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Worse gas mileage after tune up

755 Views 22 Replies 11 Participants Last post by  sixsix
2001 V6 4.3 with body lift and oversize tires. 198,000. Ive always got around 16 mpg up to 192,000, then it dropped to 14 mpg.
One day it wouldn't start, turned over but wouldn't start. Moisture was getting in the cap and rotor during extreme fog conditions.( I've changed these several times over the last year, and still not 100 pct sure I've fixed that problem. Won't know till the next fog). Anyhow, when it wouldn't start I also changed plug wires, PCV valve, and replaced the plugs with iridium. I also replaced the fuel filter and air filter. The old plugs were horrible, so I expected my mileage to return to 16 mpg, or at least remain at 14, but instead it dropped to 12mpg.
(As a side note, my mileage dropped to 14 mpg when I got new tires installed, may be a coincidence, not sure). My tester reads no codes.
I have two questions, is it possible that installing iridium has actually made my gas mileage worse? Also, is it possible that IF my actual distributor is bad, it could be making the mileage worse? (If I get moisture in my rotor and cap again, my next step is to replace the actual distributor).
* Heads up, Im not a mechanic, I'm a 61 year old little lady who doesn't have the money to pay a mechanic to do this, so I'm learning about and doing lot more than I ever thought I would, or care to. I can now take the dog house off in 10 minutes, and put it all back in 30. I never wanted to get skilled at that. So with that in mind, please don't get too technical with your explanations, I'm not an idiot, but I'm not a mechanic either. 馃榿 Many thanks for your input.
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Unless tires are taller or have more aggressive tread pattern, they shouldn't affect mileage much.

Also bear in mind mileage readings can vary greatly. I've gotten 16mpg driving to a job site, and same road coming home got 12mph. I had a strong headwind. Speed and driving affects mileage too.

If you changed cap and rotor, be sure you got firing order correct with plug wires.
The Vortec cap is 3-1-5 on left side, not 1-3-5.
I made the mistake once of switching wires, but realized it immediately.
If you got them wrong, it would run pretty rough... so probably not likely
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Don't waste your time replacing O2 sensors unless you've taken readings and KNOW they are bad. Just like the plugs that made no improvement. But getting your OBDII readings might reveal some trouble.. or any codes.

You didn't specify if this was around town, or highway mileage., or speed
You also need to take regular readings

Taller tires and lift will hurt mileage. As you engine gets older, it will struggle to overcome the extra wind-load of a taller vehicle, and lost torque of taller tires.
As always.. note where it sit when fully seated.
Even take a photo.
Also note where it stops twisting when you lift it out
This will be your starting point when you drop a new one in.
VERY easy if you follow this method.
If it doesn't drop in place (that last inch), you can turn the pump deep down inside with a long screwdriver to line it back up
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