Why 03 Outback vibrating more, higher start rev + lower idle, post belt replacement?

Hi all, finally getting settled in on W Coast and just wanted to say thanks again for all your help in picking the car in best mechanical condition. The 2003 Subaru Outback H6 VDC we bought in N California before moving our here from the E Coast a) drove very smoothly b) has been a huge help in picking up all the light office furniture and supplies we had to buy for our office and home (I found the extra interior length of a wagon/crossover more helpful than the extra interior height of a small-mid SUV) . We were able to pick up every single thing we needed locally except for a couch set. It?s unbelievable what we were able to move with this car.

The car only needed the Serpentine belt replaced and wheel alignment and I also picked up a new set of tires. At 85K miles, we?ll probably also do the 90K servicing soon.

Though the car was driving very smoothly for weeks, the pre-purchase inspection garage said that the serpentine belt needed replacement - so after a few weeks, as soon as I could give up the car for a day, I had a local garage with good reviews replace the belt, and I noticed the following afterwards:

1) Higher revs upon start and lower revs later upon idle - I recall the engine used to be at about 1000 RPMs after start and very quiet with low vibration. Now, upon starting the car, the car seems to be revving at a higher RPM than it did before the belt replacement ? about 1500 RPM ? and stays there for a bit before coming down to 750 RPMs, and it?s noisier and vibrates more than before. It?s not VERY noisy and shaky, just noticeably more so than before, when it was super quiet with low vibration.

2) Twice right after coming off the highway to local streets, the car stalled at a red light. It started right back up, but I have no idea what caused the stall. It only happened 2 times, and only happened when the gas tank was lower - both times below 1/4 tank, one time when very low. One of those times the car was also on a hill. It never happened any other time.

A) Any idea what could be causing the higher revving, higher noise and vibration upon start? What could the mechanic have done to cause this? Is the timing off ? What RPM should the car be at upon start, and what should it be at after warmed up when still?

B) Any idea what caused the 2 stalls? It only happened those two times, under the same conditions ? coming off highway to streets, upon the first or second red light, with the gas tank below 1/4 tank.

The act of simply replacing an s-belt shouldn’t have any other effects. I suspect that the mechanic managed to knock something loose in the process – perhaps a sensor wire or vacuum hose. Is the CEL on by any chance? I’d have someone look over the engine and see if anything appears to be missing or unhooked. Or maybe something just happened to give up the ghost at just that time?

The rough idle and the stalling might be related to the new, lower idle speed. If it was idling at 1000 before, 750 sounds a bit on the low side – it might be barely enough to keep the engine turning over (and have a lot of vibration). Anyway, a loose wire or hose could be causing that, and would be the first thing to look at. Your owners manual (and often, a sticker in the engine compartment) should tell you what the idle speed should be.

You asked about timing being off – I can’t see an s-belt replacement doing anything to the ignition timing or the valve timing. That wasn’t a timing belt that was replaced, was it? That’s a fair chunk of change you would have laid out.

Since that engine uses a timing chain, it was definitely not a timing belt that was replaced. The correct idle speed is actually somewhere in the vicinity of 750 rpm. Later today, after I drive my car (same engine as the OP), I will double check the correct idle speed and post back with that information. But, for the time being, I tend to agree that a dislodged vacuum hose is a likely candidate here for the source of the rough idle.

Thanks, very helpful info. The idle is not very rough, it’s just a littler rougher/louder/more vibration than before, but not something you would notice unless you drove the car before the serpentine belt was changed. It was super quiet and no vibration beforehand. I don’t think it’s at all related to the serpentine belt, I think the mechanic must have knocked something loose or changed a setting.

A poster on another forum suggested that it could be a change in altitude causing this, and the car came from the mountain area of Tahoe and is now near sea level in Silicon Valley. However, in this case I think it’s unlikely - the car was driven at sea level for weeks before the serpentine belt replacement and the idle was super smooth.

It is most likely that the mechanic changed a setting or dislodged something. The idle seems to start at higher RPMs now than it did before, and then settles at lower RPMs than before after warm up.

What does the vac hose do?

Could the dislodged vac hose have caused the stall as a result of low idle speed?

What is the CEL? What does it mean that it’s on?

