Dreaded random misfire ugh!

Zaner

Member
Jan 5, 2020
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39
Oklahoma
Okay I hate to start another misfire thread but I'm a little lost. I have read and employed advice from countless threads on here without any change in my cars condition. So I'll ask on here. 2001 mustang GT 4.6. It has the misfire but it's not throwing any codes. It appears the misfire in only at idle just hearing the occasional pop sound and it running slightly rough. The second you touch the throttle and give it gas it clears up and runs great at pull strong. But the second you put it in park or you're at idle at a stoplight you can hear the popping sound that sounds like a misfire. I've replaced the fuel filter I've had the injectors cleaned. I've put two different sets of coils on it both Ford and aftermarket. I have put two different sets of spark plugs in it trying different Gaps.I replaced my idle air control underpriced my throttle body I've replaced my plenum gaskets my EGR gaskets, I did a rudimentary smoke test and couldn't see any obvious leaks. I've read some people say they had similar symptoms to me when they had a crack in their plastic intake. My intake on my 2001 have the aluminum crossover so I don't know if mine is prone to cracking or not. this blows my mind not having a check engine light and the car running so good but having the low speed and at idle misfire. I'm even going as far as to go around and check my grounds clean them up and even run extra grounds. I've gone along to all my electrical connectors through out and clean them up and put dielectric grease on them. I just don't know what to do the only thing I can think of is I occasionally a few times a day will get my battery light will come on on the dash. It'll stay on for a few minutes and then it turns off. Is it possible the alternator is causing my misfire at idle?
 
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Zaner

Member
Jan 5, 2020
15
1
13
39
Oklahoma
Okay I hate to start another misfire thread but I'm a little lost. I have read and employed advice from countless threads on here without any change in my cars condition. So I'll ask on here. 2001 mustang GT 4.6. It has the misfire but it's not throwing any codes. It appears the misfire in only at idle just hearing the occasional pop sound and it running slightly rough. The second you touch the throttle and give it gas it clears up and runs great at pull strong. But the second you put it in park or you're at idle at a stoplight you can hear the popping sound that sounds like a misfire. I've replaced the fuel filter I've had the injectors cleaned. I've put two different sets of coils on it both Ford and aftermarket. I have put two different sets of spark plugs in it trying different Gaps.I replaced my idle air control underpriced my throttle body I've replaced my plenum gaskets my EGR gaskets, I did a rudimentary smoke test and couldn't see any obvious leaks. I've read some people say they had similar symptoms to me when they had a crack in their plastic intake. My intake on my 2001 have the aluminum crossover so I don't know if mine is prone to cracking or not. this blows my mind not having a check engine light and the car running so good but having the low speed and at idle misfire. I'm even going as far as to go around and check my grounds clean them up and even run extra grounds. I've gone along to all my electrical connectors through out and clean them up and put dielectric grease on them. I just don't know what to do the only thing I can think of is I occasionally a few times a day will get my battery light will come on on the dash. It'll stay on for a few minutes and then it turns off. Is it possible the alternator is causing my misfire at idle?
Also had the battery tested and it was solid..and the battery connection are clean and tight
 

wmburns

SN Certified Technician
Aug 14, 2009
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I'm dealing with a similar problem right now on my Romeo swapped 2000 GT. Slight misfire at idle but the motor makes good power while normal driving. I believe that an ignition problem has been ruled out. Yet the ODB2 Mode6 data shows a low level (0.244) misfire in cylinder 1, 2, and 4. The misfire stays on the same cylinder no matter what is changed.

I'm reasonably sure the fuel injectors are good as they were cleaned and flow tested not that long ago.

All 8 plugs have black carbon deposits on them. However the Long term fuel trims (LTFT) are zero at idle. Rising to only +3 to +5 percent under heavy load. This tells me that the MAF is good and the PCM's fuel control is good.

I first thought the problem was coolant leaking into the spark plug wells. There was an intake leak from the coolant cross over near the #1 cylinder. This was allowing moisture to get down inside the spark plug wells. This has since been resealed and the leak has stopped. However the misfire remains.

Next ran a compression test. This is where my results are "strange". The compression was "normal" but it did take 7-8 compression strokes to get above 180 PSI. If done for only 2-4 strokes the compression was about 120-130 PSI which is low.

Since the motor has 150K miles my current theory is carbon deposits inside the combustion chamber. Perhaps binding the rings. I'm going to try running some intake value cleaner through the motor and see what effect this has on the compression and misfire rates.

I'm also thinking about removing all of the spark plugs and pouring SeaFoam directly inside the combustion chamber. Let it set so it can extended soak to loosen up any carbon binding the rings. Obviously this will make a mess when the Seafoam is removed by cranking the motor. But this seems like a reasonable way to maximize the effect of the Seafoam.
 

