2005 Mazda 3 Rough idling / running rich after warm up. No codes.

kws0623

Member
:
Mazda 3 2005
OK I work at a car parts store with a couple really good mechanics and we're all stumped by this one. When I start my car and drive it a couple miles and then return to idle my engine starts to run extremely rich and runs rough. It shakes and you can feel it in the car, it actually sounds like a little diesel. But after I drive it another 5 miles or so and the engine is at max temperature the idling goes back to normal. I've swapped out the MAF, the upstream 02 sensor, new air filter, cleaned the throttle body, new engine coolant temp sensor (i have no clue why I changed this one), new spark plugs, oil and oil filter. When you hook it up to the data stream it shows that its running extremely rich and when you give it gas it drops timing to -13 and then goes up like normal. The fuel trim is going up normally. I've checked the pcv valve coming out of the crank case, no leaks. I'm at my wits end on this one could it be a EGR valve possibly not opening or closing correctly until its completely warmed up? Could it be the VTCS not opening correctly after its warmed up. I just changed the engine coolant temp sensor and when it cools down I'm going to see if that changed anything.

I have a knock sensor someone told me might be going bad but I haven't changed that yet since I cant really see how that could possibly be messing with my fuel ratios. The car only has 80k miles on it and the piston heads are spotless, it doesn't burn any oil and still gets exactly 35 mpg on the highway, which is the same as when it was new. When the car gets past this phase it runs like new, but it takes it a while to get past it. After all the new additions i've reset the ecm and let it idle for 20 mins to let it relearn with the new equipment. Any help would be appreciated.

Thanks, Kevin
 
You might wanna check to see if the EGR is clogged or the valve is inoperable. I could see how the pcm would increase injector pulse while anticipating more air to come through EGR. Yet if EGR is clogged, your left with a rich condition.
 
Last edited:
If the egr were to be clogged why does it clear up when the car is fully warmed up. I almost changed the EGR but decided not to on the recommendation of others. They said if it was clogged the problem would persist and it wouldnt make a difference if the car was warm or not.
 
My bad I misread it initially. Basically your engine runs rich until finally fuel trims adjust and then mask the problem. So it could be anything that causes a rich condition. Leaky fuel injectors, low compression, dirty MAF, weak spark, timing off a degree, etc
Sounds like you could cross some of that off the list.
 
My thoughts: A bad oxygen sensor (especially an upstream one) or OS wiring can cause the rich-running issues you mention, as could a vacuum or intake/exhaust manifold leak. However, these typically result in a lean type code. The ECU, if receiving a faulty OS signal or lean signal would protect the engine by adding fuel.
A bad or intermittently bad ignition coil could cause rich conditions, as well.
What is odd in your situation is that there are no codes and check engine light.
 
My thoughts: A bad oxygen sensor (especially an upstream one) or OS wiring can cause the rich-running issues you mention, as could a vacuum or intake/exhaust manifold leak. However, these typically result in a lean type code. The ECU, if receiving a faulty OS signal or lean signal would protect the engine by adding fuel.
A bad or intermittently bad ignition coil could cause rich conditions, as well.
What is odd in your situation is that there are no codes and check engine light.

He mentioned that the car only runs rough when it is fully warmed up, leading me to believe that his fuel injectors are leaking. Ive heard of very similar problems on a BMW forum that turned out to be faulty injectors.

A leaky exhaust manifold and whatever else you mentioned that causes lean codes will automatically cause the ECU to dump more fuel into the cylinders to compensate, so the car will start to run rich regardless.
 
He mentioned that the car only runs rough when it is fully warmed up, leading me to believe that his fuel injectors are leaking. I’ve heard of very similar problems on a BMW forum that turned out to be faulty injectors.

A leaky exhaust manifold and whatever else you mentioned that causes lean codes will automatically cause the ECU to dump more fuel into the cylinders to compensate, so the car will start to run rich regardless.

That does make sense.
 
Turns out it was the wrong spark plugs the last owner put in the car, they were the wrong ones. They were too hot and not made for that engine. I put the new ones in and now it runs perfect.
 
Sometimes the simplest things.......

Thanks for letting us know. I would've never guessed.
 
Back