Looking for user history regarding their solution for MPG drop

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My MPG suddendy fell almost 90%, and then another 30% with the next gas tank. The dealer pretty much hinted that they would rip me off, since my car
is OBD-I (88' injector V6 Olds).
What I'm looking for is personal history of people who dealt with a problem like this and what was the final cause that got the car back from the dead. Basically, I'm looking for pointers to start self-diagnistics.
In my case - air filter is clean, spark plugs/wires are relatively fresh, thermostat & engine coolant sensor are relatively new, and report ok on the dash. I do know that the cat is somewhat clogged, but in the past in the similar condition it didn't cause the MPG drop to such extent.
Car rides very smooth, which probably means that there's no rust on brakes/wheels.
In the essence, I'm close to be making only 1/3 of normal mileage at this point, highway or not.
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snipped-for-privacy@inbox.com wrote:

Take a look underneath in the back. Do you see any gas dripping from the gas tank or wet spots on the bottom of the gas tank?
-jim
-
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Standard tune-up first. Then check the O2 sensors. And don't forget the ignition wires. My 93 Intrepid had a mileage drop. A big one. I did the simple tune-up, filters and plugs. Then the O2's (what a pain!). Finally did the wires. The wires were the single biggest improvement. I'm back up to the high 20's with 195K miles.
One tip. The Intrepid was 'surging' on the highway with the cruise on. Not bad. It was REAL bad before I changed the O2 sensors. But the wires got it all fixed.
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In article
snipped-for-privacy@inbox.com wrote:

Dirty air filters do not effect fuel mileage despite what the TV commercials claim.

Are they the right ones and correctly installed/routed?

The is absolutely no dash readout that correlates to whether or not fuel is being delivered in the proper quantities.

"Somewhat clogged?" That's like my sister in law being somewhat pregnant. It's either clogged or not clogged. Anything that changes the volumetric efficiency of the engine (plugged cat) will have a major effect on fuel economy. Measure the exhaust back pressure. Test, don't guess.


You've neglected to divulge what model this car is but if it has a Mass Air Flow Sensor, I'd be really suspicious and start looking there IF resolving the plugged exhaust doesn't put things 100% back to normal.
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On Wed, 14 Nov 2007 01:36:57 GMT, aarcuda69062

Blanket statement that is by no means always true. Not true on carbureted vehicles.

Coolant sensor for the dash guage is not the same as the one for the computer. The one for the computer will cause a very rich mixture if its terminals are corroded.

Fix this first.

88 Olds V6 he said -- could be any one of 4 different engines.
Might want to look for a broken hose to the MAP sensor.
Don www.donsautomotive.com

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According to the OP, this is injected.

I can only think of one "Olds" V-6, it had glow plugs. ;-)

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Just to be a bitch, lets look at these 2 blurbs:
" Dirty air filters do not effect fuel mileage despite what the TV commercials claim. "
" Anything that changes the volumetric efficiency of the engine will have a major effect on fuel economy. "
I know, I know, dirty ain't plugged. Tooth hurts and I have to take it out on *someone*.
Dave
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On Nov 14, 11:42 am, snipped-for-privacy@yahoo.com wrote:

I can see the efficiency at wide open throttle dropping slightly. In effect a restriction in the air filter lowers the MAX VE. But, at part throttle, one just opens the throttle slightly more to regain the desired VE. If one is driving using the desired output the VE is a given value regardless of throttle opening and filter restrictions.
From what I have seen, VE effects do affect thermal efficiency but the effect is not major except at VERY low VE like idle or very little throttle opening.
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I think the theory is that having a dirty air filter is equivalent to having the throttle not open as far.
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On Nov 13, 6:05 pm, snipped-for-privacy@inbox.com wrote:

90%??? How is that possible? You went from 30mpg to 3mpg?
Maybe your neighbor is swiping the gas from your tank.
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On Nov 13, 3:05 pm, snipped-for-privacy@inbox.com wrote:

I'm sorry for the typo! It went down 50%, not 90.

The engine is 3.8 liter, and there's MAF sensor. Don't know how many O2s, 1 or 2.
Is there a method to diagnose ignition wires, oxygen & air sensors w/ out expensive equipment?
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Yeah, it's called the "check engine" light. Anything reducing your mileage from 20mpg to 10mpg should be immediately and grossly obvious, though -- misfires if it's the ignition, gasoline odor from the exhaust for the other two.
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Check the O2 sensor.
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Speeders & Drunk Drivers are MURDERERS wrote:

What the original poster didn't tell you was that he recently switched from driving the speed limit, to driving about 30 miles per hour over it.
By the way, have you ever driven faster than the posted speed limit?
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On Wed, 14 Nov 2007 22:09:52 -0500, Murderous Speeding Drunken Distracted Driver

please don't feed the troll
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AZ Nomad wrote:

Why do you think my nym is what it is? :-)
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On Nov 13, 6:05 pm, snipped-for-privacy@inbox.com wrote:

I don't get the math here at all.

Really now...repairs are a ripoff?

A personal history??? Huh???

Finally we get to the point. If you have lost 93% of the mpg on a car that might have gotten 20 mpg then you are apparently getting less than 2mpg. The car would be running so poorly and smoking so badly that it would be undrivable. I doubt you could come up with such a loss unless you have a leak somewhere. Do you smell gasoline fumes indicating a leak?
If there is no leak then have a good mechanic such as the one at the dealer do a through analysis and figure out what the problem is. My guess is the catalytic converter is causing the problem.

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I already posted here, that it was a typo - 50% drop - from 20MPG to 10MPG in 2 steps. I do know my cat is clogged to an extent that I will need to replace it soon, but the car still runs, and doesn't backfire.
The thing is - I had a clogged cat before, I think a couple years ago, and my mileage didn't suffer. There was no difference whatsoever, but the car died one day on the highway, and the cause was the cat.
But if it *can* cause this, I guess I might need to address this sooner than I thought.
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On Nov 16, 12:25 pm, snipped-for-privacy@inbox.com wrote:

Changed the cat today, filled in a new tank. Will post results after whatever the amount of miles it would take me. Very oddly, just before I changed the converter, my mileage jumped up from 10 to 12.5MPG.
I did tell the car on several occasions, that I was about to change the cat once the payroll comes through, but it looks a bit metaphysical to me.
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On Nov 30, 6:19 pm, snipped-for-privacy@inbox.com wrote:

Nope, nothing changed. It's the same 10MPG.
I guess I'm doing regular tuneup next.
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