Help --- MPG from 20 to 15 In a Month

craptastichevy

Looking for any hints as to what the problem might be. My 94 Blazer runs just as rough as it always has, but has gone from 20 MPG to 15 MPG in the course of October 2001.
First, question, as always......no driving habits have not changed, if anything they have gotten so as to provide better fuel economy. O2 has been replaced, the carbon has been removed, fuel filter has been changed.......can't think of much other than the fuel injectors that could have gone sour.
But that brings me back to my confusion, the cars runs no differently now at 15MPG than it did at 20 MPG.
And let me add that at 2500 miles a month, every bit of fuel economy counts.

D-

brorjace

Craptastic
Eliniate the obvious by replacing the distributor cap, rotor and wires, as well as the air and fuel filters. (Basic tune-up stuff)

While you're at it, run a really good fuel injector cleaner like Techron or Redline SI-1 through the tank. Even though you've been recently de-carboned, other bits that were not completely dislodged might now be causing trouble and you'll want to get rid of them.

Check all the oils in the driveline, especially the differential fluids.

Check your tire pressures. I recommend putting in 80-90% of the max sidewall pressure in each tire.

Check to see if you have a sticking brake caliper. After driving a fair distance, get out and sniff at every wheel. Smell burning asbestos? This might be a clue. Your brakes are always "on" to an extent and you're using more fuel to overcome this. Replcae the offending caliper with a rebuilt one.

Keep us posted on your progress.

--- Bror Jace

jgmilberg

I would start with a distributor cap, rotor, spark plug wires and spark plugs. Get the good cap, rotor and wires!!!!! The cheap ones ALWAYS cause problems. I only use Delco or Accel parts, they give the best performance for the money and outlast the cheapo stuff 3 to 1.

If that stuff doesn't help, I would get the injectors cleaned. You need to find a shop with the kit that runs the car from a can of cleaner. The shop will shut off the fuel pump and install a can of injector cleaner on to the throttle body and they will let it run until the engine stops, then hook everything up like normal and the injectors will be cleaned. I have tried the dump in the tank cleaners, and they are not as good.

eharri3

Check to make sure your math is correct. Make sure you fill the tank until the nozzel shuts off, drive for awhile, fill it to the same point again, then write down how many miles you drove and divide it by the number of gallons it took to fill up the tank the second time. I'm not in any way trying to insult your intelligence, but many people check mileage incorrectly by doing things like basing their calculations on how many miles they rack up before the gauge reads E, not realizing that gas gauges can be pessimistic and there's often a big reserve left at that point.

andre1969

Not all fuel nozzles will shut off at the same level. I think the best way to get a true reading on fuel economy is to run the tank down until it's almost empty, and then fill it back up. You may still get an inconsistency from tank to tank, but since you're spreading that inconsistency over more gallons, it'll still be more accurate.

For instance, I recently purchased a 1979 Chrysler New Yorker. I bought it in West Virginia, and filled up the tank before making the trip back home. Once I got back home, I filled it up again. That run was about 157 miles, and it took just under 7 gallons to fill it back up. That comes out to about 22.5 mpg, which sounds just a *tad* optimistic for a 23 year old car that weighs over 3800 lb, with a 360 2-bbl, and running at speeds up to 80-90 mph. And just for a reality check, in case the odometer was off, I checked it against my Intrepid, which we drove out there to pick up the car. It registered about 154 miles on the trip, so it was close.

Anyway, the next time I filled up, I let it run almost empty. It has a 21 gallon tank, and when I filled up, it took about 19.8 or something close. Anyway, this time, the mpg came out to about 10-11 mpg, which was mostly back and forth to work, and a bit of highway driving.

I think the reality of what happened is the first time around, when I put the 7 gallons in, it would've taken more if I'd really tried to top it off. But I'm not that patient ;-) So it probably burned more than 7 gallons on the highway trip home, but then as a result it burned less than 19.8 on the second tank.

