alexhouston Posted August 17, 2006 Share Posted August 17, 2006 My '88 Reatta (The Riviatta) shows to be getting 12.5 MPG on the screen. Not really an issue to me except the daughter has started complaining about why does it use so much gas? I can't get it away her long enough to check the milage by miles driven and gallons put in. It used to indicate closer to 25mpg. Still runs fine and strong with no other symptoms except for the dreaded yellow light. 104K miles.Can anybody give me a clue as to how the sensor for milage works. Could it be something to do with my recent installation of the Riviera fuel sending unit? Fuel guage works perfectly. Link to comment Share on other sites More sharing options...
Guest F14CRAZY Posted August 17, 2006 Share Posted August 17, 2006 As I understand it, the MPG computation is calculated from readings from the throttle position sensor. Is the car's system getting reset everyday, like with a bad battery (just enough to turn over?). If it gets reset, it takes a while for it to get accurate again. You could go into OPTIONS and reset the MPG average, and see what happens. Link to comment Share on other sites More sharing options...
Guest EDBS0 Posted August 17, 2006 Share Posted August 17, 2006 Clean air filter, fresh oxygen sensor will go a long way to restoring mpgs.AND it takes under 5 minutes to do. Link to comment Share on other sites More sharing options...
alexhouston Posted August 17, 2006 Author Share Posted August 17, 2006 I am quite sure it is not a dirty filter or Oxygen sensor issue. Both new at last tune up recently. Link to comment Share on other sites More sharing options...
ncgirl05 Posted August 17, 2006 Share Posted August 17, 2006 My '89 has a new oxygen sensor, new spark plugs, new wires, K&N air filter and still only gets 22 to 22.5 MPG. I can't figure out what to do to get more MPG - I've seen others here report 27-30 MPG! My sweetie's black '89 Reatta with 197,000 miles gets 25.5! Am I missing something here? Link to comment Share on other sites More sharing options...
padgett Posted August 17, 2006 Share Posted August 17, 2006 Gas milage can depend on a lot of things.1) the 88 had a PROM replacement to correct the reading being low2) tune is critical to any engine3) tires must be properly inflated - nowadays this means over 304) how it is driven also makes a lot of differenceThe last needs expansion: the 3800s I have had (Reatta, Bonneville, TranSport) have all been similar: 16-18 mpg in short local hops not over 5-6 miles per leg and not over 45 mph. And 24-27 (uncorrected) on the Interstate. On long trips (all Interstate) I have seen 30 in cold weather (a/c off, windows shut, no stop between gas stations).The best gas mileage seems to be between 50 and 60 mph and drops slowly at higher speeds.12.5 mpg I would say something is definately wrong. If everything feels good and there are no error codes then the most likely suspect is the O2 sensor (or like my wife, sitting in a parking space for a 1/2 hour with the a/c on while taling on the cell phone). They can occasionally be bad out of the box.I would suggest looking at the instantaneous readings while on a long level stretch in lockup at about 50 mph. Link to comment Share on other sites More sharing options...
Vincent Vega Posted August 17, 2006 Share Posted August 17, 2006 Both my 88's have low mileage calculated for city driving. A balance of highway and city produces a better calculation. When calculated manually I average about 2 mpg better per tank than what is calculated on board. If it never gets driven on the highway don't expect to see it calculate higher than 17 mpg or so. Link to comment Share on other sites More sharing options...
padgett Posted August 17, 2006 Share Posted August 17, 2006 OK, a bit more info: the 88s has a TSB for low MPG readings compared to the actual use and a reprogrammed PROM code ANCX p/n 16123488 was issued by GM per TSB 88-6E-23. The MPG is 15-20% highter with the new Memcal. I believe that is to only Memcal now available for a 1988 Federal Reatta. With the ANCX ED99 will show a PROM ID of 9864. Link to comment Share on other sites More sharing options...
charlie1 Posted August 17, 2006 Share Posted August 17, 2006 The best Highway mileage I ever got with my 1989 Riviera was 30MPG between Ashville NC and Philadelphia when the speed limit was 55MPH. Do not like 55 but I think these cars were geared for 55MPH.Charlie Link to comment Share on other sites More sharing options...
padgett Posted August 17, 2006 Share Posted August 17, 2006 I make a boo boo - that is the ID for *my* Prom. The stock ones are different:Most stock 88s will be 2541 (5259 and 9266 are also in 9/87 book but that is very early). The updated PROM is 3534. Link to comment Share on other sites More sharing options...
