I have a 2004 Chevy Cobalt (aka. Cavalier). When new, I was
getting 30.5 mpg or so. Then about 45,000 miles the valves
started to burn. Replaced the heads under warranty, and the
mileage has dropped to about 25.5 mpg... with the same lead foot
in the same kind of traffic.

I understand that running lean will make the valves hotter.

But this thing has no guts (not that it had much before), and I
really miss the extra mileage.

The dealership says that it is set for factory settings...

Ideas?

David A. Smith

I have a 2004 Chevy Cobalt (aka. Cavalier).

I have a 2004 Chevy Cobalt (aka. Cavalier). When new, I was
getting 30.5 mpg or so. Then about 45,000 miles the valves
started to burn. Replaced the heads under warranty, and the
mileage has dropped to about 25.5 mpg... with the same lead foot
in the same kind of traffic.

I understand that running lean will make the valves hotter.

But this thing has no guts (not that it had much before), and I
really miss the extra mileage.

The dealership says that it is set for factory settings...

Ideas?

I have a 2004 Chevy Cobalt (aka. Cavalier). When new, I was
getting 30.5 mpg or so. Then about 45,000 miles the valves
started to burn. Replaced the heads under warranty, and the
mileage has dropped to about 25.5 mpg... with the same lead foot
in the same kind of traffic.

I understand that running lean will make the valves hotter.

But this thing has no guts (not that it had much before), and I
really miss the extra mileage.

The dealership says that it is set for factory settings...

Did they replace the oxygen sensors when they did
the heads? Perhaps your factory oxygen sensors
were weakened by whatever damaged the heads and
has them not responding poorly.  That would explain the
economy.

Performance may be related to a programing change
that remapped fuel curves or engine timing to effect a
long term repair for whatever caused the initial failure.

If you want to really dig into it get a wide band oxygen
sensor, stick it in ahead of the cats, and drive around
and monitor your air fuel ratio.  the engine computer
should be trying to keep it right around 14.7 when in
closed loop.  In open loop, wide open throttle it should
drop to around 12.5.
http://www.innovatemotorsports.com/

On Fri, 7 Mar 2008 08:10:59 -0800 (PST), in sci.engr.mechdlzc

dl...@cox.net> wrote:
The mileage dropped to 27+ immediately after the "valve job"
(which replaced the entire head as a "core"), and has drifted
slowly down since then.  10% loss to keep from overheating
the valves I can stand... it will help the person that gets this
car used.  But heading for 20% loss is unacceptable from
my belief-set.

How's the oil?

Do you smell gas in it from a lot of blow-by indicating
the rings or worse are shot?

Any leak down  or compression tests been
done?

Have you verified something as simple as the
crank / valve timing relationship is correct?

Have you tried pulling codes out of the
computer or at least reset it?

On Mar 6, 6:46 pm, "N:dlzc D:aol T:com \(dlzc\)"
dl...@cox.net
wrote:
I have a 2004 Chevy Cobalt (aka. Cavalier).

That was your first mistake... try a Corolla next time,
or whatever Consumer Reports recommends nowadays.

On Thu, 6 Mar 2008 19:46:26 -0700, in sci.engr.mech "N:dlzc
D:aol T:com
\(dlzc\)" <dlzc1@cox.net> wrote:
....
As MM pointed one issue is reformulation of gasoline. A
big one being the switch from MTBE to 10% Ethanol to
oxygenate the fuel. Another thing to look for is do you
live in an area that broke through the EPA air pollution
criteria during that time and was also forced to use
another reformulation for this purpose, NOx in particular.

Also any change in running while not moving, idling to
warm up/clear ice/snow, in traffic, etc can have a large
effect given your burning fuel with no movement at all.

On Mar 7, 9:45 am, m...@mine.net wrote:

On Fri, 7 Mar 2008 08:10:59 -0800 (PST), in sci.engr.mechdlzc

dl...@cox.net> wrote:
The mileage dropped to 27+ immediately after the "valve job"
(which replaced the entire head as a "core"), and has drifted
slowly down since then. 10% loss to keep from overheating
the valves I can stand... it will help the person that gets this
car used. But heading for 20% loss is unacceptable from
my belief-set.

