Misfire debug

[supton]

Coworker has a 2006 Mazda 3 (stickshift) and apparently it threw a cylinder 1 misfire code this morning. Runs ok but I guess it stumbles (if not outright misses) in the 3,200 to 3,700 rpm range. Fine outside that range. He looked up the code (has Torq to scan) and apparently someone said they had this cylinder 1 misfire and replacing the valve cover gasket fixed. We took a quick look and it doesn't look like a bad job--but it's still the middle of winter.

I said he should pull the coils (it's coil on plug) and check for oil. If none is found then he should pull the plugs (they have 40k on them) and check; and perhaps just change, if it's close to time.

What I'm not sure about: if oil is found, could he just clean off the coil (brake cleaner?), pull the plug so as to drain the oil; clean the plug off (brake cleaner?), and reassemble. That way he can fix when warmer. [I know it'll smoke at startup while burning off the oil.] This correct?

Also: any other suggestions for him to look into?

 

[LeakySeals]

If the coil is soaked then maybe there is a grommet on the VC? better make sure any valve cover repair includes them. I would also be curious about the injector. Swap it to another cylinder, see what happens.

 

[GenesisGuy70]

I'm certainly no expert but how would a bad valve cover gasket cause a misfire? The valves would still only allow a certain amount of air in or exhaust out and I don't see how what can't be more than a minor leak would change the pressure enough to alter this.

Genuinely interested if that's possible. Maybe it is! But my initial thoughts are that this seems a little far fetched.

 

[supton]

Originally Posted By: Throt
I'm certainly no expert but how would a bad valve cover gasket cause a misfire? The valves would still only allow a certain amount of air in or exhaust out and I don't see how what can't be more than a minor leak would change the pressure enough to alter this.

Genuinely interested if that's possible. Maybe it is! But my initial thoughts are that this seems a little far fetched.


I think whatever person my coworker found may be confusing valve cover gasket with spark plug tube gasket. Looking at how to do the job I could see how it might be thought of as just the gasket. youtube link for 2006 Mazda 3 valve cover gasket

 

[ARCOgraphite]

Likely oil in plug counterbore, most PCV have unmetered "fresh" air post filter soa small VC perimeter gasket leak should not affect part throttle fuel metering with lamda correction.

Odd that oil is an insulator so oil in the plug well that should not cause misfire. Are there enough PPM metal compound adds in the oil to make it conduct high tension around the plug insulator to GND? Sond like its time for an experiment.

 

[GenesisGuy70]

Real question is, did it run right when it was started back up!

That top end looked fantastic. Wonder how many miles were on that motor.

Overall, looks like a super straightforward job.

If a woman can do it, so can you!

 

[mk378]

Misfire diagnosis always starts with pulling the plugs to see if you have a broken plug and examine the deposits on the plugs, which tells a lot about the general condition of the engine. Plugs should likely be replaced with new ones if they have any age on them at all. If there is oil in the plug tubes that needs to be resolved. Swapping the coil with another cylinder to see if the misfire moves with it is a way to diagnose a bad coil. You can do the same thing with fuel injectors if it doesn't seem to be an ignition problem.

 

[supton]

Ah, didn't think to swap coils around, I'll recommend that to him. Don't know if he'll want to swap injectors. I'll pass that info along.

 

[Olas]

Originally Posted By: supton
Ah, didn't think to swap coils around, I'll recommend that to him. Don't know if he'll want to swap injectors. I'll pass that info along.


If the misfire is due to a bad spark plug then swapping coils won't make any difference. Swap the coil AND plug between the bad ylcinder and a good cylinder to see if the misfire moves location, then you'll mow for definite if the coil is the problem.

Try a test light on the coil plug and on the injector plug to see if they're receiving a signal too, even good components won't function without an input signal.

