vacuum leak = low mpg?

Some time ago I had a sudden drop in mileage. At about the same time, I had
the first ever CEL code on my car, a 2002 Buick Century V6. The code was P0442
http://www.obd-codes.com/p0442 which is "Evaporative Emission Control
System Leak Detected (small leak)"

I used to get about 23 mpg, now I get about 15 mpg (around town, non hiway).

How would I go about trying to track the problem down? I no longer have a
vacuum gauge. Plus the engine compartment on this car is really packed in tight.

I just had a thought: if I locate the vaccum charcoal canister, could I just
plug up the vacuum inlet on that and see if my mpg improves? Just because the
two symptoms occurred at the same time, that doesn't mean that the vacuum leak
caused the mpg drop. And just how would a small vacuum leak cause a drop in
mpg? Is that even possible? Thanks.

sorry, I should have said that the gas cap seems to be okay. Also, once I
disconnected the battery, the CEL is now off and the code no longer appears
but the low mpg remains.

On 10/25/2010 10:48 PM, Tom wrote:
> Some time ago I had a sudden drop in mileage. At about the same time, I had
> the first ever CEL code on my car, a 2002 Buick Century V6. The code was P0442
> http://www.obd-codes.com/p0442 which is "Evaporative Emission Control
> System Leak Detected (small leak)"
>
> I used to get about 23 mpg, now I get about 15 mpg (around town, non hiway).
>
> How would I go about trying to track the problem down? I no longer have a
> vacuum gauge. Plus the engine compartment on this car is really packed in tight.
>
> I just had a thought: if I locate the vaccum charcoal canister, could I just
> plug up the vacuum inlet on that and see if my mpg improves? Just because the
> two symptoms occurred at the same time, that doesn't mean that the vacuum leak
> caused the mpg drop. And just how would a small vacuum leak cause a drop in
> mpg? Is that even possible? Thanks.
>

Tom wrote:
> sorry, I should have said that the gas cap seems to be okay. Also, once I
> disconnected the battery, the CEL is now off and the code no longer appears
> but the low mpg remains.
>
> On 10/25/2010 10:48 PM, Tom wrote:
>> Some time ago I had a sudden drop in mileage. At about the same time, I had
>> the first ever CEL code on my car, a 2002 Buick Century V6. The code was P0442
>> http://www.obd-codes.com/p0442 which is "Evaporative Emission Control
>> System Leak Detected (small leak)"
>>
>> I used to get about 23 mpg, now I get about 15 mpg (around town, non hiway).
>>
>> How would I go about trying to track the problem down? I no longer have a
>> vacuum gauge. Plus the engine compartment on this car is really packed in tight.
>>
>> I just had a thought: if I locate the vaccum charcoal canister, could I just
>> plug up the vacuum inlet on that and see if my mpg improves? Just because the
>> two symptoms occurred at the same time, that doesn't mean that the vacuum leak
>> caused the mpg drop. And just how would a small vacuum leak cause a drop in
>> mpg? Is that even possible? Thanks.

The two things may not be related.
Bad gas mileage sometimes is due to the injectors dumping
in lots of gas because the sensors think the engine is running lean
or the regulator is shot, etc.
A vac leak could do that but you would know it cuz the engine
won;t idle. Which engine?

On 10/25/2010 10:48 PM, Tom wrote:
> Some time ago I had a sudden drop in mileage. At about the same time, I had
> the first ever CEL code on my car, a 2002 Buick Century V6. The code was P0442
> http://www.obd-codes.com/p0442 which is "Evaporative Emission Control
> System Leak Detected (small leak)"
>
> I used to get about 23 mpg, now I get about 15 mpg (around town, non hiway).
>
> How would I go about trying to track the problem down? I no longer have a
> vacuum gauge. Plus the engine compartment on this car is really packed in tight.
>
> I just had a thought: if I locate the vaccum charcoal canister, could I just
> plug up the vacuum inlet on that and see if my mpg improves? Just because the
> two symptoms occurred at the same time, that doesn't mean that the vacuum leak
> caused the mpg drop. And just how would a small vacuum leak cause a drop in
> mpg? Is that even possible? Thanks.
>
That's not a vacuum leak. And it only leaks when the system is being
tested. At max once per drive cycle.

On 10/26/2010 12:02 AM, Paul in Houston TX wrote:
> Tom wrote:
>> sorry, I should have said that the gas cap seems to be okay. Also, once I
>> disconnected the battery, the CEL is now off and the code no longer appears
>> but the low mpg remains.
>>
>> On 10/25/2010 10:48 PM, Tom wrote:
>>> Some time ago I had a sudden drop in mileage. At about the same time, I had
>>> the first ever CEL code on my car, a 2002 Buick Century V6. The code was P0442
>>> http://www.obd-codes.com/p0442 which is "Evaporative Emission Control
>>> System Leak Detected (small leak)"
>>>
>>> I used to get about 23 mpg, now I get about 15 mpg (around town, non hiway).
>>>
>>> How would I go about trying to track the problem down? I no longer have a
>>> vacuum gauge. Plus the engine compartment on this car is really packed in
>>> tight.
>>>
>>> I just had a thought: if I locate the vaccum charcoal canister, could I just
>>> plug up the vacuum inlet on that and see if my mpg improves? Just because the
>>> two symptoms occurred at the same time, that doesn't mean that the vacuum leak
>>> caused the mpg drop. And just how would a small vacuum leak cause a drop in
>>> mpg? Is that even possible? Thanks.
>
> The two things may not be related.
> Bad gas mileage sometimes is due to the injectors dumping
> in lots of gas because the sensors think the engine is running lean
> or the regulator is shot, etc.
> A vac leak could do that but you would know it cuz the engine
> won;t idle. Which engine?

