[DRWhalley]

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. 😁 Many thanks for your input.

 

[intramural]

(As a side note, my mileage dropped to 14 mpg when I got new tires installed, may be a coincidence, not sure).

What tires did you switch from and to

 

[MechBob]

Sounds like very possibly distributor gear is shot. Or,at least, distributor vents are plugged. Or both.

 

[Rod's Trucks]

I highly commend you for attempting to learn about the van you have, and being willing to get a bit of grease under your fingernails. I think it takes me more than 10 minutes to remove the doghouse, and I have done it several times.
As you have discovered, it is somewhat satisfying ( both mentally and in your pocketbook) and not all that difficult, to do the basic maintenance.

I cannot offer much advice with regard to your mileage issue, other than to say iridium plugs should not be the cause of the lower fuel mileage. Also, it is still the same distributor as previously, so there appears to be no reason for that to be the reason for a sudden nearly 10% drop in fuel economy.
I would be looking at fuel trim numbers, if available with your scanner. It may be coincidental, and running rich for some reason, like a faulty sensor or possibly a faulty fuel pressure regulator.
I am sure there are other possibilities.
Rod Johnson
Issaquah, WA

 

[DRWhalley]

What tires did you switch from and to

What tires did you switch from and to

Basically the same tire, different brand, just slightly softer tread than my older ones which were bald in places. I have 235 R15 Falcon wildpeak.

 

[Mmusicman]

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

 

[DRWhalley]

Yes, I got those right, I marked each one. It runs great, just guzzles gas. Maybe I'll replace the oxygen sensors.

Basically the same tire, different brand, just slightly softer tread than my older ones which were bald in places. I have 235 R15 Falcon wildpeak.

Right, so just a coincidence. Thanks

 

[Mmusicman]

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.

 

[Mwood]

could be a vacuum leak if the work you did disturbed any vacuum hoses in the engine bay.
maybe one of the new spark plugs wasnt fully tightened and so that cylinder is leaking some compression.
are all the new tires filled to the proper air pressure?

I second the advice on taking the doghouse off and having a look around. they are really easy to zip off once you know where the hardware is.
tricky to get back on without cracking the kick plate next to the gas pedal in half, watch for that. and its easier to line up the upper bolts for the doghouse from the engine bay side.

my van has 200k miles and gets about 20 mpg at 60 mph on flat ground. around 14 mpg in town. I drive like a.. well nevermind. I drive slowly

just a few ideas
Mitch

 

[DRWhalley]

I highly commend you for attempting to learn about the van you have, and being willing to get a bit of grease under your fingernails. I think it takes me more than 10 minutes to remove the doghouse, and I have done it several times.
As you have discovered, it is somewhat satisfying ( both mentally and in your pocketbook) and not all that difficult, to do the basic maintenance.

I cannot offer much advice with regard to your mileage issue, other than to say iridium plugs should not be the cause of the lower fuel mileage. Also, it is still the same distributor as previously, so there appears to be no reason for that to be the reason for a sudden nearly 10% drop in fuel economy.
I would be looking at fuel trim numbers, if available with your scanner. It may be coincidental, and running rich for some reason, like a faulty sensor or possibly a faulty fuel pressure regulator.
I am sure there are other possibilities.
Rod Johnson
Issaquah, WA

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.

I live on the road, so this is around town, on dirt road, and on highway, in hills, interstate and back roads. I never do just one, it's always that way, every year. I know the big tires effect mileage, but I also had them when I was getting 16 mpg, on steep mountain roads at high altitude. The plugs were needed, I have more power now, just sucking gas. Those plugs were really really bad. No codes, but I hear oxygen sensors don't always produce a code. I calculate mileage at every fill up. Ho hum...

could be a vacuum leak if the work you did disturbed any vacuum hoses in the engine bay.
maybe one of the new spark plugs wasnt fully tightened and so that cylinder is leaking some compression.
are all the new tires filled to the proper air pressure?

I second the advice on taking the doghouse off and having a look around. they are really easy to zip off once you know where the hardware is.
tricky to get back on without cracking the kick plate next to the gas pedal in half, watch for that. and its easier to line up the upper bolts for the doghouse from the engine bay side.

my van has 200k miles and gets about 20 mpg at 60 mph on flat ground. around 14 mpg in town. I drive like a.. well nevermind. I drive slowly

just a few ideas
Mitch

I double checked that the wires were on properly, but could check the plugs tightness yet again. I can do the doghouse blindfolded now, done it at least 6 times in the past 6 weeks. Lol. I had tire air pressure done at my last oil change, not since, it would be purely coincidental if the pressure went bad right as I did a service myself, but I'll check them again too. Thanks for the reminders.

And I say take that Blindfold off when you do the doghouse/engine cover.

Thanks for the advice 😜

 

[sixsix]

And I say take that Blindfold off when you do the doghouse/engine cover.

 

[gwd]

Maybe your ignition coil is cracked and leaking the spark.
Have you checked your distributor for gear wear. (pancake sickness)
Check you vacuum lines to see if they are leaking or cracked.
Get a fuel pressure gauge and see if you have the proper fuel readings.
Check your wheel aliment, maybe wheels are off spec.
Replace your PVC valve if not done yet.
Just some ideas.

 

[ADDvanced]

Basically the same tire, different brand, just slightly softer tread than my older ones which were bald in places. I have 235 R15 Falcon wildpeak.

Tires rolling resistance improves with age, bald tires have the least rolling resistance, new tires the most due to tread squirm. Wildpeaks especially have fairly large lugs/aggressive pattern, and with a softer tread compound it would be even worse. It's probably your tires, man.

 

[DRWhalley]

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. 😁 Many thanks for your input.

I figured the drop from 16 to 14 might have been the new tires I put in, but the drop from 14 to 12 happened after I did the service and I'd already been driving it for three months at that point. But thanks for the thought 🤔