 

[rshaw125]

Originally Posted By: mk378
Misfire diagnosis always starts with pulling the plugs to see if you have a broken plug and examine the deposits on the plugs, which tells a lot about the general condition of the engine. Plugs should likely be replaced with new ones if they have any age on them at all. If there is oil in the plug tubes that needs to be resolved. Swapping the coil with another cylinder to see if the misfire moves with it is a way to diagnose a bad coil. You can do the same thing with fuel injectors if it doesn't seem to be an ignition problem.


Also while you have the plugs out do a compression test.

 

[supton]

Originally Posted By: Olas
Originally Posted By: supton
Ah, didn't think to swap coils around, I'll recommend that to him. Don't know if he'll want to swap injectors. I'll pass that info along.


If the misfire is due to a bad spark plug then swapping coils won't make any difference.


Very true. But I'd swap one component at a time. He's going to pull plugs anyhow.

Quote:
Swap the coil AND plug between the bad ylcinder and a good cylinder to see if the misfire moves location, then you'll mow for definite if the coil is the problem.


I don't get why you'd swap both items at the same time. Seems to be a recipe to complicate the process.

Quote:
Try a test light on the coil plug and on the injector plug to see if they're receiving a signal too, even good components won't function without an input signal.


I guess that can be tried, assuming it doesn't have to be under power for the miss to show up. One step at a time though: new plugs and other items he has to do first, then play with coils later on.

 

[supton]

Originally Posted By: rshaw125
Originally Posted By: mk378
Misfire diagnosis always starts with pulling the plugs to see if you have a broken plug and examine the deposits on the plugs, which tells a lot about the general condition of the engine. Plugs should likely be replaced with new ones if they have any age on them at all. If there is oil in the plug tubes that needs to be resolved. Swapping the coil with another cylinder to see if the misfire moves with it is a way to diagnose a bad coil. You can do the same thing with fuel injectors if it doesn't seem to be an ignition problem.


Also while you have the plugs out do a compression test.


Seems a bit overkill. Nice to do but overkill. It started doing it today. Not slowly getting worse, just suddenly. I'm 99.9% sure he doesn't have that tool (probably can borrow it from Autozone?), but he's got enough to do this weekend. Compression test can come later.

 

[Nick1994]

Yup, swap coils with another cylinder. My mechanic did that for my grandmother's Trailblazer a few years ago and it set a code right away in the new cylinder, easy diagnosis in that case.

If that doesn't work, then swap the spark plugs with another cylinder. If no change, probably fuel related.

 

[LeakySeals]

I said injector because you said it stumbles (if not outright misses) in the 3,200 to 3,700 rpm range. Agree swapping coils first, easier. Swapping injector harder, but not that bad. Info on YouTube.

 

[Trav]

The chances of an injector giving up the ghost in a certain RPM range and not in others is very slim, that would be the last thing on my list of things to look at at this point.

 

[supton]

The coworker dug into the job this weekend. From what I understand he pulled a coil and verified no idle change on at least one cylinder, indicating bad spark (pull two and bad idle, not sure on all the steps he did--wasn't there). Pulled the coils and found two had oil on them, so off came the valve cover. He did plugs too, and found one wasted plug (his words, no pics), which coincided with the cylinder that didn't need a coil. Runs much better now he says.

So I don't know if the issue was valve cover but it's resolved now. Thanks all.

 

[Olas]

Sometimes all you need to do is step away from the Internet and put your eyes on the problem - it soon becomes apparent what's happening..

 

[supton]

Originally Posted By: Olas
Sometimes all you need to do is step away from the Internet and put your eyes on the problem - it soon becomes apparent what's happening..


?

 

[Olas]

Originally Posted By: supton
Originally Posted By: Olas
Sometimes all you need to do is step away from the Internet and put your eyes on the problem - it soon becomes apparent what's happening..


?


As soon as the owner looked at the vehicle the problem became apparent and was rectified. You can't do that asking questions on forums..

 

[supton]

But at the same time, asking questions may yield better clues. Had it been my Camry I probably would have been advised to buy coils as it's a common weak spot--if not now then later.

Never hurts to ask. Especially since it was going to be some time before he could debug.