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PCV valve was unplugged, cause of my mpg woes?


B52U

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I have a 2000 B4000 SEV6 and have been experiencing poor gas mileage for awhile and have been doing all the standard maintenance items trying to fight it.

I happened to look under the hood again today and low and behold, the PVC was completely unplugged! Would an always "open" PVC condition cause poor mileage? I assume that would be equivalent to a vacuum leak? The engine had been running ok, just with sucky mileage (15 mpg highway).
 


RonD

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PCV system is a planned vacuum leak.

If the PCV valve was connected to the hose but not in the valve cover then that wouldn't change intake vacuum at all, which is what I would suspect is what happened.

If the PCV valve's hose was unplugged then you would have noticed a high idle and CEL(check engine light) would come on, as that would be a major vacuum leak.

Vacuum holds PCV valve closed, it is only when vacuum in intake is low that the PCV valve opens and pulls in oil vapor from Blow-by.
Accelerating and heavier engine loads cause lower vacuum in the intake, which is also when you get the most blow-by.

But no to the MPG loss with this.

I would first pull 2(or more) spark plugs and check the tips, white or very light brown is what you want to see, dark color means engine is running Rich, so lower MPG.

Best MPG happens when engine coolant is above 190degF, thermostat should be a 192 or 195degF model.
Also cleaner oil above 190degF
So if you have notice temp gauge is running lower than normal(just below 1/2) then that would cause a drop in MPG.

O2 sensors do wear out, this can turn on CEL but not always, 150k miles is about as long as you should run them, 100k mile change is spec.
This would cause Rich running and browner spark plug tips

If spark plugs don't show Rich running then you could be leaking fuel, fuel filter connections, fuel lines, at engine or gas tank.
Even fuel injector o-rings
Except for the gas tank you can run finger around these and see if you get a gasoline smell.

Also check filler hose, it can crack and fuel splashes out while driving, lowering MPG
 
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B52U

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Thanks very much, the PVC hose was connected. No codes showing., The PVC was just was unseated from its hole in the valve cover so I put it back in. I very recently replaced the O2 sensors and cleaned the MAF, so I will definitely check the other items you mentioned for signs of running rich.
 

B52U

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Plug, drivers side, firewall side. Brown tip.


Second plug, drivers side front. Lighter brown tip.


Does it appear I'm running rich?
 

RonD

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No, those look newer and OK, the white porcelain on the electrode will get a lighter brown as they age, which is also OK, a dark brown or blackish color will be there when running Rich.

Google: spark plug tip images

You could just have one cylinder with a leaky injector, but have to pull all the plugs to find that.

I would check for leaks, won't be a big one, my 4.0l is lucky to get 18MPG highway
 

B52U

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Ah I misunderstood. Yes these are pretty new. I replaced them back about 5000 miles ago. No gas smell around fuel filter connections or filler hose. I will look at injectors next. Thanks for the assistance.

Incidentally while looking at my fuel lines near the gas tank I found this spring hanging from the frame rail near my leaf springs. Any idea what the heck this spring does and where it's supposed to be connected?



 

RonD

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A spring was often used to hold E-brake cable up and out of the way.

Can't think of anything else
 
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