99 V6 Mystery Intake Hose And Extreme Loping At Startup


New Member
Dec 17, 2012
Hello. Thank you for taking the time to help.

A few days ago, I started my Mustang and the RPMs went up to the 2K range immediately. After a few seconds, it dropped to below 500 and the engine nearly died before revving back up to the 2K range. I also heard a new "sucking" sound that is constant now, whenever the engine is running.

The sucking sound is coming from the cut-off hose in the center of the attached photo.

This hose has never been connected to anything since I got the car over a year ago. I changed the head gaskets last winter and have been driving the car daily since then, with no trouble from this hose. The hose has been in its current condition (cut off and unconnected) the entire time.

If I let the car idle for about 2 minutes after starting, the RPMs go up and down less and less, eventually leveling out around 700, but the engine runs rough.

This problem has not affected my gas mileage or performance while driving. In fact, both seem to have improved, even though the engine runs rougher now than before the loping started.

I put a pressure gauge on the fuel rail. At 1500-1700 RPMs (the high end of the loping range), pressure reads about 30 psi. At 300-500 (the low end), it reads about 40. When it levels out at 700 or so, pressure levels out at 35.

I have had PO172 and PO175 trouble codes since before the head gasket change, and these are the ONLY codes I get now, too. I doubt that this is related.

The engine is a 1999, and the body is 1995.

So--what the hell is this hose, where should it go, and why hasn't it given me trouble til now??

Thanks again!


  • Mustang intake hose.jpg
    Mustang intake hose.jpg
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