2018 CX-9 - Idle speed / surge

brilloAMD

Member
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2013 CX5 Touring - Metro Grey Mica
My wife and I purchased a 2018 CX-9 grand touring around 6 weeks ago (car has ~1800 miles) and at a stop I notice the tach will swing between 500rpm and 750rpm at a stop. I don't feel the idle change in speed but I'm surprised I can see it change on the tach (most cars instrument clusters aren't that sensitive). Anyone else notice this? I guess I'm surprised the idle isn't more stable.

Thanks,

Alan
 
Was the A/C on? If so, it's probably the ac compressor working the engine.

Most likely this was it...

This can confuse some people because it does not have to be a hot day, as the A/C is automatically forced ON for dehumidification whenever the vents are on the DEFROSTER, DEF/FEET, or FEET modes.
 
My wife and I purchased a 2018 CX-9 grand touring around 6 weeks ago (car has ~1800 miles) and at a stop I notice the tach will swing between 500rpm and 750rpm at a stop. I don't feel the idle change in speed but I'm surprised I can see it change on the tach (most cars instrument clusters aren't that sensitive). Anyone else notice this? I guess I'm surprised the idle isn't more stable.

Thanks,

Alan

750rpm is the normal rpm for an engine on idle...500rpm is actually quite low but if idle is bet 500-750 then That would be normal. What's not normal is when it surges past 750rpm like 1500 or 2000 when accelerator is not pressed
 
don't recall as i leave the automatic climate control on, but its certainly possible. I thought about that but still surprised to see it actually move the tach.
 
My wife and I purchased a 2018 CX-9 grand touring around 6 weeks ago (car has ~1800 miles) and at a stop I notice the tach will swing between 500rpm and 750rpm at a stop. I don't feel the idle change in speed but I'm surprised I can see it change on the tach (most cars instrument clusters aren't that sensitive). Anyone else notice this? I guess I'm surprised the idle isn't more stable.

Thanks,

Alan

it's 100% normal for the idle to fluctuate when powering electronics, AC compressor, etc
 
True. However, is this Fluctuation or Surging based on the OP's description? If Fluctuation, then its starting point should be from normal idle RPM which is 750 on my 2018 Signature. I have just over 1,000 miles and experience no Surging with Fluctuations ranging between 50-100 rpm at best with or without AC comp running. If Surging, then his cycles should be oscillatory (possibly rhythmic) and not random periodic (random Fluctuation).

To the OP:

Do you experience 'rhythmic oscillations' (probably surging) or periodic random fluctuations where RPMs simply stick at a higher or lower level for a period of time before returning to smooth idle? Do you notice this behavior after initial engine start-up and coolant reaches normal operating temperature range (Blue light goes out)? Do you notice coolant temperature range changes that coincide with engine RPM changes? Do you notice throttle hesitation on initial acceleration from a stop? Do you notice throttle hesitation on initial accelerating from any cruising speed? Do you notice any vibration coming from the engine while under heavy throttle during acceleration (like when entering the freeway)?

I'd be surprised if this were a native vacuum issue, but given the lack of attention to detail that my dealer put into their PDI, it might be possible that something was taken apart and not put together correctly during the PDI itself. There are a couple manual checks you can make on your own under the hood before going back to the dealer. Check the above questions and report back what you observe. Or, take it back to the dealer (dreaded) and let them diagnose it (with much regret).

Either way - all the best with it. I hate to hear about such problems with a 2018. Mine runs flawlessly.
 
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Still notice the fluctuation in idle between 500-750 when in drive. Turning off the AC has no impact. Only seems to happen when in D and not when in park. Im currently using premium fuel, could that be the issue? Car has 3,000ish miles now. Only seems to be an issue after warmup and seems to be random as far as movement of the tachometer. What do other folks see as idle speed on the tach at a stop light? I dont notice any driveability issues but I have felt the idle drop at times which would seem to be unusual.

What is the idle spec? 500 seems to low.
 
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500 does seem low to me. I can't really compare right now as the weather here is -29c and dropping, so my idle speeds will always be higher. Have you tried taking the car to the dealer to have the intake/exhaust systems checked (engine and cabin filters, fuel delivery systems, exhaust systems)? Maybe they can shake out the culprit.
 
what idle speed are other folks seeing? Do other people see the tach move between 500 and 750?
 
