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Intermittent Misfire


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#1
Rije

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My car starting having intermittent misfires. I've changed all the spark plugs, and just changed the wires.

The problem occurs after the car has warmed up. I'll crank it in the morning, drive 40 minutes, and the whole time it'll be fine. If I drive it somewhere 3 or 4 hours later, it'll start having intermittent misfires, and I can feel it while accelerating.

I have seen through searches that replacing the coil pack should help. I have a haynes manual, but it can be confusing at times, as I'm unfamiliar with these things.

Car: Mazda 626, 1999, 6 cyl

Could you guys help me diagnose how to fix this problem. Also, could you guys give me a rough run down on whats involved with replacing the coil pack... I've seen people talking about distributors and stuff, but I'm not sure what these things are.

I found a coil pack link through auto zone. Is this what I need?

Ignition Coil at Auto Zone

Any help, or redirection to a post that I've overlooked would be very helpful

Thanks

#2
snailman153624

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The coilpack is what provides the high-voltage current spike your spark plugs use to make a spark.

On the V6, looking under the hood, it's bolted to the right side of the front cylinder bank, and has 6 spark plug wires plugged into it, and a harness from the ECU.

I would try to troubleshoot further before throwing money at new parts, you will spend a lot and not gain much otherwise (unless you get lucky).

Misfires are often due to clogged EGR valves, which sounds more like your problem in this instance. Remember that vapor is only combustible if the fuel air ratio is in between the limits of a finite bound. Too much fuel, and there's not enough oxygen to go around to sustain combustion....not enough fuel, and the particles are too dispersed to sustain the reaction. If the EGR valve sticks open, it will screw up the fuel/air ratio, and result in random misfires.

#3
NickR

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From reading this board, I think I only remember one case where someone had to replace a coilpack. I don't trust my memory 100%, but it's certainly not a common problem. Problems with the plug wires are much more common. I had to replace mine, in my 2000 V6, at about 200,000kms.

#4
Rije

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Thanks for the quick reply guys.

I didn't realize the EGR could influence misfires that much. I have had problems with my EGR (P0401) even after cleaning my ports in my TB extensively. Maybe its stuck open.

Another note: I experienced misfires after washing my engine with a pressure washing wand using the engine cleaner at a Car Wash place. I left my engine running in order to cycle all the water out of the moving parts. I had assumed water just got inside the plug holes and I removed them to let the water drain.

Would the EGR cause a misfire in a specific cylinder? Also, would the EGR problem explain why the car only has problems after it has warmed up. Like I said earlier, if I start it in the morning, there isn't much of a problem. Only when I restart my car after driving it for a certain distance does a problem exist.

I was considering that the fuel delivery system might have an issue. I was also considering that something to do with how the car has a higher rpm due to it not being warmed up influences the misfires.

The misfires usually signal the flashing CEL when accelerating... If I keep a constant gas stream (maintain speed), then the car doesn't feel like its misfiring. At idle, you can feel the car putter, as if a misfire is happening as well, which doesn't happen if the car is in park.

Like I said, any help is appreciated. Thanks for your replies already!

#5
NickR

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The fact that the problem is related to water, and heat, suggest electrical. And removing the wires to get the water out the plug holes may have damaged the wires.

#6
Rije

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I was thinking something similar.... I guess I need to take a meter and check the coil pack.. I've replaced the plug wires (they were stock and needed it anyways, car is 130k)

I was reading through earlier posts and saw that a non-flush TB or vac leak could trigger the P0174 code I just recently flagged (system too lean, bank 2, same bank as misfire).

Would the misfire cause the lean bank? Or would the lean bank cause the misfire? I'm assuming they're related.

#7
snailman153624

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Yes, the EGR can cause misifires on a specific cylinder, as the EGR port into the intake manifold is only on one of the cylinder runners.

However, this is only one possibility. NickR has many good suggestions above as well.

#8
Rije

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I electrically tested the coil pack today... All is well with the coil... I still need to check the EGR, and potentially the fuel injector... Also, I read somewhere that the O2 sensor could be damaged that could cause these things...

If the cylinder misfire would cause the lean bank (P0174) then I could focus on that problem, or vice versa... If they're independent, then that leaves two problems

#9
rosado

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My car does the exact same thing. Have you discovered what the problem is yet?

#10
djdevon3

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Prime suspects for P0174 (rich condition causing the ECU to compensate by going lean) is the fuel pressure regulator or injector(s). 

 

A P0174 can be O2 sensor related however iIf it was a faulty O2 sensor it should set an O2 related code to go along with the P0174.  Same with a dead TPS or wiring to the TPS.  It can set a P0174 but it will set a TPS code to go with it.  Same for the EGR, there should be an EGR related code to go with it on the 1996-2002 OBD-II equipped 626 because the electronic EGR is different from the 1993-1995 version.  If it was MAF/VAF related that should trigger another code too.

 

If it's a faulty fuel pressure regulator or injectors you might not see any other codes.  If you are only receiving a P0174 with no other codes then look for components that are not monitored by the ECU.  The fuel pressure regulator is not monitored since it's a vacuum device. The PRC solenoid is monitored but the fuel pressure regulator can fail without preventing the PRC solenoid from doing its job.  It can fail without setting any other codes.  So can the fuel injectors.

 

Somehow more fuel is being fed than should be.  You have a rich condition. You have to find out why. 



#11
rosado

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Code p0171 comes up we have check so much stuff not sure what to do next.

#12
djdevon3

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P0171 is not the same as P0174.  Because a P0171 is on the front bank you have to add in the possibility of a PCV valve issue and remove an EGR issue (EGR not attached to front bank).  Also an exhaust leak at the exhaust manifold could introduce too much air onto the O2 sensor causing it to set either of those codes.



#13
chenxuwen

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Replacing the coil pack is not hard. First remove the intake tube to the airfilter box, then removing four bolts on the coil pack will put you in bussiness.

 

I recently got a misfire and found this webpage useful:

http://www.mobiloil....e_Misfires.aspx

 

I was having P0305 few days ago and that code along. I first checked the wires and then the plug. Once I replaced the coil pack, things worked.

Tnough I am 100% sure it was coil now, my multimeter says that the bad coil is OK. So I am still confused what is the role of temperature here.

 

I was originally led to believed that it was the #5 injectors because the misfire only shows up when the car is warm.

However, I read from a bunch of other posts that injector related misfires usually have more than more than one code.

 

I am just throwing in my two cents. DJ and snail are way more experienced.



#14
djdevon3

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It's all just guessing if no diagnostics are involved.  The more ideas the better.  When it comes to issues on the 626 there are usually multiple possibilities for any issue.  That's why diagnostics and testing are so important to narrow down the list of possible suspects.  It's like being a car doctor or car detective.



#15
rosado

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we finally figured it out. it was 2 bad fuel injectors. the car is running great now  ty for your help






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