I have been seeing several idle trouble threads lately and thought my solution could be someone else's as well. For the last couple winter months my weekend car has been having significant idle issues where it will sporadically creep up to 2k rpm and not drop down regardless of clutch being in or sitting at a light. It came and went without any sort of pattern to diagnose.
I tested the TPS (good), the ICV (good), balanced the TBs, adjusted the idle screws, checked cap and rotor (good), and even did a smoke test for vacuum leaks (none). I was out of ideas until I took the plenum off to clean the ICV. When disconnecting the vent hose that runs from the plenum to the valve cover, I discovered about a tablespoon or off-white sludge lying at the bottom of the vent hose. I had no idea what this gunk was but it was obviously my problems. After a bit of research, I find that it is petroleum condensate which can solidify and build up if the engine (or plenum more) doesn't get hot enough frequent enough to burn it off. Since I only drove the car two or three times a month for 5-10 minutes at a time, this stuff just built up enough to clog my breather hose and mess with my negative back pressure and cause idle issues.
The reason for it being sporadic was that the gunk would change shape as it softened or hardened based on how long I ran the engine and how much air could get past it. Also present with the tapioca like residue was around a teaspoon of clean fluid, probably condensed vapor or water waiting to get dirty and solidify into more gunk.
After cleaning it out of the plenum port and bottom of the hose, my idle has been perfectly normal and throttle response is significantly quicker coming off idle. Blipping the throttle now gives an instant response rather than a slight hesitation.
So don't throw $300 at a new TPS and ICV before checking your cylinder head vent hose for built up gunk! If it isn't your daily, either store the car for the winter or when you do drive it, run at least 30 minutes to allow the plenum to get hot enough to take care of the condensate. Several 5 minute runs a week is about the worst thing you can do to the car, particularly during a cold winter.
I tested the TPS (good), the ICV (good), balanced the TBs, adjusted the idle screws, checked cap and rotor (good), and even did a smoke test for vacuum leaks (none). I was out of ideas until I took the plenum off to clean the ICV. When disconnecting the vent hose that runs from the plenum to the valve cover, I discovered about a tablespoon or off-white sludge lying at the bottom of the vent hose. I had no idea what this gunk was but it was obviously my problems. After a bit of research, I find that it is petroleum condensate which can solidify and build up if the engine (or plenum more) doesn't get hot enough frequent enough to burn it off. Since I only drove the car two or three times a month for 5-10 minutes at a time, this stuff just built up enough to clog my breather hose and mess with my negative back pressure and cause idle issues.
The reason for it being sporadic was that the gunk would change shape as it softened or hardened based on how long I ran the engine and how much air could get past it. Also present with the tapioca like residue was around a teaspoon of clean fluid, probably condensed vapor or water waiting to get dirty and solidify into more gunk.
After cleaning it out of the plenum port and bottom of the hose, my idle has been perfectly normal and throttle response is significantly quicker coming off idle. Blipping the throttle now gives an instant response rather than a slight hesitation.
So don't throw $300 at a new TPS and ICV before checking your cylinder head vent hose for built up gunk! If it isn't your daily, either store the car for the winter or when you do drive it, run at least 30 minutes to allow the plenum to get hot enough to take care of the condensate. Several 5 minute runs a week is about the worst thing you can do to the car, particularly during a cold winter.