Vincent Vega Posted August 18, 2006 Share Posted August 18, 2006 Is the prom ID actually printed on it or stored electronically? Link to comment Share on other sites More sharing options...
Guest F14CRAZY Posted August 18, 2006 Share Posted August 18, 2006 I know for sure it's electronic, but don't have a "free" memcal to check to see if it's printed. Link to comment Share on other sites More sharing options...
padgett Posted August 18, 2006 Share Posted August 18, 2006 There is generally a silver tag over the UVProm window that is visible from the top but ED99 should display the numeric code. Link to comment Share on other sites More sharing options...
HNLCityLights Posted August 18, 2006 Share Posted August 18, 2006 Put some 100% acetone with the ratio of 1 oz to 10 gal. gas and it upped the milgage on our 88, 89, 90 Reatta and 82 Corvette. You could go 2 oz to 10 gal gas if your state puts 10% Ethanol in your gas.Gained about 5 mpg for about $0.02 worth of acetone at $3.25 @gal for 87 Octane here in Hawaii.Be careful not to spill any on your paint... Link to comment Share on other sites More sharing options...
DeeBee Posted August 19, 2006 Share Posted August 19, 2006 My '89 gets 26 city and 30-34 freeway depending on speed. On a recent 1900 mile trip from California to Idaho and back, through Oregon I got 32 all the way. The average for the first 900 mile leg was 30 mpg.On the return I got 34 through Oregon and down to most Northern California. After that, speed when up to 75-80 mph and I got 30.I put a bottle of Techron in the tank, new plugs, air cleaner (Purloator), Castrol Synthetic oil with a Mobile One filter, new plug wires and tranny service and flush. Brand new tires (Fuldas, stock sizes running at 32 pounds.Ran Cruise control. My trick was to take off the cruise on hills and back off slightly on the throttle going up to prevent a downshift. Speed stays within 5 mph of where I want it but gas mileage goes way up on the average.I'll admit to not running the Reatta like I do the Fiero. I like a more sedate ride in the Reatta. Smooth is fast, fast isn't. Link to comment Share on other sites More sharing options...
alexhouston Posted August 20, 2006 Author Share Posted August 20, 2006 Solved the puzzle.Charactoristics of city driving and a teen with a lead foot!I cleared out the trip settings and started with a Zero MPG reading. Drove down the highway and got 31mpg intantaneous mpg after a few miles. Then got closer to home with all the stop lights and traffic. Average quickly went to 12.2mpg. Teen then drove it and came back with 8.1 mpg showing.Cosidering we almost never drive this car on the highway, and most trips only a couple miles at most, I guess that explains the low average that shows up on the screen.Thanks for everyones input. Link to comment Share on other sites More sharing options...
padgett Posted August 20, 2006 Share Posted August 20, 2006 Still not right. Puttering around locally with a max speed of 45 mph, a stop light every few blocks, and no more than 6 miles per leg (tank lasts a month) I still average over 16. Link to comment Share on other sites More sharing options...
Rawja Posted August 20, 2006 Share Posted August 20, 2006 Padgett, I think you missed the salient point:<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Teen then drove it</div></div> I'd probably have a talk with the teen, but probably all that would accomplish would be getting the mileage computer reset before each trip home to cover his tracks. <img src="http://forums.aaca.org/images/graemlins/tongue.gif" alt="" /> Link to comment Share on other sites More sharing options...
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