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Gas mileage question

Hey Guys, I have a 1991 GMC Jimmy that lives in Mexico, I am there in the winter, it runs fine, starts great. No missing, no lagging, pretty good power (as far as I can tell). BUT I seem to be only getting about 8 miles a gallon? I really don’t even know where to start. It is tuned and like I said runs fine. All filters are new, etc… Any ideas would be greatly appreciated.

What kind of roads are you driving on? Nice highways, or slow going dirt roads? Flat terrain or lots of hills? Lots of stop and go’s or mostly highway cruising at 55 mph? 8 mpg isn’t good, but under some conditions that might be what to expect. '91 Jimmy’s were never very good vehicles as far as high mpg is concerned.

I’d start with a new air filter, a dirt clogged filter will kill mpg. Next some new spark plugs. When you have a new filter and new plugs check your mpg and see if it improves.

If you have the 8=cyl and 4WD, city mileage is estimated at 11 MPG. 8 MPG is not out of the question. What engine and drive train do you have?

4 liter 6cyl, 2 wheel drive. A lot of the driving was on VERY steep hills, I mean very. But only for short distances. All filters, plugs, etc are new. Thanks for the feedback, very helpful.

What is the estimated MPG for this car?

It could be a matter of (Mexican) gas quality.
Is it short trips where the engine barely gets warmed up?
Low speed “puttering”?

A bad oxygen sensor isn’t out of the question on a 22 year old car.

Based on the EPA rating, 8 MPG wouldn’t be out of line on a 4WD Jimmy. Factor in age, engine wear, and possibly a few things such as lowered tire pressure and transmission slippage and 8 could be the norm. A few steep hills can also be a mileage killer.

This vehicle has a distributor so you might consider checking the ignition timing to make sure that it’s correct. Retarded timing can hurt fuel mileage; especially with hills involved.
Advancing the timing a degree or two over stock can help things a bit but considering the steep hills this should be done with caution.
You do NOT want to hear any pinging out of that motor if you advance the timing. If you do (and it wasn’t pinging before) retard the timing back to where it was.
If the timing is checked this should be done with the test connector properly grounded; otherwise any timing check will not be accurate.

Poor mileage as a change is often associated with reduced power. Hills, length of trips and age of vehicles contribute, but without know your driving style, is mostly conjecture on our part.

I would also question the conversion from the liters you buy @ Pemex to the gallons and miles that you drive. Are you sure of your conversion?

Thanks for the tip! I am using the standard conversion of multiplying gallons by 3.75. Should I be using a different multiplier?

You’re close. It is actually 3.785 liters per gallon (US liquid). But that’s a difference of only 1%.You wouldn’t notice it in your calculation.

@PVTodd Make sure you convert the kilometers to miles by dividing by 1.609. Kilometers/liter translates to 3.80/1.609=2.36 conversion factor approximately to get miles per gallon. The gallons to liters conversion is closer to 3.80 than 3.75.

Thanks guys! I use 3.785 for the liters to gallons conversion. The odometer is in miles so no need to reconvert. Great information.

@PVTodd, 1 gallon is 3.785 liters

Does the exhaust – especially after a few minutes after a cold start – does it have any hint of a gasoline smell?

George, I am in the States and will be back in Mexico in a few weeks. I’ll check the smell. If it does smell of gas, what do you think it could be? Too rich? Should I check it after just a few minutes after starting?

That would be an indication the mixture is too rich. A slightly overrich mixture will run ok but the mpg will be less. It could be too rich both on cold starting and after it is warmed up. But too rich on cold starts, this usually this would cause a starting problem in hot weather, so I expect the problem you are having is that it is too rich after it warms up. Another symptom of too rich is a sort of black soot (dry carbon like stuff, not very greasy) at the end of the tailpipe. No harm to take a look.

It’s not unusual to have a faint odor of gasoline from the tailpipe on a cold start even when everything is ok. Check the odor about 5 minutes after starting, when the engine has had a chance to warm up a bit, then again after the engine is at operating temp. Could provide a clue at least. Best of luck.

That will be off a little. One gallon = 3.785 liters.

Try this

With an injected engine can the richness be leaned out?