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Revs dropping and rising again. Help !

I own a 2000 Outback. The problem I have with it is intermittent.
What happens is that travelling at any speed, the revs drop from 2k to 1k then back again. It hasn't done it for the last week until today.
I have replaced spark plugs and leads, fuel filter,PCV valve,air filter and all hoses are good. The check engine light doesn't come on at all. What could it be ? Anyone shed any light on what it might be ?


  • I have cleaned the IAC valve which was really dirty, car seems to be ok at the moment Only time will tell. Will let you know how things go. :D
  • ateixeiraateixeira Posts: 72,587
    Next serviceable item would be the ignition coil.

    Don't worry, it's easier than it seems. This was an $80 item for my 98 Forester.
  • Thanks for the tip. Do the ignition coils breakdown a lot and are they easy to detect ? :confuse:
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,269
    No, not a lot, but they do handle high voltage and the housing is made of plastic, so they are subject to failure. If the housing gets even a small crack, it can cause arcing of the electrical current and, as a result, lead to poor combustion due to loss of spark.
  • ateixeiraateixeira Posts: 72,587
    ...and often a misfire, so a lot of times you'll also get a check-engine light.
  • Well 2nite was going to be the evening to clean the MAP/IAT sensor, but due to Wisconsin weather and a lack of a garage will have to be postponed until tomorrow.
    Car has been running ok, but it still has a few rev drops here and there.
    Someone did tell me about using a product called ' Seafoam '
    Thank you both for the info about the ignition coil.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,269
    Seafoam is good stuff for cleaning out the fuel and vacuum systems.
  • Thanx for the info on the Seafoam
  • Well so far replaced.
    Plugs and leads.
    PCV valve
    Fuel filter
    Air Filter
    IAC valve cleaned
    MAP/IAT cleaned
    Throttle sensor cleaned
    There are no vacumn leaks and no check engine lights. Oooh I tell a lie I did get a P420 - catalyst below threshold but that went away
    The car is not exactly the same, but when the engine does decide to drop in revs, it's like the brakes going on and so sluggish. You can press that throttle and nothing happens. If you pull over to the side of the road and switch the ignition off and leave it for 5 mins or so everything seems ok again. Stumped and pi**ed off.
  • ateixeiraateixeira Posts: 72,587
    The ignition coil is still original? If so that would be the next serviceable item.

    I got a misfire code before mine went bad (at about ~9 years of age), though.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,269
    You did not mention if this car has a manual or automatic transmission. If it is an automatic, have you tried shifting the transmission to neutral when this happens to see if the problem continues? That may help to isolate it to the engine or the transmission.... Also, in what conditions does it tend to happen - are conditions similar each time or seemingly random?
  • Sorry the car has an automatic transmisssion. The conditions that this happens is varied = foggy wet weather, hot weather snow rain etc anything in fact.. As in driving condition any again uphill, downhill fast slow.
    A very good idea to put the trans in neutral, will give that a go next time it happens.
    Thanks for taking the time to answer me.
  • Yeah I had been reading the unfortunate problem you had as well and last night I was looking at coils. I haven't had a misfire code at all though. My Outback is 10 years old now.
  • Well last night coming home from work, the car did start to stutter again. So I put it in neutral. This did stop the stuttering only because no drive was engaged. The engine revs did still rise and fall but not so much as there wasn't any load on it.
    So to come to a decision engine or transmission. It is the engine. :(
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,269
    Great! At least you narrowed it down!

    The P0420 code may be indicative of an issue with one or more of the oxygen sensors (though I wouldn't replace those on a whim given their cost); I am not sure exactly how feedback from those sensors plays into the engine's operation, though - perhaps fibber2 (Steve) could help you out in that regard.

    At this point, I am leaning toward the throttle position sensor.
  • Yeah, know what you mean about the price of those sensors. The only thing that I can really think of is maybe something along the electrical route, but of course I don't know what.
    A bad earth - maybe ?
    A faulty ignition switch or a shorting relay ? A loose/broken/breaking wire somewhere.
    Going to have a good look tomorrow if the weather is good, the rest of this evening going to study a few wiring diagrams to confuse myself even more.
    Actually you mention the TPS. That's located at the top of the pedal and comes in one unit. That is a good idea. Thanks for the prompt in that direction 'cos the PCM needs that for timing and fuel injection. Smart one areally good idea. :D
  • i recently had a problem with my legacy intermittently losing power momentarily. Turned out to be the crank sensor. I don't think anybody suggested this one.

    Dealer fixed it for 350 total.

    Hope that helps!
  • $350. That's not bad with a dealer fixing it too. Did they have to do a lot of investigating. The one thing I don't want to do is spend loads of money on labor and then the problem still there. :surprise:
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,269
    Did your car throw a CEL when this happened? I would be very surprised if the crank or camshaft sensors could fail (even intermittently) without a code logging in the ECM. That said, both the crank and camshaft sensors are easy to access and not overly expensive to replace.

    Bad ground is also a possibility. I have heard quite a bit of discussion amongst enthusiasts about Subaru vehicles being poorly grounded. That should be easy enough to eliminate as a possibility, though, just by running a grounding strap from some point on the engine block to the firewall or fender.
  • No my car didn't throw a CEL. I think I'll give the grounding strap a try though for elimination purposes.
    I'm in the middle of looking at wiring diagrams to see if the PCM has a ground along the line somewhere.
    Also want to try and checkout the ECT sensor somehow, as I've heard that this could be a major cause. :confuse: .
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