Help: RPMs bottom out after high revs

[TheSavyNeal]

Hey everyone, just picked up my 1999 Boxster today!

The only issue I have with it is that my RPMs will bottom out when I either A) have it up at 5-6k RPMs and let it come back down (if I come to a red light) or B) when the AC is on and I stop after normal driving.

The guy I got it from was very knowledgeable and had replaced quite a few things and said he thinks it was narrowed down to a sensor. Has anyone heard of this issue before or have any idea what exactly I might have to replace?

Thanks in advance!

[Timco]

I don't recall if these have a throttle positioning sensor or TPS, but start with removing and really cleaning the throttle body. That will smooth out an idle.

[woodsman]

What do you mean when you say 'bottom out'?
Is that roof black or blue? Love the Sport Seats- they are a rarely seen option on Boxsters!
And for the record, if it's an E-gas car it's got a TPS.

[thstone]

Great looking car - that paint is beautiful for a 15 year old car!

What did the mechanic who did the PPI say about this problem? That would be the most trusted information.

I hate to say this, but relying on what the seller told you is .... well, let's just say that the information he provided might be questionable. I say this because if the problem had actually been diagnosed down to one particular "sensor", then he would have told you the exact part number that was needed to remedy the problem. Even more likely is that he would have fixed the problem before selling. Since he didn't provide this info, then he probably had someone look at the car for 2 mins and they said, "Its probably a sensor" and this is what he passed on to you.

For reference, 99 Boxster's have a throttle cable, not E-gas, so there is no TPS (throttle position sensor).

The idle control valve mounts to the side of the throttle body and maintains the idle RPM's. The ECU will adjust the idle RPM to account for the added load on the engine when the a/c is on.

In my experience, there are a number of components that influence idle RPM's and might be the source of the symptoms that you are experiencing (MAF, ICV, TB, air leak, ECU, temp sensor, etc).

My suggestion would be to have a competent mechanic look at the car to find the source of the issue.

Maybe someone else has had this exact problem and can help with a more specific fix based on their experience.

And if you were/are a US Navy Seal, thank you for your service!

[TheSavyNeal]

Originally Posted by thstone

Great looking car - that paint is beautiful for a 15 year old car!

What did the mechanic who did the PPI say about this problem? That would be the most trusted information.

I hate to say this, but relying on what the seller told you is .... well, let's just say that the information he provided might be questionable. I say this because if the problem had actually been diagnosed down to one particular "sensor", then he would have told you the exact part number that was needed to remedy the problem. Even more likely is that he would have fixed the problem before selling. Since he didn't provide this info, then he probably had someone look at the car for 2 mins and they said, "Its probably a sensor" and this is what he passed on to you.

For reference, 99 Boxster's have a throttle cable, not E-gas, so there is no TPS (throttle position sensor).

The idle control valve mounts to the side of the throttle body and maintains the idle RPM's. The ECU will adjust the idle RPM to account for the added load on the engine when the a/c is on.

In my experience, there are a number of components that influence idle RPM's and might be the source of the symptoms that you are experiencing (MAF, ICV, TB, air leak, ECU, temp sensor, etc).

My suggestion would be to have a competent mechanic look at the car to find the source of the issue.

Maybe someone else has had this exact problem and can help with a more specific fix based on their experience.

And if you were/are a US Navy Seal, thank you for your service!

Thanks for the insight! I went against my better judgement and just had a fellow Porsche enthusiast here look at it with me and he agreed that it could be the MAF or throttle body being dirty. The guy was an honest person, sometimes you can just tell these things, and he had a very detailed log of fixes he's done.

The MAF has been changed out twice and it does have a K&N air filter in there. My friend said to keep things simple I'd probably be better off switching to a normal filter to make sure it isn't messing with the MAF and between that and cleaning the throttle body I should have a good start anyway.

Haven't done any work on Porsche's before but I'm going to give it a try next week after finding a writeup on how to clean it out. Any tips/tricks would be appreciated.

I am currently in the Navy, not as a SEAL. I came up with the name a while back just as a play on words. Don't want to be accused of claiming anything I'm not >.<

[TheSavyNeal]

Originally Posted by woodsman

What do you mean when you say 'bottom out'?
Is that roof black or blue? Love the Sport Seats- they are a rarely seen option on Boxsters!
And for the record, if it's an E-gas car it's got a TPS.

I mean it stalls out. It idles just fine but when the RPMs come down from being around 5k+ it'll dip below the normal idle speed and sometimes stall.

Yep, it's a black top. The guy just bought it new and it has the glass in there too!

I love the seats too but they are very worn Debating getting the leather redone because the driver seat is torn in a couple spots.

I am 50/50 on keeping it or fixing it up for a profit. I got it for only $6k so I'm fairly sure I can touch up a lot of the cosmetic stuff and make a little out of it. Not sure if I can justify spending $2k getting the seats reupholstered though..

[Spinnaker]

Originally Posted by TheSavyNeal

I love the seats too but they are very worn Debating getting the leather redone because the driver seat is torn in a couple spots. Not sure if I can justify spending $2k getting the seats reupholstered though..

https://colorplus.com/

[TheSavyNeal]

Originally Posted by Spinnaker

https://colorplus.com/

The size of the tears in the seat are beyond repairing them myself. Definitely bookmarking this though, thanks

[TheSavyNeal]

Here's a video of exactly what's going on.

My idle will dip low and sometimes stall out completely after you rev it and let it drop. Also if you turn the AC on or anything that uses a lot of power the idle will drop with it. You can also see the lights dim when the idle drops.

https://www.youtube.com/watch?v=ZLNgH7pZbP4&feature=youtu.be

[BIGJake111]

Originally Posted by TheSavyNeal

Here's a video of exactly what's going on.



My idle will dip low and sometimes stall out completely after you rev it and let it drop. Also if you turn the AC on or anything that uses a lot of power the idle will drop with it. You can also see the lights dim when the idle drops.



https://www.youtube.com/watch?v=ZLNgH7pZbP4&feature=youtu.be

As for the lights dimming I have noticed this on both my boxsters with window operation among other electrical uses, current car has a virtually brand new battery and starts up almost immediately but the lights still dim as windows are raised or lowered. My assumption is that it is normal but I still find it odd.

[TheSavyNeal]

True, I've seen it before but it does it with pretty much anything drawing power. The amp even cuts out for a second when the idle dips. But the main issue is it stalling out. If I down shift in a turn and it stalls it kind of freaks me out that I have no power steering at X mph