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Revs dropping and rising again. Help !

motoman984motoman984 Posts: 17
edited July 2014 in Subaru
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: 10,868
    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.
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • 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: 10,868
    Seafoam is good stuff for cleaning out the fuel and vacuum systems.
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • 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: 10,868
    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?
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • 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: 10,868
    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.
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • 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: 10,868
    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.
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • 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: .
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,775
    Sorry, I didn't see that I was being paged the other night.

    I doubt that it applies here. P0420 - Cat efficiency below standards - is a code usually triggered if the downstream oxygen sensor doesn't think that the cat is properly cleaning up whatever hydrocarbons are being produced.

    The front sensor directly relates to engine performance. When I had trouble with one of those, there was stumbling and other driveability issues as the system kept tweaking mixture, timing, etc., to reduce emissions.

    Whatever does get produced should be cleaned up (assuming it is not excessive) by the cat. The second O2 sensor is then a monitor to be sure that the cat is doing the job. I don't believe that It does anything to engine performance - just monitor and report.
  • timbodeytimbodey Posts: 6
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  • Even if the CAT got really clogged ? maybe :sick:
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,775
    Last time I experienced a really clogged cat, the engine ran rough all the time. Stick an obstruction into the tailpipe and see what I mean. No power if it cannot breath. Plus, I'd think that the rear detector would throw a constant code, not the one brief time that you reported. Of course, I could be wrong!
  • ateixeiraateixeira Posts: 72,587
    My 1980 Mustang 3.3l straight six had that problem. It basically stumbled and choked. Never ran properly until I swapped the cat out.
  • motoman984motoman984 Posts: 17
    Scary thing is that the car doesn't run rough all the time and even when it does, you'd think it would give a CEL
  • motoman984motoman984 Posts: 17
    The last week it has been brilliant and hasn't missed a beat, shouldn't of said that without touching some wood first. Oh well the missus is gonna use the car this weekend hee hee hee :D
  • Car has been fine now for ages, with only about once every 2 weeks missing a beat. I just turn the ignition off and back on and it runs brilliant for the next couple of weeks. The only extra sympton I smell now is a smell of rubber or plastic burning.
    Have had a look underneath and can't see anything. My next project is to change the coolant before winter and at the same time change the ECT sensor while I'm at it
    Have heard that this has a major part to play in cooling and vehicle running. No my vehicle isn't overheating, well the temperature gauge doesn't say it is.
  • I looked underneath the hood as I could see smoke eminating from it, the engine did smell very hot, I saw smoke rising up and got on the ground and looked.
    Oil was leaking from the sump pan gasket on to the exhaust and that's what the smell was. Why would this be happening? it's like it is under pressure or something like that
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