"Random Engine Misfire"
 

This code keeps popping up at intermittent intervals. It can happen twice in one day, lay off for a month, and then come back again. This is what happens:

When coasting, say, on a freeway downhill, or an off-ramp, there will be an engine stumble, the dreaded "check engine light" comes on, and it'll either clear up of itself, or I have to rev a bit to get the engine running right again.

Had it to the dealer, who I gave a Bennie Frank to for him to tell me that the code was "Random Engine Misfire". (Gee, what a helpful code.) He then decided that it was the #1 spark plug lead, changed it out, and charged me $128.00 .

A month later, same misfire, same code. Took it back in, the guy says, "Well, it COULD be the Mass airflow Sensor, or it's either your plugs, coils (which NEVER go bad), or spark plug leads/boots. Since it's intermittent, we can't tell which, so just let it fail outright, and we'll know then."

(Nice, eh? What if it "fails outright" on the freeway at rush hour, knot head?)

Since then it's pulled this trick twice, I got a bit annoyed, and called up a Car Quest parts shop dealer, who had exact the same thing happen to HIM; and it wasn't the plugs, or wires; it was the coils. ("Those never go bad!")

Now, here's the crazy thing: the coils, from the dealer, are $100 a crack; from Pep Boys they're $18 each, and from everyone else, everything in between!!!

Questions for the group:

1. Does this sound like a bad coil?
2. Why the great discrepancy in price for coils? Are the Borg-Warners junk??
3. Is it the Mass Airflow Sensor?

Thanks to the group in advance.

"It's drivin' me crazy, it's drivin' me nuts!" (Wm. Burroughs)

Car: 2001 Mazda Protege 2.0 LX, loaded, California ULEV version

-Mileage 01

Are you getting hesitation or rough idle at stops? (Stops as in, at a red light, wtill in gear, not parked).

If the car comes to a complete stop after one of these misfires, yes, it'll keep missing until I rev the engine; then all is well again for a month, two weeks, ten minutes, it's totally unpredictable.

Forgot on original post: mileage on car is 135000; the CarQuest parts guy with a similar problem (and a bad coil) had 140000 on his when it started acting up, but it was a Toyota.

I just went through this. The problem with mine was an EGR. $211.00 if you do it your self.

diagnosing a vehicle over the internet is never an easy task. When faced with this situation I always start with getting my vehicle back up to date with all maintenance work needed.

Cleaning the EGR is not a bad idea either but I'd be willing to bet it's a faulty lead/boot and if neither of those it would most likely be a bad coil. They DO go out.

The reason the price is so different is because of quality. If you want an OE coil at a better price contact Ken@protegegarage.com and tell him Matty sent you from mazda3club.com. He'll hook you up.

In my case, when we pulled the EGR there was a piece of metal holding the EGR open. It got there when I was doing a mod to the EGR pipe so it would reach my header.

Thanks, Roddimus...!

Hey, here's something odd I found..."start with the cheap stuff," I always say...so I replaced the plugs, and when I measured them with a DVM, 3 were exactly 4500 ohms...and one was 4000.

Hm! I wonder if there was a resistor breaking down under high, then sudden low draw...and I THINK it was the #1 cylinder, which is the lead the dealer replaced because he thought he saw a small arc-mark on it. Plugs are also "never bad"...in 40 years I never saw a defective one, but that doesn't mean it can't happen.

...and I am surprised to find that the 2.0 ULEV California model HAS a EGR!! I had heard that with the two cats, an EGR was unnecessary to meet smog.

This is wrong? The 2.0 DOES have an EGR??

..and yes, diagnosing over the Net IS an exercise in psychic activity...but I do keep this thing maintained fairly well; if it needs it, it gets it. I put on 22,000 a year, and live on the freeway...where is just where you don't want a conkout.

Well, it hasn't done it since the plugs were replaced, and has a LOT more pep. Hmmm...we shall see. If it does it again, we'll try coils, I guess.

EGR has a lot more to do than just emissions.

The spent exhaust gas is also piped back into the intake to help control pre-ignition. In warmer climates higher intake temps and poor gas quality can cause fuel to detonate early. By reducing the amount of fresh oxygen in the mixture you remove any possibility of the fuel burning. The cylinder feels dead and the non-existent combustion process allows the combustion chamber to cool. Once properly cooled the EGR closes and fresh air goes back to the c.c. until it starts pinging again.

OK, now it's getting really wierd on me.

1) Pull up to a stop light, it idles fine for 2 seconds, then shakes and misfires; putting it in neutral, I gun it, it straightens up, and the engine light does not come on. Later on, same act, and the light DOES come on, then goes off after 4 engine start/stop cycles.

