Bad MPG

Yankee7985 · February 8, 2004

I've got a 95 Z71 TBI 350 with 170,000 miles and am getting truly lousy mileage. I used to get about 375-400 miles per tank, but now I am now only getting about 275-300 miles. In the past 2 months I have replaced the following: Timing chain set, catalytic converter, plugs, wires, fuel pump, fuel filter, EGR, dist. P/U coil, cap/rotor, wires, TPS, and ignition module. Any suggestions as to the problem would be helpful.

wisconsinz71 · February 8, 2004

Have you ever calculated true mileage by filling it, setting the trip to zero the refilling it so you can see how many gallons for the miles driven? The reason I ask is if you are only going by the miles you used to get per tank, what if the fuel level sender ohms reading changed a little and now your seeing a different gauge reading. If you truely lost MPG then I would think back to when it occured, if it was suddenly was there anything that happened that you can track it back to as a possible cause?

Jeremy WI · February 8, 2004

Have you checked it over for vacuum leak or an intake leak?

OUMike · February 8, 2004

Yankee,

you are describing the EXACT same problem I had with my 94' 4.3L. I've replaced fuel filer, PCV, plugs, wires and air filter. My mileage had gone from about 400 to tank to 200 to tank (had old parts), then back to about 320 to tank after I replaced everything. I recently set my trip to zero, drove and took a reading. I'm still getting about 16.5 MPG per tank. I'm not sure what I was getting before, but I'd be my home it wasn't any better than that. I can't explain the loss in "MPT" (Miles Per Tank), but that isn't an acurate way to check mileage. Your truck is fine.

PS...sounds like we got some new members...welcome

JayMan · February 8, 2004

It sounds like you replaced everything but the O2 sensors. An O2 sensor that is aging will lower fuel economy but not set the MIL until it actually fails. In the meantime, your fuel economy takes a nosedive and you could even damage the catalytic converter.

The two guys that used to host Shadetree Mechanic on TNN, now hosting Two Guys Garage on Speed Channel, have always recommended an O2 sensor change at least every 75,000 miles, more often if it's showing obvious signs of decay.

With my 1990 Toyota 4Runner, which never got more than 16 MPG highway, I had to change the O2 sensor in 1995 as the fuel economy dipped to 12. With the new O2 sensor, back up to 16. I had to change it again in early 2000, then I sold it.

I don't use the expensive dealer O2 sensors unless I have no choice. A place like NAPA carries Neihoff or another good brand, and every Neihoff O2 sensor I've ever purchased from them was made by Bosch of Germany.

Also if you have intake valve, combustion chamber, and injector tip deposits, that can really effect fuel economy. Try a few cans of a good "total" system cleaner like Greased Lightning, STP, etc.

RyanZ71 · February 8, 2004

Thats a good point there Jay about the 02 Sensors. I also suggest they be replaced every 75,000-85,000 miles, and they do make a difference both in fuel economy (maybe not a huge one but a difference none the less) as well as the emissions. On the newer cars the most common reason cars that were decently maintained would fail the emissions test would be a bad 02 sensor. 9 times out of 10 they would pass with flying colors after the owner had the 02 sensor replaced.

Yankee7985 · February 9, 2004

I forgot. I have also changed the O2 sensor. The only leaks are from one broken bolt on each one of the exhaust manifolds.

99-2door · February 9, 2004

I forgot. I have also changed the O2 sensor. The only leaks are from one broken bolt on each one of the exhaust manifolds.

If your leak at your manifolds is big enough that could be your problem. If enough gas is getting through to cause erroneous readings at the O2 sensors. That could make them think that your truck is running lean and cause it to richen the mix unnecessarily. Goodbye gas mileage. Something to check into anyway.

JayMan · February 9, 2004

I forgot. I have also changed the O2 sensor. The only leaks are from one broken bolt on each one of the exhaust manifolds.

If your leak at your manifolds is big enough that could be your problem. If enough gas is getting through to cause erroneous readings at the O2 sensors. That could make them think that your truck is running lean and cause it to richen the mix unnecessarily. Goodbye gas mileage. Something to check into anyway.

Excellent point. From the exhaust manifolds to the catalytic converters, the system has to be absolutely air tight. A slight leak will completely confuse the O2 sensors. With more air entering the exhaust stream, your O2 sensors think you're running lean and add more fuel.