By on July 16, 2012



Ramin writes:


I am not a TTAC member, but I read it almost daily. I suppose I should join soon. Anyway having read your “piston slap: we need your help” post, I have one that has been stumping me for about a year now:

The car is an 08 Impreza STI. For the past year or so, the power steering struggles and whines. It is much worse when the car is cold, doubly so when the weather and the car are cold. There is no belt squeal. I have tried flushing and bleeding, both with factory fluid and also with the Lucas stop-leak stuff. Modest initial improvement only lasts a little while. Subaru forum posts suggest the STI cooks its PS fluid because the fluid lines route near hot turbo components. However it seems now even with fresh fluid, the problem persists, leading me to suspect a component has gone bad. I don’t want to drop over $600 for a new pump. Are there any tricks you know of, like for example, replacing a particular gasket? Or, better yet, some advice on narrowing down exactly what the culprit is (short of replacing the whole freaking pump)?

Thanks a lot and keep up the good work – I love the site and what you all have done with it.

Sajeev answers:

Thanks for your kind words.  I never thought that an unemployed (Lincoln) forum moderator could eventually be the ring leader of this crazy Piston Slap thing I created. Apparently my unemployment period was good for me, and perhaps it enriched/enriches your life. So there’s that.

I am a little concerned you put Lucas Stop Leak in a system that never leaked: stop leak products tend to gum up areas that don’t need gumming.  But that might be unfounded, go ahead and verify on the forums. In general avoid stop leak products unless you 1) have a leak and 2) really don’t give a crap about the leaky vehicle in question.

I think the knowledge you gained on the forum is right.  The fluid lines are in an unfortunate location, and Turbos make a TON of heat.  There are two things I’d recommend:

1. Switch over to a racing grade Power Steering fluid, if it has a higher boiling point than the stock stuff. Several oil companies supposedly offer a fluid with a higher boiling point, as googled here. I will not speculate or endorse one over the other, and I am sure the forums have already covered this.

2. Protect those steering lines! You need to shield them from the turbo’s heat.  I would use an insulating heat shield wrap for the lines, and possibly make a sheet metal sleeve in this general area, to further help isolate the lines from the Turbo.

Why am I saying this?  Because I’ve raced a couple of late model Corvettes, and they do suffer from clutch fluid problems in a Texas summer with a hamfisted AutoJourno behind the wheel.  So do them both. The same thing applies here, especially when we talk the heat of a Turbocharger in a tight Subie engine compartment.


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5 Comments on “Piston Slap: The Heat Is On!...”

  • avatar

    Air is getting in the system, probably an o-ring. When it is cold, the o-ring shrinks, and the problem intensifies. This is common on Subaru vehicles, but you can fix it yourself with a new o-ring, or with silicone sealant:

    Power Steering

    Reports verify complaints about the Subaru having power steering concerns. Most common of which is the leaking issue before its power steering pump finally breaks down. Drivers trace this problem to Subaru’s power steering pump being fragile, specifically in places with high temperature. The pump uses a plastic fitting, and an o-ring is used for its sealing. This is where the leak springs from. You can fix this by yourself by removing the fitting, cleaning the contact area with car cleaner, and placing some grey silicone around the o-ring before reassembling. If the problem persists, do consult your professional mechanic.

    Meanwhile, the issue on the power steering making noises, which happens when the power steering fluid is aerated after a driver replaces the timing belt or the alternator, can be solved by replacing the aerated fluid with fresh fluid. Such “noisy” problem must be dealt with at once since it can make the car harder to steer.

  • avatar

    Agree with DIYer 100%….almost certainly an air leak on the suction side of the p/s pump.

  • avatar

    Good point. Perhaps the fluid boiled over and air got in as it cooled?

    • 0 avatar

      More likely high underhood temps take their toll on the o-ring(s) in the suction side of the p/s hydraulic circuit (or possibly a return hose). Biggest clue is the OP’s statement that situation is worse with cold engine/outside temps. Cooked O-ring (or hose connection) shrinks in the cold, allowing air to be sucked in, aerating the fluid and causing noise and poor p/s performance.

  • avatar

    Hi everyone, I am Ramin. I just registered to post my thanks and to give an update on the situation.

    Since I posed the question to Sajeev a couple months back, I tried to seal up that top o-ring referenced by DIYer. I failed miserably. When I restarted the car, there was much more noise and shuddering in the system. I must have made it worse and now air was definitely getting sucked in through that spot. At one point, two days later, there was so much foaming in the system that it overflowed the reservoir and spilled all over one of the headers – smoke and all. It freaked me out, and I broke down and bought a new pump. The Subie pumps come as an assembly with the referenced plastic fitting and new o-ring. I installed it a couple days later, flushed new fluid through, and the problem went away.

    Now my attention is focused toward keeping the second pump from failing. Sajeev, thanks for your tip re: racing fluid. I will look into it as well as the heat wrap. I have seen forum threads where folks are actually putting fluid coolers in. I don’t know if I want to go that extreme, but anyway there you have it.

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