Expert: Sean Halliday - 6/14/2007

Question
QUESTION: I have a BMW 325e that I recently purchased to work on.  The problem with it is that it will not idle.  It starts up and revs to about 3000 rpm, while the "fuel economy" gauge runs all the way up, and then it drops off to about 500, where it stalls, but catches itself just in time to bounce back to 3000.  The last owner said he had problems with it not idling but this is unreasonable.  I also notice a loud hiss coming from the direction of the air intake manifold.  Some have told me that while the manifold may be aggravating the problem it isn't the real issue.  They tell me to replace all the sensors, starting with the idle sensor.  I tested the MAF and throttle and they are okay.  Can leaky hoses and intake manifold cause this huge revving?
Thanks

ANSWER: Yes,it sounds like a leaking manifold.The most common is the hose under the manifold which you cant see.Best way is to remove the manifold and do all the hoses and gasket at the same time.I wouldnt change all the sensors but look at the idle air control and make sure the MAF sensor is definately ok.

---------- FOLLOW-UP ----------

QUESTION: I replaced a number of the hoses, and the intake manifold gasket.  I checked all the sensors for proper resistance, and it seemed okay.  It idles now without revving.
Now upon acceleration it seems to choke.  Worst is when you take your foot off the gas pedal the vacuum drops off and the car almost stalls before it realizes what has happened and begins to idle again.  Its electronically controlled timing, so I dont need to check that.  I had replaced all the spark plugs but the outside of the engine is very dirty and I noticed one of them got gummed up last time I put it in.

Answer
The gummed up plug is a clue.Have you done a compression test because a gummed up plug is a sign of worn valves/blown head gasket.(leak down test).The compression across the cylinders should be 100 psi approx if lower or very low such as 70 or 80 is worn valves and need replaced.The engine will not idle properly with low compression.It will choke also and i would change the fuel filter or check the fuel for color to make sure it isnt dirty.I assume you checked timing ,belt and crank sensor?....cap and rotor....