High RPM misfire? Check your alternator! - Maintenance and Repair Forum

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High RPM misfire? Check your alternator!
Thursday, August 24, 2006 7:50 AM
Doesn't seem to make a whole lot of sense, but it's true.

I have a 99 Escort, and from the time I got it (around this time last year), it's always had a little bit of a misfire at higher RPM's, generally under hard acceleration (like getting on the highway). Nothing major, just enough to feel the engine stumble a bit. I went through all the normal troubleshooting...plugs, wires, repeated fuel injector cleaner treatments, etc. I even considered replacing the stock coil with a MSD Blaster, though I never got around to that. When the "typical" fixes didn't take care of it, I just figured it's one of things you learn to deal with on a car that has over 100K miles on it. (Insert crappy Ford joke here)

Last week, my alternator died. I hadn't noticed any previous problems except an occasional slow crank last winter, which I wrote off to the cold. So now I put on a brand new alternator and suddenly my misfire is gone.

WTF, right? But the more I thought about it, the more sense it made. Ignition systems don't have a "static" current draw. Older MSD instruction manuals will tell you that (for an example) a MSD-6 ignition will draw about .7amps of current per 1000 RPM. That means the higher the engine revs, the higher the current draw. Stock ignitions are no different. If the ignition is looking for enough current to run it and it can't get it, the voltage will drop. Voltage drop at the ignition equals voltage drop at the coil primaries. Voltage drop at the coil primaries = HUGE voltage drop at the coil secondaries = reduced spark intensity = misfire.

My theory is that the entire time I've owned the car, it has been running on a failing alternator, or one that wasn't providing "quite" enough voltage/current to keep the ignition happy at high RPMs. So if you're experiencing trouble with a high RPM misfire, connect a voltmeter to the main power supply to the ignition control module (don't ask me which one it is, I don't know...that's what schematics are for ) and watch the voltage while you drive around, especially as you really wind the motor up. Your solution might be in the last place you'd expect to find it.







09:f9:11:02:9d:74:e3:5b:d8:41:56:c5:63


Re: High RPM misfire? Check your alternator!
Thursday, August 24, 2006 8:29 AM
JimmyZ wrote:I have a 99 Escort


BAN HIM!!!

And oh yeah, wrong forum too.......



J/K, will keep that tip in mind.




Currently #4 in Ecotec Forced Induction horsepower ratings. 505.8 WHP 414WTQ!!!
Currently 3rd quickest Ecotec on the .org - 10.949 @ 131.50 MPH!!!

Re: High RPM misfire? Check your alternator!
Thursday, August 24, 2006 8:35 AM
Alternator theory is true.. Happend to me on my old 97 2.2











~2014 New Z under the knife, same heart different body~
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Re: High RPM misfire? Check your alternator!
Thursday, August 24, 2006 8:39 AM
Roofy wrote:And oh yeah, wrong forum too.......
Moved to maintenance...dunno why I didn't put it here in the first place.






09:f9:11:02:9d:74:e3:5b:d8:41:56:c5:63

Re: High RPM misfire? Check your alternator!
Thursday, August 24, 2006 8:41 AM
Interesting, that may the phantom problem that I could never seem to fully cure from my winter car (93 Plymouth Sundance). After replacing virtually every possible cause, the problem decreased dramatically, but still reared its ugly head every so often. Perhaps that is the cause. Good info.



Re: High RPM misfire? Check your alternator!
Thursday, August 24, 2006 10:24 AM
Common problem its a ford
Re: High RPM misfire? Check your alternator!
Thursday, August 24, 2006 10:38 AM
rich weeks wrote:Common problem its a ford
This coming from someone that owns a 2.2 Auto Cavalier.

POT: Yo, you're black!
KETTLE: No @!#$!

This is good information that applies to ALL cars except diesels, if people would be smart enough to get the immature Versus, "my car can beat up your car" mentality out of their heads.







09:f9:11:02:9d:74:e3:5b:d8:41:56:c5:63

Re: <b>High RPM misfire? Check your alternator!</b>
Thursday, August 24, 2006 2:32 PM
<b>JimmyZ</b> wrote:Doesn't seem to make a whole lot of sense, but it's true.

