S40 Intermittent RPM Drop

KPs40 · Post by **KPs40** » 16 May 2009, 09:55

Wife's 2005 S40 T5 has started dropping rpms when the engine is lightly loaded at around 2k rpms. Problem seems to be when engine is at a mid-range operating temperature. Never happens when engine is cold, and only once when completely warmed up.

When problem occurs, pressing pedal to the floor will clear it up . If you maintain light pressure on the pedal when the problem starts, the engine will begin to alternate between the low rpm's and the rpms that it was turning before the symptom appeared. The best way to describe this is that the engine is "surging".

I am thinking a temp sensor somewhere, but which one? Coolant temp sensor, boost sensor, or another one?

Post by **MadeInJapan** » 16 May 2009, 12:55

Welcome to MVS.
It could be a sensor- but maybe not.
You don't mention your miles and when the spark plugs were last changed. Sometimes basic maintenance items can cause your issues. If you think it's a sensor, have your trouble codes read with a san tool and see what comes up.

KPs40 · Post by **KPs40** » 16 May 2009, 15:06

MadeInJapan wrote:Welcome to MVS.
It could be a sensor- but maybe not.
You don't mention your miles and when the spark plugs were last changed. Sometimes basic maintenance items can cause your issues. If you think it's a sensor, have your trouble codes read with a san tool and see what comes up.

54k miles. Plugs have never been changed, but I would expect to get more miles out of them than that. Am I being unrealistic in that expectation?

Post by **MadeInJapan** » 16 May 2009, 21:17

Really, 50K miles is about all you can expect even with OEM plugs- (official maintenance schedule may say more than this but I've really never seen these things last much longer than 50k). I would change them out now...look in your owner's manual and get the right ones- there are alternatives to the Volvo plugs (but of course, they are probably best for your car)- make sure they are gapped properly (also in your owner's manual). I would tell you exactly which plugs work well with your car and what the gap is suppose to be but without owning an S40 myself, I'm really not sure.