Pulsating Idle - Intermittant

[openwater]

1999 Boxster Base - 89,500 miles. This car is driven about 4,000 miles/year in a temperate climate - San Diego

Five or six times over the past three years I've experienced the following issue.

This happens completely out of the blue... One day an absolutely rock solid idle at 780RPMs, the next this craziness. No CEL. No pending fault codes on the OBD2. The MAF (OEM) was replaced about 10,000 miles ago, no obstructions in the intake, air filter is clean. Acceleration feels "normal", but let your foot off the gas even slightly and the car lurches like it either air or fuel starved. Warm or cold engine... no difference.

This condition usually lasts about two or three days, then goes away completely. I was thinking of puling the throttle body and cleaning the idle control valve but the intermittent nature of the issue has me guesing...

Thanks in advance!

Steve

[Loren]

Most likely a dirty throttle body and/or bad ICV (Idle Control Valve). Try cleaning both first.

[logray]

+1 to Loren's comment!

[openwater]

It took a week to get the gasket in stock but I finally tackled the throttle body and idle control valve today. As advised, the cleaning did the trick! A couple of notes for those who follow...

• I thought that I needed to remove the hard plastic air intake body from the engine compartment to get access. It only needs to be loosened and the engine mounts removed.
• A magnetic wand is a must to make sure you catch the lower right throttle body bolt when reassembling. Its blind, and in a tight spot.
• It's easier to remove the left side engine mount at the engine side than the throttle body side.
• All the bolts/nuts on this project are 10mm.
• I found it easier to leave the rubber tube on the lower right, rear side for last when disassembling. My clamp was positioned pointing down so there was no way to get to it until I could translate the whole body. Also, at some point someone forgot to feed the tube through the engine bracket and I was developing a wear spot. Make sure you feed yours back through.
• I used "Throttle Body and Intake Cleaner" on the whole set up.
• The Idle control valve has a curved "door" inside one of the openings. When I first filled the valve with cleaner I didn't realize the "door" moved. But after a few minutes of cleaning and soaking I realized that it has a spring action that closes it. At first the movement felt "gravelly'. The more I soaked, the more fluid the movement became. I used a q-tip to move the "door" back and forth. Finally, it felt smooth.

Questions:

1. When disassembling I managed to slip with the wrench and crack the hard black tube that runs just to the right of the throttle body (as you face the engine). It appears that there is a cable inside. What is this and what does it do? I put it back together with duct tape for now, I'll wait for final reassembly of the engine compartment trim until I hear back from one of you.
2. The idle is fine now. Seems to be hunting a bit though.... will the computer take some time to learn the new flow pattern? When the car is warm and I idle for a minute or so it stabilizes at the familiar 780rpms, but when stopping briefly it wanders between 800 and 1000.

Thanks in advance!

Steve

[logray]

Nice work.

Can you post a pic of #1?

#2 you probably have an intake leak if the idle is wandering, or your idle air control valve still needs more cleaning.

[openwater]

This Boxster lives outside and it's raining... so I grabbed this pic from the internet. Not a great angle, I can get a shot of mine tomorrow. It's hard plastic tubing with a cable? inside. It starts somewhere near where the coolant reservoir comes through the trunk. I'm thinking oil dipstick. Hmmm. Easy enough to test in the morning.

FYI - after driving about 20 miles tonight the idle settled down and is as steady as ever at 780 rpms.

Thanks again.

Edited September 6, 2011 by openwater

[openwater]

Yep.... part 996 107 183 03. Oil level dipstick guide tube.