CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

2002 Ford varying rpm on cruise control

This Is a 2002 Ford focus se 4door wagon dohc Vin 3 automatic transmission.
So in cruise control going any faster than 35mph and having a varying 500 rpm and the speed drops up and down 2-4mph as well I do feel it in the car.
I replaced my output speed sensor but that wasn’t the issue.
No code is showing up either on the obd2 reader.

If you have a vacuum diaphragm type of cruise control system…you may have a leak in that diaphragm and or a leaking hose that provides vacuum to this unit… check and see…its easy…

Assuming its a vac system…remove the hose to the unit…squeeze the diaphragm in and put your finger over the hole where the hose was…it should hold steady… Then look at any and all hoses associated w the system and report back. Just some suggestions…

I don’t think this car has a vacuum type of cruise control.

The cruise control needs to know both the engine rpm and the vehicle speed. You might need to replace the other sensor. When you are holding a steady speed does the speedometer hold steady as well?

The speed does not hold stead the speed bounces along with the rpms an example is in cruise control driving 45 set bouncing from 43-48 somewhere along those lines and I’m driving on straight clear roads no hills so that is not what’s causing this.

What about when you are driving a steady speed without CC. Does the speedometer hold steady? or does it bounce around like it does driving w/CC?

So you are telling us that your dash gauge cluster is showing rapid fluctuations of both rpm and speed…and obviously these are not real events?

If that is the case you have an issue in either the connections of or the power to that system… I would look for one unifying factor…like power. Because speed and rpm are nicely separated systems getting their info from two different sources.

You may have a ground or battery connection issue. Have you looked or considered this?

When I’m driving at a steady speed without CC the rpm stays fairly the same or as best as it can.

These are real events as in the car feels like it’s losing power when the rpm drops and the speed physically drops the car and when the rpm bounces up I can feel the car pushing the gas in cruise control and the car physically goes faster so its not the instrument cluster I can physically feel the car drop down when rpm drops and push up in power when rpm raises

Perhaps the throttle body is not operating smoothly.
At 18+ y.o. it’s probably due for a cleaning.

I will get to cleaning and see if that helps but I should say this car only has 85k miles so it doesn’t have that much usage for it being 18years old

That’s the equivalent of a 90 year old person with one foot in the grave and the other foot on a banana peel.

Cleaned the throttle body, it didn’t make a difference.

Cruise control is a feedback loop.
The system can become unstable if there is too much “loop gain” or phase shift.
In more plain English, if the throttle is too sensitive,
or there is too much time delay between changing the throttle position and the engine responding, the speed could wobble up and down.
So, does the response to the gas pedal feel at all unusual?

Two things that can make the throttle more sensitive to changes are over advanced ignition timing and low EGR flow.
I suggest you check ig timing and make sure EGR passages are clear.