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I have a 2001 Subaru Legacy L station wagon with 143k miles. I do not know when the timing belt was done, or when the spark plugs were replaces last. I only know that the brakes, tires, and one of the catalitic converters were replaced recently.

The issue I am having is after driving for more than 20 minutes while the AC is one, the engine heats up. If I turn the AC off, and drive slightly slower the temp slightly goes down and stays just above the center temp. Also, I have noticed if I am driving below 40-50 MPH the RPM stays below 2000-2500. But when I get on the highway and drive 50+ MPH the RPM is steady on/over 3500.

Can any one recommend what to look into? Also, is the high RPM at higher speeds is a normal thing with this aged car? (all my previous cars didn't behave like that...)

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While a manual transmission in the Subarus seem to run high at highway speeds (according to this website, 3200 @ 70mph is normal ... this to me who runs a vehicle which maintains ~1800-2000rpm at 70mph seems high, but nothing like what you are experiencing), running over 3500rpm at 50mph seems quite a bit excessive. This could be the cause of your heating issues. I don't think it has anything to do with the A/C, as it is probably just making the problem more noticable, but not causing it.

You could be experiencing one of two issues as I see it:

If you have a manual transmission, your clutch may be majorly slipping ... I don't think this is the issue. If the clutch is working correctly, for any selected gear at a specific engine speed, you can expect a specific vehicle speed. It's mechanical; it's mathmatical. It will be the same between different vehicles of the same model and year. The only way it is going to be different is if your clutch is slipping. In your case the clutch would have to be slipping like a mad beast. This would probably be very obvious to you and soon enough the clutch would not be working at all.

If you have an automatic transmission, it may not be going into overdrive. This I believe is probably your issue. If the transmission is not going into overdrive as it is supposed to, it will be running at the higher RPM and will be working much harder than normal. When an engine is forced to work hard for a long period of time, it heats up and will generally start to get heat soaked. Once it gets heat soaked, it will not be able to shed heat as easily and thus you'll be seeing a rise in engine temperatures. I notice you also say it is heating up, but not that it is exactly "overheating".

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  • It is an automatic transmission. - And it never really overheats in the sense that it reaches the red line. But it is just barely touching the red. I always lower the speed and turn the AC off at that point. How can I check the overdrive issue?
    – KingsInnerSoul
    Jul 11, 2014 at 16:46
  • @KingsInnerSoul ... first of all, is there a switch for it? If so, is it on or off? The switch for these are usually located on the shifter, but might be on the dash. If no switch, it is completely controlled via the gearshift lever. To see if it's the overdrive, while driving around 50mph, click the switch to turn off the O/D (or pull the stick back into drive). If you feel no difference in engine speed or pitch change, it's not making it to O/D. You'd then need to take it to a shop for tranny repairs.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 11, 2014 at 17:32
  • What would be a fair price for a tranny repair?
    – KingsInnerSoul
    Jul 11, 2014 at 17:34
  • That's subjective to location and the actual repair to be done. It's also off-topic for this site, as the information becomes stale quickly. If you'd like to ask through the chat, let me know. Just put @Paulster2 in front of anything you type, and it will get to me.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 11, 2014 at 17:43

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