Bad MPG

99yukon350 · December 18, 2006

I know it is winter and we have a different fuel mix but now I am getting around 11.5-13 (127,000 miles) Last winter I don't think I got below 14 driving to the same places. I have no codes coming up. However, lately I have noticed after a 25 mile + drive about a minute after I shut it off it sounds like the gas tank is popping. It sounds like the old metal portable gas tanks if they are not vented properly. I cant determine if that sound is the gas tank or the muffler. Would poor venting cause a decrease in mileage without a code?

I am going to start adjusting the tire pressure now to see if I can find the right match. I cant go by the door number because I have LT 265/75/16 load C tires on it now. Any suggestions on PSI?

I may also have an eletrical short somewhere. every now and then the truck will die and I won't have any gauges. I used to think it was the battery because it wouldnt stay tight but I have a new battery with new bolts on the cables. Would this add to the bad gas mileage.

I am going to replace the air filter even though it fairly new. I changed my oil/filter about 1200 miles ago so when it comes time again I will run seafoam thru it. Plugs, wires, cap, rotar, fuel pump, fuel filter were all replaced in Janurary when the intake gasket and water pump were replaced.

Thanks

C & A s Dad · December 18, 2006

Really the only thing you mentioned that could affect you mileage is the LT tires. I am no expert but have ready many posts on a RV forum saying that switching from P series tires to LT's can hit your MPG pretty good. I saw where some guy had calculated the extra weight but using the actual increase in tire weight and the rpms to arrive at the increased rolling weight. Not sure how accurate this was, but an extra 7 lbs of weight per tire added a great deal of rolling weight.

Most LTs are going to weigh 4 to 8 lbs more than the same size P tire.

I would also tell you that you should consider you O2 sensors if you have not replaced them. Even they may not be setting a code, they can affect your MPG.

99yukon350 · December 18, 2006

The tires I have now actually weigh less the last set. Those were a D rating and these Cs I have now weigh about 8 lbs less than those ones did. I actual got my current tires last winter. I just don't remember how I had the PSI then.

Bish · December 18, 2006

I switched from stock P265/70R16 to LT265/75R16 LRC and my mileage did not change and I use excel to check every tankful. I generally run mine at 40psi unless I am towing in which case I run max (50 I think).

Winter gas causes my mileage to drop 1-2 mpg plus. Warm up time in winter also kills mileage.

Ditto on the o2 sensors also. I gradually lost about 2 mpg over a year. No codes set but I replaced the sensors as I was over 80K and my mileage went up IMMEDIATELY after replacing them.

67ChevyRedneck · December 18, 2006

Could just have been a bad tank of gas too.... I check almost every tank in my truck and about 2 months ago I got about 13 mpg, I've never gotten much below 15, so I was a bit worried. The next tank it was back up to 15.... and I did a lot of highway driving last week and got an average of 17 on that one.... I figured I got some crappy gas?

99yukon350 · December 18, 2006

All sumer it got between 16-19mpg. the last time I remember 16 was pulling a half loaded trailer of wood going 75mpg @3000 rpms back in Nov. I did just find out that the place i filled up now uses 10% ethanol in their fuel. Even so most of the places I filled up last year were 10%. I am going to look for a place around here that doesnt have 10%.

I was thinking of switching out the 02 sensors. What brand? Bosch? Would replacing the muffler help any? It has never been replaced.

Should I run more PSI in the front than the back? That is what I remember one place telling me. If so how much of a difference?

Giant · December 19, 2006

on the electrical dying on you... Check the ground cable from the battery to the block. I had the same thing in my 94 Cadillac DeVille Concours. I've never felt a worse drop in my stomache then when I associated my caddy with 'electrical' problems. Scared the s**t out of me. Then I found it to be a bad ground. To fix it I bought a new cable but couldn't get it on. I couldn't get the old cable off of the long rod that went into the block. Just by taking the rod out (with the cable still on it) and putting it back on again must have knocked enough rust loose to make it work again.

Just a thought.

C & A s Dad · December 19, 2006

All sumer it got between 16-19mpg. the last time I remember 16 was pulling a half loaded trailer of wood going 75mpg @3000 rpms back in Nov. I did just find out that the place i filled up now uses 10% ethanol in their fuel. Even so most of the places I filled up last year were 10%. I am going to look for a place around here that doesnt have 10%.

I was thinking of switching out the 02 sensors. What brand? Bosch? Would replacing the muffler help any? It has never been replaced.

Should I run more PSI in the front than the back? That is what I remember one place telling me. If so how much of a difference?

<{POST_SNAPBACK}>

On the 02 Sensors. Bosch is our (CARQUEST) Supplier and we have been very happy with them since changing to them about 3 years ago. They are exact OE Form, Fit and Function and supply many of the OE suppliers. NGK is also a major OE supplier but they do not seem as readily avail in the aftermarket. One other thing I really like about Bosch is they "burn in" test every sensor at the factory. So do not be alarmed if it looks like they have been used when you take them out of the box

Bish · December 19, 2006

I went with Bosch when I replaced mine at about 80K. I'm over 183K now and the Bosch are still holding up nicely.

z85violator · December 19, 2006

how long are you warming your truck up in the morning?

my 5.3 uses more gas warming up for 5 mins then if i just jump in and go(also warms up faster)

99yukon350 · December 20, 2006

I usually don't let it warm up more than a minute unless there is a thick coat of frost on the windshield. I have a few miles to get to the highway and is normally up to temp by then.

I noticed today my volt meter went down. (I have been watching it closely lately)I actually noticed today when the dash lights didn't look very bright. I 1st checked the dimmer and it was ll the way up. The meter was on the big line before the orange, (half way mark between 9 and 14?). I shut radio and heater off didnt do anything. Drove 10 miles on the highway to get home and it didn't move up or down.

99yukon350 · December 21, 2006

This morning the truck wouldnt start. It would click but not turn over, I hooked up the battery charger and that didnt help. I only got to check the wire on the starter, it looked clean and it was tight. I havent done anything else yet, I'm just finishing up my last semster in college so I don't have the time.