Backfiring Problems!
 

I have an 87 GT with speed density, GT40 heads, Cobra Intake, 65mmThrottle body, Roller rockers, long tube headers, flowmasters, 3.55 gear. When I accelerate I get a heavy backfiring sound immediately after I have changed gear at low RPM's. It happens in second and third gear and sometimes in fourth. It's driving me crazy! My local speed shop says it could be (1) loss of vacuum pressure (2) O2 sensors are inoperative, or (3) Convert to Mass Air. I don't have much faith in my local shop because their first attempt to remedy the problem was to change my fuel pump to a larger one. $300 Canadian dollars later she's still Backfiring! Please help, what should I do? Is my engine in jeopardy because of this?

Okay, first thing's first. I don't know what "vacuum pressure" is (sorry, but that statement makes NO sense). If the shop is referring to MAP being high or low, that's one thing. Having the aftermarket parts on an SD vehicle makes it difficult to pinpoint. You DO have the stock cam, right? Are those rockers 1.6:1 or 1.7:1?

If the EGOs (oxygen sensors) are bad or not swinging as they should, or even if they're fixed, there WILL be a trouble code for it. I'd pull the codes and post them here, if I were you.

On the other hand, I think you should just convert to mass air and be done with it. I did so on my '87 notch, and have never even once looked back. BTW, my car has an automatic, and before the conversion would knock horribly when above 3500rpm or so (sounds *similar* to your case), regardless of timing (within reason) or octane. It was mostly due to the inaccurate calculation of load caused by using 3.73 gears. Everything is right as rain now, and the mileage has gone up! FWIW, this car has almost 150,000 miles, still has great compression, and uses maybe a quart between changes (less since synthetic).

You might want to look into that fuel pump and/or fuel pressure regulator, though. I believe you have a lean condition brought on by:
1. Improper load calculation,
2. A vacuum leak (pretty common),
3. Lack of fuel delivery (pump/regulator),
4. Excessive EGR flow,
5. Timing is too retarded (a few shops STILL don't know how to do it right on EEC-IV systems!), or
6. The ubiquitous air pump check valve.
If I missed anything, it's because I've been up for quite some time, and I missed my nap.

Hope this helps. Good luck.

Thanks for the help Capri306. I have a stock cam, 1.7 rockers, 88,000 miles, I also use a quart(synthetic) between changes. I know my mechanic pulled all my pollution control. He has advanced my timing and added a 160 degree thermostat(which I feel hasn't helped my car any) if anything it's running worse. Are there any reputable Mustang speed shops in the Detroit area that can help? Plus, I do want to convert to mass air. Once agin thanks for your help and sorry to keep you up so late.

No prob, dude. Glad to help fellow 'Stangers out.

That's pretty normal, really. Even for a brand new vehicle, that's well below the limits of worry.

No doubt it is! You REALLY need to find a TRUE speed shop. It really sounds like they have NO idea what they're doing. In fact, get that 160 T-stat out of there ASAP! It's doing great harm to your engine, as low temperatures greatly increase wear on an engine and do squat for performance. If anything, colder T-stats can HURT performance, as you've already seen. The computer will take longer to get into closed loop -- or worse, never will -- leading to driveability problems. Also, you're not getting the full timing advance you think you are if the temperature isn't where it should be. I've been using a 197* T-stat for a long time now, and have seen nothing but gains from using the stock temperature.

Removing your emissions equipment is another BIG no-no in my book, and really set off some red flags for me. Anyone who truly knows EEC-IV and getting performance out of it knows that the emissions equipment is part of the performance puzzle, not a blocker to it. Your "mechanic" should go back to school and learn a few things about electronic engine controls.

FYI:
EGR only comes on during cruising and light acceleration (NEVER during WOT blasts, cold strategies, or idling). EGR actually lowers combustion chamber temperatures, and disabling or removing it just hurts mileage and the longevity of the engine, not to mention the constant "Check Engine" light and hard times troubleshooting EEC codes. Did I fail to mention that removing some things will cause the computer to go into what is called FMEM mode? It's where is sees a total failure of a sensor and substitutes a known "good" value for it during calculations. That substitute is only an approximation at best, and only worked during engine operation under one single condition. Try to get the EGR back to working order, and be SURE to have the two little coolant lines going to the throttle body put back on if they were detached.

Now as for the Thermactor system (Ford speak for air injection/air pump). It's only absolutely necessary (really) if you have catalytic converters on the vehicle. Its sole purpose is to divert and pump air to the correct location and at the correct time to do 3 things: 1. Get the cats up to light-off temperature so they start working, 2. Divert the air to the atmosphere if conditions dictate (prolonged idling for example in some systems) to prevent them from overheating, and 3. Help the catalyzation process by injecting air between catalyst beds. If you don't have catalytic converters, it's not a BIG issue sicne cats are the only reason it's there, but you WILL run into trouble codes if everything's not there.

If he cut anything else like the charcoal canister or PCV lines, I'd start drop kicking his butt in the parking lot. ;)

Oh you bet! You're in Motor City for cryin' out loud! Okay, if you want some of the better shops (and I'm sure I'll be helped on this....) I'd try Alternative Auto first and see if you can't get some help from Lidio. He's a GOD of automotive tuning and performance.

As far as mass air conversion, you can do it yourself very easily. I did it with a MAF sensor and computer from a friend who stripped a crunched 'Stang, and a wiring harness from an Escort at a junkyard (hey, always knew 'scorts were good for something!). I followed the directions given at corral.net's tech article on MAF conversion. A very interesing article. If you DO end up going to a junkyard, be sure to snip a few extra EEC-IV pin connectors while you're there. Trust me.

Hope this helps! Good luck. :)

Hey Capri thanks for all that smog stuff knowledge. I had no idea about alot of that EGr stuff. Not like i can remove my smog stuff out here in cali anyways but its good to know :)

Np, dude. I like infecting everyone with knowledge. ;) Get this, I live in BFE, Michigan. There are NO emission laws here whatsoever, but for some reason I still find the concepts envolved with emission control fascinating. I have to give props to one of my auto-tech profs for turning me on to that stuff.

Just to show you how much of an automotive engineering dweeb I am, I do "experiments" with my carbureted 4-bangers just to see how much mileage I can squeeze from them. I brought an '85 LX up to 32mpg, and my record-holder is my current '84 "winter-beater" 626 is running 35-36mpg, and climbing. It's nice getting 500+ miles to a tank when you're not burning the high-octane stuff in the summer through the 'Stang. I MISS DRIVING IT ALREADY!!!! :( :( :( I guess it's an even tradeoff, really. :rolleyes: FYI, I'm working on intake temperature vs. mixture effects right now.