mouseandcat Posted May 21, 2018 Share Posted May 21, 2018 Hi all, wondering if you all can tell me if what I'm noticing is abnormal behavior. Observation: While driving on the freeway the other day, I notice my RPMs bouncing up and down cyclically between ~750 and ~1250. This seems to happen when: - moving along > 15 mph - when shifted to any of the gears 1-5 - clutch disengaged (i.e. clutch pedal pushed down) [if moving and clutch disengaged but not shifted into gear, I don't observe this bouncing up and down between RPMs] It does this in a regular interval, going down from the RPM it was at while clutch engaged then when I disengage the clutch, it goes down to 750 then back up to 1250, then back down to 750; about a 2-3 second cycle (750 > 1250 > 750). Context: 2005 Legacy i2.5 (130K miles) got rod knock replaced the short block from the dealer ($$$, but we love the car). The was 3 weeks ago and I have driven the car 150 miles since (typically use less than 250 miles per month in recent years). It seems really odd and I didn't notice it prior to driving the car (had it for a year, new to us, from family). Also the idle is a bit rough, seems to go down below 750. Head gasket was replaced 1 year ago. Anyone ideas as to why this is? We have an unlimited mileage warranty for a year so I can take it to the dealership but just want to do some sleuthing first. Link to comment Share on other sites More sharing options...
Brighton96 Posted May 21, 2018 Share Posted May 21, 2018 Probably Throttle Position sensor. My 2nd gen was doing the same thing for a while and once it threw a code I was able to get it fixed. The TPS is just a variable resistor and when it starts to fail, the ecu won't know where the throttle is and will act really weirdly. The symptoms will usually only be noticeable at lower speeds though. Link to comment Share on other sites More sharing options...
RumbleRumble Posted May 26, 2018 Share Posted May 26, 2018 Look for a vacuum leak. Exhaust leaks before the 02 sensor can do this as well. Link to comment Share on other sites More sharing options...
mouseandcat Posted May 30, 2018 Author Share Posted May 30, 2018 funny, the pattern went away. i usually only drive it on the weekends, so after sitting for a the week, i used it for a few errands, and i couldn't replicate the rpm bouncing up and down. strange. if it's a vacuum leak, i assume it should stick around? if its part electronics part connection, i know enough that it may come back up if i hit a bump hard or condensation causes some kind of short...i'll update as soon as i can replicate it. thanks! Link to comment Share on other sites More sharing options...
RumbleRumble Posted June 1, 2018 Share Posted June 1, 2018 in theory yes. could be that when they took your engine out and swapped over parts they could have stretch a vacuum line. You can the clean your IACV but make sure you buy a new gasket. Link to comment Share on other sites More sharing options...
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