Infernal combustion question

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
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Sell it.
There is no way to know if the change is from the replacement, or fuel reformulations. And while the difference is a big thing to you, the dealer will say it's within the normal tolerances. DJ
wrote:

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wrote:

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

I try and employ as many US people with my dollars as I can. Failing that, I try to keep my money on this continent. It helps keep us from being employed by selling each other insurance.
David A. Smith
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On Thu, 6 Mar 2008 19:46:26 -0700, in sci.engr.mech "N:dlzc D:aol T:com

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.
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Dear me:
wrote: ...

No, we have had the same reformulated gas for decades, and I went from MTBE to Ethanol a few times before the "valve job", without this loss of mileage. (We'd do each for "half a year".)

It "never" snows here in the desert. I am very regular in warm-up routine. Traffic sux with the snowbirds here, but this car and I have been through three seasons of that without this effect showing up.
Thanks, though.
David A. Smith
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wrote:

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 /
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Dear Mike H:
...

I doubt they changed the sensors, but running lean might not have affected them. Something to have checked for sure.

That is what I suspected, but running this much more fuel should have the "cat" running really hot, no? Tail pipe "residue" has not seemed to have changed... maybe a tiny bit of "light brown" on top of the galvanized pipe surface.

http://www.innovatemotorsports.com /
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.
Sounds like I need a good mechanic not associated with GM to verify what was done, and where is is at. Anybody know one in Arridzona?
Thanks for not telling me I should have bought an import (even though many parts in it are imported).
David A. Smith
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On Fri, 7 Mar 2008 08:10:59 -0800 (PST), in sci.engr.mech dlzc

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?
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On Mar 7, 9:45am, snipped-for-privacy@mine.net wrote:

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

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

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.

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

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
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Go to AutoZone and ask to borrow (for free!) their OBD-II reader. If you drive your car over to AutoZone, they can probably show you where to plug in the reader to pull any codes that may come up.
Take the car to an independent mechanic this time. Ask the AutoZone folks to recommend one. I've had bad experiences with dealership laborers... which is why I now try to do as much work on my cars myself as possible. Failing that, I take it to a good local mechanic.
I was going to recommend a compression check but it looks like you did that already... are you sure Dealer installed new valves, or just ground them down and put them back in? Another compression check might be in the future for you...
Michael
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dlzc wrote:

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.
-jim

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Dear jim:
...

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

OK. I have a list of things to check. (Including that they even did replace the head with the valves.) Thanks to all for the suggestions.
David A. Smith
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On Sat, 8 Mar 2008 17:11:20 -0700, in sci.engr.mech "N:dlzc D:aol T:com

You keep saying this, yet you don't point to anything they did to achieve this. Just replacing the head doesn't do this. What was the root cause of the lean running causing burnt valves and how was it changed?
To be honest given your posting history, this almost appears to be a troll posting as you given the lack of pertinent info.
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On Sat, 08 Mar 2008 19:45:54 -0500, snipped-for-privacy@mine.net wrote:

...
==>You keep saying this, yet you don't point to anything they did to achieve

Hmmmmm...an anomymous poster - " snipped-for-privacy@mine.net" - is calling a regular engineering contributor of some merit, a 'troll' - this sounds more like a comment from a fugitive from an interest group composed of old ladies, not from another engineering contributor, whose posts also show merit.
For what its worth: the most elementary racing engine gambit is to increase the mixture richness to cool valves when combustion temperatures start eroding valve seats. And it is not difficult to tilt the closed loop mixture control in a digital engine controller.
Sure, there is room for confusion. There is lean for economy. there is richer for power. And there is even richer for cooler combustion temps.
Brian Whatcott Altus OK
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"N:dlzc D:aol T:com (dlzc)" wrote:

No not really. First of all there are 2 types of valves (intake and exhaust) and without knowing any specifics on which valves burnt or how, the notion that a lean condition caused the valve problem seems to be just wild speculation on your part. Nor is there any evidence that correcting a valve burning lean condition would reduce gas mileage. If the engine had been running lean enough to burn valves and they corrected whatever was causing that lean condition then you should now be getting better gas mileage.

If they said they did that - they probably did (what motive would they have to lie). However, your interpretation of why they did that may be wrong. The fact that they replaced the entire head suggests that there was some manufacturing defect in the original head.
    And then there is the question of whether any of this means anything at all. You had a problem in the middle of winter. You check gas mileage as a consequence of the repairs - you notice its down by 3mpg from the last time you checked. Maybe the last time you checked was the middle of summer. Maybe if you wait till July you will find the mileage is back to 30mpg.
-jim

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Dear jim:

Mileage started decreasing. Mentioned to the dealership, they spent more than the usual time, came back with "compression low... not spark plugs, might be head gasket, valves, or piston rings... we will let you know in a bit". Came back with "it is valves".
The only way intake valves get burned is if they are left open during combustion, or there is no coolant flow near them.

More fuel gets expended for each cycle of the piston, how could it not affect mileage?

I respectfully disagree with the diagnosis, but do agree that I *assumed* that the dealer "enriched" the mixture when they might not have. They may simply have chosen the wrong cause for the lack / loss of compression.

Agreed.
Perhaps. But they did identify failed / failing valves.

Late summer. Still 100+ degF in the daytime.

Mileage has steadily declined through the "winter", and now that it is back up to 80 degF, the mileage is not increasing. I get an instantaneous and long term trend on mileage (via computer), and have confirmed that it is a good indicator with the miles travelled between fillups. I checked it when I first got the car, and again when the dealer said "oh, you cannot trust it"... with this last observation to them.
There had never been more than a 1mpg variation summer-to-winter prior to this "incident". I steadfastly refuse to run A/C, even when it is 110+ degF in the summer, since my house is never as cool as my car... mkaes it even nicer to be home.
Thanks for your persistence. Lets hold off for two weeks, and see what the diagnosis is.
David A. Smith
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"N:dlzc D:aol T:com (dlzc)" wrote:

Ok that much is more or less true. But bad valves springs, bad loose or worn valve guides, bad cam , or carbon build-up are all possible causes.

Yes if it not in tune that's true. But running too lean will mean "More fuel gets expended for each cycle of the piston" it will also mean a lot more air is going thru with that additional fuel.

That wasn't a diagnosis. It was an explanation of why your explanation is unlikely to be true.

But you arrived at the conclusion that the furl trim was the cause. Perhaps it was and perhaps that is still what is causing the poor mileage.

Ok so I believe you do have a problem that is decreasing gas mileage. That is something you should push them on, because contrary to your previous observation there is a correlation between long engine life and good gas mileage not the other way around. As long as your car is getting good gas mileage you can expect it to have a long life (if you also do the regular maintenance). At this point the dealer is hoping to convince you to ignore it in the hope you will make it out of warranty before whatever it is that is wrong does the engine in.
-jim

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wrote: ...

Diagnosis: a damned air filter. Mileage climbed from low 25's to 28 and still going up.
Absolutely stupid. Sorry for all the "storm". When I actually did oil changes myself, I'd replace the stupid thing every other oil change. But if it is not a schedule published by GM, it does not get done. Depending on the dealer.
David A. Smith
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On Thu, 6 Mar 2008 19:46:26 -0700, "N:dlzc D:aol T:com \(dlzc\)"

Compression low, ignition retarded, or both? A compression gauge may cost $25 - a timing (xenon) light nearer $40 They would tell you something, and the effort is not excessive.
Brian W
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