EFI questions

[Bluevette]

In the past few weeks my 302 EFI has started to run funny. The engine is a rebuilt 89 GT motor with a c-303 cam and Cobra R intake and computer, MSD ignition T-5 transmission. About 5 or 6 weeks ago it started backfireing when I downshift. I installed new manifold gaskets but saw little if any improvment. Now the car is starting to Idle iratacily and die sometimes when I stop quickly.
I have about 5500 miles on the car and until these problems started I had no problems with the car. It seems to have plenty of power and feels fine when accelerating and cruising but it is really starting to bother me.
Anyone have any ideas?
Thanks

[hssss]

Look at the IAC, it can be cleaned with spray carb cleaner, also for a small vacume leak.

[Dwight]

Have you checked the TPS ( throttle position sensor)? Max voltage should be .997 vdc. What is rpms at idle? Vaccum leaks will also cause the problem you described. I had a 5/16" hose on a 1/4" barb and it took months to find the leak. A couple of good books to buy;

How to Tune and Modify Ford Fuel Injection by Ben Watson isbn 978-0-7603-0503-4 retail $19.95

The Office Ford Mustang 5.0 Tech Ref & Performance Handbook by Al Kirschenbaum isbn 0-8376-0210-6 retail $34.95


Dwight

[Dennis Kelley]

Maybe Header gaskets? Good luck let us know what it was.

later

dennis

[wtm442]

I second everyones thoughts ... vacuum leak and maybe an exhaust leak.

[Rwillia4]

If is is not a vacuum leak...
I would check the O2 sensors. If you ran low lead gas or other funny additives you can trash the O2 sensors quickly which are a major player in the correct fuel/air ratio. When they are bad the idle is typically the first thing to go, by not staying steady and progressively surging more. When bad they usually make the engine run rich, which could cause the backfiring (but there needs to be "spare" oxygen getting in to cause ignition). I think you maybe chasing more then one problem as previously stated.