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Old 07-03-2023, 10:44 AM   #1
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2005 Mazda 3 Randomly speeds up / down

When going steady speed, my 2005 Mazda 3 Manual slows down / speeds up a tiny amount every few seconds / minute. No RPM changes when it happens.

Manual transmission
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Old 07-03-2023, 12:35 PM   #2
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Re: 2005 Mazda 3 Randomly speeds up / down

Welcome to the forum. Please quantify the amount the vehicle speeds up/slows down. If it's enough to be noticeable, but you don't notice a change in the engine speed, that could pretty much only mean the clutch is slipping. But realistically even then I would expect you'd notice a change in the engine speed as the engine becomes loaded or unloaded.

When stopped and in neutral or with the clutch pedal fully depressed does the engine idle at a constant speed or does it speed up and slow down at about the same rate?

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Old 07-03-2023, 01:12 PM   #3
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Re: 2005 Mazda 3 Randomly speeds up / down

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Originally Posted by shorod View Post
Welcome to the forum. Please quantify the amount the vehicle speeds up/slows down. If it's enough to be noticeable, but you don't notice a change in the engine speed, that could pretty much only mean the clutch is slipping. But realistically even then I would expect you'd notice a change in the engine speed as the engine becomes loaded or unloaded.

When stopped and in neutral or with the clutch pedal fully depressed does the engine idle at a constant speed or does it speed up and slow down at about the same rate?

-Rod
Itís very slight, but the rpmís donít change. Last time clutch went bad RPMs would go up fast without acceleration. No issues when idle.
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Old 07-04-2023, 02:37 AM   #4
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Re: 2005 Mazda 3 Randomly speeds up / down

Cruise on or off? What gear? Flat terrain or rolling?
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Old 07-04-2023, 09:13 AM   #5
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Re: 2005 Mazda 3 Randomly speeds up / down

So is "very slight" 1 mph, 2mph, less than 1 mph?

When a clutch is bad and slipping considerably, yes, the RPMs will raise quickly without the vehicle speed changing appreciably. But if the clutch disc is in the early stages of "worn" or is contaminated with engine or gear oil, or possibly even just glazed, the symptoms may be less obvious.

I suppose one other thing you could try to rule out the rest of the driveline would be to shift to neutral while at highway speeds on a level or even slight downhill, unpopulated highway and see if the vehicle speed seems to pulse in a similar fashion. If not, and the engine speed is truly also not changing, that leaves the only option as the interface between the engine and the driveline (ie: clutch).

-Rod
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Old 07-04-2023, 09:46 AM   #6
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Re: 2005 Mazda 3 Randomly speeds up / down

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Cruise on or off? What gear? Flat terrain or rolling?
Cruise off. Every gear I assume, but not sure. Flat terrain
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Old 07-15-2023, 09:06 AM   #7
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Re: 2005 Mazda 3 Randomly speeds up / down

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Cruise off. Every gear I assume, but not sure. Flat terrain
Since rpm do not change, cruise off I would guess that either small variation and elevation or your right foot are to blameÖ Try it with cruise control on and in different gears and report back.
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