Repair Topics / Fuel Economy

Car Fuel Economy Questions and Answers

Question: 1999 Ford Taurus LX Wagon mileage 90,000 . I am a fellow licensed mechanic, up here in Alberta. This car was given to me with blown head gaskets. I replaced them and new head bolts, all plugs, wires, serpentine belt, put in new Synthetic oil like previous owner had since new. It does not use a drop of oil and runs like a charm. It had the engine service light on when I first restarted it, probably from antifreeze on the O2 sensor. Replaced them and light went out. The previous owner claimed it only got 20 MPG since new. I have driven it through 3 tanks of gas now, and he is right. The last tank had a new air filter in it but no difference. It has lots of power. I believe this car would beat my 68 Mustang with a 302 auto in a race. What do I look for to fix this mileage problem? It has no Codes at all and I changed everything except the fuel filter as the previous owner changed it a month ago.

Answer: Depending upon the type of driving that you do, twenty miles per gallon (MPG) may not be too bad. Check with other owners of similar cars and see what kind of mileage they are getting. If the twenty MPG is low, check the Mass Air Flow Sensor .

Question: 1999 Lincoln Mk VIII mileage: 128,000. I have just had the transmission rebuilt in this car and now I have developed two problems that did not exist prior to the rebuild. The first is that I get an intermittent "check engine" light. This comes on usually after 3 minutes and goes off and on with no pattern as to engine/speed. The second is that my gas mileage has gone from 22-24 mpg avg. to 16.5 or less. The error codes and the computer have been reset but the light keeps coming on and the mileage is horrible. The shop replaced one of my oxygen sensors at their expense believing they had somehow damaged it during the rebuild. My transmission shifts as if new and there seems to be no problem with the rebuild. The engine is running smoothly and has plenty of power and all normal startup computer checks read OK. Any thoughts on where to look/things to try would be helpful.

Answer: We think you have a Mass Air Flow Sensor that is out of range and needs to be replaced.

Question:   2001 Ford Focus, miles : 79,000. My wife contends that operating the vehicle at low Rpms, almost to the point of lugging, results in better fuel economy. I have been under the impression that the smaller vehicles were to be driven at the higher RPM's to maintain performance and minimize long term maintenance problems, (sludge, plugged injectors, etc.). Can you help us with this impasse?

Answer: Lugging an engine at low speeds is not particularly good for an engine. On the other hand, high RPM can cause accelerated wear. The key here is moderation with regular maintenance as recommended by the manufacturer.

Question: 2004 Honda Accord. Does running your A/C reduce your gas mileage?

Answer: Yes it does. The air-conditioning system uses horsepower from the engine to run the air conditioning compressor.

Question: 1999 Nissan Altima mileage: 134,000. Since March of this year, I am keeping records of my car's mpg averages. I began to notice a significant drop in fuel economy. In the city, the averages were between 22 and 24 mpg, and in the highway, the average was 26 without reaching 27. All this with new aftermarket spark plugs, wheel alignment in all four wheels, correct cold air pressure (32 psi front, 29 psi rear), new set of spark plug cables, a nearly new distributor cap and rotor, and an rpm adjust made at a local dealer. I was using an oil that wasn't energy conserving, 15w 50 synthetic, the one I was using since the car was new, and I decided to use 10w 30 synthetic, so fuel economy improves, and it remained the same. Here in Puerto Rico, heat is a problem and that is why many mechanics including dealers, use oils that aren't energy conserving. But, well, even with the 10w 30 oil, the problem was there. I read on the Internet, that the oxygen sensor is related to fuel efficiency and that the best sensors are usually replaced at 100,000 miles. So I decided to replace the oxygen sensor with an equal aftermarket oxygen sensor, the same one my car had since new. And, the result was good.

The highway mileage improved, but not exactly to the 34-35 mpg average, I obtained in the past. I obtained 31.4 mpg controlling the speed, the acceleration, and the braking in a long trip. The problem is that I still obtain between 22.5 and less than 24 mpg in the city, using the car under the same roads I used it when it was new. This car is capable of delivering 26-27 mpg under these conditions because I calculated its fuel efficiency when it was new. Some people around me say that this is not a big problem, due to the fact that the engine has nearly 134,000 miles and there is wear, even after using synthetic oil since the car was new. Also, this car is automatic not standard. In spite of this, the fact is that I am running at least 20 miles less with each and every fuel tank, having all of the above checked, and this happens consistently. This means more fuel expense, and I think that probably something can be done to put the fuel efficiency of this 1993 Nissan Sentra (automatic) closer to where it is suppose to be, which is 26 mpg in the city and 35 mpg in the highway.

