Poor gas mileage


#1

I have a 1990 Pontiac GrandAm SE that just hit 50,000 original miles. It is a 4-cylinder and only gets about 22 miles per gallon freeway.



The other problem is

When I start the car for the first it the engine sounds fine. As I drive and the temperature gauge reaches 110-115 degrees the engine starts to rev up and down when say stopped at a redlight. The reving is loud enough to be heard when standing outside the car…

Are the two problems related?



Had a tuneup 2 months sgo. EGR Vakve and a bigger valve located just underneath airfilter tray replaced.


#2

The idle revving sounds like a problem and when the solution to it is found that may improve the mileage. Sorry I don’t have a suggestion for that, but someone likely will.

I don’t know how you drive, but most all drivers can do a little modification of how they drive and gain significant mileage increases. Keep the speed down to the speed limit and where safe maybe lower. You will get much better mileage at 55mph than at 70mph. Slowing down by taking the foot off the accelerator when coming up to a stop or traffic light will also help.


#3

The low fuel mileage and the engine surge may be related. The first thing to have checked is for a partially stuck open thermostat. If the thermostat is stuck partially open, it takes longer for the engine to come up to operating temperature, if it does at all. Or it may come up to the hairy edge of operating temperature where it switches between open loop and closed loop mode.

The other thing to have checked is the coolant temp sensor for the computer. Again, if this sensor has decided to go flaky, it can cause both reduced fuel mileage and engine surge.

Tester