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Old 05-12-2013, 02:34 PM   #1
DIY
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misfire diagnosis

I currently have a misfire issure with my car. It is not always predictable and I have a few Ideas what it could be. Please let me know your thoughts.

SYMPTOMS: for the last 2 weeks my car has not had the 02 sensor working, about 6 days ago the misfire started, 3 days ago I finally managed to put a new 02. problem continues.
Today I pulled the plugs and half way up the tips are white (evenly across all 6) so i'm its a bit lean.

BEHAVIOUR: cold start, and mid RPM range the car stops reving or the RPM start to drop. never dies out at idle. only under load but it can be heard when reved in nutral occasionally.
when the car is still cold it sometimes has a hard time accelerating or just doesnt, and the RPM stays the same. it also has a verry rough idle. when it warms up its much smoother in the bottom end but, sometimes when hitting 3-3.5k RPM the same thing happens. It can be a constant stall of RPM or I get a TON of jerking because it fires, then doesnt, then fires again constantly.I can be driving on the highway at constant speed and the car starts losing power and gains it back a split second later, again with the jerking

REPLAEMENTS:fuel pump was replaced to a Walboro in the fall
fuel filter replaced in fall
injectors replaced last year (but are rebuilt ford 19lb)
distributor cap, and ignititon coil probably need to be replaced but I have yet to think the ignition system is a problem.

currently I'm thinking the lack of o2 sensor put so much strain on my fuel pump (since it was just running full rich) that its dead, or all of my injectors are dirty equally.
as its a walboro it had the tendancy to whine, sometimes audiable but more often when I was at the track. (when your strain your hearing it always whines lol).. currently I dont hear it any more, but if it was dead, it would just be dead right?

since the problem gets better when the car warms up I was thinking this could be the distributor becuase the gap would be smaller but the recent appearance of the half white plugs make me think its a fuel issue

any suggestions are appreciated!
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Old 05-12-2013, 10:41 PM   #2
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buuuuump
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Old 05-12-2013, 11:10 PM   #3
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I use to have an intermittent problem that sounds somewhat similar. The car would idle fine but when I went to drive it at anything more than really light throttle loads it would misfire and buck and jerk pretty hard. When ever it did happen, I was able to get it to go away for a couple days (or a couple weeks depending on the car's mood lol) by unhooking the AFM and coolant temp sensor when the car was off and trying to start the car a couple times (it shouldn't start with those things disconnected). Then hooking everything back up.

I never did actually diagnose the problem fully because I switched over to MS and all of the problems went away.... maybe you should do the same lol

However seeing as it went away when I got rid of the stock DME (all of the wiring and sensors stayed the same except the IAT sensor), the problem could have been with the DME itself. They have been known to flake out from time to time.
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Old 05-13-2013, 06:54 AM   #4
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Have you tried unplugging the icv?

Sounded like an issue i had a few years back with my icv.
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Old 05-13-2013, 08:39 AM   #5
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I had a similar issue last year. Shitty spark plug wires was the culprit.
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Old 05-13-2013, 02:08 PM   #6
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I think I have a spare dme ill try that first tonight,

If its the icv ill be really surprised because I don't think it could change the power that much, but I got a spare one of those too, ill try that too.

I'm still not convinced its the ignition system so ill try that last...

And I really gotta get that megasquirt goin this year.. Maybe after the next school !!!

Thanks for the suggestions!!
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Old 05-13-2013, 05:04 PM   #7
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I know you said your pump and stuff is new, but have you checked the fuel pressure anyways?
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Old 05-13-2013, 05:46 PM   #8
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Quote:
Originally Posted by craz azn View Post
I know you said your pump and stuff is new, but have you checked the fuel pressure anyways?
Not yet, I'm gonna see if I can find a 10$ pressure guage and tee fitting at crappy tire tonight..

Could it be the relay too I guess

Ill report back in hours lol
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Old 05-13-2013, 09:10 PM   #9
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I don't think your o2 has anything to do with this. You say that all plugs seem to be evenly running hot. You changed your fuel pump but did not check your FPR. if the diaphragm is ripped it can be a hooligan. Pull the vacuum line off the FPR and see if its full of gas or damp at least.

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Old 05-13-2013, 09:30 PM   #10
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Checked the fuel pressure with a 20$ home depot made pressre gauge.. But that doesn't seem to be it .. Stayed constant the whole time .. Ill see if I can pick up a temporary dme tomorrow, apparently the spare one I have is a 44 pin? I dunno its not a 55 ..
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Old 05-13-2013, 09:56 PM   #11
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Oh okay, thats good. If you need a dme i have a 173 sitting around you can use if you feel like making the trip here.

Have you checked the CTS? when the dme one goes bad they won't take throttle when the car is cold.
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Old 05-13-2013, 10:29 PM   #12
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Cts?

And thanks for the offer but I think ill be heading east
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Old 05-13-2013, 10:40 PM   #13
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Think he's referring to the Coolant Temp Sensor

I've also got a 173 if you need an extra to try
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Old 05-14-2013, 08:47 AM   #14
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Yes coolant temp sensor is what I was talking about.
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Old 05-14-2013, 09:32 AM   #15
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IMO, the AFM is your top suspect. Are you getting any error codes? Always try free fixes first; follow this guy's "Refurbishing your AFM" procedure:

http://www.the944.com/afm.htm
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