Your right...it doesn't make sense. A fully charged battery, which is capable of delivering a whole bunch of amps on demand, should have no problem delivering enough voltage and amps to run your ignition system just fine, even if the alternator output momentarily dropped to zero. Maybe when you changed the alternator, you inadvertently jostled a connector, wiring harness or some other electrical device that may have had an impact on your misfiring problem. Also, high rpm anamolies that may seem to be ignition problems can sometimes be fuel related, such as a weak fuel pump causing inadequate fuel delivery at higher rpms. Of course, your theory may be right. I'm really not trying to be argumentative here....just another viewpoint to consider before anyone goes out and buys a new alternator to correct their misfiring problem......




Re: <b>High RPM misfire? Check your alternator!</
Thursday, August 24, 2006 8:08 PM
Lees, I agree. Something to think about is that your battery should only be around 12 volts. A health alternator should be mid-14s. I'm guessing that for this to be an issue, you would have to already be straining the alternator.


"The Blue Bullet"
Re: <b>High RPM misfire? Check your alternator!</
Friday, August 25, 2006 8:47 AM
Quote:

I have a 99 Escort

Re: <b>High RPM misfire? Check your alternator!</
Friday, August 25, 2006 11:45 AM
Lees04Cav wrote:Your right...it doesn't make sense. A fully charged battery, which is capable of delivering a whole bunch of amps on demand, should have no problem delivering enough voltage and amps to run your ignition system just fine, even if the alternator output momentarily dropped to zero. Maybe when you changed the alternator, you inadvertently jostled a connector, wiring harness or some other electrical device that may have had an impact on your misfiring problem. Also, high rpm anamolies that may seem to be ignition problems can sometimes be fuel related, such as a weak fuel pump causing inadequate fuel delivery at higher rpms. Of course, your theory may be right. I'm really not trying to be argumentative here....just another viewpoint to consider before anyone goes out and buys a new alternator to correct their misfiring problem......
A fully charged battery puts out 12V. A properly functioning charging system runs (ideally) at 13.8V, but generally fluctuates anywhere between 13.5 and 14.5V. Supplying reduced voltage to the coil reduces the amount of current drawn by the coil. Reduced current draw results in lower magnetic saturation levels which results in reduced (or nonexistent) spark intensity. A drop of 1.5V-2.5V (as in the example you're giving) will result in a drop in spark voltage of between 1000 and 5000 volts, depending on the application.

So yeah, the new alternator fixed my misfire.
jbody03eco wrote:
Quote:

I have a 99 Escort
This, once again, is the most ridiculous, pointless and off-topic comment anyone could make, and only goes to demonstrate why there is so much "hate" for this "scene". I'd like to point out that you drive a Cavalier (or one could assume, based on your screen name). THIS is why some people have long since stopped giving advice in these forums. What I gave is perfectly valid advice that applies to a J-Body as much as to my Escort, or my old Hondas or my Windstar, or my old Saturn. Boneheads like you always have to turn a simple, useful piece of advice into some kind of "us against them" bull@!#$ and it's getting old. Next time, if you don't have anything constructive to offer, try keeping your yap shut.
I wrote:http://www.j-body.org/forums/read.php?f=1&i=333506&t=332969&start=25#333506

I remember today that what's left is alot of people driving crappy econo-box cars are pointing and laughing at other people driving equally crappy econo-box cars, like there's any superiority in levels of mediocrity.






09:f9:11:02:9d:74:e3:5b:d8:41:56:c5:63


Re: <b>High RPM misfire? Check your alternator!</
Friday, August 25, 2006 8:18 PM
TIP: Autozone and Advance can put an AMP probe on and test the output of your alternator for you. they will call it a charging system test.



Re: <b>High RPM misfire? Check your alternator!</
Saturday, August 26, 2006 4:27 AM
This site has some good tips and confirms what JimmyZ has said. i.e. diodes failing can reduce alternator output although the alternator still gives some output.

Somewhere on here it states that GM alternators regulate voltage between 14,8/9 to 13.8 dependant on temperature



http://www.forparts.com/techdiode.htm


Alont
Re: <b>High RPM misfire? Check your alternator!</
Saturday, August 26, 2006 12:15 PM
actually this can apply to diesels too, if the glow plugs arent warming up the cylinders properly then it might cause a misfire



Re: <b>High RPM misfire? Check your alternator!</
Tuesday, September 05, 2006 10:39 PM
Maybe this could be made a sticky. This ended up being my problem and this theory worked out for me..

My alternator when cold usually pushes out about 14 and when hot about 13.5
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