Recently acquired my 1990 740 turbo (175k miles, automatic), and have been spending lots of time burying myself under the hood to really get to know my car. I replaced the fuel line when I got it (split due to age/ sitting for a year +), and at the time noticed a three prong plug hanging down in the left side of the engine under the throttle body. I figured it was just something Volvo left in for manufacturing efficiency because I know sometimes car manufacturers will use the same harness for all models, loose ends or not, so I ignored it. Since then I've been trying to figure out what has been going on with my idle. Hot or cold, will float between 750 and 1000 rpms, and on hot start, idle will start at 500rpms until I put it in gear, then it sits at around 1100-1200rpms. I went through the PCV and AMM until I decided to check out the throttle body. I looked for the wire leading up to the TPS, and I realised the three prong plug hanging down belongs there. I've been driving it around without it plugged in for 2 months and I haven't been having any problems, so start-up problems, no stalling, no stumbling, good acceleration, etc. The only thing I will note is that on a few occasions where I would stab the gas pedal then let off quick, the rpms would drop low, like 200rpms low (nearly sounding like it was gonna die), then jump back up to 750+ rpms. I recently plugged it in, and alas, no difference, whatsoever. What's going on?? I'm at a loss.

Second, I noticed there was a lot of engine noise, and sounded like an exhaust leak. under the car at the bottom of the exhaust downpipe there was a hole the looked like was punched there, and is threaded. So I screwed in a 1/2" long 5/16 bolt to plug it. Anybody know what the function of the threaded hole is? Im certainly more quiet now.

thanks,
Bicyclelifestyle
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Drink Beer, Ride Bikes, Drive Volvo.