Low MPG, even on highway

Bigbadbowtie08

Original poster
Member
Dec 19, 2011
31
I had to stop 3 times from atlanta to destin for gas. Was getting 13.4mpg highway. I was being easy on the gas too, trying to save as much gas as possible.

Fairly new plugs (AC Delco)
Clean air filter
Sea foam treated the fuel system
new oil
fairly certain that the Cat has been replaced once already.
I'm going to the dealership tomorrow to get the service history print outs.

I'm getting about 180 mptank.
I tow a landscape trailer for work and have noticed it seeming sluggish more recently.
I have 104,xxx miles on the trailblazer as of right now.

Hopefully someone has some ideas of what the heck is wrong... because I know something is wrong for a fact.
 

Sparky

Member
Dec 4, 2011
12,927
What is your temp gauge reading at?
 

Fire06

Member
Dec 18, 2011
7,223
O2 sensor? Were the plugs ACdelco iridium's, air filter, throttle body cleaned , Just winging a few things here , thermostat and temp sensor
can't think of anything else.. Good luck
 

AtlWrk

Member
Dec 6, 2011
674
Sparky said:
What is your temp gauge reading at?

:iagree: though you'll need to see actual temps on a scan tool. You can be under temp enough to run rich and clog your cat without throwing a code or it being noticeable on the dash gauge.

If the root cause for replacing the cat was never corrected it reasons that the new cat would suffer the same fate. A simple back-pressure test can confirm or rule out a clogged converter. You can also pull out the front 02 sensor and take a short drive. It's a good indication of a clogged cat if it's noticeably peppier without the front sensor in the manifold.
 

Bigbadbowtie08

Original poster
Member
Dec 19, 2011
31
forgot to mention cleaned the throttle body (which was rather clean)
and I have a scan gauge mounted below my head unit. no codes at all, temp at about 209 yesterday.
 

Bigbadbowtie08

Original poster
Member
Dec 19, 2011
31
Just talked to a friend that works at the local Castrol oil change place..
He ran a dianostics on the truck and said it isn't the catlytic converter. He told me to replace my current AC Delco spark plugs with
AC Delco Iridium/Spark Plug and also to replace the AC Delco spark plug wire boots.

He told me that this should solve my problems.
I had the Transmission flushed August 2010 at 73,785 miles

(Currently at 104,2xx miles)

Thoughts?
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
The AC Iridium plugs (41-103) are the recommended plug.
I'm not changing the boots on mine unless there is obvious damage. Just my opinion.
 

MacMan

Member
Mar 3, 2012
194
Wooluf1952 said:
The AC Iridium plugs (41-103) are the recommended plug.
I'm not changing the boots on mine unless there is obvious damage......
:iagree:

But even changing the plugs to the recommended Iridiums is NOT going to gain you much more than a few tenth's of MPG, unless there is some serious misfiring going on......and you'd know RIGHT AWAY if that was happening.

Carrying a lot of dead weight in it? What rear end gear ratio does it have? What speeds are you (honestly) driving at?
 

AtlWrk

Member
Dec 6, 2011
674
Bigbadbowtie08 said:
He ran a dianostics on the truck and said it isn't the catlytic converter.

What diagnostic did he run? Was it a back-pressure test? I ask because the PCM really has no way to detect a clogged cat as long as it is adequately cleaning what exhaust it does pass. Having had the cat replaced once already just raises a red flag for me. Did you get any service records?

What does your long term fuel trim (LTFT) look like?
 

Bigbadbowtie08

Original poster
Member
Dec 19, 2011
31
I was only able to get half of the service records.. Northpoint Chevrolet claimed the other ones were "too old".. Um wouldn't you have everything in a database?

Also one of the last two... starting from the front.. the very last rear two plugs I havent changed. Well one plug is changed however the bolt snapped on me and is flat against the hole that it goes into. It happened when I was tightening up the coil and it was still loose... then Pop..

I'm seriously P.O about this.. autozone was of no help at all. I don't have a replacement bolt..
It's nice to have a very large paper weight in the driveway... Shoot me!
 

Bigbadbowtie08

Original poster
Member
Dec 19, 2011
31
sh0rt update.. I tried again today using a 90* drill extension and had no luck. I'm so frustrated I can't begin to put it into words. Why do the 4.2 L engines have to be so far stuffed under the passenger area. Guess I'll take it to a machinist.. once I find one!
 

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