Yes, it does idle fine. Also, the last time there was snow, I had the car
idling with the tailpipe right above some snow. The snow didn't get blackened.
The engine is 3.1L V6.

Here's how it originally began: I was driving along, then braked. The braking
felt soft. I looked at the dashboard and the CEL was on. That's also the same
month when my mpg went bad. (I've been calculating it at every gas fillup.)
The braking has not been soft again, since that one time.

If as Steve says it's not a constant leak anyway, could some third unknown
factor have caused both the CEL and the lowered mpg?

Thanks for both replies.

In article >, says...
>
> On 10/26/2010 12:02 AM, Paul in Houston TX wrote:
> > Tom wrote:
> >> sorry, I should have said that the gas cap seems to be okay. Also, once I
> >> disconnected the battery, the CEL is now off and the code no longer appears
> >> but the low mpg remains.
> >>
> >> On 10/25/2010 10:48 PM, Tom wrote:
> >>> Some time ago I had a sudden drop in mileage. At about the same time, I had
> >>> the first ever CEL code on my car, a 2002 Buick Century V6. The code was P0442
> >>> http://www.obd-codes.com/p0442 which is "Evaporative Emission Control
> >>> System Leak Detected (small leak)"
> >>>
>
> If as Steve says it's not a constant leak anyway, could some third unknown
> factor have caused both the CEL and the lowered mpg?
>
> Thanks for both replies.

The third reason could be a known unknown.
Means something unrelated to the evap CEL is causing a drop in mileage.
The CEL and lower mileage are pure coincidence.
But that engine is idling fine, and has good OBDII indications when
something is wrong.

Plug/wires are a possibility. Bad injectors don't have to throw a code.
Or you changed gas, or have an overfilled trans or low tires.
But my first guess is you're not measuring MPG correctly, or weren't
before.
That covers just some of the known unknowns.

The third reason could also be an unknown unknown.
Of course we know nothing about that. It's all unknown.

Tom > wrote in :

> Some time ago I had a sudden drop in mileage. At about the same time,
> I had the first ever CEL code on my car, a 2002 Buick Century V6. The
> code was P0442
> http://www.obd-codes.com/p0442 which is "Evaporative Emission
> Control
> System Leak Detected (small leak)"
>
> I used to get about 23 mpg, now I get about 15 mpg (around town, non
> hiway).
>
> How would I go about trying to track the problem down? I no longer
> have a vacuum gauge. Plus the engine compartment on this car is really
> packed in tight.
>
> I just had a thought: if I locate the vaccum charcoal canister, could
> I just plug up the vacuum inlet on that and see if my mpg improves?
> Just because the two symptoms occurred at the same time, that doesn't
> mean that the vacuum leak caused the mpg drop. And just how would a
> small vacuum leak cause a drop in mpg? Is that even possible? Thanks.
>


Your mileage problem is completely unrelated to that code. EVAP "leaks" are
NOT the same as intake air "leaks".

EVAP codes are completely irrelevant to mileage and engine performance--
unless there's a deeper problem that is being misidentified by the computer
as being an EVAP code. In those unlikely cases, it's usual to find a
relevant TSB from the manufacturer.

Somebody needs to--among many other things--check your engine computer's
various parameters, such as fuel trim, injector pulse width, and the
several sensor values, before this issue can be resolved.



--
Tegger

>
> Here's how it originally began: I was driving along, then braked. The braking
> felt soft.


could mean you have a sticking brake caliper..

Mark

Tegger > wrote in
:

> Tom > wrote in :
>
>> Some time ago I had a sudden drop in mileage. At about the same time,
>> I had the first ever CEL code on my car, a 2002 Buick Century V6. The
>> code was P0442
>> http://www.obd-codes.com/p0442 which is "Evaporative Emission
>> Control
>> System Leak Detected (small leak)"
>>
>> I used to get about 23 mpg, now I get about 15 mpg (around town, non
>> hiway).
>>
>> How would I go about trying to track the problem down? I no longer
>> have a vacuum gauge. Plus the engine compartment on this car is
>> really packed in tight.
>>
>> I just had a thought: if I locate the vaccum charcoal canister, could
>> I just plug up the vacuum inlet on that and see if my mpg improves?
>> Just because the two symptoms occurred at the same time, that doesn't
>> mean that the vacuum leak caused the mpg drop. And just how would a
>> small vacuum leak cause a drop in mpg? Is that even possible? Thanks.
>>
>
>
> Your mileage problem is completely unrelated to that code. EVAP
> "leaks" are NOT the same as intake air "leaks".
>
> EVAP codes are completely irrelevant to mileage and engine
> performance-- unless there's a deeper problem that is being
> misidentified by the computer as being an EVAP code. In those unlikely
> cases, it's usual to find a relevant TSB from the manufacturer.
>
> Somebody needs to--among many other things--check your engine
> computer's various parameters, such as fuel trim, injector pulse
> width, and the several sensor values, before this issue can be
> resolved.
>
What are the odds here we're talking about a modified ROM?


--
(setq (chuck nil) car(chuck) )

On Oct 26, 10:55*pm, Mark > wrote:
> > Here's how it originally began: I was driving along, then braked. The braking
> > felt soft.
>
> could mean you have a sticking brake caliper..
>
> Mark


....that could explain the bad mileage.