How your oil filler cap may cause poor fuel economy, low MPG
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- Опубліковано 11 лис 2017
- How your oil filler cap may cause poor economy.
Applies to many cars, particularly those with MAF sensor snorkel
Air drawn into the crankcase by the PCV (positive crankcase ventilation) valve is normally drawn in via the crankcase breather inlet tube which is attached to and sources fresh filtered air from the air intake snorkel between the intake throttle plate and the MAF (manifold air flow) sensor.
The result of having a loose oil cap is that some of the air entering the engine drawn in by the PCV will not go through the MAF sensor as it normally should
Instead being drawn in via the loose oil cap, and thus not be accounted for in fuel mixture control that uses the MAF sensor to determine the volume of air flowing into the engine.
This is similar to a leaking air intake snorkel or defective MAF that will cause incorrect mixture calculations.
In addition to this as the air drawn into the crankcase via the leaking oil cap is not filtered as it is if drawn in via the intake snorkel that non filtered dirty air will contaminate the engine oil and cause it to be prematurely compromised in it ability to lubricate the engine.
And this condition is worsened by air back flowing in to the snorkel from the crankcase that contains oil vapor that should be going into the PCV valve and not the air intake, the result is the oil vapor will contaminate the throttle plate causing a build of dirt on it and that may cause poor idle control in addition to the poor mileage from the first condition. - Авто та транспорт
On the odd chance that anyone watching this video doesn't happen to read the description that explains why this can cause poor fuel economy, here is the reason again
Air drawn into the crankcase by the PCV (positive crankcase ventilation) valve is normally drawn in via the crankcase breather inlet tube which is attached to and sources fresh filtered air from the air intake snorkel between the intake throttle plate and the MAF (manifold air flow) sensor.
The result of having a loose oil cap is that some of the air entering the engine drawn in by the PCV will not go through the MAF sensor as it normally should
Instead being drawn in via the loose oil cap, and thus not be accounted for in fuel mixture control that uses the MAF sensor to determine the volume of air flowing into the engine.
This is similar to a leaking air intake snorkel or defective MAF that will cause incorrect mixture calculations.
In addition to this as the air drawn into the crankcase via the leaking oil cap is not filtered as it is if drawn in via the intake snorkel that non filtered dirty air will contaminate the engine oil and cause it to be prematurely compromised in it ability to lubricate the engine.
And this condition is worsened by air back flowing in to the snorkel from the crankcase that contains oil vapor that should be going into the PCV valve and not the air intake, the result is the oil vapor will contaminate the throttle plate causing a build of dirt on it and that may cause poor idle control in addition to the poor mileage from the first condition.
Whats your opinion on finite thermo cycling being a large portion of an engines determined life? ( I try to sound smart when i drink)
I have a leaky valve/tappet cover gasket. Could it also be drawing in air and causing the same problem? The car is also idling at 1300 instead of 700.
@@newsgetsold
I doubt it, the issue here is not like a vacuum leak of an intake for example it is just that the source of the air incoming that is around atmospheric anyway is not coming through the MAF, or is not being accounted for in the ECM
The high idle you have would indicate some other fault.
@@AuMechanic Okay thanks. I just thought the oil filler cap is also in the tappet cover so a leak elsewhere on the cover would have the same result?
@@newsgetsold
It would need to be a fairly open leak enough the "vent" the crankcase as the air is drawn into the PCV (positive crankcase ventilation) valve by vacuum.
A loose filler cap provides enough leak path to do that but a small leak of the gasket isn't large enough to draw so much air in it bypasses the MAF sensor on the intake snorkel.
I replaced oil cap on a vauxhall meriva and noticed the difference in power.This was after a friend mentioned it.
Although no oil was leaking out of the cap it could not have been tight enough for some time, although it did not feel loose at all really.
For the cost it is worth replacing every couple of years if you are keeping the car for any length of time.
The difference in power i noticed the next day i drove the car,i reckon i might have been losing about 10hp on a 1.4 16v petrol engine.! No doubt i will get better fuel economy now too,even though it is good at the moment.That £7 i spent on the cap will pay for itself in a few weeks of better fuel economy i should imagine.
It's one of the overlooked things on an engine, but it could cost the driver a fortune in wasted fuel over the years!
Cheers mate, for highlighting the issue!! 🙂👍
Thanks for the comment.
The other issue with this is sucking un filtered air (and dust) into the crank case too so its worth checking.
@@AuMechanic Yes, indeed. Thank you.
The fuel cap is another one to check on older cars. The seals go on those eventually. If it feels loose or you can smell petrol around it, it's probably shot. Petrol vapours leave behind dirty, oily looking blotchy patches on the inside of the flap as well.
