extremely reduced gas mileage-any ideas?

[lsu-kev]

hey guys- i have a 2005 hr admiral on w-22 chassis. i live and drive in flat south louisiana. no hills. just recently my gas mileage has dropped from around 9 mpg to 4 mpg. seems to be running maybe a little rough when accelerating from a stop but once i get up to speed it runs smooth. one exception is when cruise is set it sometimes revs up for a minute or so and then will settle back down to about 2200 rpm. any ideas on what to check or change first-plugs, fuel filter, air filter? any advice is greatly appreciated.

[cannon man]

first get a code reader n see if it set any codes. check plug wires n air filter. if ok i would think about o2 sensors. have you had the brake recall done? trans shifting ok?
good luck, dave

[tomwalt]

I had an O2 sensor cause this on a chevy 350cid. I did have an error code that pointed me in the right direction.

Other thoughts - A lot depends on how much the motor is run. Is the gas old or have you recived a lot of rain that could have got into the tank? Dry gas or fuel injector cleaner may help.

[lsu-kev]

dave-i have had the recall done and the trans seems to be shifting fine. on cruise it does seem to rev up and go back to normal on rpm frequently which to me is odd because i am driving on flat ground.

[JonBrazel]

Get in touch with me if you would like to use our Proscan service to help diagnose your issue.

[max49]

quote "seems to be running maybe a little rough when accelerating from a stop but once i get up to speed it runs smooth."

"Classic burnt plug wire". I'd check #5 first and #6 second. 9MPG to 4MPG sounds pretty extreme

[Topflite51]

02 sensor(s) are the usual cause of this.

[GMRHost]

The last time I had a drop this extreme, it was the Map Sensor. AMAZING difference after replacing it. But check your codes first- that's what they are there for!

[VanDiemen23]

also check the water temp sensor - if it's broken and set on cold the engine may run enriched all the time as if it's on the cold start program.

I've also had TWO A/C compressor failures that produced this problem. If the idle's high enough the engine will run, maybe roughly, even though the compressor is consuming something like 40 HP. On one of my vehicles it continued to refrigerate the air, even though it was nearly locked up. Serpentine belts can transmit a LOT of HP without slipping.

[lsu-kev]

is there a way for a knowledgeable person who can do a few things but is not a mechanic to check the codes?

[Mekanic]

Like the other(s) posters pointed out Coolant temp sensor, Map sensor, O2 sensor(s) any of these can cause the BIG MPG drop.
It wouldn't hurt to check the plugs wire too.

[KIX]

Quote:

Originally Posted by lsu-kev is there a way for a knowledgeable person who can do a few things but is not a mechanic to check the codes?

Yes, buy a code reader..........they are relatively inexpensive these days. A basic one can be bought for under $50 I fact some chain type auto parts stores will read your codes for free in hopes of selling you parts.

[ctpres]

Sounds like our problem in early stages. Finally failed long enough for shop to see. No codes, analizer showing rich one side and lean on other. After almost full day of tech time, still no positive results. Look at cat. converters and see if one discolored more than other due to heat. If you try to maintain speed up long incline it will really get bad at times I can't even pass slow moving vehicle. Ours under warrenty, so if you are not, wait a few more days and I will report here.

[DriVer]

Quote:

Originally Posted by KIX Yes, buy a code reader..........they are relatively inexpensive these days. A basic one can be bought for under $50 I fact some chain type auto parts stores will read your codes for free in hopes of selling you parts.

There are NO reliable code readers for the GM 8.1L engine other than the GM Tech II scanner. Code scanners will return a limited amount of DTCs however they will never return the critical information that GM Tech II scanners display.

To the OP, and in my experience, extremely bad gas mileages could be influenced by a bad or going bad O2 sensor(s). The 8.1L only has 2 of these unlike some automobiles that may have 4.