Rough Idle and low rpm misfire
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Old November 25th, 2009, 10:45 PM   7 links from elsewhere to this Post. Click to view. #1
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Rough Idle and low rpm misfire


I've been struggling with misfires ever since my intake manifold cracked. I replaced the manifold and still have problems. The misfire occurs during idle and low to medium rpm load, but not during WOT. It feels like shaking and sputtering.

Plugs have been gapped at .54, .56, and .60 with similar if not identical results. They are currently at .54.

The plugs have been replaced three times. The first time I replaced them, the misfire was gone and the car performed perfect. I took the driver side boots out after a short drive to see if there was any fluid down in the holes (this was actually BEFORE I replaced the manifold) and when I put them back in, the car misfired again. I took the second driver side boot out and repositioned it and put it back in. The car ran perfectly again... for about 3 days. Then it misfired again.

Since then I have replaced the intake manifold, the spark plugs, and the coils, paying special mind to that second coil and plug. I've repositioned it, wiggled the wires, replaced the coil and boot three times, and replaced the plug several times. The misfire won't go away this time.

I use only 93 octane and have been running fuel cleaner every time I fill up to clear up any clogged injectors.

I've also checked all readily reachable vacuum lines.

EGR is working properly as far as I know (I put a vacuum on the solonoid and it stalled out.)

Nothing is working. I don't know what to try anymore.
My mechanic can't figure it out either.
The problem only seems to get worse.

My car is a mostly stock 03 Mustang GT with 120,000 miles and FLowmaster mufflers (just got the mufflers yesterday so it has nothing to do with the problem.)
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Old November 25th, 2009, 10:59 PM   #2
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Welcome to the forums. Did it throw a code?
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Old November 25th, 2009, 11:34 PM   #3
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I had a friend with a similar situation and it turned out to be one bad spark plug wire. No way of knowing (to the eye) until he put new wires on there. You already did that much just try a new set of wires.
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Old November 25th, 2009, 11:55 PM   #4
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We dont have plug wires on new edge cars.

From what I can tell from searching 3 or 4 forums this is a common problem with the 99-04's. No one really has an answer. Some guys have full tuned, blown, built motors and still have the slight miss around 1500 rpms.

Some people say its the tune that ford put in, but theres other people with new tunes that still have the problem. So far I've never seen anything bad come out of it, mines done it since I bought it also. I'll look around for a few of the thread I was talking about and post them in here later.
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Old November 26th, 2009, 12:14 AM   #5
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No codes thrown.
Also forgot to mention I cleaned the MAF.
It didnt used to misfire.
It performed like a dream when I got it.
I'm thinkin bad wiring somewhere since the one reposition set it straight for awhile.
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Old November 26th, 2009, 12:43 AM   #6
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Is my engine supposed to miss at low RPMs? - Mustang Forums at StangNet

99 GT bad miss at 2000 rpm - Mustang Forums at StangNet

Low RPM miss? No codes showing... - Mustang Forums at StangNet

Question about MAF & Baro - strange miss here & there - Mustang Forums at StangNet

Thats just a quick search on stangnet cause I had it open in another window
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Old November 26th, 2009, 09:38 AM   #7
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Thanks for the links, Slonlow. There's some good stuff in there. One popular solution was to replace the 02 sensors. Maybe I'll try that next.

Update: I popped the hood after a drive this morning and it looked like the first driver side fuel injector had gas on it while all the others looked dry. If the injector is clogged, could it overflow up out of the injector and past the o-ring like that?
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Old November 26th, 2009, 02:06 PM   #8
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Mine does it to sum days it'll run good while others ittll miss fire or spudder at low rpms when I get in it it seems to have all of it's power, iv changed plugs coils traced wires o2sensors ect.ect. It's drivin me crazy!!!
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Old November 26th, 2009, 05:55 PM   #9
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ani ideas for fixes or sources of the problem?
What all can cause a misfire?
And any answer to my injector question?
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Old November 26th, 2009, 07:21 PM   #10
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Well, to be honest, if you want a real answer you should get a wide band 02 sensor and a tuner so that you can data log.

That would tell you your issue real quick.
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Old November 26th, 2009, 07:44 PM   #11
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Originally Posted by HMUSN View Post
Well, to be honest, if you want a real answer you should get a wide band 02 sensor and a tuner so that you can data log.

That would tell you your issue real quick.
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Old November 26th, 2009, 08:09 PM   #12
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Ive been having th esame problem for forever. Ive taken my car to my mechanic 6 times and he still cant figure out the problem.

So far Ive ran several bottles of fuel cleaner, mechanic did a fuel system flush, 8 new plugs, replaced a clogged injector, and I think he retuned the computer or somethin...Also replaced a sensor...

It threw a code for the #8 injector and we replaced that, but it still has the same problem with no code...

It just misses and sputters...it sucks lol
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Old November 27th, 2009, 11:59 PM   #13
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okay. i guess i'll try the wide band o2 sensor and tuner when I get the cash.

Any input on the injector issue? could that be a probable cause of the misfire?
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Old November 29th, 2009, 12:47 PM   #14
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If you have a clogged injector, that might cause you some problems.

I'm having the same problem as you. I thought it might be my injectors so I ran some injector cleaner through my car along with some good 93 oct gas. I put two bottles of slick 50 fuel system cleaner in. That didnt work...I guess theres more to it than my injectors.

It might be worth you trying it though. Its about 12 bucks a bottle. You might have some build up and that would clean it out.

Worth a shot!
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Old November 29th, 2009, 01:43 PM   #15
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I just noticed a new symptom. when i yank on the throttle, it hesitates and makes an air sucking noise before actually revving. its a real quick thing. maybe a quarter of a second. but definitely noticeable. The air sucking is the air intake, but i never noticed this before, and i never noticed the hesitation before. is that normal?
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Old November 30th, 2009, 08:21 AM   #16
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Idle air control vavle possibly?
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Old November 30th, 2009, 09:06 AM   #17
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Did you pull the injectors and look at them? Try cleaning them outside the car. Did you try replacing the boots?
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Old November 30th, 2009, 09:41 AM   #18
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IAC is working properly. I have not looked at the injectors. That can be a messy job. I'm not even sure how to tell if they are clogged or dirty or working properly. any tips?
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Old November 30th, 2009, 10:09 AM   #19
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Mine did this too. Worked fine when i bought the car and then all of a sudden it started to misfire and sometimes it wouldnt start and t would backfire on startup. It pissed me off because i would never know when it wpuld do it and sometimes it would go away for like a month and then come back. Turns out it was my alternator. I took it off and had it checked and it wasnt putting out enough voltage. I replaced it and havent had a problem since. Try turning your ac on all the way and then checking your battery and the alternator should be putting out around 14.1-14.4 volts.
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Old November 30th, 2009, 01:27 PM   #20
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Thanks for the tip! I tested the alternator and it was sporadically pulling values from 13.0 to 13.7 volts. Are these values too low? Could this be my problem?
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