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  #1  
Old 05-12-2006, 08:39 PM
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Misfire/backfire on heavy acceleration

I did a search, couldn't find similar situation.
97 Ford F150 XLT 4.6L 4x4
Cold air intake
ECM reprogrammed with power programmer
Engine was replaced 2 years ago
Plug wires and plugs replaced 20K miles.

Here's the problem:
Heavy accleration - Misfire, backfire/poppingthrough intake beginning around 4000 to 4500 RPM and hesitation. After hot day - hesitation/misfire begins about 3300 RPM. Starts no problem. Idles no problem.

No engine codes or engine light.

Found a loose plug wire on right side coil pack 2 weeks ago after problem started. Pulled wire today - black carbon buildup on one edge of that plug wire and base of coil back where wire plugs into pack.

Replaced fuel filter last weekend. Fuel pressure at rail increases during throttle up. Idle pressure around 15 to 20 psi. Immediately increases to 40 PSI or so on throttle increase.

Any Ideas?
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  #2  
Old 05-12-2006, 09:06 PM
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15-20 psi KOER is low. I like to see 40-45 psi KOEO or running. Do you have access to a scan tool to where you can see what your long and short term fuel trims are, and what your BARO reading is?
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Old 05-12-2006, 10:19 PM
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boy your idle pressure is low. and when the fuel pressure increases when reved would show that the fuel presure regulator can raise the pressure up to 40,still low, tipicly 10psi higher than idle. double check your idle fuel pressure then remove the vacuum line from the regulator and it should rise about 10 psi. should be about 40-45psi at idle without looking up in bok. If idle is in the teens and removing the vacuum line can give you fourty than I would say the regulator is bad. My initial thought was a dirty MAF sensor but you got a little out of spec fuel pressure readings. If easy a deadhead pressure could be nice.
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Old 05-13-2006, 12:13 AM
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My idle fuel pressure was around 30 psi before the new fuel filter. I may have had air in the lines when I checked it right after replacing the filter, but dble checked it just now...28 to 30 PSI at idle. increase to 32 on throttle increase, back to 28 and level at 30 after returning to ide.

Removed the vacume line - 38 to 40 PSI. Good vacuum as well.

My code reader is the superchips. It reads and clears TC's and programs the ECM. Will not read any of the sensors.

I also cleaned the carbon from the coil pack and plug wire.

Could I have a fouled plug from the loose plug wire on the coil pack? Could a fouled plug cause this w/o a TC? (#1 cylinder)
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Old 05-13-2006, 01:40 AM
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Disconnect the MAF, drive it and see what happens.
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Old 05-13-2006, 08:52 PM
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Quote:
Originally Posted by Quintin
drive it and see what happens.
That's the same line that got me a 2nd child.

Well I messed with the right side coil pack, cleaned off the carbon off the #1 plug wire, now I have a miss at idle that was not there before. No codes yet.

What should the outcome be by disconnecting the MAF? The truck should die right away, shouldn't it?

Anyone want a truck, cheap?
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Old 05-13-2006, 08:54 PM
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If the MAF is contaminated or giving skewed readings, disconnecting it will tell the PCM to go back to a known good set of parameters to run off of. If it does okay with the MAF disconnected, then you probably need a mass air sensor, or you can try cleaning it to see what happens.
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Old 05-13-2006, 11:20 PM
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Disconnected MAF. Idled fine. Would not run above idle. It stalled as soon as I gave it any throttle. I let it run for about 5 minutes.

I cleaned my K&N when this started as well. I had the truck running with the filter off to dble check air flow. The truck stalled anytime my hand got near the bottom edge of the air intake tube.

Funny thing about this whole thing is my gas mileage has improved a little.

I just got back from the store. The thing runs smooth at 1200 to 1500 RPM at 65 MPH - no problem. I can't feel any bumping or surging. As soon as I try to accelerate, it surges. Full throttle, misses, surges, and begins backfiring through intake at 3800 - 4000 RPM like clockwork. A local shop says thats an indication of lean conditions.

MAF maybe not doing well, or the other sensor in the tube?

Thanks for the replies so far. This has me stumped. Please give some advice...What Next?

Last edited by az97xlt; 05-14-2006 at 10:45 AM.
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  #9  
Old 05-14-2006, 02:11 PM
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Throttle Position Sensor

Anyone know if this is accurate:
According to the service bulletin 98-23-10, the truck should run and revert back to the throttle position sensor/RPM to maintain fuel mixture and drive ability with the MAF disconnected. The truck didn't. Any throttle application, it died.

Could a bad throttle position sensor be causing my problems? If so, is there a way to check it without a scan tool?
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  #10  
Old 05-14-2006, 03:56 PM
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have you checked the fuel presure while it is backfiring
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  #11  
Old 05-14-2006, 07:54 PM
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Yes. no detectable drop in fuel pressure. At the driveway, I only have a VERY SHORT window to see it. It does not misfire at holding at high idle, only during power up, and under load.

On the road, I can maintain the behavior for a couple of seconds. I don't have a pressure gauge that I can read while in motion.
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  #12  
Old 05-14-2006, 10:38 PM
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Try removing the power programmer software. I bought a Power Programmer III (Hypertech) last spring and had horrible misfire under hard acceleration in the 93 octane mode as soon as I installed it. I replaced my plugs and wires (1999 F150 Supercab 4x4,4.6 liter, 82000miles) which I believe were the originals (I bought the truck used) and noticed that the misfire went almost completely away. At WOT it was still there around 4500rpm's, and it was also very prevalent when towing my boat, even at lower rpm's.

When I removed the PPIII software completely, all misfires went away. Currently I am using it again in the 87octane tune and find that when towing or accelerating hard I still hear some detenation unless I use high octane fuel.

Just my two cents,
Mike
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  #13  
Old 05-15-2006, 11:22 PM
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Went back to stock - no change
disconnected MAF - stumbled tried to stall on acceleration/pumped gas and accelerated the same.

What next?
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  #14  
Old 05-17-2006, 01:50 AM
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Hmmm, If I had to frigg this long for a problem, I'd be spending money. OEM plugs & wires to start with, then the coil pack, no matter how clean you got the terminals.
Could this be an ignition-related problem ? When my buddy put a blower-laden 3.8L V6 in his Fiero, he had a similar problem. Needed a higher-output coil to fire properly at high rpm & high throttle opening. Never missed or backfired after that.
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  #15  
Old 05-21-2006, 03:54 PM
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Thumbs up Thanks for the help

Its fixed. I didn't want to throw money into it searching for a fix. I HATE just changing parts until it works. I replaced plugs, wires and coil packs (threw money and parts and hell it worked). Runs like new.

Thanks for the assistance and ideas.

Last edited by az97xlt; 05-21-2006 at 03:57 PM.
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Old 05-21-2006, 03:54 PM


 
 
 
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