[If this was an EGR issue, I think it wouldn't run OK for 2 seconds at a stop...I mean, if the EGR was stuck open from cruising sped to a halt, it wouldn't idle OK for those 2 seconds at a full stop, would it? I mean, the EGR is closed at a full stop, right? What would make it snap open?]

2) The thing is "coding" (check engine light) now about every 50 miles. Took it into the dealer, who read "Random engine Misfire" and a new one, "Precat not hot enough". The tech took a look at the second code and said, "Garbage. The CPU's so mixed up right now I wouldn't trust that code at all." Dealer suggested I replace the coils. Coils on order.

3) Tonight I managed to get the CE light to go out by disconnecting the battery for a few minutes. When I hooked it back up, the light stayed out after startup, _but the engine idle was miserably low_. Gunned it a few times, and it settled on normal. Turned her off, started back up, idle normal.

Thinking I was on to something, I disconnected the battery again for a few minutes, hooked it back up, started the engine, _and the idle settled down to a miserably low level again, almost stalling_. Gunned it a few times, and it slowly settled back to normal; tried loading the engine down (lights, A/C, etc), and the idle stayed rock steady.

Is it possible we have a bad Idle Air Control Valve, or a set of data coming from a bad sensor (02 sensor?) that is causing the CPU to order the idle valve to "rich up" the mixture by cutting the air down?

:dunno:

I think I'll go with the coils next. Less work than cleaning the EGR. Unless someone has a better idea. But I'm beginning to doubt this is the coils. Has anyone heard of a diagnostic code being totally off? The dealer still insists that "Random Engine Misfire" relates only to the plugs, plug wire or coils.

Man, I'm going bonkers.

Dealers Says "Code Accurate"
 

Talked to the dealer again today...says he has no history on this sort of problem w/this model...and said that the diagnostic code could not be wrong since it had "coded" this way several times.

Going to replace coils this week.

If ANYONE has had this code "P0300"-"Random Engine Misfire" pop up, I'd like to hear from them.

Coils fixed my (similar) problem. Do not be decieved by a bad EGR in the future - it is a known issue for us and may make you think the new coils are bad...

Inspecting your new spark plugs again will help - is one cleaner? Or carbon-choked?

Just my 2 cents on plugs; I used to build up hot rod engines for guys around the neighborhood. On two seperate occasions, they bought Champion plugs, and 1 of the 8 would not fire at all. Replacing the bad one fixed it both times. A couple of years later a friend tuned up his '72 Monte Carlo and finished up with an ignition miss. He brought it to my house and I screwed an AC plug into it and vroom vroom.
I've never bought a Champion again other than for the lawn mower.

Good luck with your P5. What about a weak injector couldn't that cause a random misfire?

New Engine Light Code
 

Thanks for the info, Rusty.

And, oh, yes, the old plugs looked OK..no fouling when I changed them, but will look again.

We seem to have a new problem; code "P0421"; "insufficient pre-cat efficiency"; speculation is that the pre-cat could have been damaged by the engine misfire problem, which happened while the experts piecemeal-troubleshot it over 2 months.

Or, at 135,000, it's just gotten worn out. One independent said that Mazda Cats are skimpy in the Platinum and Rhodium deposits, so they wear out quicker. I don't know.

The engine seems to run fine after replacing the coils, wires and plugs, but now THIS devil popped up, and the dealer is quoting me $500-$700 to fix THAT, and some independents are quoting $1400, as _they_ want to replace BOTH oxy sensors at the same time!

I thought this was supposed to be an economy car? If there IS such a thing in California...California Air Resources Board has sure done a number on us consumers.<sigh>

I'm supposed to go back to the dealer on Monday and have the system analyzed _again_.

Any advice appreciated. I'm an old guy who is beyond replacing Cats myself, but my wallet is sure taking a beating.

Pre-Cat Fried
 

OK, today the dealer says I have a fried pre-cat. Todays fees, just to diagnose, $88. Precat is on order at $500.

Merry Xmas.

I keep thinking that is the dealer had "shotgunned" this thing at the get-go, instead of trying a piece here and there, and replaced the plugs, wires and coils all in one whack, it would have hurt, but maybe I'd still have a pre-cat.

But at 136,000, maybe not for long.

:dunno:
I HATE these computer-controlled California cars. CARB (Calif. Air Resources Board) are a bunch of six-figure salaried useless sacks of liquid :buttslay:

My only consolation through all this is that these posts will help other users, and that Mazda dealers might look in here once in a while and learn something to add to their "knowledge base" (see above icon).

Don't hate on the emissions equipment and regulations until you visit Mexico City!

Bleah!