Rough idle with oil cap on

[m60kaf]

My 99 2.5 non egas boxster runs real rough and is sluggish

Replaced MAF which seems to have made it a whole lot better

However idle is no appauling, +/- 250 rpm and occasional stall and as you come to junctions it can occasionally pop and bang and hesitate although most of the time its fine

If I remove the oil filler cap there is suction on the cap - u can hear it woosh - but it is only light

take the cap of and the idle is fine

Does this point to anything?

[JeTexas]

Sounds like a dirty throttle body. Opening the oil cap causes a vacuum leak that gives it enough air to idle.

Do a search for throttle body cleaning procedures.

[yeeha68]

Bad idle and oil cap suction usually point to a bad AOS (Air Oil Seperator) A common problem for those of us with older cars. Do a search for the topic and you will find a ton of info. It is 1-2 hr fix/replacement.

[Cassiebox]

correct - assume bad AOS. But then also clean the TB as it most likely is coated w/ or has a small pool of oil in it.

:)

[m60kaf]

Thanks guys

Stripped down all the intake piping and no splits

But a good smearing of oil in the aos output the j pipe and the connector into the intake and the back side of te TB

So I guess this isnt right there should just be air in there? and I need to peoplace the AOS?

You certainly have to clean the TB after this problem - very messy!

[1schoir]

Thanks guys

Stripped down all the intake piping and no splits

But a good smearing of oil in the aos output the j pipe and the connector into the intake and the back side of te TB

So I guess this isnt right there should just be air in there? and I need to peoplace the AOS?

You certainly have to clean the TB after this problem - very messy!

Definitely should only be air in there. Replace the AOS and clean everything up.

When you're done, double check the amount of suction when you remove the oil filler cap. If it's reduced from the level you have described, you will know you solved the problem 100%.

Regards, Maurice.

Edited January 31, 2009 by 1schoir

[Stefan]

On the 1999, the throttle body is not involved in the idle. It is controlled separately by the idle control valve (ICV) which is easy to remove (2 torx bolts) and clean. It is common for them to get gunked up over time and stop operating properly. I would check that first as it is the easiest to do.

[m60kaf]

OK so I changed the AOS - needed doing - leaking from its joints and rubber pipe on bottom shot

Anyway - still have the original problems.

If I put my old maf on takes forever for the revs to rise when you press accelerator so sure thats failed.

With my new MAF on it runs like a rocket and revs like a Porsche - but idles apauling - you rev it it stalls - can drive but pops and bangs at junctions.

If I disconnect the MAF it runs fine (fine as in idles OK and drives OKish - but slow response on the throttle as you would expect with no MAF)

So am I most probably looking at an air leak after the MAF or a trip to the dealer for some magic programming for the MAF (I've read software updates can fix MAF issues)

[1schoir]

OK so I changed the AOS - needed doing - leaking from its joints and rubber pipe on bottom shot

Anyway - still have the original problems.

If I put my old maf on takes forever for the revs to rise when you press accelerator so sure thats failed.

With my new MAF on it runs like a rocket and revs like a Porsche - but idles apauling - you rev it it stalls - can drive but pops and bangs at junctions.

If I disconnect the MAF it runs fine (fine as in idles OK and drives OKish - but slow response on the throttle as you would expect with no MAF)

So am I most probably looking at an air leak after the MAF or a trip to the dealer for some magic programming for the MAF (I've read software updates can fix MAF issues)

Which version of the MAF did you install, early or late?

Regards, Maurice.

[m60kaf]

OK so I changed the AOS - needed doing - leaking from its joints and rubber pipe on bottom shot

Anyway - still have the original problems.

If I put my old maf on takes forever for the revs to rise when you press accelerator so sure thats failed.

With my new MAF on it runs like a rocket and revs like a Porsche - but idles apauling - you rev it it stalls - can drive but pops and bangs at junctions.

If I disconnect the MAF it runs fine (fine as in idles OK and drives OKish - but slow response on the throttle as you would expect with no MAF)

So am I most probably looking at an air leak after the MAF or a trip to the dealer for some magic programming for the MAF (I've read software updates can fix MAF issues)

Which version of the MAF did you install, early or late?

Regards, Maurice.

Same part number but manufactured in 2003

Edited February 15, 2009 by m60kaf