Please help with terrible gas mileage

Hello. Ive got a 91 S-10 Regular cab with the 4.3V6 in it (TBI). Its got right around 178Kmiles on it. It seems to run pretty good, but has a couple of issues I noticed. One is when on the highway, I used to not have to depress the accelerator almost completely to the floor to get it to drop a gear and pass. (Automatic), And it seems like there is a little less power once the truck warms up as opposed to being cold.

Anyways, Im getting around 270 miles to a tank of gas. I just read that the expected mileage is 17/22mpg for this engine- 20 gallon tank. Im getting around 14. LOL I used to get around 350. Heres what I have done.

The fuel pump went out. Replaced it but the mileage was bad way before then. Thought that was hopefully the issue, ended up not being. The distrbutor cap, rotor, spark plugs, plug wires, pvc valve, and alternator are about a year old. The O2 sensor is about 2.5 years old. Surely they last longer than that. I replaced the air filter about 2 months ago. I dont know what else to check/replace.. Could it be that the spark coil or whatever else is just not putting out a good spark??? The engine never knocks or pings, so I didnt think there was an issue with timing. Ive used injector cleaner several times. Could they be the issue? The both have gas coming out of them when running. What about engine compression. Could the engine just be wearing out? Should I do a compression check?

I just dont know what else to do. Its been a great truck. Never really given me any problems. But I need to get the gas thing figured out. 270 miles on a 20 gallon tank sucks. I hope you guys have some suggestions. Thanks a lot

Hello. Ive got a 91 S-10 Regular cab with the 4.3V6 in it (TBI). Its got right around 178Kmiles on it. It seems to run pretty good, but has a couple of issues I noticed. One is when on the highway, I used to not have to depress the accelerator almost completely to the floor to get it to drop a gear and pass. (Automatic), And it seems like there is a little less power once the truck warms up as opposed to being cold.

Anyways, Im getting around 270 miles to a tank of gas. I just read that the expected mileage is 17/22mpg for this engine- 20 gallon tank. Im getting around 14. LOL I used to get around 350. Heres what I have done.

The fuel pump went out. Replaced it but the mileage was bad way before then. Thought that was hopefully the issue, ended up not being. The distrbutor cap, rotor, spark plugs, plug wires, pvc valve, and alternator are about a year old. The O2 sensor is about 2.5 years old. Surely they last longer than that. I replaced the air filter about 2 months ago. I dont know what else to check/replace.. Could it be that the spark coil or whatever else is just not putting out a good spark??? The engine never knocks or pings, so I didnt think there was an issue with timing. Ive used injector cleaner several times. Could they be the issue? The both have gas coming out of them when running. What about engine compression. Could the engine just be wearing out? Should I do a compression check?

I just dont know what else to do. Its been a great truck. Never really given me any problems. But I need to get the gas thing figured out. 270 miles on a 20 gallon tank sucks. I hope you guys have some suggestions. Thanks a lot


Have you checked base timing?
What kind/brand of spark plugs?
Is the engine run up to proper tempt?
Any check engine lights?
Is the engine running up to par?

Have you checked base timing?
What kind/brand of spark plugs?
Is the engine run up to proper tempt?
Any check engine lights?
Is the engine running up to par?

Thanks for the reply. No actually I havent checked the engine timing. I figured and I guess I shouldnt have that if it sounded good and wasnt pinging or knocking that the timing was ok.
As for spark plugs I have Bosch Platinums in it. Not the Platinum 4s just the standad ones. I used to have the fours but they never made a noticeable difference so when I did the basic tune up I changed them out. The Engine is running at the right temp and has a new recommended thermostat in it. The check engine light has only come on once and it was coded for the MAP sensor about three years ago, which was replaced.

As for the engine running. Its never really given me a prblem. I used to get around 350 miles to a tank and the only thing that the engine has ever done and has so the entire 5 years Ive had it is have a lobe type idle. When at a stop it just kind of vibrates a little and you can see it in the steering wheel and antenna. If you give it a little bit of gas it goes away. It has lawys done this though even when the gas mileage was a lot better so I never figured it to be a culprit.

Thanks for the reply. No actually I havent checked the engine timing. I figured and I guess I shouldnt have that if it sounded good and wasnt pinging or knocking that the timing was ok.
As for spark plugs I have Bosch Platinums in it. Not the Platinum 4s just the standad ones. I used to have the fours but they never made a noticeable difference so when I did the basic tune up I changed them out. The Engine is running at the right temp and has a new recommended thermostat in it. The check engine light has only come on once and it was coded for the MAP sensor about three years ago, which was replaced.

