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Legacy 3.0r Spec B 2008 white smoke from exhaust and under bonnet

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was on a fairy long drive yesterday in my new 2008 subaru legacy 3.0r spec b sports tourer, dropping my dogs off at my parents, as my wife have come on a short trip to Spain. The car has been converted to run on LPG aswell as petrol at about 50k miles, the clock is on 78k miles now.

I was in the fast lane of the motorway stuck behind someone going slowly, with an annoying BMW behind me waving his hands, I think because I had to tap the break to cancel the cruse control.

When the guy in front of me moved over, I put my foot down to get away from the BMW. Then when I was up to a fairy high speed, I suddenly noticed a large trail of what looked like white smoke behind me.  I immediately slowed down, but could not see that the car was overhearing. Then I encountered traffic that brought me to a standstill, where I could see white smoke/steam coming from under the bonnet. Again the temp appeared normal.

The smoke gradually died away as I drove at a sensible speed the rest of the way to my parents.

I did fear that it was burning coolant from HG failure, so I immediately checked my coolant level when I got there but it was up at the full level. My brother then wanted to see the car, so I took him for a quick sprint could not see any white smoke during the drive, but when we got back, I lifted the bonnet and could see smoke/steam coming from down at the back of the engine. More precisely, there are two dip sticks, one front and one back, it was coming from what looked like under the back dip stick.

I have checked under the oil cap, and both dip sticks, the oil level was normal, and the oil looked clean, as I have said before, the coolant level was on the full line.

I was running on LPG for the whole drive, apart from when I took my brother for a spin, then I was running on petrol.

Also the LPG has a flash tube, and was fitted at about 50k miles.

Given that the car is only on 78k miles in such a solid engine, I am struggling to believe that it is HG problems,  and as I have said, I cannot see any contamination. As soon as I get back from Spain I am planning on doing a compression test to rule out HG.

On the way back from my parents (100 miles) I didn't see any white smoke on the journey, but when I popped the bonnet when I got home , there was some coming from that same location.

I have had the car a week and this is the first time I have seen this. I do wonder though, as this is a new car for me, that maybe I have been putting my foot down a bit too much, which the car was not usually used to.

Sorry for such a long post, just wanted to get all the info in. 

I also have a video of the smoke coming from under the bonnet, which I will put up shortly.

Also forgot to add that on the way home, the engine light came on, so I pulled over to scan and got code: P0420 (catalyst system efficiency below threshold bank 1)

Many many thanks in advance. 


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If it's an automatic, which I assume it is the rearmost dipstick (passengers side, correct?) is for the transmission fluid. Check the level when the engine is running, in park. Two different levels, one for hot reading (after a run) the other for cold (just recently started the car). The level should be between the two markers. If it is a manual, not a clue what that dipstick is for.

The code could be many causes. Is there any sign/sound of exhaust leak?

I have no experience with LPG modded vehicles, so no comment on that. Good luck anyway.

Check also the cv boots - grease thrown out over the cats/exhaust might appear to be smoke/steam.

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Yes, you are right, it is an auto. OK I'll check the levels both hit and cold. As for an exhaust leak, I cannot see any signs. If the smoke is coming from under that dip stick. Then maybe some sort of transition flued leak. Could have split something when putting my foot down on the motorway? 

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