Hello, first post but looking for advice.

To outline the issue, I started to notice it when coasting to a stop - if I am slowing down, say from 15mph and put it in neutral, just before coming to a stop the revs jump to around 1500rpm and then go back down. It does not do this with the clutch down and a gear selected, only if it is in neutral regardless of clutch pedal depression.

Additionally, when starting the car it goes to 2k rpm and then drops back down, if cold, to around 1200 and then slowly back down as it warms up, however, it is idling high - once warmed up it idles at around 800-900.

I have replaced the throttle body to no effect, I also disconnected the battery while doing this and cleared the pcm via OBD, no change. I note that the car indicates it is trying to idle at around 720rpm when warm but is sitting much higher.

I also note that when putting a tiny amount of throttle, the expected rpm will vary in a linear curve as per how much throttle is applied, but in reality there is no throttle adjustment between idle and 1800, the throttle pedal is behaving like an on off between those areas. OBD indicates a constant 12-13% throttle when nothing is on the pedal, I believe this is relatively normal, as soon as 14% is hit, which is about as light as I can actuate, it jumps up to 1800.

The clutch switch seems to work as expected, as per OBD, and no error codes persist. I have once some time ago had an egr error code after hitting a deep puddle but this did not return after clearing. I have occasionally seen a clutch switch out of range error but this is intermittent, ie I clear it and it doesn't return for weeks.

I'm thinking maybe this is a vacuum leak but before I go investigating I wondered if anyone could point me in the right direction. The thing most confusing me is why having the car in a gear makes a difference to rpm when slowing when the clutch pedal is fully pressed.

I'm not too worried, the car still drives fine, but I'd like to solve the issue if possible as it's one of those irritating issues that doesn't want to go away. Thanks for any advice.

Suspect Catalyst performance is dropping off. Or a sensor is going down

If the sensors sense unburnt fuel or poor catalyst performance it will increase the revs to compensate.

You could try new plugs and leads and check the air filter is clean and dry, before you start changing catalyst parts, just in case. Unless you know you've already done them

Can't remember when I did the plugs but will check them, air filter, oil filter and oil all done relatively recently, had the air filter out when I swapped over the throttle body and it was all clean.

 

Any idea if there are specific sensors I should check? Exhaust/cat issues wouldn't shock me - the rear exhaust hanger rusted through recently and I did a bodge fix with a couple of hose clamps but I doubt exhaust has ever been changed

12 hours ago, ffb09 said:

Can't remember when I did the plugs but will check them

And leads

 

12 hours ago, ffb09 said:

Any idea if there are specific sensors I should check?

There's only two. One before the catalyst, one after

Check the first one first