Because the evap emissions system detects the leak and complains. I believe the actual mechanism is to maintain a slight vacuum in the fuel tank; if the cap is leaking, the ECU will detect that it can't maintain the slightly lower pressure.
Evaporative emissions are apparently a big deal in terms of pollutants.
Dana
Come on JD, since when did you start to just assume things are correct that don't make sense ?
I think this says more about their test equipment than it says about the car. If it would really fail the check engine light would come on (at least on 96 and newer cars) I remember the checks they did on my
95 mustang. I too had a locking gas cap and they were always messing around with it with the test equipment. they always DID get it passed. (Stant cap)sure, pressure and vacuum leaks , but this is the fuel tank we are talking about. The only thing happening here is the fuel being pumped out at a high pressure to the pressure-regulator/injectors. How can an open/leaky cap be of any influence to that ???
I've used a locking stant cap on my 95GT for 5 years and always got good mileage and it always passed emissions.
yeah, I'd like to see that. Remove NO-SPAM from email address when replying
Because the manufacturer build in a system that checks if there is a fuel-leak. I think they have to by law so the emissions-gestapo can easily see if your car complies. Has nothing to do with the engine running bad.
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yup, here in Arizona they advise people in the middle of summer to not fill up during the day due to the gasoline vapors escaping and it causing smog. (ozone I think, not sure)
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Because OBD2 regulations require monitoring of the evaporative emissions system. Same reason the light comes on for the evap purge valve.
They don't make sense to me either, however I'm not a fuel system expert; I merely accept what experts tell me until I find out differently. The mileage increase was a shocker to me as well. I wish I could explain it.
Again, I stress that I am not a fuel injection system expert. I'd sure like to hear from one who could explain all of this to us all.
According to this tech, Stant caps that 'pass' emissions testing, weren't tested at all. Most quickly gloss over this as a matter of routine, without ever actually checking the cap for proper operation. Every aftermarket locking gas cap he has tested has failed, so he does not recommend that they be used. How this takes a leap to reduced engine performance is still a mystery to me, however I'm positive there is a connection somewhere. Going from 14 to 20+ mpg. on a 5.0 Ford engine just by changing a fuel cap is no small feat. I'll eventually find out why; until then, I'm merely passing along the information.
-JD
No, no. I mean how many different Ford keys I would have to carry on a ring to be able to work all possible combinations.
Hmm... lemmie guess here... Two dozen maybe?
Now I'm curious!
| > my guess would be....none......? | >
| > K. | >
| | No, no. I mean how many different Ford keys I would have to | carry on a ring to be able to work all possible combinations. | | -- | YOU are the real piece of work in this post. I think you are | a couple of drumsticks short of a picnic there bud. - SVTKate
Thank You Dana, I was trying to get Brent to answer that one LOL Dang kids with their arms waving around, you were probabaly top of your class in school too :)
I read this whole string with great interest. My Mustang is a 90 GT, and I installed a Stant locking cap after about the first year of owning it. In all these years my car has not only passed emmissions, but done so with flying colors. Also, the "check engine" light never came on as a result of the cap being loose. In regards to mileage, my big decrease came after installing aluminum heads, 24lb injectors, a larger pump, and a few other related items. At no time did I ever notice a drop in mileage from the locking cap. (I always used to get significantly higher than the projected mileage for my car for both city and highway, and this didn't change with the cap.)
I certainly cannot refute the input of experts, but my experience contradicts the notion that all locking caps are inherently bad. I can't even say my cap has really been tested each time, but considering I've had a variety of shops test it over the years its hard to imagine not checking it is such a common practice.
My 2nd car (daily driver) also has a locking cap, and I didn't change it until about a year after buying it. I always keep track of my mileage, and have seen no change. Anyway, with so much being said on this subject, I just wanted to share my experiences. Best regards, everyone.
Randy
We're living in a world that's been pulled over our eyes to blind us from the truth. Where are you, white rabbit?
A '90 Mustang GT used the EEC-IV computer and did not use the OBD-2 emissions standard so it would not have the capability to sense a leak in the gas cap. The cap would have been tested during an emissions inspection if it was required by the jurisdiction. OBD-2 was introduced in the Mustang GT in '96 with the 4.6L engine and EEC-V computers.
about 8
Dang, now maybe I should start a collection ;-)
| > Now I'm curious! | >
| | about 8 | | | | -- | YOU are the real piece of work in this post. I think you are | a couple of drumsticks short of a picnic there bud. - SVTKate
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