I have a 2000 F150 with a 4.6L engine and a 4R70W transmission. I have noticed that removing or loosening the gas cap seems to enhance both performance and economy although no actual testing of either of these has been done. Other than the truck being designed to be operated with the cap in place and tightened, are there any mechanical deficiencies created by allowing the fuel tank to breathe without restriction? I have noticed that the "Check Engine Soon" light will come on after a bit of driving with the cap loosened but it will disappear soon after tightening the cap.

Thanks in advance.

George

 

Yes the technology states,make sure the gas cap is tight.

 

Thanks, Dennis, for the reply. I appreciate your loyalty to the technology. However, if that is the same technology that supports the rest of the stock aminities, it leaves room for questions. So, are there any mechanical deficiencies created by allowing the fuel tank to breathe without restriction?

George

 

Okay, Here's a long answer


PRESSURE TESTING AN EVAP SYSTEM

Note: The contents of this article describe only basic functions and testing of general application pressure operated components used in the manufacture of automobiles. The purpose of this article is to provide basic guidelines associated with testing and understanding engine related pressure components. For specific component testing, refer to the manufacturer’s published data for your vehicle make and model.

Since the introduction of the Mityvac® P/N 04050 Silverline Plus‘ Automotive Test Kit, the automotive industry has increased the requirements for emission testing throughout the world. This has presented the modern technician with a new method of testing that requires the use of a hand-held pressure pump kit. The type of pressure used in modern vehicle systems is generally a low pressure (below 25 psi) and is found in very sensitive devices that convert pressure to an electrical signal for the vehicle’s computer system. The computer processes this electrical signal and uses it to make any necessary adjustments, keeping the engine running at its optimal performance. One of the most common of these devices is known as an Evaporative Emission Sensor. This sensor actually measures the amount of fuel vapor pressure that is in the fuel tank of the vehicle. By testing the sensor one can eliminate the possibility of the Evaporative Emission Sensor being defective, detect a broken fuel vapor hose or the fact that the fuel cap is loose or missing.

 

An excellent explanation. Thank you for taking the time to submit it.

If I understand the bulletin correctly, the EVAP System is to assure that emission control is maintained, and that is determined by a sensor that sends a signal to the CPU to assure optimum performance under the "controlled emission" circumstance. A broken hose or loosened cap will tell the CPU that the emission control has been disturbed which will activate the "Check Engine Soon" alarm. It does not suggest something harmful to the engine itself is taking place. Is my read on this correct?

George

 

Your read is right. No indicators are noted to prove mechanical problems associated with driving with a loose gas cap. But you may want to consider these issues stemming from the loose /no gas cap scenario. The check engine light WILL come on if the cap is loose, so how will you know whether its just the cap or you are now having other engine/emission control problems? Unless you hook up a decoder real often, you wont know. Had any people standing close to your tank lately flipping cigar ashes ? That could get exciting. And the tanks are capped tightly for a reason, it helps keep gas vapors from the air we breathe, some towns are requiring the cap tests on their emissions check. And, the occasional high pressure car wash wont be good for your fuel mileage / performance if water is forced in to your uncapped/loose cap tank, all my opinion only, good luck with your quest for optimum performance,

 

Greg, your reply is gratefully received and your observations are noted and met with a substantial amount of agreement. Thank you very much for the directness of your post. My complements to all for an excellent forum.

George

 

I think you should tighten your gas cap. That will eliminate a lot of the problems.

 

jreed. Your thought is well taken. I agree that the cap should always be tight. My question arose from a hap-happence of the cap for whatever reason being left in position in the tank neck but not tightened. Some eighty miles of driving occurred before the "Check Engine Soon" light came on. Knowing that the last thing done to the vehicle was to take on fuel, it was normal to start the trouble shoot at the gas tank where the loose cap was discovered. After tightening it, the "Check Engine Soon" extinguished in less than two miles. I noticed that during the time that the truck was driven that the cap was not tight that the fuel gauge moved slower off of full and the performance was if not increased was definitely more together or smoother all around. The answers to my question tells me that there is no reason to expect that any harm came to the vehicle as a result of the loose gas cap.

Thanks again to all for the replies.

George

 

I can't vouch for the numbers, but I would guess about 90 percent of the problems that the CEL lights up for are not "life threatening". The CEL is an emissions light. If the oil pressure or engine hot light comes on, I would be concerned! The rest of them are nuisances, but need to be fixed. Eventually!