91 306 h/c/i hesitation? HELP - Mustang Evolution

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Old 09-12-2012, 11:52 PM   #1
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91 306 h/c/i hesitation? HELP

I have a 91 five speed fox 306 forged pistons afr 165 heads F cam cobra intake lt headers off road x pipe 24lb injectors pro m mass air calibrated for 24 lbs 255lph walbro new calibrated tps voltage checked on mass air. Prob is I can cruise around under 2k rpm fine all day but as soon as I gun it the car starts to backfire and hesitate. I can back off to about half throttle and it will rev out but slowly. I had autolite 3924's in it which fouled out pretty quick. I now have a set of e3's in it (mechanic said they don't foul as easy) any input would be appreciated. Sorry for the rant I just want a drivable stang!!
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Old 09-13-2012, 03:23 AM   #2
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Re: 91 306 h/c/i hesitation? HELP

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Originally Posted by 2003GtVert View Post
I have a 91 five speed fox 306 forged pistons afr 165 heads F cam cobra intake lt headers off road x pipe 24lb injectors pro m mass air calibrated for 24 lbs 255lph walbro new calibrated tps voltage checked on mass air. Prob is I can cruise around under 2k rpm fine all day but as soon as I gun it the car starts to backfire and hesitate. I can back off to about half throttle and it will rev out but slowly. I had autolite 3924's in it which fouled out pretty quick. I now have a set of e3's in it (mechanic said they don't foul as easy) any input would be appreciated. Sorry for the rant I just want a drivable stang!!
You need to check your fuel pressure and timing.
The Autolite plugs should have worked just fine. It sounds like your mech is a plug salesman and not a problem solver.
If the plugs were fouling, then it is due to over fueling, oil consumption, or ignition misfire. Changing to another brand of plug is not going to cure any of those issues.
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Old 09-13-2012, 05:58 AM   #3
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Quote:
Originally Posted by 2003GtVert
I have a 91 five speed fox 306 forged pistons afr 165 heads F cam cobra intake lt headers off road x pipe 24lb injectors pro m mass air calibrated for 24 lbs 255lph walbro new calibrated tps voltage checked on mass air. Prob is I can cruise around under 2k rpm fine all day but as soon as I gun it the car starts to backfire and hesitate. I can back off to about half throttle and it will rev out but slowly. I had autolite 3924's in it which fouled out pretty quick. I now have a set of e3's in it (mechanic said they don't foul as easy) any input would be appreciated. Sorry for the rant I just want a drivable stang!!
Did you have the car tuned at all? I had a similar issue and it ended up the air/fuel mixture was outta wack
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Old 09-13-2012, 10:45 AM   #4
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Re: 91 306 h/c/i hesitation? HELP

Thanks for the replies gents, as far as the plugs go I thought the same thing myself but I didnt want to argue with the mech. As far as tuning goes I have not had it tuned no, but im getting conflicting stories as to whether it should run on the stock ecu or not. I spoke to two reps at AM and they both say it should run fine. Others say especially because of the heads it needs a tune. Either way i'll get it over to a tuner/dyno, Thanks again.
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Old 09-13-2012, 11:26 AM   #5
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I don't think it should need a tune. 94-95 ecu's are more finicky than 89-93. So sometimes they may or maynot. But either way they should at least be able to pin point / resolve any issues.
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Old 09-13-2012, 04:58 PM   #6
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Your air/fuel ratio is all outta wack. The A9L ECU is capable enough to handle the changes you have but remember, the ECU was still programmed to assume a lot of characteristics that are now much different than stock. And when you change injectors, the ECU is tricked into thinking they are still 19# injectors with a mass air meter to match. Do yourself a favor and get a tune. You won't regret it and your car will run like it should.
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Old 09-13-2012, 05:02 PM   #7
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Definately sounds like fuel issues. My 87 gt did the exact same. Cruise around no problem, ease into it and it was fine. The moment you stabbed the throttle it would pop and crack and not go any faster until you eased off te throttle.

