'99 Mercury Grand Marquis
 

My family and my mom needed to make a trip from Russellville, KY to Indianapolis, IN over the weekend to my aunt's funeral who passed away last week. This is under a 300 mile drive and we decided to go in mom's '99 Mercury Grand Marquis. Since mom doesn't drive I was at the wheel and decided to see what I could get out of a full size car with a V8 engine. I filled up here in Russellville (topped off), drove to Indianapolis all 4 lane or interestate, drove approximately 30 miles of city driving/funeral procession driving inside the city of Indianapolis, drove back to Russellville, and topped the tank back off at the same station. The low fuel light came on about 25 miles prior to getting back to the gas station where it all began. Total trip was 540.9 miles on 17.181 gallons of gas for 31.48 MPG. Wish I knew what the mileage would have been without the 30 miles of city driving. This was accomplished by setting the c/c at 55 MPH and doing my best at timing out traffic lights in the Indianapolis area. This was my dad's car prior to his death and he had told me he normally got 28-29 MPG on the highway at speeds of 65-75 MPH so I just had to see what I could do with it. With the c/c set at 55 the total trip length was about 5 hours each way. Both on the way and on the return trip I probably never had to disengaged the c/c more than 6 times while on the 4 lane and interstate highways including interstate city driving in Indianapolis, IN and Louisville, KY. Pretty easy trip just sit back and steer.

My 2000 GM does that as well. 29/30 hiway all day long.

Sadly all that I have tried to make it do more has failed, hydro, fuel vapor, messing with the PCM..tuneing it for 27: 1 air/fuel all failed to make a dent..29/30 every time..

Rich

I'm surprised you weren't getting detonation at 27:1 ratio. Do you have a Scan Gauge? If so was it running in open or closed loop at 27:1?

Sorry forgot, it was 17 to 1...

And yes at 20:1 it was running crappy...

And it seems even thou my wideband A/F readouts said it was running at 17:1 the injectors cycles were up from 8/10% to 14/16% at this setting.

So the CPM was adding fuel but staying at 17:1.. can not figure it out how these numbers work out...

Found that the post cat O2s were causing this and with a device cancling them out I could now run 17:1 and keep the injector duty cycle at 8/10%...but still could not see a better MPG even then.

I hope to retest that soon.

RIch