Engine idle drops when oil cap is removed. Guessing this isn't normal? (1 Viewer)

[dcx13]

Like the title says, when the motor's running and the oil filler cap is removed, there is a noticeable/audible drop in rpms. Put the cap back on... doesn't need to be screwed in... and the rpms go back up. There's no noticeable change in overall performance during driving, and no smoke/exhaust signs of an issue. There is a bit of smoke/condensation/vapor at start up, but goes away once everything's up to temp. Engine temp lives around 180ºF +/- (install of a Koso gauge has been great for this!).

The only other oddity, with similar effect on rpm, is when the brake pedal is depressed, there's an increase of roughly 200rpm.

Because both are changing rpm characteristics, it leads me to believe there's some sort of a vacuum issue. But the oil cap off weirdness has me really scratching my head on this.

Any thoughts?

 

[mct75]

That is normal. Don't take the cap off while the engine is running!

 

[riffman12]

I was under the impression that this is normal

 

[dcx13]

That is normal. Don't take the cap off while the engine is running!

Well, yeah. I was doing a look around the engine bay, and noticed some grime around the cap area. Took it off to wipe it clean. As long as it's normal, then I won't worry about it (or take the cap off while running).

The brake pedal induced rpm change still has me a bit baffled.

 

[mct75]

Basically the PCV system is able to suck in a lot more air through the hole for the oil cap which messes with stuff.

When you press the brakes, the 200 RPM increase is potentially an issue with the brake booster. If it stops fine I would just drive it.

 

[dcx13]

Basically the PCV system is able to suck in a lot more air through the hole for the oil cap which messes with stuff.

When you press the brakes, the 200 RPM increase is potentially an issue with the brake booster. If it stops fine I would just drive it.

Booster was replaced right after I go the truck. Was driving from LA to SF, and brake pedal went from normal to block of wood. Did troubleshooting per FSM, and it confirmed a dead booster. Did the work myself, again per FSM, and everything works fine. Unless there's some minor tweak I missed?

 

[mct75]

The booster may have a leak in it (I'm not an expert at these things) but I wouldn't be concerned. Someone else who knows better may chime in.

 

[93yotasr5]

Oil cap removed is letting in unmetered air.
This is normal.

 

[blkprj80]

The grime around the oil cap is probably because some people take off the cap, invert it, and lay it on the table or on the radiator support. Any oil that is in/on the threads will run down and flow across your cap gasket. Then when you put it back on, it will make an oily mess around the cap.

I noticed this when I was changing my oil one time. Now I just leave the cap with the oily side down and place it on a dog food sack or something disposable. Valve cover stays much cleaner now.

 

[LandCruisinMy93]

Totally normal, you're introducing unmetered air into the intake via the PCV.

 

[Tools R Us]

The grime around the oil cap is probably because some people take off the cap, invert it, and lay it on the table or on the radiator support. Any oil that is in/on the threads will run down and flow across your cap gasket. Then when you put it back on, it will make an oily mess around the cap.

I noticed this when I was changing my oil one time. Now I just leave the cap with the oily side down and place it on a dog food sack or something disposable. Valve cover stays much cleaner now.

Or,, the cap gasket is hard like a brick, most of them we see are, a new gasket is cheap.

 

[jonheld]

The brake pedal induced rpm change still has me a bit baffled.

How much of an RPM change? It is normal to see a small change in idle when you pump the brake pedal.

 

[dcx13]

How much of an RPM change? It is normal to see a small change in idle when you pump the brake pedal.

At most, about 200rpm.

 

[93yotasr5]

22re--oil cap removed, runs wierd - YotaTech Forums

 

[Stevesnj]

The crankcase is Positive crankcase. PCV valve is a Positive Crankcase Ventilator to equalize the pressure. If you adversely affect the positive pressure, like removing the oil cap while running, too much air is vented.

 

[BILT4ME]

The crankcase is Positive crankcase. PCV valve is a Positive Crankcase Ventilator to equalize the pressure. If you adversely affect the positive pressure, like removing the oil cap while running, too much air is vented.

Yes, but no.

The engine is actually under vacuum due to the throttle body pulling air from the crankcase, making the term Positive Crankcase Ventilation, meaning it is venting any positive pressure on the crankcase. Since these engines run tight (little blow-by on the rings) then the engines typically run in a vacuum state.

If you remove the oil cap, you allow FREE UNMETERED AIR in, which is then sucked into the throttle body, messing up you air/fuel ratio, then causes the computer to adjust timing and fuel output to compensate.


Yes, any time you step on the brakes, the engine idle goes up just a little for a similar reason. In that case, you are TAKING AWAY air, which causes the air/fuel ratio to change, and the idle increases until the computer adjusts.

Both situations are completely normal.