'99 3.1 issues

My '99 got about 29/30 on the highway up until about 2 years ago and now its barely getting 23 on hwy. It has 100K miles on it. Is there anything I can do here? I brought it in for a tuneup and the shop said plugs were fine and wires were good. No need for a tuneup! Original plugs and wires. Oh well...they are the experts. I had intake gasket replaced at 60K and probably needs to be replaced again. Could that zap mileage? Any advice here? Thanks

Was this mileage figured out in the warm weather or the cold weather? Cold weather will automatically reduce fuel economy sometimes a lot.

Have you recently replaced the tires? different brands of tires can also cause loss of fuel economy.

Have you checked the air in the tires? With the cold temps the tire pressure will drop significantly. generally 1 psi per 10 degrees. 10 to 12 psi (which is about the normal loss) can reduce economy by 12-22%.

Unless there are some symptoms of a problem than I would not be to concerned until you have the same results in the spring when it warms up.

One last thought, check to see if the fuel pressure regulator is leaking. Typically you would notice an extended crank under certain conditions if it was leaking.

There is alot more to a tune up then plugs and wires. Most shops are full of retards. plugs, plug wires, air filter, fuel filter, clean coil(s), clean throttle body, run fuel cleaner. etc etc...

What temp is the car running at. If the thermostat is stuck open it will cause the car to run rich. Same with an o2 sensor. At 100k if you have not done a fuel filter it may be time.

Same with the throttle body. If it has significant carbon build up, you will see poor fuel economy. On the highway I get well over 30 mpg in my Ls with the 3.1. As long as I stay around 55-65. Anything above that and my mpg shoots way down.

If the car is at 100k and the throttle body has never been cleaned it will be rediculously dirty. You will have to take it off the intake manifold and clean it with either 406 BG throttle body cleaner, lacquer thinner and brake clean. Use a brush. Clean the IAC valve with the pintle pointing down. If any cleaning agents get into the valve it will mess it up. The TPS is not to be cleaned or touched by any chemical. That is an electrical part, not mechanical so any agents will mess it up.

Well look into that stuff and let me know. I have more things but those are a good start.

Thanks Air and SC2. The operating temp is at about half on the analog gauge. I had the fuel filter changed when I brought it in for a tuneup that they said I did not need. I am going to have my intake gasket replaced by a friend of a friend when the weather warms up. This will be the second time it as replaced but I understand now the gasket is steel and rubber and should last for the remaining life of the car. SC, mine is also an LS and no, I have not had the throttle body cleaned ever. I only run hyperlube cleaner thru the tank periodically. It runs good but it seems since the gasket was changed at 60K, the mileage has plummeted. When I get gasket replaced, I will have the guy clean out the throttle body and look at the injectors. Love the car! No rust at all!