How's the oil?

Unsurprising. OIl changed every 3500-4000 miles (which was pretty
often at 300+ miles a week).

Do you smell gas in it from a lot of blow-by indicating
the rings or worse are shot?

Have not checked. Big head, small space under hood.

Any leak down or compression tests been
done?

Compression check identified burned valves, when mileage dropped from
30+ to 28 or so. So yes, this could all be one big decline, say low
oil pressure / blocked passage, and a "meteor strike" needs to be
scheduled to put this thing out of its misery. And get me out a long-
term loan.

Have you verified something as simple as the
crank / valve timing relationship is correct?

I trusted the GM service guys when they installed it. Which is why I
am thinking I need a second opinion.

Have you tried pulling codes out of the
computer or at least reset it?

No means, no experience. Last car I worked on was a 1955 Ford with a
1964 T-bird engine in it. Mileage wasn't good there either. ;>)

Basic question, does this seem like too big a drop in mileage to
"protect the valves from running lean"?

David A. Smith

The mileage dropped to 27+ immediately after the "valve job" (which
replaced the entire head as a "core"), and has drifted slowly down
since then. 10% loss to keep from overheating the valves I can
stand... it will help the person that gets this car used. But heading
for 20% loss is unacceptable from my belief-set.

I have a 2004 Chevy Cobalt (aka. Cavalier). When new, I was
getting 30.5 mpg or so. Then about 45,000 miles the valves
started to burn. Replaced the heads under warranty, and the
mileage has dropped to about 25.5 mpg... with the same lead foot
in the same kind of traffic.

I understand that running lean will make the valves hotter.

But this thing has no guts (not that it had much before), and I
really miss the extra mileage.

The dealership says that it is set for factory settings...

Ideas?

David A. Smith

On Mar 7, 9:45 am, m...@mine.net wrote:
On Fri, 7 Mar 2008 08:10:59 -0800 (PST), in sci.engr.mechdlzc

dl...@cox.net> wrote:
The mileage dropped to 27+ immediately after the "valve job"
(which replaced the entire head as a "core"), and has drifted
slowly down since then. 10% loss to keep from overheating
the valves I can stand... it will help the person that gets this
car used. But heading for 20% loss is unacceptable from
my belief-set.

How's the oil?

Unsurprising. OIl changed every 3500-4000 miles (which was pretty
often at 300+ miles a week).

Do you smell gas in it from a lot of blow-by indicating
the rings or worse are shot?

Have not checked. Big head, small space under hood.

Any leak down or compression tests been
done?

Compression check identified burned valves, when mileage dropped from
30+ to 28 or so. So yes, this could all be one big decline, say low
oil pressure / blocked passage, and a "meteor strike" needs to be
scheduled to put this thing out of its misery. And get me out a long-
term loan.

Have you verified something as simple as the
crank / valve timing relationship is correct?

I trusted the GM service guys when they installed it. Which is why I
am thinking I need a second opinion.

Have you tried pulling codes out of the
computer or at least reset it?

No means, no experience. Last car I worked on was a 1955 Ford with a
1964 T-bird engine in it. Mileage wasn't good there either. ;>)

Basic question, does this seem like too big a drop in mileage to
"protect the valves from running lean"?

David A. Smith

Doubtful the mileage drop is due to anything
intentional or that they did anything to "protect
the valves from running lean".

I would have someone check the timing belt.
More likely they did something unintentional - like
re-install the timing belt one tooth off.

Dear jim:

"jim" <"sjedgingN0sp"@m@mwt.net> wrote in message
news:1205019164_452@isp.n...
...
Doubtful the mileage drop is due to anything
intentional or that they did anything to "protect
the valves from running lean".

If you alter the mixture, by adding a litle more fuel, the valves
will run a tad cooler.