Rough Idle and low rpm misfire - Page 2
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Old November 30th, 2009, 01:43 PM   #21
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A second test with different multimeter comfirms that the alternator is putting out approximately 13.6 volts with ac on full.
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Old November 30th, 2009, 02:27 PM   #22
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I also just noticed a sound coming from the rear of the engine. Comparable to a japanese motorcycle at idle in tone, but not volume. maybe a vacuum leak i cant see?

Another idea that crossed my mind is maybe a clogged catalytic converter.

Last edited by Gamble812; November 30th, 2009 at 08:33 PM.
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Old December 9th, 2009, 08:59 PM   #23
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My car (with a similar problem) is under the knife atm. My mechanic said hes thinkin its the front o2 sensors. I would definitely check into those. When I get it back I'll let you know exactly what he did to fix it and hopefully then for you, you can do the same to fix it.
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Old December 10th, 2009, 01:31 AM   #24
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yes those are pretty low valuse for a good running alternator. It should be putting out between 14.0-14.6 volts with a load on it. The alternator is causing either a complete misfire or a low spark and thats what causes the studdering and backfiring
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Old December 10th, 2009, 01:33 AM   #25
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when my car was doing this i tried changing the spark plugs and dong everything i could think of and still nothing. Try doing a diagnostic scan on you odometer. i also thought it could be my off road exhaust causing the o2s to send a code throwing my timing off so i tried the spark plug non fowler trick and still didnt fix it

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Old December 10th, 2009, 01:34 AM   #26
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your odometer can tell you trouble codes your car is spitting out
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Old December 10th, 2009, 03:36 AM   #27
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I would think the fuel injectors at low rpm they are not putting out enough fuel but at higher rpms the pressure is enough to cause it to flow properly. Fuel injector cleaner is not gonna fix that problem. have is scanned to see if the injectors are pulsing correctly. My buddy is having the same issue but i can't get to it till the 13th of dec. I will post when i fix the issue.
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Old December 10th, 2009, 03:44 AM   #28
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also forgot check intake again you can use a dye to look for a leak you could have a leak under the intake that your not aware of.
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Old December 10th, 2009, 06:29 AM   #29
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A few questions:

Did you or a mechanic change the intake manifold?

Where the O-rings replaced on the injectors?

Was one of the injectors dropped accidentally?

Was die-electric grease placed on the wiring connector for the injectors and coils?
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Old December 10th, 2009, 06:43 AM   #30
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check vacuum, then check/clean your COPs. My car had a problem that sounds very similar, I noticed my cops were dirty when I changed my plugs and half assed cleaned them and I have not heard the miss in a while. Not sure if it cured the problem but its a start.
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Old December 10th, 2009, 10:25 AM   #31
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i checked all that stuff too when my car was doing it and it didnt fix shit you have to replace your alternator because its not giving off a good enough spark to run the engine at idle or lower rpms
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Old December 15th, 2009, 03:37 PM   #32
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i replaced the intake manifold myself. I did replace the o-rings and i made sure I did not drop them or get them dirty at all. I was very careful about that. I also replaced the COP's in the last few days with no improvement. I bought a little handheld computer that gives live PCM data and graphs the data and whatnot. The driver side o2 sensor had a nice little sine curve that tightened up as i accelerate. The passenger side o2 sensor had a rough sine curve that turned into garble BS when i accelerate. Also the long term fuel trim on the passenger side did not change hardly at all when accelerating, while the driver side LTFT varied greatly. However, the driver side LTFT sits right at 25% when at idle while the passenger side is around 0%, varying just a tad. When i plugged the thing into my brother's trans am, both o2 sensors had an identical sine wave, and the fuel trims fluctuated at identical intervals. both fuel trims sat around 0% at idle on his car.

I dont know much about fuel trim or o2 sensors, but it seems to me that this could have something to do with my problem. If anyone thinks it may help, I think i can actually upload the data to my computer and post it on the forums.

Last edited by Gamble812; December 16th, 2009 at 12:46 PM.
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Old January 13th, 2010, 09:45 AM   #33
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Update: Not the front o2 sensors. Replace them and it fixed the fuel trim issue i was having, but it still has the rough idles and misfire through mid range rpms. so here's a list of what its not:

spark plugs
COP's
o2 sensors
MAF
TPS
EGR
alternator

I guess at this point it can only really be a vacuum leak or dirty/bad injectors?
Whats the best way to test for a vacuum leak? And where can i get the equipment for it? and how much does it cost?

A guy at autozone said he saw the same problem with a mustang gt a few weeks ago and it ended up being a small imperfection in the headgasket causing a slow leak. This is the LAST thing I wanna go and try, so any suggestions or opinions on this?
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Old January 13th, 2010, 12:26 PM   #34
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swap injector with another one, like on opposite bank, and see if problem follows.pay attention to lower o-ring. have you done a compression or leak down test?
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Old January 13th, 2010, 01:25 PM   #35
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im having this Legendary missfire problem also. but on my svo.. im going to test for leaks, and the only way i had come to comclusion was cap of the TB. while at idle and listen for hissing. or if it keeps running then i know i have a major prob. And with your injectors.... i do not believe in injector cleaner when your running 93 octane gas.... think about it. i used gasoline to clean my parts on my car.. and it worked like magic. you really think the injector cleaner will work??? idk that's just . i had a prob with my injectors on my 302 in my bronc. got em supersonic cleaned (if i remember what its called) for like $13 each injector. and omg what a difference! ran soo much better.

also i was thinkin maybe test your ignition coils??? i havnt worked on your year mustang, but one thing that you can add to your list of "things they are not" test your coils. get a DVOM and measure resistance. compare it to specs and see what happens. a weak spark could cause it.


i can get you them specs from SomeData right now. hold on.
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Old January 13th, 2010, 01:36 PM   #36
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I tested the coils and they are within specs. I even bought a new coil and swapped it around all the cylinders one by one with no improvement. I was thinkin the same thing about the injectors. Always run 93 through em so no point in using injector cleaner.

Did a compression test and all cylinders were good.
fuel pressure test was also good.

I don't even know where to go from here.

also, besides throttle, what affects short term fuel trim?
Cuz now i have a stft problem on one side.

Last edited by Gamble812; January 13th, 2010 at 01:44 PM.
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Old January 13th, 2010, 01:43 PM   #37
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if you are interested there's a TSB for this missfire that shows IN DETAIL ways to go about it. but PM me cuz im not puttin it on here.


***TSB- Technical Service Bulliten (if you didnt know.)
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Old January 13th, 2010, 01:43 PM   #38
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Originally Posted by Gamble812 View Post
I tested the coils and they are within specs. I even bought a new coil and swapped it around all the cylinders one by one with no improvement. I was thinkin the same thing about the injectors. Always run 93 through em so no point in using injector cleaner.

Did a compression test and all cylinders were good.
fuel pressure test was also good.

I don't even know where to go from here.
What makes 93 octane cleaner than 87? actually you will find more additives in 93. 93 will varnish up injectors as easily as 87. fuel inj. cleaner removes this varnish that is put there by the gas/additives so you should use a inj cleaner every now and then, 93 is not self cleaning.

have you swapped injectors?
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Old January 13th, 2010, 01:46 PM   #39
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I did not swap injectors. That would do nothing as the car is throwing no codes so there would be nothing to follow.
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Old January 13th, 2010, 01:48 PM   #40
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Originally Posted by Gamble812 View Post
I did not swap injectors. That would do nothing as the car is throwing no codes so there would be nothing to follow.
......The Miss to that cyl..........
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