Diagnosing Check Engine Light without tool?

[mark2000]

Thanks all. I didn't know about that ECU reset which could affect the idle. I think I'll do the plugs and wires first since they're the cheapest. The coil packs are more expensive and require special order where I shop. But that will be my next step.

I'll also check the hose for cracks. It looks complicated to remove, is there some sort of sensor in there that I should beware of when removing it?

As I understand, it's on top of the EGR valve. Maybe I should check that too, while the hose is off.

But as a bare minimum, if I just want to figure out what the problem is - theoretically - should I be able to just replace spark plug #4 - and its wire - and theoretically, if that was truly the only issue - everything should work and the problem should be gone, right?

Thanks so much all!

 

[CheeseHelmet]

It's actually the precat...

Nope, that's just what you call it. I happen to call it a primary cat (iirc that's what a Bosch book on engine management I read called it) and Mazda calls it a "Warm Up Three Way Converter". I think we all know what we're talking about here, let's not worry about semantics.

Thanks all. I didn't know about that ECU reset which could affect the idle. I think I'll do the plugs and wires first since they're the cheapest. The coil packs are more expensive and require special order where I shop. But that will be my next step.

The car shouldn't idle poorly after an ECU reset. It's probably idling poorly because your ignition system is on its last legs or your EGR valve is starting to go.

I'll also check the hose for cracks. It looks complicated to remove, is there some sort of sensor in there that I should beware of when removing it?

As I understand, it's on top of the EGR valve. Maybe I should check that too, while the hose is off.

It's not hard to remove, just loosen two hose clamps and slip it off. The EGR valve is located in a really awkward-to-reach place under the throttle body, I wouldn't recommend messing with it until you have your ignition problems fixed. For all you know, the EGR valve is still working properly.

But as a bare minimum, if I just want to figure out what the problem is - theoretically - should I be able to just replace spark plug #4 - and its wire - and theoretically, if that was truly the only issue - everything should work and the problem should be gone, right?

No.

The reality of the situation is that everything is old and starting to wear out. The insulation on the wires is breaking down, the ignition coils are producing a weak and erratic spark, and the gap on the spark plugs are probably out of spec because the electrode has worn away.

Don't be cheap, we see lots of people around here who neglect their ignition system and then post threads asking how to fix the dreaded P0421 DTC ("Warm-up catalyst system efficiency below threshold" aka. your precat is ******). If you'd like, replace the plugs and wires for now but I suspect you won't notice too much of an improvement until you finally replace the ignition coils.

BTW: I just checked rockauto and I can get everything (plugs, wires, coils) for $105 shipped. That's pretty reasonable imo...

 

[mark2000]

It is a very reasonable cost-- I just can't wait that long for shipping. I don't commute in the car-- Its entirely for pleasure use two weeks without hockey or grabbing a trunk of groceries isn't doable

I guess I will just get the plugs, wires and coils and do them all at once. I just hope it fixes the problem.

The weird thing, I thought, was that performance got significantly worse when I switched the coil packs around. It just seems odd that they'd both go bad at once. I did jiggle the long spark plug wire during the job. Maybe that was enough to do it in for good... 'twas awfully brittle and some of the wire's costing slipped around.

PS: are most spark plugs "gapped" lately or will I have to do that myself? I am not looking for performance parts here. Just regular will do-- for the amount I actually use this car.

 

[CheeseHelmet]

The weird thing, I thought, was that performance got significantly worse when I switched the coil packs around. It just seems odd that they'd both go bad at once. I did jiggle the long spark plug wire during the job. Maybe that was enough to do it in for good... 'twas awfully brittle and some of the wire's costing slipped around.

It's not weird, you're right about making it worse by playing with the wires. If those wires have never been changed then they're most definitely bad. Since it sounds like those have never been changed, I think it would be a pretty safe assumption to say that the ignition coils have never been changed either.

Like a lot of parts, ignition coils are a wear item and do need to be changed to keep the engine running properly. If you're getting all the parts locally you can try just replacing the wires and plugs but if I was in your shoes I'd just replace everything. If you have to pass an emissions test every year I wouldn't take a chance.

PS: are most spark plugs "gapped" lately or will I have to do that myself? I am not looking for performance parts here. Just regular will do-- for the amount I actually use this car.

If you get the NGK plugs (which you should, they work very well) then no, they come pre-gapped. It wouldn't hurt to check them before putting them in, but I've never needed to gap them. There are two types you can choose: the ones that come with your car (NGK BKR5E-11) or you can get so-called "Long Reach" plugs (NGK ZFR5F-11) which seem to make the engine run smoother under load. The gap should be set to 1.0-1.1mm (or 0.040-0.043 inches). According to this, those plugs I mentioned come pre-gapped at 1.1mm.

 

[mark2000]

It seems it was the plugs and wires.

I replaced the Bosch ones that were in there with NGK BKR5E-11s and everything seems to be running very well now. Engine is quieter, smoother, and stronger in acceleration. Rough idle is gone.

Didn't do coils at this point, but they're even easier, so that shouldn't be a problem when the time comes.

The biggest problem in the job was removing spark plug #4! It was so tightly jammed in there, that the tool I was using broke. I had to fashion a longer handle on the tool in order to get enough torque to twist the plug out. When screwing the plugs back in, I did it by hand until they stopped, and then did one small light turn with the tool, as I've read lots of advice saying not to screw them in too hard.

Thanks so much for everyone's help! I'm so relieved this didn't turn out to be something more catastrophic (like a CAT issue or something I couldn't do myself).

 

[tweety2300]

Your lucky you didn't strip the threads. They make grease for spark plugs to keep that from happening