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What is this vacuum hose and could it contribute to a rough idle?

Go to solution Solved by logray,

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Hi, I am relatively new to the Porsche world. I bought a '99 996 about 2 years ago, and it has been running like a charm until recently. Earlier this year I moved from FL to NC, where winter exists. Around the time the temperature started getting near 40 degrees (which rarely happened in Florida), the engine starting exhibiting a very rough idle when starting cold. I got a code for a bad o2 sensor, but fixing that didn't seem to resolve the problem. No other codes, so I started going through a checklist of things to replace: MAF, coil packs, spark plugs. I cleaned out the throttle body. Nothing seemed to help the situation.

Right before Christmas, my alternator died. I got a new one on Friday, and put it in today. In doing so, I noticed that there was a small crack in this hose:


What is this hose and could it be responsible for rough cold idles? Also, what does it connect to and how hard is it to replace it?

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The vacuum hose you point out is for the brake booster.

A leak in that hose is an intake leak, contributing to symptoms that generally accompany intake manifold leaks post MAF and pre-lambda sensor (which typically results in fuel added programatically).

For rough running, I would start by taking a look at your fuel trims. If they are way out of whack, solve those problems before tackling the really hard stuff.


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Thanks. I ran the codes, it didn't indicate that the fuel mixture was off. Also, I forgot to indicate, but as the engine warms up, the idle returns to normal. It only runs rough when it's started from cold, with an ambient temperature of 40 degrees fahrenheit or below. When it's 60 outside or the engine is already warm, it starts up fine and the idle appears normal.

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Remember that on a cold start there are a couple things going on. Secondary air into the exhaust, higher idle, among other things.

One of which I would pay attention to are the O2 senors. Before they've heated up and can start giving feedback to the DME, the computer is going to be dealing out a different map than when it is warm.

If you have an intake leak and it stumbles at cold start then smooths out once it warms up, one possible explanation is while the engine is cold the ECU is not adding enough fuel and so it is running lean (or perhaps it is adding too much based on what is has learned previously to compensate for the air leak and so it is running too rich, keep in mind when engines are cold and they first start up the injectors are open longer). Then once the O2 sensors warm up and start providing feedback the ECU adapts the fuel mixture to compensate for the intake leak and smooth things out as best it can (but I would bet a few dollars even yours is not running "well" once it is warm, but you've just grown accustomed to it or never sat in one that is running well).

Just one possible explanation though.

Certainly, since it's easy to do, cleaning the intake and IACV can help it run more smoothly, if they are dirty (be careful not to introduce more intake leaks when you put stuff back together). There is an oring behind the throttle body that is cheap and easy to replace, over time they get hard and don't seal as well.

And when you say your trims are fine, I might have a second look at those, as I would be guessing with a cracked brake booster you are going to be running lean and the ECU is adding more fuel to compensate.

Oh, and did anyone mention MAF here? (perhaps part two...)

Happy new year!


Edited by logray
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Wow! Many thanks! I cleaned out the IACV and it is running a lot better (and starting it in a high 30 degree temperature was a significant improvement) but like you said it still does not feel like 100%. Once it is fully warmed up things get better, but this is the first Porsche I have ever sat in so it could very well be that the problem was always there and I just had no idea.

I really appreciate your assistance. Now to find what part number that hose is. How far back does it go? It looks like it disappears into oblivion; hopefully the far endpoint is still reasonably accessible...

I have already replaced the MAF. I have no idea if the old one was the original one and I have 115k miles on this thing, so I figured it wouldn't hurt.

Happy New Year!

Edited by SteveMeckman
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IIRC the brake booster hose runs the length of the car and into to the brake booster near the front left wheel. I don't remember it having a junction anywhere, and with the engine in place it would certainly be very difficult to replace.

Perhaps it would be possible to use some epoxy or use a hose repair/splice kit in lieu of replacing the hose?

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Any vacuum leak can contribute to a rough idle. Look closely at that connector. I think the hose is inserted into the elbow and sealed with an "O" ring. Find a new elbow first. Then you can see how/if it comes apart.

Check pic #21


Edited by fpb111
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Where exactly is the crack on the hose? That hose inserts into the plastic elbow, which has an o-ring inside. The hose is then locked by a plastic c-clip on the elbow. The o-ring on mine has hardened and I could hear hissing sound (air leak) where the hose meets the elbow. O-ring replaced and problem solved.

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What do I do with that hose when I drop the engine? Replace it, I suppose. If it runs the length of the car, on top of the engine, I am assuming that the engine will need to be dropped a little to provide the clearance to replace it.

The elbow seems fine, the crack is right behind the intake inlet. I am not worried about popping the hose out of that elbow, my dilemma is trying to get to the other end of the hose. Sounds like more work than I am willing to do right now; I only have jackstands.

Picking up some epoxy this evening. Hopefully that'll take care of things.

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  • Solution

A good 2 part epoxy should hold, at least temporarily. A good permanent fix is a new hose.

The hose routes along the top of the engine bay and snakes down by the transmission then under the body underneath belly pans and then behind the wheel well liner where it pokes into the frunk and into where the brake booster lives near the master cylinder.

Here's a pic of that hose from my car. You can see how much of a pain it would be to do on jack stands and with the engine in car. Impossible, no. Difficult, yes.



Edited by logray
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