Hey everyone. I need some advice and soon because this truck is becoming a gas tank vacuum cleaner. I just put gas in the truck last night, drove back to my apartment and parked it overnight (Gas mileage was around 12. Short 2 mile trip for gas). After letting it idle for 5 minutes, I drove to campus and back 3 more times today and to lunch at a total of about 15 miles. All these rides were at slow speeds (20-40mph) and were short trips less than a few miles long with some warm up idling if the truck was sitting long enough for the temp gauge to be below 120. No rough acceleration, but some hills and some stop and go driving. Temp gauge takes a long time to get to 190, but the T-stat is only a couple months old and I replaced it myself. At the end of the day, I checked the MPG meter and it was around EIGHT MPG!!! The engine starts and runs just fine and I also just changed the oil last week with Mobil1 High Mileage. The trans shifts fine, maybe a bit sloppy as it does when it is very cold (10-20F) because it doesn't get a chance to warm up. All fluids are new; tune up was done last year with all new plugs, wires, cap, rotor. I banged on the catalytic converter and there was no rattling or rust. No check engine light or any warning lights for that matter. Brakes were recently checked several times and other than a bad parking brake, there are no issues with dragging. Are there any parts which I might need to consider replacing soon? The O2 sensors are original as far as I know. I was considering ordering Bosch sensors from Amazon, but I wanted to ask first so I don't waste money on something that will not make a difference.

 

What year is your blazer?

 

Welcome to the club, I'm chasing the same gremlin, I'm getting 7-8 mpg too. I'm going to do a tune up again,it's been less than a year, and replace the catalytic converter since I wanted to anyway. Hope you find the problem or if I do before I will let you know.

 

bigkhans: 1998

gojorg: But you have 156k on yours with the original converter?

To add to my previous post, the trans isn't slipping. Just sloppy when cold. I changed the fluid a month ago and it is still like new as it always is (I change it once a year. Never goes darker than pink).

 

I believe it is the original one, I bought it with 131,000 and only done regular maintenance and a tune up about 11 months ago.

 

Don't go by the computer. I have a driver information center in the Regal and it's consistently 1-2mpg off. Short trips, idling for extended amounts of time, the hills and stop and go that you mentioned - they all contribute to low mpg in our heavy trucks.

We were getting 25-28 average mpg in the black Regal before we sold it to my Wife's dad. Now that he has it and only drives 10 blocks to work and lets it sit and warm up before he goes, he's lucky to get 13mpg out of it (according to the computer).

Have you had this low gas mileage in the exact same conditions, exact same weather, exact same warm-up/idle time, taking the exact same route with the exact same weight in the truck?

Better to drive, then fill up, keeping track of miles driven. Divide by gallons used to fill back up. And really, even if it is 8mpg, how many miles are you driving at school? Are you really spending that much more money?

The only thing I would even remotely attribute to low gas mileage as you're describing is your truck never entering closed loop. What does your temp gauge read? Just because you put the thermostat in yourself doesn't mean it's not a faulty thermostat. I hate to put even more ideas in your head, but that's the only possibility I see.

 

If anything the DIC is optimistic. Last tank it showed about 9 gallons used and the gas pump kept going all the way to 11. Doesn't normally do that. Not leaking gas as far as I can tell and I have a new gas cap that is very snug. The speedometer is 2mph too slow because I have 235/75R15 tires when the truck calls for 235/70R15, so the MPG meter thinks I am going farther per gallons used, which means I might be getting less than 8.

On the highway, I get between 16-18 at 65mph and higher around 50-55. In town normally I get around 10-12.

It is alot more money. Each commute is 3 miles one way and I have classes (sometimes 5 per day, 2 of them consecutive) spaced out throughout the day 5 days a week. I can easily burn through half a tank a week at 10-12mpg, but so far the truck is going to be at 1/4 of a tank by Thursday. I eat lunch out if I have an hour between classes and I go back to my apartment if there is more time so I can get work done. Basically, I am the classic totally inactive in campus activities commuter student who unless I am walking to or from the truck, I am either in class or not on campus at all.

I let it warm up just enough so I am not burning through gas idling for a long time (5 min if the temp is above 30, 10 if above 15, rarely 15 if it is below zero) but so the heater works so I have defrost and the fluids are warm. The truck is parked outside all night and when I start it up when it is really cold it has faint taps for a minute or two (done that for 50k+ miles. Nothing wrong with the motor) and then it is smooth after a few minutes. Sometimes when I drive it the temp gauge isn't even past 1/3 by the time I park the truck as it has been for 2 years. Sometimes depending on the route it will get to 190 as usual, but most of the time it doesn't.

Weather conditions are about the same as normal for this time of year, weight is about the same or even lower than last year this time, warm up time is actually reduced by half (I used to let it go for 10 min, now it is 5-7). There are 3 or 4 possible routes and the ride to or from campus doesn't involve hills on most of them. 1 or 2 do, but they aren't too steep or long.

What do you mean by closed loop? What conditions need to be set for that?

EDIT: Just in case it matters, I have replaced all the vacuum lines a few months ago and disabled the one going to the T-case to prevent any ATF getting into the climate controls if anything happened. I have a Posi-Lok so it doesn't need to be connected anyway. The only mods are as shown in my signature and everything is tight, but I think there may be a vac leak somewhere since the HVAC dash vents seem to lose airflow when I accelerate and then it comes back when I let off the gas. This is with the fan on MAX. Idles perfect at 550rpm and I get comments about how quiet it is.

 

I replaced the front O2 sensors today. Drove it down the highway and got around 18 average at 65mph and the instant econ was higher. Went down in town, but hopefully over the entire tank the mileage will be better. I dumped a bottle of Techron in the tank again, so hopefully that will help.

 

where is the front o2 sensor? is that the one in the intake pipe?

 

No, there are 2 front sensors and they are in the Y pipe under the truck. One per side and they are placed before the pipes curve and combine to one and then it is connected to the catalytic converter.

Just FYI, I did do that erase code function, which reset the I/M data back to "not ready" and it actually feels different. Not nearly as much hesitation. I only gave it like 1/3 throttle earlier from a stop just after I did the reset and it took off like a jetliner. I mean went all the way to 4k RPM before shifting into 2nd and seemed smoother on acceleration. Trans also seems smoother for some reason, but not sure what that has to do with new O2 sensors.