I did something that two different local Nissan dealers did not, which was replacing the oxygen sensor for an exactly new oxygen sensor and it worked, so that was part of the problem. But, the car is still below its average. One more thing is that this car had its accelerator cable adjusted by a local mechanic, not a dealer. Can this affect its rpm? Sometimes, after driving and for example, stopping at a parking lot, the car seems to remain at a higher rpm than usual, instead of reaching a lower rpm level. This happens, but not all the time, maybe 50 percent of the time. Other thing is that in the mornings when the car is cold, I heard a sound similar to the one you can hear on a blender immediately after start up, and it disappears quickly, happening only when the car is cold. In the past, the car made a similar sound and the starter was replaced, but somehow, this sound is different. The catalyst hasn't been replaced, is running with the same one it has since new. The other thing, are well. The car runs and accelerates well, but the accelerator feels hard when I compared with the past. This is since the accelerator was adjusted, what mechanics here call the accelerator leg, and other call it the accelerator cable. I don't know if that adjustment is making a difference, but I am going to try with other spark plugs I have, to see if there is a difference with the ones my car is using right now. Since I am using these spark plugs, I haven't heard the usual engine knock that this car makes in heavy traffic, using premium fuel all the time. And, because the owner's manual relates the small amount of engine knock with fuel efficiency, maybe that is another fact.

Also, I don't know about the rest of the sensors this car uses, (if they are working well), but I do know that there are many other sensors or parts related. (I have the big manual also). I don't know about the new gasoline's that are being sold, but here in Puerto Rico everybody uses premium 93 fuel all the time. Well, that is all the information I have. Maybe others have bigger problems, but one of the virtues of this car is its fuel efficiency, and if it looses it, the car looses a great part of its value. This is not a Porsche, or a BMW M3, or one of the other monsters around, but is a reliable car, that requires little maintenance, and, it usually delivers better fuel than the average by far, which I read that for 1999 the mpg average was 23.8. Pablo in Puerto Rico

Answer: First, we would replace your aftermarket spark plugs with the same brand that was original equipment from the factory. There is a good chance that your catalytic converter is partially plugged if it has not been changed in 134,000 miles. If the timing belt has been replaced, verify that the cam timing is correct. We have worked on cars brought into us with complaints similar to yours and found that the timing belt had been installed one notch off. Rather than correcting the problem some mechanics take a short cut and adjust the distributor to compensate for the incorrect belt timing. This works, but mileage is compromised. Finally, do a compression and leak-down test to find out the condition of your engine.

Question: 2002 Ford Thunderbird mileage: 71,000. Hi guys. I have a gas consumption problem with this car that I have had since I purchased it a year ago with 55,000 miles on it. I only get 200 miles out of a tank of gas. It has an 18-gallon tank. This is involving mostly expressway driving, very little city. I average around 14 mpg, which is abysmal. Taken it to several Ford dealerships, which test and say according to their diagnostic computer, I should be getting 26.7 mpg. The car runs perfectly, except for an occasional shudder coming from the transmission. There are no gas leaks at all. The dealerships don't know what's wrong with the car, so I am hoping you can steer me in the right direction.

Answer: Try replacing the Mass Air Flow Sensor and the fuel filter.

Question: 2003 Ford Pickup F-150 mileage: 70,000.  My truck has two gas tanks. Front tank is slightly larger than the back.  Noticed a drop in fuel economy lately (past few months).  Started checking, front tank gets the usual 17-mpg.  Back tank varies from 11 to 13.  No fuel leaks (at least not when it is sitting in garage).  No fuel smell.  Runs good.  Any suggestions?   I checked your previously answered questions but didn't see anything quite the same.  Thank you for your time!

Answer: Your problem with the fuel monitoring system, check the oxygen sensors and the mass air flow sensor.

Question: 2001 Nissan Fronteer 2WD mileage: 67,000.  Help me Bob and Ken!  I am having a TERRIBLE gas mileage problem.  In the last 8 months, my fuel economy has dropped from 16 mpg to 12 mpg!  I have spent over $600 having the problem diagnosed to no avail.  Repairs that were made include Adjust throttle linkage, Scoped and set engine, throttle body gasket repair, timing adjusted, tune-up, new plugs, new O2 sensor, new catalytic converter, vacuum lines tested, emissions tested, and exhaust system check and fuel injector cleaning.  After putting the truck on 2 different diagnostic computers, at 2 different places, 2 different mechanics have stated the engine was in great shape to have so many miles.   I know that with so many miles that I'm not going to get the same gas mileage as I did when I bought it (24 mpg), BUT 12 MPG IS CRAZY!  Please see what you guys can come up with.