Regarding poor fuel economy, while the O2 sensor will cause the fuel trims to adjust to compensate for the un-metered air that is bypassing the MAF, that will not compensate for instantaneous air flow metered by the MAF sensor, the results of which is that fuel consumption will increase in the same way it does with a dirty MAF sensor.
Thanks for the tip. Ill have to check mine out as my suv has been using alot more fuel than normal. But i have no codes popping up or check engine light. But my exhaust definitely smells like its running very rich! Is this possible from a bad cap? Thanks mate.
It only effects the instantaneous AF ratio not the overall AF ratio, and you would be able to see it in the fuel trims from the ECM as it has corrected for the air coming on via the leaking cap.
So in short MPG will only increase a bit, not a huge amount enough to smell it rich at the exhaust.
You need to plug in the OBD and read the fuel trims to see what's going on there.
@@AuMechanic ok, Thank you very much for the info. Ill have to check that out. I was thinking maybe a bad O2 sensor or something? but i was thinking that would of triggered the check engine light on. But maybe not. Thanks again mate. Have a lovely day.
Ordered my new dexos oil cap today. Will be here I no a week. It’s tight when attached. But the seal is loose when I remove it. My oil filler neck feels loose as well. It’s a Chevy equinox. Any idea how to replace the filler neck that the cap screws into?
In short, no.
That's a bit beyond the scope[e of the video.
You just need to make sure it seals one way or another.
I have the same issue. There are a bunch of Permatex products. Do you know which one would be used to run a bead around the tube where it sits on the valve cover?
think it was prolly some schlub at firestone who slapped the wrong oil filler cap on my pontiac. replaced it today.
In my 2020 Chevy Sonic, it is the oil filler tube that is loose and not the oil cap or gasket. I want to run a bead of some sort of sealant, like Permatex where the tube sits on the valve cover, but I am confused as to which one is the correct one. Can you recommend something? Thx.
Heat tolerant silicon maybe.
Be careful using silicone on modern cars, only use O2 sensor safe silicone or you risk contaminating the O2 sensor/s.
i have azera 2009 & this damn cap is cost me replacing the vales sealing thinking the leakage is from vales sealing. then after few years the alternator broke due to the oil leakage again from the same area! then i found that the cap is not closing tight & leakage is coming from it! so this cap need to be genuine part & closed tight. mine is the type making clicking sound so if it's turning fast not slow it need to be changed. i'm just starting loving EVs more & more🥲 specially the one with the range extender🙂
Would a bad oil cap cause high oil consumption... Please reply back well greatly appreciated it 🙏
It is possible, if the filler cap is allowing enough air in the breather baffle may allow some oil to get into the intake snorkel (flexible pipe) between the air box and the throttle plate where the crankcase breather feeds from.
You would be able to see this if you take of the intake snorkel off and look at the throttle plate it would have a significant amount of oil residue on it.
Or the oil is leaking out from the cap which you would see oil on the top of the engine around the filler cap
Or some other external leak which would end up leaving some oil stains where you park it.
If the engine is all clean and dry of oil externally and it is not leaking from the engine visibly then it may the oil control rings on the piston which would.cause exhaust smoke under hard acceleration at revs, or from the valve stem seals which would be smoking at the exhaust when idling for a long period or a puff of smoke from the exhaust after decelerating down a hill and then accelerating again.
@@AuMechanic thanks for the reply. Will look into everything 👍 you said
I found that with my Holden VY SS Commodore which has a Worn oil filler neck in the valve rocker cover.
The knob which the thread of the oil filler cap screws onto is worn so I fitted an O ring underneath the oil filler cap as a temporary repair to tighten it up & it works.
I seem to be getting far better fuel economy now !
Glad to hear it.
Hmm, news to me but willing to try it out as I have a shitty oil cap.
Hi Merlin
Read the description, it covers which engines this will apply to and some detail in exactly why this occurs.
Believe me it makes a lot of difference.When i changed the cap on a meriva 1.4 16v
The cap did not feel loose but was probably not as tight as it should have been this meant the car was drawing in air though the cap.
When i got in the car next day the car pulled a lot better after fitting the new cap.
My bmw would trough a code for this. Dam BMW
Would this trigger Check Engine light?
Yes potentially it could, if it is more extreme it may cause a fuel trim code and too much oil drawn in to the intake could cause some fouling that may result in a code.
my mechanic told me that if you loose your oil filler cap it won't suck air, put push a lot of air out as part of the pvc system
He's got it backwards.
The vacuum of the PCV is greater than the blow by in a normal engine.
My oil cap is hard as a rock and almost needs an impact wrench to open. That's also no good.
Might be an idea to replace it before it breaks and bits of it go into the top of the engine.