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
Do you have the AC on when this happens?
Tester
Are you certain the car’s speed is changing but the engine rpms isn’t? If you are sure about that, the most likely cause w/ conventional manual transmission configuration, the clutch must be slipping.
Usually when this happens it is very obvious that the engine rpm is not matching up the speed. The problem could be a problem in the speedometer or rpm-display for example.
I agree with George that a slipping clutch is the likely cause if the symptoms are exactly as stated.
1 Like
Tester
More than likely, yes.
The RPM does not do that like the video sent. RPMs do not change, it feels very subtle
RPMs do not change when this happens though
Then what you might be sensing is the air conditioning compressor cycling on and off because the system is low on refrigerant.
Tester
2 Likes
Try turning off your AC and see if it still happens…as Tester mentioned… If that doesn’t change anything then:
I would check it with a scanner, you can watch your VSS speed as well as the abs speed sensors (if equipped) to see if they are going up/down as you described… Also use a GPS speedometer to verify… Your speedometer is not as correct as most people think, they are normally set 1 to 2 mph higher then your actual speed, I have checked this on many different years, makes, and models of vehicles… But the scanner will be correct if not malfunctioning… That is why police vehicles have certified speedometers…
You could have a VSS starting to go bad… or the dash gauge (speedo)… or whatever is between the two nowadays… lol
I don’t think a clutch is going to slip not slip then slip not slip over and over and over again without the RPMs changing and the vehicle speed only change very little… When a clutch slips either you are giving it gas and it slips or the vehicle start slowing down because it is slipping…