OT: Another 3.8 GM engine question

First off, I drive 120 miles/day - same stretch of road AND I know how to check mileage - over thousands of miles driven.

FIrst car was a 1992 Pontiac Bonneville with with 3.8 engine at 190,000 miles - got 27-28 mpg on highway at 70 mph. (car totalled in accident)

2nd car was a 1999 Pontiac Bonneville with 3.8 engine at 110,000 miles, got 27-29 mpg on highway at 70 mph. (engine and transmission went out, sold car)

3rd car IS a 1992 Buick Lesabre with 3.8 liter engine at 110,000 miles, I am currently getting 23 - 24 mpg on the highway at 70 mph. I have tested this over 6,000 miles and I can only get 23 - 24 mpg????

Why dones the 3rd car get the same mile per gallon? Well maintained, using the same engine oil and driving on the same road?

Thanks!

I think I'll talk to my mechanic friends about the MAF, O2, and TPS as John_Bud suggests.

I also have a '92 Buick Regal with the same engine (smaller car) but I don't really know what mileage I'm getting because the odo is inop. Based on my daily commute and the fact I need to refuel about every 10 days, I figure I'm getting in the mid to high 20's.

One thing I notice about the larger GM cars with 3.8 (Bonneville, Eighty-Eight, LeSabre) is that on any kind of downward slope the vehicle will pick up speed. On a level flat area you can take your foot off the gas and the deceleration rate is much slower than other vehicles. That's not the case with the Regal. I assume it's due to transmission and final drive ratios.

I also had a '94 Bonneville SSEi (supercharged) that did not "coast" as well as the Olds and it only got about 24 mpg, but that was because it was so much fun to hit the go pedal!

At 100,000 miles you should check the O2 sensors (actually you should probably just change them out). The TPS or throttle position sensor may also be off a bit and the computor will richen the fuel across the board. Also the Mass air sensor (if it has one) may be dirty and getting a false reading.

Did you check the odometer for accuracy? Some are off.

Best of luck getting the most mpg -

jb

The spark plugs I took out were the original GM plugs with 100,000 on them, the gap was very large - like 75-80 thousands...could that cause the coil to work harder and burn out? New plugs and wires are in it now.

IF you have ever seen a bonneville and the lesabere, they have the same basic shape and size, so I didnt think that was a variable.

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