hesitated and lost power instantly. It seems like a backfire from the exhaust. If not, What can it be? The car have the following modifications: 65mm throttle body, 70 mm mass air, mac cold air, pulleys, 1 5/8 shorty headers, mac pro chamber, 2 1/2 mac catback and 3.73 gears. Any help or advices will be greatly appreciated.

Re: needs help with backfiring
 

It could be that a fuel injector is bad. I had the same thing happen with my 90, but it usually happened under load. The dealers have some way of flow testing the injectors without major work (they only charged me an hour of labor to do it). It was never a loud backfire, but it was a popping noise coming from the exhaust.

Re: needs help with backfiring
 

An Actron CP9015 code scanner can help you out.
In addition to the Key On Engine On test, you can further diagnose a problem by running the cylinder balance test(this tests each injector individually while the car is running and will tell you if it is malfunctioning).

I have become a big believer in this diagnostic tool. It doesn't always tell you everything, but it sure beats guessing,

good luck...it's $30 at Sears or any parts store...

Re: needs help with backfiring
 

Check your fuel pressure. This sounds like a classic case of a failing fuel pump and/or regulator. Yes, leaning out causes backfiring.

Do a little Googling, and you can find out how to perform a cylinder balance test. All you need is a paper clip and about five minutes of your time. Good luck.