2001 Legacy/Outback Shaking at Highway Speeds: Help!

booba2

Help! I have a 2001 Subaru Outback. When I drive at highway speeds, my car will begin to shake. I can hear the engine working harder and it just rides rough in general. It comes on slowly and goes away slowly on its own. It seems to do this more frequently when I use regular as opposed to mid grade gas.

In the last year, I have had my head gaskets redone, a new driveshaft put in, new spark plugs and wires, and a new knock sensor. I do not have a lot of money to spend on getting this diagnosed or fixed, so if anyone knows anything that can help me, I would greatly appreciate the information. Thanks.

ateixeira

No check-engine light?

Next thing I was thinking was the ignition coil, but unburned fuel should trigger a CEL.

xwesx

Shaking.... like the something is out of balance? Usually, if the engine, it will come across as a vibration. If it were something in the wheels, steering, driveshaft, etc., it would actually feel like a shake - so, I guess, more intense/violent.

Taking a WAG by your description, I suspect the driveshaft is out of balance; probably something to do with the center bearing or the bushing surrounding it. Did the problem happen before your had your driveshaft replaced, or only since?

I had something similar happen on a '96 Outback, but it only manifested itself in high-torque situations, such as when accelerating hard, while turning, coming from a stop (like taking a right turn from a stop light). The car would shake a little, plus I would hear a LOUD thumping from under the car. I replaced the front half of the driveshaft and all was well. The rubber bushing that surrounds the center bearing had begun to fail.

booba2

This shaking started before the driveshaft was replaced...actually it was going on for a while and I was putting off taking it in until I heard a loud "clunk clunk" when I started from a stop. That's when I decided I couldn't wait any longer and took it in...I thought that the whole thing was connected, but I guess not since the "clunk clunk" went away after a new driveshaft, but not the problem on the highway.

In response to the first response: no check engine light on. I suppose it is possible that the actual bulb for the check engine light burned out, but doubtful.

ateixeira

Check that light - when you insert the key a test sequence should light it up.

If it's burned out I'd get a code reader and pull the codes.

xwesx

Okay. That "clunk clunk" was probably the same issue I had with mine, so replacement of the front half of the driveshaft (which includes the bushing & bearing) would have resolved that.

booba2

I checked that light and it isn't burned out. I asked about fuel system cleaners at an auto parts store and got one to try. The man there said that many Subarus call for premium octane gas, so he suggested that I let it get really low, then add the cleaner with a full tank of premium and see what happens. I figured it couldn't hurt and buying one can of cleaner is much cheaper than just having it looked at.

ateixeira

OK, let us know if that makes a difference.

booba2

Well, I thought it was making a difference, but I was mistaken. It shook on the way to work twice now since I put in the cleaner and premium fuel, so now I feel like I am back to square one.

fibber2

I just read thru the posts. It just suddenly happens when you are driving at some moderate speed steady state, when you are accelerating, or only at very high speeds? We need more on the conditions.

Automatic or manual?

When it suddenly begins happening, try shifting into neutral (or if manual, depress the clutch). Note - only do this when you are alone on the road - not in traffic! If it is related to balance of a half shaft or center drive shaft, it should continue to shake even as the engine rpm dies off to idle. If it stops abruptly, it is more likely an engine/tranny problem.

booba2

This is a manual shift car. It happens when I am at steady, highway speeds. I am usually going about 75 mph when it starts, but slowing down does not make it stop faster. And it isn't really a "sudden" thing. It starts gradually and goes away gradually on its own. On my way to work, I transfer highways, so that I am going about 70-75 on the first highway, slow down to about 50, then speed back up to 70-75. On may way to work, if it does this, then it usually happens after I speed back up on the second highway, so I have wondered if that sets it off, but it has also done it to me at other random times in other random places, but always at highway speeds.

I actually do have an appointment for it in about a week for someone to look at it.

booba2

I took your advice and put the clutch in when it was vibrating, and it did not have much of an effect. I'm not sure how many of these posts you read, but it was doing this to me before my drive shaft was replaced. Although, I was thinking that it must not have been doing it when I had my head gaskets done, otherwise I would have mentioned it to the mechanics that did those. Is it possible that when that was done, that there could have been a mistake or could something have gotten overlooked that would cause this?

fibber2

Boy, now you have me scratching my head!

I didn't see this as a shaft balance issue as it doesn't happen consistently at around the same speed and conditions. A permanently out of balance component (tire with a broken belt, bent rim, shaft missing a weight) usually gives a pretty repeatable performance. If it is occuring and you depress the clutch, one would expect the shake to continue. But again, you should be able to induce it easily by just hitting the right speed.

If it was an engine misfire (injector problem, ignition failure), you would expect to eventually get a CEL. I know that even with an automatic, it is possible to load the engine down on a hill and make it 'lug' a bit prior to getting a downshift. That produces a bit of shake, but it disappears as soon as the downshift occurs. It is a form of misfire, but not enough to throw a code. But depressing the clutch at speed would make that instantly go away as the load drops and the RPM drops.

One thought. Yours is an '01. Mine car was a very early '02 that came thru with '01 brakes. I hate the brakes on this car. They are very prone to 'warpage' (actually an uneven material transfer to the disk that results in thickness variation). I can scrub this off and smooth out the braking, but under some conditions it comes back. On some long, high speed drives I've had 'transfer' occur for unknown reasons. Minor contact friction building up heat??? Net is that sometimes when I do get on the brakes, the front end shakes, and I can sometimes feel it even when not touching the pedal. Slow down or stop for a while, and it goes away. Inspection doesn't indicate a non-retracting caliper, and brake wear appears to be quite normal. I've got around 35k on this set of pads, and they still look OK. But it does sound a little like what you describe.

You might want to have your mechanic do a thorough check of the calipers (sliders and piston retraction), look for uneven wear, etc.

Long shot, but without driving it, it is hard to diagnose.

jd_24

With my 01 OB wagon (automatic) air pressure differences in the front tires can increase vibration very similar to a tire or two out of balance. Simple enough to check.

fibber2

You have to be careful about that one. Enough of a pressure difference on the front two can result in the front differential speed averaging below the rear. This can overheat the center diffy wet clutch and burn the ATF.

jd_24

Agree! I found that I had about about 4 psi difference in the two front tires. (I got lazy in checking) A very minor out-of-balance issue felt way more pronounced. The two rear were OK and at spec. One front tire wass at spec.