Backfire through intake under hard throttle - Forums at Modded Mustangs
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post #1 of 40 Old November 25th, 2012, 01:19 PM Thread Starter
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Backfire through intake under hard throttle

Hey guys. I picked up a 95 GT two days ago, and I've been going through the car fixing small problems. This one is turning out to be a large problem. The car has a BBK intake, and flowmaster cat back. Otherwise stock.

Symptoms:
Drives fine under normal(conservative) driving
It will rev fine all the way to the limiter if you rev it slowly
If you stomp the pedal from an idle it will rev perfectly until 4,000rpm then start "missing?" and backfire through the intake.
If your driving normally and stomp the pedal it will pull fine until 4,000 and do the same
Bogging the car at slow speed in any gear and going full throttle, it will just pull no problems, until...you guessed it, around 4,000rpm, then hesitation, missing, studdering, and popping from the intake.

Things I have done:
New plugs gapped at .049
New cap and rotor
Run premium fuel
Checked timing at 10* advanced to 12* with no change.

Do you guys have any ideas?
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post #2 of 40 Old November 25th, 2012, 02:32 PM
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Try cleaning the MAF, changing the fuel filter, check your spark plug wires for arcing, check the egr, check for vacuum leaks, etc.

All these things are items that should be addressed at some point anyway, so its a good place to start.

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post #3 of 40 Old November 25th, 2012, 08:27 PM Thread Starter
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I cleaned the MAF with spray cleaner, it seemed to help with the hesitation a bit, but the backfiring is still there. I dont have time to work on it between class and homework this week so Im dropping it off to my mechanic tomorrow. They are gonna run diagnostics on the entire car, so I should know something by Tuesday evening.

Thanks for the help stang.
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post #4 of 40 Old November 25th, 2012, 08:35 PM
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I'm going to go with fuel related issue.

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post #5 of 40 Old November 25th, 2012, 09:30 PM Thread Starter
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Me too, or timing advance under load. I don't think it is an internal problem, like a rod, or valve or spring because it runs perfectly until 4000 or so RPM. Even got 21mpg last tank.
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post #6 of 40 Old November 27th, 2012, 07:46 PM Thread Starter
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Ok guys...update

So far this is what has been done or checked:

New plugs Motocraft
New wires correctly ordered via hanes manual MSD
New dizzy cap and rotor MSD
Cleaned Maf with Maf cleaner
No CEL
Ignition module tested high and low rpm ok
Coil tested high and low rpm ok
Fuel pressure tested ranges from 39-45psi depending on load


If you ease the throttle it will run fine all the way to redline
If you stab the throttle in any gear, at any speed, or neutral, it will pull hard up to 4300 rpm then start missing and backfiring through the intake.
It has a slightly rough idle...very slightly rough

Drives fine at any speed in any gear under moderate throttle conditions.

My mechanic says that its floating the valves and recommends a cam, lifter, pushrod, rocker arm, and valve spring swap. But he isn't sure.

Please, any advise will help.

Tomorrow I will clean the EGR.
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post #7 of 40 Old November 27th, 2012, 07:55 PM
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Along with the EGR try checking for vacuum leaks.

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post #8 of 40 Old November 27th, 2012, 08:24 PM Thread Starter
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I will try to check for vacuum leaks as well, but man there are a ton of vac lines on this thing!!! Im gonna try the carb cleaner trick around the intake mani to see if the manifold gasket is ok.

The thing that gets me is that it isn't throwing a code. No CEL. Runs fine till 4300rpm, and even then it runs fine if I gradually get it there, but if I put the pedal to the metal it acts up. Exactly the same place in the rpm range every time!
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post #9 of 40 Old November 27th, 2012, 10:10 PM Thread Starter
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I just read that the engine coolant temp sensor controls something in the ECU. My low coolant light flashes occasionally. Think its worth looking into?
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post #10 of 40 Old November 27th, 2012, 10:29 PM
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Most likely the sensor in the overflow is shitting out. I always unplugged mine.

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post #11 of 40 Old November 27th, 2012, 10:45 PM Thread Starter
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Ok, thanks, but would that cause a malfunction with the ECU and possibly cause my problem.

Its throwing me off that it happens at exactly the same rpm every single time, and no CEL!
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post #12 of 40 Old November 27th, 2012, 10:50 PM
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No. That's just a sensor for recognizing if you don't have enough coolant in your overflow tank. Nothing to do with the computer.

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post #13 of 40 Old November 27th, 2012, 10:52 PM
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See if you can get a fuel pressure gauge with a long hose that you can have a passanger watch while you drive the car. Watch your pressure while driving.

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post #14 of 40 Old November 27th, 2012, 10:58 PM Thread Starter
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I have one here at home. My mechanic told me today they checked the pressure and it was 39-45 depending on the load.
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post #15 of 40 Old November 27th, 2012, 11:04 PM
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Well if he did a driving test already, than that rules that out.

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post #16 of 40 Old November 27th, 2012, 11:05 PM Thread Starter
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Can a bad PIP in the dizzy cause a lean backfire from the intake?
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post #17 of 40 Old November 27th, 2012, 11:10 PM
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Not sure. That thought did come to mind, but I have no idea.

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post #18 of 40 Old November 27th, 2012, 11:13 PM Thread Starter
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I swear it seems like timing to me because it happens at exactly the same point. But now I have noticed the idle getting a bit more rough. Just did the cylinder balance test and got 9 flashes so the injectors are working like they should.
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post #19 of 40 Old November 28th, 2012, 09:11 AM
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I have the same problem...do you unhook your battery?

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post #20 of 40 Old November 28th, 2012, 10:44 AM Thread Starter
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Not sure what you mean Tyler. Do you mean disconnect the battery to clear codes? If so. No I havn't done that yet.
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