1st time caller, long time viewer. Bought a 97 Discovery for my daughter, seemingly well maintained. After a couple of months the truck started "surging" after about 20 - 30 minutes of normal driving. The engine RPMs surge between 1700 and 2000 RPM about every 3 seconds. I also noticed that the temperature guage showed normal for a while after starting down the road, but when the surge started it was bottomed out cold. Changed Mass air flow sensor, fuel filter, plugs and wires. Going for coolant temperture sensor next. Anyone have any other ideas?

 

Surge is probably due to dirty idle air control valve (stepper motor). Lot of write ups on process to clean it. Just be sure not to change valve position when you have it out.

 

It only surges when warmed up and under load. I'm curious why the temp gauge goes to dead cold at the same time.

 

Surging and the coolant temp gauge is attributable to the Vehicle Speed Sensor. Yours needs to be replaced.

 

Wasn't doubting you guys, but already had the CTS and didnt want to change a whole bunch a parts all at once. The surging is unchanged but the temerature gauge stayed normal thoughout the drive while it was surging. Just so you guys can understand my ignorance, I'm used to working on a 1957 Ferguson tractor, but did own a 74 MGB back in the early 80's (I am mildly familiar with the Prince of Darknesses electrical prowess). Can you guys help me decide which I should try first? The $200 VSS or the $50 stepper motor? BTW another new symptom came up that I waasn't aware of. When the vehice was sarted for the first time yesterday (has been setting for about 3 weeks), the RPMs went to 2100 and stayed there. I waited to see if when the truck warmed up if it would idle down. It did not. I turned it off, resarted, and RPMs were normal (800-900ish). Started this morning, same deal. Started this afternoon, RPMs normal on first and subsequent starts. Thanks, guys!

 

First, normal startup rpms should be a grand and then settle at 675 - 700 once the engine starts to warm up. It's like an automatic choke. Your rpms at 800-900 are too high, can be adjusted down by relieving the throttle cable tension using the thumb screw at the throttle lever.

This I had to copy from the Service Manual so you can decide for yourself.

Vehicle speed sensor
The vehicle speed sensor is located on the transfer
box behind the transmission brake. The unit has a
multirole as the vehicle electronic speedometer
sensor and as a sensor for ’surge damping’, (a
function which smooths out engine response to sharp
accelerator movement and reduces vehicle ’bucking’).
The sensor is an additional input for engine overheat
protection