Check to see if you have too much (or too little) oil in the engine. I had issues with idling and the dealer fixed it by removing the excess quart jiffy lube put in and replacing their filter with an OEM one.
 
what idle speed are other folks seeing? Do other people see the tach move between 500 and 750?

Yup I've noticed this on mine but honestly don't find anything wrong with it as I think this is quite normal as a 250 variance in RPM is insignificant and also possible due to the torque lock converter of the Skyactive transmission (only notice this when stopped at a stop light and engine idle speed changes a bit) and/or compressor or aux fan engaging/disengaging. What I'd be more worried about is the vehicle surging based on some reported instances here in the forum...so far haven't experienced this on mine
 
mine will do this on light brake pedal pressure, when the Istop light is flashing...its almost like its deliberate to make you look down to enable enough pedal pressure to engage stop.....
 
My wife and I purchased a 2018 CX-9 grand touring around 6 weeks ago (car has ~1800 miles) and at a stop I notice the tach will swing between 500rpm and 750rpm at a stop. I don't feel the idle change in speed but I'm surprised I can see it change on the tach (most cars instrument clusters aren't that sensitive). Anyone else notice this? I guess I'm surprised the idle isn't more stable.

Thanks,

Alan



Bingo! Just started getting the surging effect at 1,000 rpm and only on very light throttle demand while the vehicle is downshifting. Yep, you called it!

I'm going to start doing some isolation road testing on this problem. This vehicle has always shifted very smoothly and this was not how it behaved before. This was also not how any of the other CX-9's that I test drove behaved before making my purchase. This is new behavior and it seems to have come on rather subtly. In addition, it now lugs down from 1st gear to Idle at or below 1,000 rpm. So, you fee a slight 'thump' in the transmission (very subtle) when coming to a full stop - but only intermittently. This is definitely not normal. If it were normal, this behavior/characteristic would have been present from day one and in all other CX-9s that I test drove prior. This new behavior just began when before all shifts were extremely smooth regardless of engine speed or transmission speed.

I'm thinking the problem might be in the new Torque Converter design, quite honestly. I hope I'm dead wrong. In fact, I'd be rather happy if I were wrong.

iSKYACTIV-Drive incorporates a new 80% lock-up ratio that extends engine rotation with transmission rotation (extended lock-up synchronization). This takes place low on the RPM band and that's what causes me to think there might be a problem in the torque converter transitions at lower RPMs - producing the "lurching" effect, "lugging" effect, "sputtering" effect, "throttling" effect, etc., as the engine speed gets closer to idle and the lock-up remains in effect. iSKYACTIV-Drive was Mazda's attempt at a sort of hybrid-transmission. One that yields the benefits of CVT, Auto and Manual transmissions. This was supposed to be a smoother shifting transmission as a direct result, as just one of its proclaimed benefits.

Again, I could be wrong and I hope that I am. But, the "Full Range Lock-Up Clutch" design could very well be the issue and the new multi-disc clutch could be the culprit - or something closely related (connected) to it.
 
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what idle speed are other folks seeing? Do other people see the tach move between 500 and 750?


At or below 1,000. Coming to a full stop. Or, at or below 1,000 while coasting (no throttle) down a slight decline. No symptoms when climbing a slight incline (or hill) and coming to a stop. All shifts are smooth down to a full stop and engine idle. Also, when the vehicle is heavier (as with a full load of passengers or immediately after refueling the tank) symptoms are greatly reduced to the point of barely being noticeable at all.

Symptoms Increase When:

- Vehicle is lighter
- Vehicle is costing (no throttle)
- Vehicle is on a decline (slightly downhill and descending)
- At or below 1,000 rpms

Symptoms Decrease When:

- Vehicle is heavier
- Vehicle is being accelerated (continuously advancing throttle)
- Vehicle is on an incline (slightly uphill and climbing)
- Above 1,000 rpms

Manifested Symptoms:

- Subtle engine throttling/surging/oscillations in rpm
- Subtle transmission lugging
- Subtle transmission shuttering
- Subtle gear dump from 1st to Idle (feels like subtle thump somewhere in drivetrain)

All engine/transmission behavior above 1,000 rpm is normal (smooth and responsive). Engine/Transmission behavior at or below 1,000 rpm on low to no throttle demand is becoming "sloppy," "lagging" with subtle "thumping" feel in undercar area.
 
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