Personal computing discussed

Moderators: Captain Ned, emkubed

 
synthtel2
Gold subscriber
Gerbil XP
Topic Author
Posts: 493
Joined: Mon Nov 16, 2015 10:30 am

Subaru EJ knock sensor false positive

Tue Apr 18, 2017 10:54 pm

Subaru EJ engines that accumulate a lot of miles often seem to get a problem where the knock sensor false-positives on the sound of piston slap and retards timings by ridiculous amounts for no reason. It's usually a sub-2000 RPM high-load thing, so really annoying for automatic transmissions, but not such a big a deal for manuals. A common fix is to relocate the knock sensor by a few inches to the transmission housing (which doesn't tend to cause problems).

My EJ223 ('99 Impreza OBS) with 275,000 miles has this problem in a very bad way. I've relocated the sensor (and it helped), but on a bad day, ~2/3rds torque is still about the max, regardless of RPM. An OBD-II reader confirms timings are to blame; full throttle can in some conditions drop timings 30 degrees behind where they're supposed to be. If I unplug the knock sensor, everything is awesome (I have an unfortunate amount of experience with a knock-prone EJ25D, and am sure I would know if things weren't awesome).

The thing that keeps bugging me is that good days and bad days don't happen in quite the pattern I'd expect. What I'd expect is that the problem should exist when the engine is cold (more about oil temp and IAT) and disappear when the pistons get hot (since they expand to fill up more of the gap). Instead, the problem stays gone for a while after a good run up to temp. For instance, the engine was in top form for all parts of a few 10-minute drives around town last week, because it had spent a few hours on the highway two days earlier. Having sat for a week after that, it's now starting to have trouble again. This would make some sense as far as burning off deposits in the cylinders -> reducing compression and wall temps -> eliminating knock, except that knock isn't the problem, and I'm at a loss to draw a line between that behavior and piston slap. FWIW, the amount of audible piston slap does vary as expected with performance.

As far as an actual solution, unplugging the knock sensor seems to be a pretty good one. I know what abuse that EJ25D tanked (and was still tanking 100,000 miles later :o) and don't find the idea concerning (I've got ears well-tuned to do the sensor's job if need be), but I'm still not entirely comfortable doing that without running 89+ octane, so it's effectively a bit cheaper to run with the glitch still in effect.

So, more out of curiosity than any practical concern, does anyone have any ideas on what the root cause of the weird good day / bad day pattern might be?
 
bfg-9000
Gerbil First Class
Posts: 154
Joined: Tue Mar 01, 2016 9:17 pm

Re: Subaru EJ knock sensor false positive

Wed Apr 19, 2017 12:31 am

May not be piston noise but valve noise causing it especially on such a worn engine, and if the problem occurs when hot but not cold. When hot the oil thins enough to make the worn hydraulic lash adjusters tick. Try thicker oil.

The traditional way to desensitize a knock sensor is with lots of teflon tape and only loosely tightening it. Some people will put a 90deg fitting or washers between it and the block.
 
synthtel2
Gold subscriber
Gerbil XP
Topic Author
Posts: 493
Joined: Mon Nov 16, 2015 10:30 am

Re: Subaru EJ knock sensor false positive

Wed Apr 19, 2017 1:10 am

It does have a lot of valvetrain noise, but that's mostly at a much higher (unlikely to be confused with knock) pitch, and doesn't follow the same patterns. It's pretty much always there - the phase II EJ22s moved to solid lifters, and it's safe to say these aren't in perfect adjustment.

A low-tension sensor fit with tape in the middle sounds like a great idea. Thanks.

Who is online

Users browsing this forum: Google [Bot] and 39 guests