As for the engine running. Its never really given me a prblem. I used to get around 350 miles to a tank and the only thing that the engine has ever done and has so the entire 5 years Ive had it is have a lobe type idle. When at a stop it just kind of vibrates a little and you can see it in the steering wheel and antenna. If you give it a little bit of gas it goes away. It has lawys done this though even when the gas mileage was a lot better so I never figured it to be a culprit.


A good place to start is to drop the bosch plugs in the trash can and install the AC-delco spark the book calls for. Then check and set the base timing.
Also a good scanner check on the computer may show something.
Good luck
MT

Thanks for the reply. No actually I havent checked the engine timing. I figured and I guess I shouldnt have that if it sounded good and wasnt pinging or knocking that the timing was ok.
As for spark plugs I have Bosch Platinums in it. Not the Platinum 4s just the standad ones. I used to have the fours but they never made a noticeable difference so when I did the basic tune up I changed them out. The Engine is running at the right temp and has a new recommended thermostat in it. The check engine light has only come on once and it was coded for the MAP sensor about three years ago, which was replaced.

As for the engine running. Its never really given me a prblem. I used to get around 350 miles to a tank and the only thing that the engine has ever done and has so the entire 5 years Ive had it is have a lobe type idle. When at a stop it just kind of vibrates a little and you can see it in the steering wheel and antenna. If you give it a little bit of gas it goes away. It has lawys done this though even when the gas mileage was a lot better so I never figured it to be a culprit.

HERE'S WHAT I KNOW...... UNLESS YOU ARE RUNNING IT ON A TRACK OR HEAVY OFF ROADING, THE BOSCH 4'S HAVE A TENDENCY TO CAUSE THE ENGINE TO FLOOD. BOSCH PLUGS AREN'T THE GREATEST TO BEGIN WITH. I'M WITH THE LAST POST, SWITCH TO AC DELCO, CHECK YOUR COMPRESSION, AND CHECK THE TIMING.

ALSO, HAVE YOU DONE ANY OTHER WORK TO YOUR TRUCK (I.E. LIFTING, LOWERING, CHANGING WHEEL SIZE?) THOSE CAN ALSO HAVE AN EFFECT ON MILEAGE

A good place to start is to drop the bosch plugs in the trash can and install the AC-delco spark the book calls for. Then check and set the base timing.
Also a good scanner check on the computer may show something.
Good luck
MT


Ok, you got me thinking about the plugs. I went out and checked and I have in there Autolites. They say either Premium or Platinum, I cant tell. So I dont have Bochs after all. I did at one time have the 4s but no anymore. For me to check the timing. Do I get me a basic induction timing light?? I dont have one but can get one. I do have a compression guage set though.

One other thing I forgot about was that the last time I changed out the distributor cap and rotor, they were carboned up pretty bad. The inside terminals on the cap and the rotor had a lot of carbon or blackish/greyish type material on them. I had purchased the best ones they had. Should the cap and rotor be doing that in such a short amount of time?? If not, what would cause that? Not enough spark from the coil maybe? Also the plug wires I have are 7mm. Should I have went bigger? LOL I drove all day today and am down to a little less than a 1/4 tank and only at 240 miles. Geeze

Ok, you got me thinking about the plugs. I went out and checked and I have in there Autolites. They say either Premium or Platinum, I cant tell. So I dont have Bochs after all. I did at one time have the 4s but no anymore. For me to check the timing. Do I get me a basic induction timing light?? I dont have one but can get one. I do have a compression guage set though.

One other thing I forgot about was that the last time I changed out the distributor cap and rotor, they were carboned up pretty bad. The inside terminals on the cap and the rotor had a lot of carbon or blackish/greyish type material on them. I had purchased the best ones they had. Should the cap and rotor be doing that in such a short amount of time?? If not, what would cause that? Not enough spark from the coil maybe? Also the plug wires I have are 7mm. Should I have went bigger? LOL I drove all day today and am down to a little less than a 1/4 tank and only at 240 miles. Geeze



THE FOURS IS WHAT CAUSED THE CARBON ON THE CAP AND ROTOR... NOT ENOUGH SPARK TO USE UP ALL THE GAS, OR IS IT NOT ENOUGHGAS TO USE UP ALL THE SPARK? ANY WAY..... I WOULD PROBABLY TRY GOOD TUNE UP, ALSO CHECK YOUR FUEL FILTER, AND SEE IF YOU GET ANY BETTER.