Answer: Several things should be checked; fuel pressure regulator and mass air flow sensor.  If you have not had the computer scanned, you should do it now having all five modes checked.

Question: 1999  Z28 Camaro mileage: 84,000.  Hi, I have a Z28 Camaro.  When I first bought the car a few years ago, I was getting great mileage (20-25) gal.  My car has a carburetor, but has all of that emission equipment for California.  I recently changed the o� sensor, set the timing, and adjusted the carburetor, so I was wondering what happened to my mileage (10 miles per gallon now).  So please answer my question, and give me some info on the computer controlled q-jets.

Answer:   Your car is equipped with a computer controlled engine management system.  An independent auto service center can scan your computer for fault or error codes.  The information should pinpoint the malfunctioning components. The mileage problem you are having may indicate you are ready for major tune up.

Question: 2002 Dodge Grand Caravan mileage: 60,000.  On a resent long trip, I experienced poor fuel mileage.  Before the trip I did a tune-up, new plugs, wires, air filter, PVC etc.  Once we started, I noticed that we were getting about 23 MPG but after 20 miles, the mileage would drop to 12 to 14.  When the engine was cold after stopping for lunch or dinner the mileage would start at 23 then again drop to 12-14 after 20 miles.  I notice the tail pipe was black.  Doing a key on engine light test, I found the following codes.  1-2, 5-1, 5-5.  This said the O� sensor (replaced at 82K) was detecting a lean mixture.  After coming home, I took the vehicle to my local repair shop where they could not find any codes stored.  The car is still getting poor mileage.  It must be a sensor, where should I start?  Thanks, Ben   

Answer:  You may have a faulty oxygen sensor or coolant temperature sensor.

Question: 1996 Pontiac Bonneville mileage: 167,000.  In the past month and a half, my mileage has gone from 450 miles per whole tank to 300-330 per whole tank.  I believe it is a 15-16 gallon tank.  At first, I thought it was due to colder weather when I started getting about 50 miles less per tank.   But now (mid December) I am suddenly getting 100-130 less per tank.  What can I check for and what are some possible problems that might be causing this?  Thanks!

Answer:   If a major tune-up has not been done recently, this could be the cause of your problem.  Replace the spark plugs, plug wires, fuel and air filters, etc.  Also, replace the oxygen sensor.  In addition, service the fuel injection system.  Clean the carbon deposits from the throttle bore and idle passages.   Check the catalytic converter to make sure it is not plugged or restricted.

Question: 1999 Dodge Neon mileage: 115,000.  5 speed. Had this engine rebuilt at 100,000 miles about 5 years ago.  This spring, the mileage dropped from 18-20 mpg around town to 10 mpg.  New 0� sensor last spring.  Computer/ECU 2 years ago.  New coolant temp sensor this summer.  Why the drop in mileage?

Answer:    Several things should be checked; fuel pressure regulator and mass air flow sensor.

Question: 1999 Chevrolet 1500, miles: 145,000  I have replaced plugs, alternator, belts, changed oil, cleaned injection, and replaced fuel sensor, and my car is consuming too much gas.  It uses almost 3/4 of a tank of gas to drive 50 miles.  How can I repair my car so it doesn't use so much gas? 

Answer: Check fuel pump and Mass Air Flow Sensor.

Question: 2001 Plymouth Neon mileage: 85,000.  Gas mileage has suddenly dropped from 250 to 125 or so per tank.  Some power loss when pull off from a stop but other than that runs well.   Dealership mechanic says it checks out OK. Sounds OK.

Answer:  Check the catalytic converter to make sure it is not “plugged” or restricted.  Your car is equipped with a computer controlled engine management system.  An independent auto service center can scan your computer for fault or error codes.  The information should pinpoint the malfunctioning components.

Question:   1997 Jeep Wrangler, miles: 78,200 That gets between 18-20 miles per gallon. Is there any way to improve the mileage?

Answer:   Your Jeep is equipped with a computer controlled engine management system. The fuel mixture is constantly monitored and adjusted for optimum mileage and performance. The only way to improve mileage is change driving habits i.e., slow starts, constant speeds etc. Keeping your Jeep in top running condition also helps.

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