MPG

[mikecarnut8]

Purchased a 199 Reattat with 48K miles a month ago.  Changed all the fluids and the gas and air filter. The car runs great but the millage is poor.  I am only getting 14MPG with mostly highway driving.  I took it to a shop to get it checked out and their computer could not find any problems.  Just in case the plugs were changed and the injectors were cleaned.  The milage went up to 17MPG.  Any ideas why the milage is so poor when the car is rated 18 to 27 MPG.  Or are the manufacture milage ratings this far off.  Any suggestions would be appreciated.

Thanks for all the input.  Let me try and respond to all the comments.  I an dividing the trip milage by the gallons of gas to fill the tank to get the MPG.  Most of the driving I am doing is on the interstate with the cruis set at 73 doing about 2K RPM.  The computer was showing no codes.  The car runs at the middle of the gauge.  I have used about half

a tank of gas that was in the car and filed up with new gas.  Could you expolain about advance ( can I use a timing light to check?) What is MAF reading and how do I check this.  What is short and long term fuel trims.  I assume that closed long and lockup TCC is taking it out of overdrive.  I keep it in overdrive and live with the acceleration.  Hope I have covered all the suggestions everyone has made.

Edited June 22, 2022 by mikecarnut8
Responding to comments

[ship]

I don't have a specific answer other than your mileage is in fact kinda bad...old gas maybe contributing to it?  Any codes present?  My '91 with 110K miles can get 30 miles mpg highway if I keep it under 70 mph.

Edited June 21, 2022 by ship

[BlakesReatta]

The MPG per car can vary wildly. I have had two 1990s that never got better than 18 mpg. My convertible only gets like 18 mpg. I have a 1988 that gets about 24 mpg on the highway. I had a 1988 that I sold that got 27 mpg. Some people will argue it depends on the day of the week the car was made based on who put the engine together.

[Padgett]

Kind of driving makes all the difference. I have seen 27 mpg at a steady 55-60 mph. Should be turning right at 2000 rpm at 70 mph in cruise. Nood to look at the timing advance, MAF reading, Short and long tern fuel trims, coolant temp, and is in closed loop & lockup (TCC) ?

[Ronnie]

My '88 gets around 20-24 driving around town and on mostly two lane roads. Don't use interstate if I can avoid it. I have a toggle switch that keeps the torque converter unlocked unless I do get on the interstate. That hurts gas mileage a lot but I prefer the better throttle response you get with the converter unlocked, and I'm willing to sacrifice a little gas mileage for that. Should note that my Reatta is mostly a weekend driver and I don't burn a lot of gas in it like daily drivers do.

[Ronnie]

Also, you might want to calculate the gas mileage the old way and see what you get. My gas mileage reading on the CRT starts out low after having the battery disconnected and improves as I put more miles on it. It's just an average and the more you drive at highway speeds the better your average mileage will get.

[2seater]

As mentioned above, get familiar with the built in diagnostics and monitor the various items that affect mileage. A plug in scanner can’t provide any more information than what is built in. Probably the number one item to change is the O2 sensor if the history of the present one is unknown. One other item is the original ratings were based on straight gasoline, not the presently common 10% ethanol, which does decrease mileage 2-3%

Edited June 21, 2022 by 2seater

[BlakesReatta]

So I saw on Facebook today other people complaining about gas mileage decreasing in non Reatta vehicles. One reason provided is the higher content of ethanol in our gasoline now, which is something like 15% at present. Ethanol does not burn as efficiently as ethanol free. Could be another reason why your mileage is lower. Last year, I was consistently getting 27 mpg in my 88.

Edited June 27, 2022 by BlakesReatta

[ship]

Ethanol still at 10% here in New England...don't know about the rest of the country.

[Padgett]

By law the maximum must be posted on the pump.

[BlakesReatta]

[BlakesReatta]

I'm not sure how much it varies from region to region. Common belief is that the Midwest is ethanol country because of corn production. Many stations sell E85 for use in Flex Fuel vehicles. I know that California has different blends of gasoline due to smog control, which is more expensive to produce.

[ship]

My understanding is that if you put E15 in a non-flex fuel car you will be in for a heap of mechanical problems.

[Padgett]

I suspect the ECM could be remapped for E15 but you might need bigger jets.

[2seater]

As long as the existing fuel injectors have enough capacity, the O2 sensor will still try to achieve stoichimetric mixture. I would believe this will manifest itself by the block learn or long term adjustment increasing above  the target 128 number. That works for closed loop but cold and open loop might be bit lean.

 

Rough calculation for the stock 19# injectors indicates they are good up to a bit over 190hp @ 85% duty cycle on plain gasoline. As the percentage of ethanol increases the amount required increases as well.

[Padgett]

Think the L27 (91 Reatta) has larger injectors.

ps the computer maps also have to command "more".