THE FOURS IS WHAT CAUSED THE CARBON ON THE CAP AND ROTOR... NOT ENOUGH SPARK TO USE UP ALL THE GAS, OR IS IT NOT ENOUGHGAS TO USE UP ALL THE SPARK? ANY WAY..... I WOULD PROBABLY TRY GOOD TUNE UP, ALSO CHECK YOUR FUEL FILTER, AND SEE IF YOU GET ANY BETTER.

OK thanks. Im going to pull the cap off this weekend and check again. Ive had these standard plugs in for about a year now. So we'll see if the cap is in bad shape again. Im also going to pick up a timing light. I did replace the fuel filter when the pump was changed out about 8 months ago. But I had the bad fuel problem before then.

Any idea what tells the truck to downshift when you depress the accelerator? When I used to be on the interstate and wanted to pass it would, now I have to almost press the pedal to the floor to get it to. Wasnt sure if that was adjustable or not.

Over the years I've also dumped a bottle of Gumout into about a 1/4 tank of fuel and ran for a couple of days before filling up to clean the fuel system periodically.

You might try a transmission filter change. That's the first thing I'd check with it not shifting smoothly. Otherwise, I'd probably think the shift down linkage is getting rusted up and binding, not letting it shift easily. You can locate the linkage going into the tranny and spray it up good with pb blaster. I've had a similar problem on a past car and it turned out to be the shift down linkage, especially if you have to really mash the gas to get it to accelerate. I would try the tranny service first though, it probably needs it! Hope this helps.

You might try a transmission filter change. That's the first thing I'd check with it not shifting smoothly. Otherwise, I'd probably think the shift down linkage is getting rusted up and binding, not letting it shift easily. You can locate the linkage going into the tranny and spray it up good with pb blaster. I've had a similar problem on a past car and it turned out to be the shift down linkage, especially if you have to really mash the gas to get it to accelerate. I would try the tranny service first though, it probably needs it! Hope this helps.


cool thanks for the advice. I prb want clear on it though. It actually shifts very smooth. You can almost never feel it upshift or downshit. Its only that you have to depress the gas pedal almost to the floor to get it to downshift when you want to pass. I thought this was controlled somewhere. Maybe the TPS sensor or something I dont know. Just used to be that when cruising on the highway you didnt have to go almost completely to the floor to get it to downshift to pass

I think the TPS is also a good candidate. However, if it is, I would think the "check engine" light will also be on, tripping the code from the computer. But only one way to find out about that is to get it scanned. Other things to check include tranny fluid level, type, vacuum and actually there are modules on the transmission that might not be performing up to par. But once again, module failure should be picked up by a routine diagnostic. Hope this helps.

You might try a transmission filter change. That's the first thing I'd check with it not shifting smoothly. Otherwise, I'd probably think the shift down linkage is getting rusted up and binding, not letting it shift easily. You can locate the linkage going into the tranny and spray it up good with pb blaster. I've had a similar problem on a past car and it turned out to be the shift down linkage, especially if you have to really mash the gas to get it to accelerate. I would try the tranny service first though, it probably needs it! Hope this helps.


IF YOU DO GO WITH THE TRANS FILTER CHANGE, YOU MIGHT WANT TO CLEAN THE SHIFT SENSORS DOWN THERE WHILE YOU GOT THE PAN OFF. NOT SURE WHAT THEY LOOK LIKE ON THESE NEWER CARS, BUT ON MY 84 THEY WERE TWO LITTLE BEAN SHAPED THINGS WITH TWO LEADS ATTACHED TO EACH. AFTER I CLEANED THEM OUT, I DID NOTICED A LITTLE MORE PEP AND A LOT OF SMOOTHER SHIFTING, AND PASSING ON THE HIGHWAY WAS A LITTLE EASIER

The stuff on the cap prongs and whatnot was caused by moisture and the tendency for any electrical part to oxidize. Electricity enhances this tendency which is why it gets puffy and powdery. I wouldn't say autolites are bad plugs, certainly better than any bosch plug so I would say they're ok. If the thing is not mis firing or running rough some basic things to check are: vacuum leaks, O2 sensor(lazy ones won't set a light but can increase fuel consumption), compression, timing, catalytic converter(might cause some hesitation/lack of power, especially since you mention it lacks power a bit after it warms up), um.. I can't think of anything else offhand. I konw the throttle body base gaskets can leak over time and may not set a light but can fatten the a/f up. Get a can of carb cleaner and spray wherever vacuum is and listen for the engine to flare up a bit. Pinpoint the leak, if any are there, and fix it. Oh, a tp sensor can cause problems without a light. At idle the thing should be about .4v-1.2v and at WOT it should be 4v-5v. With it hooked up and backpinned you should see a nice steady increase in voltage with no dropouts.