EJ25 Liberty RX (Legacy) hiccups at cruise

Folk,

This has been a problem on and off for a while now. It came on again a couple of Km after a refill at a station we don't normally go to.

The engine behaves fine under acceleration, but within a few seconds of cruise, will start to hiccup, losing power totally for half a second, and repeat that irregularly *every few seconds*. It's quite jerky - annoying almost to the point of being undriveable, and I'm also worried that it might be doing damage - pinking? If you alternate between slight acceleration and lifting the foot, it doesn't seem to happen - until you stop changing speed for ~2 secs. The acceleration response is fine, so I don't think it's the fuel filter.

I suspect that the ECU has detected cruise, and is experimenting to find how much it can lean out the mix... but is going too far. I suspect the O2 sensor. Given that this is expensive to replace, is it possible/easy to expect to *try* a replacement before committing to paying for it? Apart from just replacing it anyhow, is there any other option?

Has anyone else had this symptom or can suggest an alternate cause?

Clifford Heath.

Reply to
Clifford Heath
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Reply to
bigjimpack

Don't think so - we've been through 5-10 tanks of gas from our regular sources since it started happening again recently. Also used injector cleaner, which does seem to help a bit, maybe. We only ever buy premium gas in any case. Dirty gas will foul the filter and cause problems on acceleration, but this is at cruise.

Clifford Heath.

Reply to
noyb

o2 sensor is a frequently mentioned item that explains all sorts of strange driveability problems.

My own experience, however, was that a timing belt replacement caused me hell like you're experiencing. Not exactly the same, though. My stumbles and bucks were intermittent and on accelleration (reliably at

4k rpm when it happened--and maintaining 4k rpm as a cruise speed would exhibit bucking that was annoying, but not undriveable) and on deceleration around 1250 rpm. Turns out a bad timing belt tensioner led to a flattened a tooth on a new timing belt when was slipping periodically by one tooth, causing my woes.

-- Todd H.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA
Reply to
Todd H.

That's good info Todd. I'm trying to understand exactly what the effect was caused by. Surely if the timing belt slips the timing will be off permanently? Or was it just sending a spasm down the belt by jumping on the tensioner only?

Clifford Heath.

Reply to
Clifford Heath

It seemed like it was slipping back and forth, and the ECU was able to adjust to compensate...but only to a point.

Took nearly all of 2007 to get to the bottom of it.

-- Todd H.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA
Reply to
Todd H.

Hi:

I had driveability issues recently caused by a bad O2 sensor. It was more prevelant at startup when cold than at cruising speed, but it did sometimes happen at cruising speed with a warm engine. Usually it happened when I lifed up off of the gas. It was as you describe, though, bucking and surging. And it was intermittent. And the Check Engine Light wasn't triggered as you might expect.

Initially it was diagnosed as an ignition coil - later, correctly identified as a bad O2 sensor. FWIW... Good luck!

Reply to
Fred Boer

Sounds like it might have been climbing up on top of the tension idler, then dropping back into the notch, and causing tension changes that advance both cams (or retard - is it on the trailing or leading side?). I guess a tight/failed idler bearing will do that.

I need to do the cam cover gaskets anyhow, maybe the idler is easy to do at the same time. Tackle the O2 sensor only if that doesn't cure it.

Clifford Heath.

Reply to
Clifford Heath

My situation was caused by a defective (brand new, OEM, Subaru) timing belt tensioner not pumping up to tension on a brand new belt after the engine had been hand turns the designated 3 or whatever times (specified in a TSB on the issue) when a new belt was isntalled. A tooth on the belt was flattened in this initial startup sequence and wasn't found until months later.

-- Todd H.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA
Reply to
Todd H.

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