06 mazda 3 2.0L engine idle intermittently goes up and down while driving or at idle!

6

69buick

Member
This is a completely stock 2.0 L with no recent engine work. Twice in two weeks while driving the car the engine rpms have gone out of control with no codes. While driving at 40 the engine rpms started swinging up and down from 2500 to 1500 no foot on the gas but the car was trying to accelerate and decelerate with the rpms. Also after that it stopped cycling the revs but it had a few periods of the idling sticking at 2200 rpms with car in neutral. The second time two weeks later driving at 25 mph and at idle the rpms would just go up and down continuously from about 2500 to 1200 and would not stop until I shut it off and restarted again no codes. I'm surprised no codes as I thought all throttle related sensors like the gas pedal switch had redundant sensors in them to inform the CPU there is an issue. This is also an intermittent problem two failures now in two weeks now. Car runs perfect otherwise. Ideas suggestions please besides giving into the dealer which will be real fun on the wallet since its a very intermittent issue.

Thanks
 
Forgot to add this car has 126000 miles. Any input appreciated as I'm pretty handy with wrenches and voltmeters. I'm not sure where to start as this doesn't sound like a normal idle probem like a IAC valve or dirty throttle to cause such a wild rpm pattern.

Also the second time this happened I did notice passenger front fender was very hot and saw activity in coolant reservoir with fluid. Temp gauge was normal though. This was right after is was cycling the rpms.
After long ride today on highway no issues and fender was cool. What is going on?
 
Last edited:
Just noticed coolant level is way below the minimum level while cold. Will fix that. Is there a sensor that could be acting up due to low coolant level that would effect my erratic rpm issue ?
 
If you have an air bubble in a coolant passage, it could be causing the coolant temp to give the ECU some bad data, and that would definitely affect idle (idle mixture and rpm is adjusted while the engine is warming up.)

I've noticed that the MZR's seam a little prone to developing an air pocket when flushing coolant/low level. What I found works really well is to jack up the passenger side of the car about a foot. This angles the coolant passages in the head and puts the coolant overflow tank higher in the air, encouraging bubbles to go where they're supposed to go. Hope that helps.
 
philrab said:
If you have an air bubble in a coolant passage, it could be causing the coolant temp to give the ECU some bad data, and that would definitely affect idle (idle mixture and rpm is adjusted while the engine is warming up.)

I've noticed that the MZR's seam a little prone to developing an air pocket when flushing coolant/low level. What I found works really well is to jack up the passenger side of the car about a foot. This angles the coolant passages in the head and puts the coolant overflow tank higher in the air, encouraging bubbles to go where they're supposed to go. Hope that helps.
Click to expand...

Thanks for the reply and tip for air bubbles. I have heard of that before for removing stubburn air pockets. I will have to investigate where I lost my coolant. The cycling up and down of the rpms happened when the engine was fully warmed up both times. The pattern of rpm cycling was like clock work, rev up 2 to 3 seconds and then back down 2 to 3 seconds and repeat over and over again at idle or driving. Never heard anything like that before from low coolant being that repeatable.
 
It's not easily repeatable, but I've seen this before. The issue is, while the engine is definitely warmed up, the only way the ECU knows that is by measuring coolant temp. If you have an air pocket, the coolant temp sensor suddenly sees a different temp (probably much lower, since it's not bathing in hot coolant) mistakingly tells the ECU to switch back to a cold idle map, and you have what you need for idle fluctuation. The odd part to me is, that usually throws a CEL.

I'm not 100% sure that's your issue, but I'm not spitballing either. I had this happen to a friend, and he fought me for days about fixing his coolant leak and doing a good bleed. As soon as he did, voila.
 
philrab said:
It's not easily repeatable, but I've seen this before. The issue is, while the engine is definitely warmed up, the only way the ECU knows that is by measuring coolant temp. If you have an air pocket, the coolant temp sensor suddenly sees a different temp (probably much lower, since it's not bathing in hot coolant) mistakingly tells the ECU to switch back to a cold idle map, and you have what you need for idle fluctuation. The odd part to me is, that usually throws a CEL.

I'm not 100% sure that's your issue, but I'm not spitballing either. I had this happen to a friend, and he fought me for days about fixing his coolant leak and doing a good bleed. As soon as he did, voila.
Click to expand...

I can certainly follow that logic. Perhaps when the idle speed kicks up for cold idle map by false temp sensor reading that would causes the water pump to run faster due to increased rpms causing a coolant surge within the engine. After that surge the temp sensor gets submerged in coolant again and triggers the hot idle circuit. Interesting to see how low my fluid is. I will top off tonight.
 
Well topped off coolant tank and bleed off any air with passenger side raised. It only took about 0.4 gallons of fluid to get to max level which is bascially just what the resevoir holds so I'm skeptical that the low coolant is my issue with rpms as it was not low by very much IMO.
 
Update and a question.

Well so far so good but as this was intermittent issue I'm not sure it's really fixed. I have a question on scanning the car. My issue did not set of a check engine light and I also have a code reader/erasor. Would a dealer diagnostic scan dig deeper into my issue and possibly show what went wrong keep in mind these rpms issue are 2 -3 weeks old now? The reason I ask is others drive the car but are now hesitant too. If I spent 100.00 and a dealer scan picked something up it would bring peace of mind. If a dealer scan only picks up issues if a code was thrown then it would be a wasted trip to bring it in.
 
I had this same problem on a Hyundai Coupe about 10 years ago. Everyone kept saying it was the IAC valve but even after replacing that it didn't make any difference. The problem went on for months as it was intermittent. They finally found it was a faulty throttle position sensor. Once they replace that, problem solved.
 
Please log in or register to reply here.

Similar threads

D
Threw a U0101 code: Lost Communication With Transmission Control Module (2010 Mazda3)
Replies
5
Views
5K
Mickeye5
M
M
Mazda3 Check engine light, error codes…First timer
Replies
3
Views
2K
edmaz
E
T
P0171 Code (2012 Mazda3)
Replies
1
Views
2K
edmaz
E

Latest posts

Back