Pedal constant, speed varies slightly

[David 84 911]

Anyone know why at a constant speed, say 60 MPH, why my speed would seem to consistantly rock up and down by 1-2 MPH?

Oxygen sensor?

Cheers!

[svandamme]

a steady pedal doesn't guarantee steady speed, there's many things affecting this

pedal itself
linkage to the back of the car
road surface variations
road angle, half a degree up or down
wind
cars or trucks in front of you ( drafting ) even if they are 50 meters away...

minor changes in all those parameters, all result in changed speed with steady pedal

besides, the speed itself isn't the most accurate either, how did you measure the change in speed in the first place?

the 84 has an electronic speedo signal if i'm not mistaken, if so , could be minor electrical signal noise
if it's with a speed cable, could be something slightly loose or worn cable..

1-2 mph at 60mph , i don't think that is noticable by feel or out of the ordinary.... if the rpm's are steady , and you don't notice any stutter in the engine, or misfiring...

[David 84 911]

all things being egual or constant (ie: road, other traffic etc....) it almost feels like a slight rocking or accel/ decel sensation, I feel it but do not notice aything on the speedometer. It has been doing this since I got her about 20,000 miles or so and no change.....Just one of those things I would like to figure out...I know I am due for a new fuel filter.

[svandamme]

that's a different story then , i'll let the 84 engine connaiseurs take over , if figured you noticed the speedo going up and down, with no other symptom

[cbeers]

is this related to a speed range or a range in RPMs?

In other words, are you experiencing a surge senstation or actual changes in road speed.

And does the sensation/speed change only happen at a certain rev range, say 3-3500k?

-Chris

[nhromyak]

Could be a lazy O2. Or a vacuum leak. Seems strang to be happening at this rpm though...

It could also be a worn AFM or burned out resistor in it....

[David 84 911]

Thanks. The faster I go the less it does it.....
Sorry for my ignorance but what does an AFM and a resistor do? I'm a gear head wanna be

[Huh]

AFM=air flow meter. It is a heated wire that measures the air across it then uses the MAP from the chip to give the a precise amount of fuel for optimum combustion.

[nhromyak]

The AFM in the Carreras (84 - 89) use a "barn door" to measure air intake and vacuum. The resistor sweep of this barn door has resistors as opposed to a coiling of wire like most rehostats (or whatever they are caleld).

Usually the sweeping arm can "wear-out" a section and produce a bad spot. Bending the arm and changing it's sweeping area will fix these issues. There is an article somewhere.

However, I don't seem to get most issues, I get weird ones. I had one resistor burn out (I think from a bad alternator/regulator spike) and I have a small empty spot on my AFM barn-door sweep. Changing the sweeping arm did not affect this "empty" spot.

I hit this spot on acceleration right around 2400 rpms and deceleration from higher rpms (I think). It's been a while since I noticed the problem area. I get a slight pop on deceleration, and a hicup on acceleration.

[svandamme]

don't 84's use a vane type air flow meter???

[Huh]

So it uses the same thing as the VWs with Digifant engine management. When did Porsche go MAF (mass air flow)? Yes then it is an air flow meter that uses a vane with two arms on it. The second arm is to help reduce the surging from air rushing in. I haven't know them to go bad very often. Can you use a VW part if it needs replaced? May be cheaper. I can find that stuff all day long too.