Or maybe these things really got that kind of mileage? But still, 10-11 city and 22.5 highway seems like an awfully big spread!

eharri3

Which is why you should avoid emptying the tank but instead use the same gas station at least twice to get an accurate idea of mileage. The innacuracy of the gas gauge combined with the reserve amount in the tank can throw figures off.

andre1969

Craptastic, did they switch to the winter formula oxygenated gasoline where you live yet? Personally, I've never experienced a drastic decline in fuel economy when our region switches over to it for the fall and winter, but I've heard horror stories about it hurting fuel consumption, goo-ing up fuel pumps, etc.

Or has it gotten drastically colder around the same time frame that your fuel mileage worsened? I don't notice it so much with my Intrepid, but just about any older car I've ever owned would get worse mileage when the weather got bad. I guess because they'd take longer to warm up, the choke would be on longer, and stuff like that. As your Blazer ages, it may become more succeptible to cold weather.

craptastichevy

Just as an update, i have been tracking my mileage for the past 15,000 miles for my work, so I think it is safe to say that any and all errors have averaged themselves out the last 5 1/2 months. I apprciate all the ideas and opinions, thanks bror jace, the blazer gets to go back to the shop again this weekend......once again, thanks for the help, hopefuly it will be another problem solved.

D-

craptastichevy

One more problem, this one is fresh from this mornings commute. Almost all my driving is Highway driving, and during acceleration, my car revs very high but doesnt show any chage in speed, then all of a sudden takes off like a bat outta hell. It is like the transmission is slipping, or something is misfiring, but nothing was picked up by the computer, when I had it in the shop.
Well, I am sure it obvious at this point that I am not all that car savy, so once again, I appreciate all the suggestions.

D-

andre1969

I've seen that tranny problem on several different occasions, and on several different cars. My friend has a '98 Tracker that does what you mentioned, the revving real high without any real acceleration, and then suddenly taking off. The tranny shop changed his fluid and filter, which fixed it for awhile, but it's starting to come back. I had an '82 Cutlass Supreme that did tha same thing. In both vehicles, there was just too much junk in the fluid, mainly debris from the tranny deteriorating, and clogging the filter. I had the Cutlass rebuilt, but its simple tranny was only something like $650.00, back in 1993. The shop said my friend's Tracker would be about $1,000.

Come to think of it, my '68 Dart had the same symptom. In its case though, one of the cooling lines to the radiator got crimped shut, it blew a seal, and the tranny fluid leaked out fairly fast. It would run fine until the fluid got low, and then rev without doing anything. You might just be low on tranny fluid.

brorjace

Yes, I've noticed a decline in mileage when the seasons switch over. It's a fact that with cooler air you get more power and less mileage but I never considered the switch to a "winter-mix" fuel formula. That could play an important role as well.

Still, I get drops in mileage like 3-6% and NOT the 25% reduction craptastic is talking about.

That was a good point about measuring your mileage. That's why you should take at least 4-5 consecutive tankfuls and use the average of them all for your mileage. With the variations in filling the tank, mileage can vary wildly from one tank to the next. I know that's the story with my car.

Craptastic, is your vehicle 4WD? Did it get engaged without you knowing it? You might also want to check the tire pressures ... even if they look fine. Those are just a couple thoughts I had.

I have a friend at work with a vehicle just like yours. After a new O2 sensor, professional de-gunking, new fuel filters, cleaned EGR valve, she was still getting stalling ... especially on hills and they found low fuel pressure so I told her to make sure they replaced the fuel pump. That seemed to fix HER problem, at least.

As for the slipping, it sounds like a classic aging auto-tranny problem. Pull your auto tranny's dipstick and take a whif of the fluid. Of course it will smell like oil, but does it smell burnt? If so have it flushed and replaced immediately.

Also, there's another trick to getting the most out of a slipping transmission. That is you take a bottle of GM's limited-slip differential additive (comes in a 4oz. bottle) and add one of these to your tranny. It's not a cure, but if your tranny is just slipping slightly (even with new fluid) you might be able to put off an expensive repair for a year or more.

--- Bror Jace