'89 RS 305 TBI / auto

Lately it seems the mileage per gallon has gone down in the car. It was averaging about 16 MPG mostly city and the past 2 tanks have been 14.5 and now 14 MPG. Its kinda strange...car has a full tune up lots of new parts, high flow cat and 3" catback, and an open air element.

Any ideas? It is getting a little bit colder so im wondering if that is contributing some.

Should we not be worried about this 1-2 MPG change? What do you mostly stock 305 TBI guys get around town?

This is my girl's car btw.

 

One explanation is that the weather is getting cooler which is keeping your ecm in open loop longer as well as the cooler weather makes the cool compensation fueling stay richer longer, blah blah blah. All kinds of ecm stuff happens when temps go down. Also some say "winter" gasoline is different which messes with the o2 sensor accuracy etc. Bosch o2 sensors actually run rich compared to delco sensors. And lasty, the mods you have listed may not directly hurt milage but if your putting your foot in it more to reap the benefits of the mods your milage will go down no doubt.

Brian

 

It's my car that Stevo's talking about. I haven't been driving it with any heavier of a food than I usually do. The only thing that has changed is the cooler weather and even then it's not that cold (50s/60s). It was consistently 16.6 city and now it's been consistently 14.4 for 3 tanks, but the last tank I only got 14.0. I actually hope it's something wrong and fixable, because I really don't like being at the gas station that much.

 

I get some pretty lousy gas mileage in mine as well but I do have a high mile unit and I have started modding it as well. So I am looking forward to even worse gas mileage LOL . I do not know what average fuel mileage would be with the L03????

 

I always notice a decrease in fuel economy when the gasoline switches over to the winter blends.

 

I get 20-22 MPG avg. with mine, this is combined city and hwy. but I have a 5-speed.

 

I calculated the mpg today and it was 12.66 .... there is no reason it should be THAT low. I do have remote start so I let it warm up nicely in the mornings, but no differently than I have been for the past few months. Perhaps this is related to my stalling problem.... hmmmm

https://www.thirdgen.org/forums/tech...-randomly.html

 

More than likely, yes. Especially if it's a tq convertor problem.

 

On a trip from Philly to Williamsburg, VA, the RS got about 25-26 mpg. Thats highway. In the city, where I do most of my driving, I get around 15-18 mpg.

But 12 sounds like there is a problem.

 

Well, we'll see what the new msd blaster coil does for it i just put it on this morning and just filled up the tank. Car hasnt had a stalling problem in quite some time now. It was getting bad at one point you had to put the car in neutral at every stop but now it stalls maybe once every week. Thanks for the help.

 

Well when the engine is running and the car stationary, you get 0 MPG. With the engine cold and the PCM in cold Enrichment mode it practically dumps in extra fuel.

Proper way to get good cold weather fuel mileage without hurting your engine.

Put your jacket on, your driving gloves. Crank up the engine. Scrape ice from windsheild. Put car in gear. Drive off without using much throttle for the first couple of miles. Let everything warm up while driving. Idling a cold enigne for extended periods is bad on the engine.

 

I actually just did some reading up on cold starting. A few things i was not aware of myself and ill feel kinda guilty too because i always told her to let it warm up all nice before she drives it.

My theory all along...engine is cold, oil is thick...if you dont let it idle and warm up and get in and drive getting higher up in the rpms to me would seem like increased wear because the oil is so thick when cold. I guess not that much cause itll go up with the pressure as you go up in the rpms. I am aware that the computer is in open loop which it is dumping more fuel to get it warmed up faster so i guess getting it out of that mode will give better gas mileage and properly warm the engine up faster. I actually got the remote start for the car so she wasnt freezing everytime she got in the car in the morning and so that it was warmed up properly without increased engine wear.

Reading more about it, i was also under the impression that letting a car idle used less gas then turning it off and then turning it back on. If a car is idling more then 10 seconds it uses more gas then simply turning it off and starting it back up.

Guess she'll have to suck it up and get in that cold car in the morning if she doesnt want to visit the pump as much or have increased engine wear on that 209k mile L03!

Thanks.

 

Not really the time of year for this, but in the wintertime, plug the block heater in over night (on a timer...you only need it for an hour or two). Result = warm block, warm coolant, warm oil. You probably won't notice much difference on the temp gauge as block heaters generally raise the temps about 50° above ambient temp. The heater will start working quicker though. It's an easy way to save on gas mileage, not to mention wear and tear.

No need to warm up your car in the driveway. Actually letting a car sit idling in the driveway is the SLOWEST way to increase the engine temps. Hence why it's harder on the engine to let it sit there.

Fast355's advice is good too.

 

Next time you're at your gas station take a look at the pumps and see if you don't see a "this gas contains X ammount of ethanol" my truck absolutly sucks gas when it's on the ethanol blended fuel and it's getting harder and harder to find uncut gasoline these days.

 

I should have replied awhile ago. Once summer came, the gas mileage improved some to about 15 mpg city. We then fixed a couple vacuum leaks (MAP sensor, etc) and it went up to 18 mpg city. Moral of the story? Don't overlook those brittle, 15 y/o plastic vacuum hoses.

 

I have always gotten better MPG in the winter...