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Protege5 rough Idle help

Hi, hoping to get some help for an issue I'm having on my 2003 Protege5. Recently replaced water pump, timing belt, valve cover gasket and ever since its been idling really rough. It still drives really nicely, never hiccups or drives rough but whenever I am at a stoplight the idle is super rough. There is a Check engine code for a "Random Missfire" that keeps coming up. I just replaced the sparkplugs, coils and ignition wires but none of that has fixed the problem. The issues seem to get worse as the car warms up, when its cold it idles pretty smoothly, but then gets much worse as it warms up. Also as the idle gets rough the engine shakes as well

I'd love some thoughts on what it could be as I'm not really sure what to check next.

Thanks!
 
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2003 Mazda Protege5
Did you happen to disconnect the battery for any of that work? That can cause a really rough idle but it wouldn't cause the CEL... Did you gap your spark plugs? May be worth checking fuel pressure and looking for vacuum leaks.
 
Hey Spencer, thanks for that advice, I'll check fuel pressure and vacuum leaks tomorrow. Also planning on checking and cleaning ICV and EGR,

Just to clarify, are you saying that me changing the coils and plugs without disconnecting the battery could cause the rough idle, or that disconnecting the battery could cause the idle?

Thanks!
 

pcb

The Diagram Dude
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2002 MP5
When the battery is disconnected it resets the ECU and the car needs to relearn how to idle the car again but as mentioned, it shouldn't cause a misfire but it is possible.
(It takes about 100 miles of driving for the idle to settle down.)

If your car runs fine except for the idle then it is probably just your IAC relearning how to idle the car.

The car waits for the engine to warm up before handing over control of the idle to the IAC.

Give it some time and see if it clears up.
 
Ah, I wasn't aware of the ECU re setting, the idle magically seemed half better yesterday, so maybe it is just the IAC re-learning the idle.

I'll keep an eye on it over the next few days and see how she handles.

thanks
 

pcb

The Diagram Dude
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2002 MP5
My car runs REALLY rough after I disconnect the battery.
It comes very close to stalling but I have never thrown a code.

The car needs to practice idling at different engine and air temperatures, as well as other things and the idle, gradually becomes more stable.

The service manual doesn't mention anything about it and there have been many threads started about.

I learned about it on this forum so I was prepared for it but stalling your car in traffic can be dangerous.
 
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2003 Mazda Protege5
That's about 90% why I sold mine... live and learn
Goes to show that you should read up on the forums for your car
 

pcb

The Diagram Dude
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2002 MP5
That information should be right in the owner's manual.
I wonder how many P5's ended with all kinds of unnecessary new parts or at the mechanics or even sold like what you went through.

I'm pretty sure that when our car was brand new it would idle perfectly after an ECU reset.
It's when the engine starts to drift from the original factory specs with time that it needs to adjust to get it to run smoothly.
 
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2003 Mazda Protege5
I might actually go write it in the manual somewhere, assuming it lives to see another owner lol
 
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2003 Protege DX 5MT
My car was idling really rough after driving when I first got it, culprit was a filthy air filter. I think it had cylinder misfire as well. Clean your EGR, IAC, and maybe replace PCV or clean it.
 
been driving the car this last week to allow the IAC to re-callibrate. The idle got a bit better (not stalling anymore), but is still idling rough. I re-checked gap on plugs today (good) and also cleaned Mass air flow sensor. Cleaning it didn't seem to fix anything so I unplugged the sensor when the car was running to see if I could detect any change (trying to see if that was the culprit), when I unplug it the car runs exactly the same as with it plugged in, however if I try to plug it back in when running the car dies.

I've tried to do a deep dive online, but have read two varying answers from people, some say if the car dies when unplugging it then the sensor is shot, but I've read others say that if the car doesn't die then the sensor is shot

Any insight?
 
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2003 Mazda Protege5
I heard that if you unplugged it and it didn't die then it's bad, so I replaced it. and it made no change. And it's expensive, so I wouldn't replace it. Not yet anyhow
 
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2003 Mazda Protege5
That was almost 2 years ago, I wish I had returned it (simply didn't think about it) and it would've paid for me to buy the car back lol
 
...I've tried to do a deep dive online, but have read two varying answers from people, some say if the car dies when unplugging it then the sensor is shot, but I've read others say that if the car doesn't die then the sensor is shot ...Any insight?

First things first. Get a can of MAF sensor spray cleaner -DO NOT USE CARB CLEANER- then take MAF completely out and spray the heck out of the sensor wire, do not touch it with anything. Let it dry, re-install and check operation. Car should run with MAF disconnected but will not start without it. Make sure you have no leaks in the ducting between the MAF and the throttle body.
 
Will it start if the MAF is bad?
Should not.

A dirty MAF can make car run like poop. Biggest cause is over oiling cone filters, that's a definitive MAF killer. I proactively clean the MAFs in all my car at every other oil change, just on GPs.
 
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