Zaner

Member
Jan 5, 2020
15
1
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39
Oklahoma
Hey thanks that's a lot of good information and it does sound very similar to what I'm going through. When I had my coils and spark plugs out last time I did exactly what you mentioned. My local Walmart sells a sea foam off brand type liquid. I dumped it down the spark plug holes and let it soak oh, and I also put a can in the gas tank and also let the car idle with a vacuum source unplugged and fed the seafoam in there as well little by little. Then I let the car sit overnight. It's smoked like a freight train for about 15 minutes after I got it started back up. But it didn't seem to make a difference in idle quality. I've read so much information online about people with 4.6 2 valve engines that always experience this idle misfire and or low speed misfire. and it seems like a lot of the threads go unanswered the people never talk about what they did to fix it. I ordered a new alternator just throwing another part at it to see if it'll fix it since I get an alternator light so often.
 

Zaner

Member
Jan 5, 2020
15
1
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39
Oklahoma
another mustang guy in my area told me I probably have a crack in my plastic intake even though my smoke test didn't reveal one and I can't see a crack. he said sometimes they're cracked on the underneath side and the crack is so fine that sometimes even smoke can't get out. He said he's had a couple of mustangs behave like mine that had a fine crack in the plastic. My car has the pi revised intake on it already I think it was done during a recall because it has the metal coolant crossover. I get the feeling they werent as prone to cracking..
 

Zaner

Member
Jan 5, 2020
15
1
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39
Oklahoma
Just to add further information to the list. I've done lots of research and I've read a few online threads I believe there's two of run across so far or people had low-speed misfires and idle misfires and we're going through similar things that we are. And ultimately it ended up being the alternator, both threads said they dropped it off to a mechanic and they traced it back to an intermittent cam signal that cause the car to sporadically idle funny and it was traced back to the alternator putting out some type of interference. Since my battery light flickers on and off so much even though my battery is solid. It just makes logical sense for me to try and alternator and see what happens
 

wmburns

SN Certified Technician
Aug 14, 2009
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I generally try to avoid changing parts on a WAG. I prefer to test and replace what the tests lead me to believe what is truly bad.

Regarding the cracked intake leak. There are tests for that. You have already mentioned the smoke test. But if there is a cranked intake causing a vacuum leak this should show up in the Long term and short term fuel trims (LTFT STFT). For example if the LTFT are positive (adding fuel) at idle but decrease as engine RPM's rise, that's a clue there's a vacuum leak.

I'm a firm believer that every trouble shooting session should begin with a review of the battery and charging system. If this were my car I would want to know why the battery light is coming on.

Howto perform charging system voltage drop test
 

Zaner

Member
Jan 5, 2020
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Oklahoma
Awesome thank you for the link and info. I generally am of the same school of thought that it's best to test before throwing money or unnecessary parts at a problem.I didn't give all the details but I bought this 2001 mustang GT a month or so ago as a project that had been badly neglected by the previous owners. Everything that can be wrong with it is wrong with it they even replaced some of the intake manifold bolts with wood screws. It was a real Frankenstein and I'm just getting little things taking care of overtime. The real reason that I'm replacing the alternator other than the battery light is that I believe the alternator and alternator bracket is from a totally different vehicle maybe a crown Victoria or something. Because the upper alternator bracket doesn't even lineup or bolt it into the intake the way it's supposed to. Also the alternator is about three different colors it's been spray-painted orange and black and silver so it's a real mess. I just bet the board because of all those issues I ordered a new alternator bracket and alternator even if it doesn't totally fix my issue at least it'll look better and be bolted on securely.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
the alternator is about three different colors it's been spray-painted orange and black and silver so it's a real mess
It seems to me like the painted alternator might be a more important clue than you realize. After all the alternator is grounded via the case. So a painted alternator can (and does) affect the quality of the alternator's ground. Without a good ground the alternator's full output will not make it to the rest of the car. Further the voltage regulator will not have an accurate voltage reference to match to.

The most important alternator ground point are the lower mounting ears where it bolts to the block. Recommend cleaning the mounting ears as well as the engine block where the alternator bolts to.

There is some grounding via the support bracket. IMO it's good practice to also clean these grounding points as well.

A voltage drop test could be used to verify the quality of the ground before and after cleaning.
 

Zaner

Member
Jan 5, 2020
15
1
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39
Oklahoma
It seems to me like the painted alternator might be a more important clue than you realize. After all the alternator is grounded via the case. So a painted alternator can (and does) affect the quality of the alternator's ground. Without a good ground the alternator's full output will not make it to the rest of the car. Further the voltage regulator will not have an accurate voltage reference to match to.

The most important alternator ground point are the lower mounting ears where it bolts to the block. Recommend cleaning the mounting ears as well as the engine block where the alternator bolts to.

There is some grounding via the support bracket. IMO it's good practice to also clean these grounding points as well.

A voltage drop test could be used to verify the quality of the ground before and after cleaning.
Wow I had no idea but it makes perfect sense!
 

Zaner

Member
Jan 5, 2020
15
1
13
39
Oklahoma
As an update im happy to announce the alternator cleared up all my idle issues. I put a new alternator on last night and have driven the car all day without any of the old symptoms...the cars never run do good. Guess i need to listen to the dummy battery light telling me there's an issue in the future lol