I was driving my '97 '965 (135k) the other day when the check engine light popped up. Autozone pulled the code --- RPM sensor. Bought the sensor and some NGK plugs and installed it the next evening, removing the old OEM volvo plugs and gapping the NGKs to .030.

The car runs fine, although the RPMs (idling in neutral and in gear) have dropped about 300 RPM. I'd say she's idling in neutral around 600 RPMs now.

What do you guys think caused this?

BTW --- the center electrode tip on the NGK is VERY thin, about 1/3 of the mass of the OEM volvo plug. The NGKs are not platinum plugs.

THANKS!

TIP: Use a QUALITY torx bit (#40?) when removing the RPM sensor bolts. I sprayed them down slightly with PB blaster first and let them sit. It's a tight fit for the top one, you'll almost need a 1/4" ratchet. An allen-style wrench would work/fit better. And remove the bolt, the one you can see where the ground wire is attached too first. In case you goof one of these bolts up, this is the one you want to goof up --- as you could always get out with a pair of vice grips if necessary. Be carefull, the torx bolt heads strip very easily. My RPM sensor had basically fallen apart due to to heat/age.