Turned out to be a bad fuel pump, the screen was clogged and causing the pump to overheat.
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Old 09-13-2012, 05:54 PM   #8
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Thanks again guys. I'm gonna put some better fans on it to keep it cool and get a tune. I'm running all the gas out of it, I filled it up with gas. Although it did sit for about a year prior to that but the pressure gauge on the rail shows 40 + lbs of pressure. I'm gonna drop the tank and check the strainer just to be safe.
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Old 09-13-2012, 05:56 PM   #9
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Quote:
Originally Posted by 2003GtVert
Thanks again guys. I'm gonna put some better fans on it to keep it cool and get a tune. I'm running all the gas out of it, I filled it up with gas. Although it did sit for about a year prior to that but the pressure gauge on the rail shows 40 + lbs of pressure. I'm gonna drop the tank and check the strainer just to be safe.
What's it show on the gauge when you jam the throttle wide open all at once under the hood? My car was 40 at idle, stab it to wot and it would drop to 10 lbs and hold there until I let off.
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Old 09-13-2012, 06:42 PM   #10
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Re: 91 306 h/c/i hesitation? HELP

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What's it show on the gauge when you jam the throttle wide open all at once under the hood? My car was 40 at idle, stab it to wot and it would drop to 10 lbs and hold there until I let off.
Im not able to watch the gauge while under load (driving) but while standing over the engine and reving it by hand it will still sputter and back fire but the pressure never drops that much.
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Old 09-14-2012, 12:25 AM   #11
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Re: 91 306 h/c/i hesitation? HELP

I think the problem is that the word "Tune" is used improperly 99.9% of the time on the forums here and most other forums.

I agree with dreamstang saying it should not need a "Tune" when the meaning of tune is used in the context of not needing a dyno, aftermarket chip, or aftermarket tuner tuning. The modifications should be well within the capabilities of the stock ECU to handle, which has been proved time and again by many others with similar or even more mods. You need to ensure that your engine timing and fuel pressure is set correctly.

I also agree with aegischief in saying it could use a "Tune" meaning a professional tune and using an aftermarket tuning aid and he is correct about the aftermarket MAF's tricking the ECU. Changing the parameters in the ECU to correspond with the injectors/MAF you are using is without a doubt better than just tricking the ECU. That should not be needed for your mod level, but it is the best way to do it.

Before I went to that expense I would tune it back to stock specifications in respect to the timing and fuel pressure. Your sputtering and backfiring is most likely due to one or the other, or possibly both being out of specs. Make sure your spout connector is good and plugged in properly too. I assume you removed it when setting your timing and then replaced it after you were finished?
Also make sure the vacuum line is connected to the fuel pressure regulator and does not have a hole in it somewhere along the line. To set the fuel pressure (if you have an adjustable regulator), you start the engine and let it idle. Remove the vacuum line and plug it, set the pressure to 38-40 psi, re-install the vacuum line and you should be good to go.

You should not need a dyno, chip or tuner tune for a head change. The only time you really need a tune of that type is when you go with forced induction or nitrous injection.

My 86 GT has been converted to MAF and has basically the same mods as you have listed and idles & runs perfectly with just a normal tune up.
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Old 09-14-2012, 07:29 AM   #12
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Quote:
Originally Posted by Trojan Horse
I think the problem is that the word "Tune" is used improperly 99.9% of the time on the forums here and most other forums.

I agree with dreamstang saying it should not need a "Tune" when the meaning of tune is used in the context of not needing a dyno, aftermarket chip, or aftermarket tuner tuning. The modifications should be well within the capabilities of the stock ECU to handle, which has been proved time and again by many others with similar or even more mods. You need to ensure that your engine timing and fuel pressure is set correctly.

I also agree with aegischief in saying it could use a "Tune" meaning a professional tune and using an aftermarket tuning aid and he is correct about the aftermarket MAF's tricking the ECU. Changing the parameters in the ECU to correspond with the injectors/MAF you are using is without a doubt better than just tricking the ECU. That should not be needed for your mod level, but it is the best way to do it.

Before I went to that expense I would tune it back to stock specifications in respect to the timing and fuel pressure. Your sputtering and backfiring is most likely due to one or the other, or possibly both being out of specs. Make sure your spout connector is good and plugged in properly too. I assume you removed it when setting your timing and then replaced it after you were finished?
Also make sure the vacuum line is connected to the fuel pressure regulator and does not have a hole in it somewhere along the line. To set the fuel pressure (if you have an adjustable regulator), you start the engine and let it idle. Remove the vacuum line and plug it, set the pressure to 38-40 psi, re-install the vacuum line and you should be good to go.

You should not need a dyno, chip or tuner tune for a head change. The only time you really need a tune of that type is when you go with forced induction or nitrous injection.

My 86 GT has been converted to MAF and has basically the same mods as you have listed and idles & runs perfectly with just a normal tune up.
Well said. +1
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