small rpm changes while driving at constant speed

T

truav8r

Veteran Member
Joined
Mar 13, 2003
Location
Albany, NY
TDI
2003 Jetta GL TDI Silver Auto
While driving at a constant speed on level roads, lately I've noticed the engine tends to vary its rpm slightly. I've noticed this for the last 6 months/10k miles or so. It's kind of subtle (no more than a 25 or 50rpm fluctuation according to the gauge). My wife doesn't notice it, but I do. I can definitely feel it a bit, and see it on the tach. This happens with the cruise control engaged as well, though not quite as stong of a fluctuation. Is this normal? I don't think this has been happening since day 1. I like to think I'm pretty much in tune with how the car should feel and sound. Seems to be more of an annoyance than anything. Mileage is still in our normal range: 37-40mpg on local roads, 45-47 on the freeway.

All maintenance is up to date and by the book. D1 oil and filter changes every 10k, fuel filter every 20k, and Dennis (metalman) did a complete timing belt and intake cleaning for me about 2 months ago (this was definitely happeneing before the timing belt, and I thought the vag-com routine after the timing belt would smooth things out) and he also replaced the EGR for me (it was leaking oil). I run BP fuel from a busy station almost exclusively, with 1 oz. of Primrose 405c at every fillup. Car currently has 102k, and otherwise runs like a champ.

Any ideas?? Is my fuel pump dying? Or check injection quantity? All help appreciated. Thanks.
 
mrGutWrench

mrGutWrench

Top Post Dawg
Joined
Aug 29, 2002
Location
Carrboro, NC
TDI
'03 Jetta Wagon, 5-speed, 563K Miles (July '23)
truav8r said:
While driving at a constant speed on level roads, lately I've noticed the engine tends to vary its rpm slightly. I've noticed this for the last 6 months/10k miles or so. It's kind of subtle (no more than a 25 or 50rpm fluctuation according to the gauge). My wife doesn't notice it, but I do. I can definitely feel it a bit, and see it on the tach. This happens with the cruise control engaged as well, though not quite as stong of a fluctuation. Is this normal? I don't think this has been happening since day 1. I like to think I'm pretty much in tune with how the car should feel and sound. Seems to be more of an annoyance than anything. Mileage is still in our normal range: 37-40mpg on local roads, 45-47 on the freeway.

All maintenance is up to date and by the book. D1 oil and filter changes every 10k, fuel filter every 20k, and Dennis (metalman) did a complete timing belt and intake cleaning for me about 2 months ago (this was definitely happeneing before the timing belt, and I thought the vag-com routine after the timing belt would smooth things out) and he also replaced the EGR for me (it was leaking oil). I run BP fuel from a busy station almost exclusively, with 1 oz. of Primrose 405c at every fillup. Car currently has 102k, and otherwise runs like a champ.

Any ideas?? Is my fuel pump dying? Or check injection quantity? All help appreciated. Thanks.
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__. My '02 sedan did that occasionally for all it's life; my '03 also does it but not nearly as frequently. My '03 (wagon) doesn't get as good mileage as the sedan either (but, then again, it doesn't have tires on it either ...:( )
 
D

DPM

Top Post Dawg
Joined
Mar 16, 2001
Location
Newtownards, N. Ireland
TDI
2019 Rav4 AWD Hybrid, Citroen C4 BlueHDI
If road speed is truly constant, changes in engine speed imply slippage or a failure to lock-up of the torque converter clutch.
 
T

truav8r

Veteran Member
Joined
Mar 13, 2003
Location
Albany, NY
TDI
2003 Jetta GL TDI Silver Auto
DPM said:
If road speed is truly constant, changes in engine speed imply slippage or a failure to lock-up of the torque converter clutch.
Click to expand...
The transmission is working rock solid. In fact, I forgot to mention that I also had Dennis change the tranny fluid as well. The tranny has received daily workouts since new. I don't quite drive it like I stole it, but I do drive it like I'm borrowing it for a while :)

The rpm movement is minor enough that I don't think it stems from the tranny. Something is just making the engine itself rev up and down slightly. Or as MrGW notes, this could be a normal thing that I've just started noticing lately.
 
L

lemke

Member
Joined
Jun 5, 2005
Location
D-Town
TDI
Jetta, 2000, galaxy blue
my 00 tdi will do this when driving at a constant speed in 1st gear
 
mrGutWrench

mrGutWrench

Top Post Dawg
Joined
Aug 29, 2002
Location
Carrboro, NC
TDI
'03 Jetta Wagon, 5-speed, 563K Miles (July '23)
truav8r said:
(snip) The rpm movement is minor enough that I don't think it stems from the tranny. Something is just making the engine itself rev up and down slightly. Or as MrGW notes, this could be a normal thing that I've just started noticing lately.
Click to expand...
__. On mine, it was a slight "opposite of hesitation". It wasn't so much that the rpm changed, it was that the power obviously went up for a "hiccup". The rpm may have changed fractionally and the speed may have changed fractionally (I think in an upwards direction), but it was more a "felt" thing. I'm guessing a slug of oil from the turbo collecting in the intake, then getting sucked into the engine causing a slight increase in power ("fuel" not from the fuel injection), then the fuel injection electronics getting it back under control and resuming normal speed. At least, that's what it felt like.
 
T

truav8r

Veteran Member
Joined
Mar 13, 2003
Location
Albany, NY
TDI
2003 Jetta GL TDI Silver Auto
MrGW, I think that's a very good description of what I'm seeing. The rpm bumps up ever so slightly, only to settle back down within a fraction of a second.

As I said, the car runs great otherwise. I suppose I'll just watch it for a while, and if there is no change or other ill effects I will chalk it up to the personality of an engine with 100k miles that has it's sights set on 300k :)
 
A

acktdi

Veteran Member
Joined
Nov 1, 2001
Location
Las Vegas
TDI
Golf GLS Reflex Silver
My golf had the same problem, it was caused by low transmission fluid level.
 
W

weasel

Deactivated Member Account
Joined
Sep 12, 2000
TDI
None.
What's the mileage ?? Fouled injectors can "drip" fuel and might cause this little power bump. My dad's garage oil furnace did this. Sounded like little explosions.
 
N

nick02

Veteran Member
Joined
Aug 22, 2004
Location
Johnson City, Tenn
TDI
2002 Jetta silver
I would consider wind. Mine does it also. Only at specific areas. I finally realized during stormy conditions these same areas were very pronounced. Nick
 
Powder Hound

Powder Hound

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Joined
Oct 25, 1999
Location
Under a Bridge, Crestview, FL, USA
TDI
'00 Golf 4dr White 5sp, '02 Jettachero 5sp, Wife's '03 NB Platinum Gray auto(!)
Any change in engine load will cause this phenomenon. Such small changes in load will be caused by the alternator or a/c compressor changing load, most likely the alternator.
 
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