Rough idle when oil cap is removed (3.0L)

[ErikG]

Hey all,
the title pretty much says it all.

A little background, new to me car, 88k miles, loses a little bit of oil(<1 qt per month) fairly positive its burning it as there are no major looking leaks anywhere, however there is a slight smell of burnt oil wafting from the engine bay after running it for long periods.

While topping off the oil, i checked it, added oil, started it with the oil cap off and noticed it ran terribly. After waiting a moment, checked that oil was now at correct level. I put the oil cap back on and started it up and it ran perfect.

Is this normal? If it is not, what systems should i be looking at?

Thanks!
-Erik

[5ktq]

PCV system is broken.

[ErikG]

Ok thanks, i kinda figured that what the issue was but thought i would put it up here. Its had some vacuum issues in the past so this may be related. Ill have to find some time to dig into it.

It wont damage anything if i continue to run in this way correct?

-E

[imnuts]

Same thing happened to my car, running rough when the oil cap is off, and I'm fairly certain it's normal. You're making a huge vacuum leak when you take the cap off basically. My PCV system is less than 2 years old, so I doubt that is the reason.

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[pablolizarraga]

I'm trying to get a better understanding of the CPRV's functioning as it's been cited as culprit for seeping valve gaskets. I've been smelling oil shortly after driving car faster than normal.

This is what my car does and what's logged on VCDS-lite misfire counters. You can't really tell in the video but there's a slight change in idle when I remove and replace oil cap. No misfires with oil cap on or off. Normal?



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[ErikG]

-imnuts, thanks for the input. I get that there would be a potential for creating a vac leak with the cap off, but this is always how i would check my other vehicles and they never ran rough with the oil cap off. And if the PCV valve was allowing air through it and into the intake, i would think it would just increase idle due to a leaner running A:F ratio. But maybe its so much extra air, its too lean and spuddering. According to the PCV entry on wikipedia, the PCV value should be closed at high manifold vacuum(idle) and open under lower manifold pressures.

Pablo- its really interesting that your engine load % changes with the cap off vs on. (16% on vs 13% off) But yours seems to run fine with the cap removed, mine barely stays running. Ill have to get a video up here. Your system seems normal from what i have seen in other cars, save for the engine load change.

[pablolizarraga]

I noticed that as well. I assumed it was normal. I really haven't gotten that familiar with what the measurement blocks mean.

Why is the change in engine load odd?


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[MoparFreak69]

It's normal. The pcv system puts the crankcase in a slight vacuum to help extract the fumes.
Why are you checking your oil with the car running?

[pablolizarraga]

Wasn't checking oil. Wanted to verify the CPRV was functioning properly. Heard that removing oil cap while idling was one way to check. Seems like mine is working.

Maybe I'm burning up residual oil that I've seen just below valve covers. Going to clean that up as best I can and see if I'm still smell burning oil.


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[ErikG]

I run the engine after adding a bit of oil for a brief moment, let it settle and check it. High school shop teacher said it was a good idea to do that and it kind of became a habit, but its probably not needed. I do understand that the PCV puts a slight load on the engine so that is probably the difference in readings.

The fact that my car wants to quit running when the cap is removed is telling me that the PCV valve is likely stuck open. At idle, PCV valve should only be slightly open, as the carbon rich air from the crankcase will cause the car to run richer and the extra air make it run lean. As the RPMs go up, the valve opens more because there is more blowby past the rings in the cylinders.

[b0geyman]

Mine has done this on my first engine and I thought it was normal. I had to do an engine swap a a year ago and have not checked it this way but I will do now to see if it is a common issue. I thought a PCV would throw a code or at least would increase your LTFT and STFT due to the added air.

[b0geyman]

I checked my second engine and it did not idle rough when I removed the oil cap! So I am guessing it is indeed some kind of a minor issue on the 3.0l engines since my old engine always have done it without any further complications.

[MacFady]

In addition to running rough, is the cap difficult to remove while the engine is running, how about the dip stick?

[QuattroVAG]

Subbed. My car does the same when I pull the cap


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