Also, what is the role of the vac hose?

It’s always good to check the simplest solutions. I’d like to bring the car in to the same mechanic to check these asap.

VDCdriver, can you post the correct idle speed? I suspect it’s supposed to be around 750, but that a hose is loose or sensor is slightly off.


I’m sorry for the delay in getting back to you. Due to some health issues, I didn’t drive my car for a few days, and thus, I was unable to check the RPMs for you.

Here is what I found, with an ambient temperature in the mid 60s:

When the engine is first started, the idle speed tends to be in the 1300-1500 range.
After a minute or so, it goes down to ~750 RPM.
When fully warmed up, the idle is 500 RPM.
All of these readings were taken while in Neutral or Park, with the A/C off.

So–it sounds like your idle speed is now in the correct range, and it may not have been idling in the correct range previously. However, I hope that you noted that I mentioned the ambient temperature during the time that I checked idle speeds. If the ambient temperature in your area is now lower than it was when you first got the car, that could certainly account for a difference in “cold” idle speed. Of course, once the engine is warmed up, the idle speed should be the same, no matter what the ambient temperature might be. Based on your reporting of idle speed, it sounds like your engine is currently idling in the correct speed range, providing that it does drop down to 500 RPM once it is warmed up fully.

As to the rougher running, I still think that a likely candidate is a disconnected vacuum hose. A disconnected vacuum hose can definitely lead to both rough idling and stalling. Modern cars do not stall unless there is something wrong with the engine or one of its ancillary components.

The CEL is the CHECK ENGINE LIGHT. If yours is illuminated, that means that the OBD (Onboard Diagnostic) system has detected a problem and has stored a trouble code. The stored codes can be read, free-of-charge, by many auto parts stores. You can post back here with any trouble codes that might be found, and someone will be able to interpret them for you. The code will be in a format like “P1234”. A steadily illuminated CEL is not a major concern, but if it is flashing, that means that a serious problem has been detected.

Since your problems began after the serpentine belt replacement, I would suggest that you go back to the mechanic who replaced the belt in order for him to verify that all vacuum hoses are still properly connected. There should be no charge for this if he is an honest mechanic who is interested in retaining customers.

Incidentally, my '02 Outback VDC 3.0 H-6 is also at ~86k, so the condition of the engines should be fairly similar, assuming that yours was well cared for. I can report that the engine still burns no oil in the 3,500-4,000 miles that it goes between oil changes and it has been rock-solid reliable.

Since you are due for the 90k service imminently, if the vacuum hose theory is not the solution to the rough idle problem, you might want to defer any repairs until after the 90k service, in order to see how the car is running after the 90k service. I want to point out that the spark plugs on that vehicle are supposed to be replaced at 60k. Unless you know for sure that they were replaced at 60k, I would suggest that you have the plugs replaced at the 90k service, as that will likely improve your gas mileage a bit and could help to even out the idle. The problem with used cars is that they rarely come with accurate records of their maintenance. If the previous owner decided to “cheap out” and not replace the plugs at 60k, that could account for the rough idle.

The 90k service is essentially the same as the 60k service, with the exception of the plugs, but if they were never replaced or if you can’t verify that they were replaced, 90k is definitely the time to do this. Also, the 60k and 90k maintenance both call for inspection of the transmission fluid and the differential oil. I would suggest that you do as I do, and have the trans fluid and the differential oil changed, along with the coolant and the brake fluid.

Just be prepared for a bill of ~$400.–$500. for the full 90k service + spark plug replacement. I use the Subaru credit card for all of my retail purchases, and as a result, I earn enough vouchers (called Subaru Bucks) to get my major services done free-of- charge. If you don’t have this credit card, I strongly suggest that you apply for it!

Post back with any other questions, and I will attempt to help you. If I can’t provide an answer, I’m sure that someone else can.


Are you still there???

When someone asks for help, and when I respond in great detail in an attempt to help him, I find it somewaht galling to see that the OP does not seem to return to the thread.

Mountainwalker–Since it has been several days since I attempted to assist you–if you are still among the living, it would be nice if you would post back with your comments.

To add to VDC’s note: We like to hear back not, just to make us feel good, but to find out if the advice worked. This is a learning process not just for those asking questions, but also to those answering them.