GothingNC

I was topping up my oil after the oil change and something weird happened with the idle while the oil cap was removed.

Car was fully warmed up after a 30 minute drive and when I removed the oil cap the idle dropped about 200 RPM as it normally does. So I am adding the oil and about 2 minutes later the idle smooths out rises up to 1,100 RPM while the cap is still off and it smells a little rich.

So I turn off the car and wait a few seconds and the idle is back to normal at 900 with the cap back on.

Any ideas why this may have caused this?

John

whalebird

basically your creating a vacuum leak thru the intake that vents the crankcase and oil tank.

GothingNC

Ok, that makes sense.

So a vacuum leak can make the motor appear to be running rich?

John

Ed Hughes

I think most engines exhibit this just prior to a major failure.

User 4221

Bad man, bad man.....

GothingNC

Gee, Thanks Ed

John

Amber Gramps

........so taken to it's logical conclusion

if John stops topping off the oil eventually this won't happen?







Nice

whalebird

Ed's right. I've driven a few riding mowers, tractors, weedeaters etc. into the ground. Right before the engine lets go, they get this real nice, high revving, power surge which prompts me to give it more and more. BOOM.
All kidding aside, the motronic compensates for this (tries to at least) and your base idle is out of whack. the AFM, idle control valve, etc all get screwed up. Also, there is a phenomena know as a lean-rise. You see it when tuning a carb on any given engine(including farm implements) where you lean it out and it pics up RPMs. CIS adjustments can produce this as well. I can't recall the layout of all the components and give you a play by play of why this happens, but it is normal. During an oil change, some will clamp off the breather hose to the oil filler neck by the oil cap.

GothingNC

Whalebird,

Thanks for the info, it make sense.

Usually the motors that have blown up on me in the past were running perfectly fine and self destruct just after I am thinking about how well it's running.

Twice with the wife's car (Honda and VW) and once driving my sister's(Ford)

John

whalebird

Thats exactly what I mean - they make good power, then BOOM.

pingman

I do not want to hi-jack the thread, but a question occured to me while reading so I thought I would ask. Is better to add oil with the engine running or not? Next little question, should you add oil to a warm or cold engine? Me thinks to add cold oil off the shelf to a warm running engine could cause a thermal shock of sorts. Your thoughts ladies and gentlemen. Mark

theiceman

always engine running while idlig on level ground at operating temp to check.. then I just pour the oil in as I am looking through the rear glass at my guage .. it is dead on accurate ( better make sure yours is before trying though ) . I think 1 or 2 litres added to a tank full of oil is no big deal persoally . it is not like i am taking it out of the freezer to put in ..

Ed Hughes

There's no harm in adding ambient oil to a warmed up engine. Driving thru a big puddle at speed would do a whole lot more.

I don't think i've ever added oil to my engine while it's running. When I check the level, I'll know exactly how much I may need to add. I'll then add with the engine off. Just a statement, not that there is a right or wrong way. It is just that when you know the car, there is no need to check the stick after adding, except if you're way low.

GothingNC

Iceman,

That's the way I do it.

I add 8 quarts after the oil change (with the engine off) and take it for a drive to bring it up to operating tempature. It usually takes another 1 1/2 quarts to get it to the middle mark on the dipstick and it matches the oil level gauge.

John

theiceman

yup that is what i do exactly .. and I change tohe oil every 5,000km so there is no need for me to ever add any. i always add the full 11.5 litres .. to get to full on the stick ..