Hey Guys,

My 86 mustang loses power and backfires under a load. In neutral it revs through rpm range fine. Take it out on the street and throttle
on it, starts to sputter a bit and when the boost starts to come in, it loses power and backfires. Replaced plugs, ignition coil (MSD streetfire coil),
found and fixed vacuum leak by MAP sensor. Fresh fuel in tank(93). Timing good. Fuel pressure 40psi at idle. Car still runs when MAP unplugged.
I cant figure out what is causing the problem.

Also, recently replaced valve cover gaskets, did not replace upper intake gasket (it looked good)

Engine mods are- 66mm turbo, 255lph in tank pump, 75mm throttle body, 24lb injectors, MSD coil, MSD cap and wires, Aeromotive fuel regulator, double check valves on MAP, 3 inch to 3 1/2 turbo back exhaust

 

Posted in this thread because I dont believe the turbo has anything to do with it. Car does have a tune. Was running great. Fueled it up one night then the problems started.

 

Forgot, Also have wideband O2 in car.

 

What does the wb do when it happens? My car did that after i loaded a tune with my laptop plugged into an inverter in the car at the same time. Some time of electrical issue. As soon as boost came in the pcm calculated 200 load and went dead missfire rich.

 

Maxes out at 10 right after back fire. Ima change my fuel filter tomorrow. Make another pass and see what happens. I'm running out of ideas.... Couple more things to change, rotor and ignition module...

 

You have a way to datalog it?

 

No. Wish I did. Car was running great, pulling 12.5 on wideband in full boost. Changed valve cover gaskets, got fuel and boom. It takes a dump

 

Btw, youtube wideband sensor test. Sensors go bad and give bad readings.

 

I have a friend witha tweecer for sale

 

I'm interested. Message me about it

 

The backfire sounds like timing, could be caused, or partially caused, by a bad MAP sensor....I've also seen TFI modules go bad and cause similar symptoms.

 

So, as a silly question how stock is the valvetrain? I had an engine that acted like this and the timing chain was horridly stretched out and it had eaten a cam lobe (flat tappet.)

 

What's your plug gap? Could be as simple as that.

 

Total stock valvetrain, gap set at 30

 

It was running great until I put fresh fuel in it. Took off from gas station and bam, starts backfiring.

 

I would look REAR HARD at the fuel, take a sample in a clear jar, let it settle out, see if there's any water, etc.

 

Thats what I was kind of thinking. I see it all the time on trucks (Im a heavy duty tech). I should have some free time this weekend to swap fuel filters, Ill take my sample from there.

 

So pulled the fuel filter today, and nothing in the fuel. Clean and clear. I'm gonna have to assume its timing....

 

Could be bad gas. Time for a drain and replace. There are a couple Mobil stations up here I won't get fuel from any more for exactly that reason. Car started running like garbage after I got gas from them.

Are you running a stock cam and timing chain? 86 is a flat tappet year on a Ford, right? If worse comes to worse that's not a bad direction to look these days. Could have wiped a lobe or be way out of timing. The symptoms you're describing sound a lot like a timing problem of some kind.