Imbalance idle noise, digital throttle, and unrelated P0420

[Chulew]

My 2008 MT5 2.5i has ~210k which is ~70k since I replaced timing belt, tensioner and pulleys. I was planning on doing this job again in the spring or summer but I'm wondering if there is any indication I should be worried now. I drive it hard but in a good way.

 

At idle when warm, I'm hearing (maybe even feeling but it could be my imagination) a sound like a knock about 6-8 times/sec.

 

Last spring, while rReplacing the plugs for the first time since I've had it, there was a bit fair amount of oil in the tubes but no smoking or otherwise obvious effects. I am getting P0420 on and off now but I let me father-in law add some mystery oil and fuel treatment which I won't do again. After 1/3 of that tank, the light came on. At 1/2 tank, I put most of a can of seafoam in with premium. About 1/2 tank later, it cleared but just after I filled it with 87 (when I was empty next), the light came on again and went back off a few days later. Now as I was scraping the bottom of that tank, the light came back on. I've filled again with my usual 87 but I'm thinking that all the change-ups to the fuel have messed with the system which takes a bit of time to correct, correct? Or maybe my cat is going.

 

One thing that I don't like about these electronic throttles it that I tend to get stuck coasting down a slight decline just between the two digital throttle steps so that the engine jumps back and forth between the two injection levels which is more wear on the drive train and engine mounts.

 

What I'm most concerned about is the knock right now since it is relatively new. Should I pre-empt any issues with an early timing job or do these engines typically not indicate failing tensioners/pulleys with sound?

[GEE-OTTO]

The DBW (drive by wire ) isn't you problem nor coasting down hill and switching between injections levels your injector duty cycle may seem complicated but if you ever look at the map the table that controls it is fairly static at low or part throttle.

 

Now the P0420 is probably your rear oxygen sensor it's been in there for 210k miles it's time to fail so order a dense replacement sensor from Rock Auto, replace and reset your ECU. If the code comes back it's your cat. All that seafoam doesnt help your aging converter so if it is bad look for a used 06-09 unit.

 

On to the engine, piston slap at idle is normal and audible. My 08 in 08 idled loudly esp during those winter cold starts. The best way to differentiate btwn slap and rod knock (not to be confused with pre detonation knock) is rod knock will have a more solid sound like it's deep in the engine whereas slap will be more like light tap and emminate from the valve covers. Also slap will go away once warm and knocK will always be present and speed sensitive. Also seafoam ruins engine oil so you should always change it after a generous round. If you didn't change it and ran for a long time it could have caused bearing damage.

 

If you don't hear an audible deep knock at high RPM or if it is rythmic throughout the RPM range it could be a pulley or tensioner loose.

[Chulew]

I got the car at 130k in 2010 and got one dealership on the carfax to tell me they had replaced a cat within the year (roughly 5 years ago now). Would the sensor be on the list for replacement for that job?

 

I will do an oil change asap, thanks for the tip. I only hear the knock at idle and after fully warm so that must be the rod knock and it does sound deep like i can feel it. I don't notice it outside of an idle; seems to go away or gets drowned out.

 

There is definitely an oscillation between power levels and it is related to speed of the drivetrain speed and being fed by momentum vs. throttle. When you gear down to a stop, I assume there is fuel injection at a rate controlled by rpm so there is no stall (from taking foot off gas) but neither acceleration? The oscillation I'm experiencing is definitely due to the fact that I'm between accel and decel but in a range where the rpm does not match fuel feed, like there is fuel injection to prevent stall during decel but in fact not enough decel speed to use up that amount of fuel and so it feeds on it (or such a rate that it is in the noise in between). It's not a huge issue but I just know that it isn't normal operation. Maybe a restricted injector or something, probably normal for 210k miles.

[GEE-OTTO]

The stumble you speak of is a known issue with manual transmission 2.5i's the dealer had a fix for it all you need to do it mention it to them and they will apply the fix to your ECU.

 

No they probably didn't replace sensors at that time as the dealer was fond of doing the minimum when it came to the catalytic converter failure back then. It happened to my 08 and several others they replace the physical unit reuse the sensors.

[Chulew]

Is the ECU update something they will do at not cost? Do I just ask for a firmware upgrade to latest and greatest? I have to get in for the brake line inspection recall. I'll do it then. Thanks.

[GEE-OTTO]

It's free.

[Chulew]

Passed inspection today (the light went off). But the guy told me that it did have a bank 1 below thresh cat code. Again, engine light just shut off on its own (it was on for past 3 days). How could it pass and still give a low code without engine light? Or was the tech reading past codes from memory?

 

I think that the light is coming on maybe as I get towards the end of my tank of gas each time and goes off about half-way thru the next tank. I haven't really tracked it but just seems like the case.

[GEE-OTTO]

Yes the ECU only checks for cat efficiency above 3/4 and below 1/4 tank so it will cycle on and off. The tech prob was referring to the stored code, when a CEL is thrown it is stored in the ECU until a hard reset (electrical purge). As long as the code isn't active they will pass it. If you are getting the bank 1 error you should look into replacing the Orange and Grey sensors.

[Chulew]

If you don't hear an audible deep knock at high RPM or if it is rythmic throughout the RPM range it could be a pulley or tensioner loose.

 

I don't hear the knock at higher RPM. Could the tensioner failing cause a timing issue big enough that the ECU can't correct for fuel/air mixture and is the rod knock a timing symptom?

 

How do I fix the knock and what if I can't/don't (it is only very faint now).

 

Here is what my highly experienced non-Subaru-specialized mechanic told me after giving him the details above:

 

• O2 dirty cause not burnin' all the fuel; something wore out in tune up.
  

 

• Needs plugs or air filter or somethin'.
  

 

• Change oil to thicker grade, additives made oil too thin, running past rings fouling plugs a bit so no burning fuel. (FYI: I just did change oil to my usual 5@30 Mobile 1 High Mileage and the light just came back on after filling tank again).
  

 

• Add a can of thick Lucas or ring re-seal or bardahl or something to thicken oil up
  
• Didn't we do compression test [4-5 years ago when we worked on it? Nope.]
  
• Below efficiency means there is too much fuel in exhaust, or too much air
  
• Could have vacuum leak, bad gasket, bad fuel filter, weak plugs USE OEM REPLACEMENT PARTS; fancy plugs like irridium don't last long (replaced plugs with mid/low grade brand last year)
  

Money is tight and I'm leaning toward oil additive and new plugs as a first measure, then sensors, then timing job, then cat.

[Chulew]

After a visit to my local dealer with the break recall, free diagnostic coupon and an estimate for 20% off cat job of ~$1000, I found an air hose that had been disconnected for some time that runs between the passenger side spark plugs to the air intake box. A few days with this reconnected has cleared the P0420. Boy am I glad!!

 

Timing belt looks ok except for a slight groove in the center of the flat side. Any idea what this might be from? I will keep and eye as I should get at least another year before it's due anyway. If the knock gets worse, I'll push it up.

[GEE-OTTO]

That hose, the PCV breather hose, i guess could have caused the P0420 it is well upstream of that loop/system. That code in general is intermitent ultimately youll have to replace to the sensor or cat. The former is less than $60 shipped and could clear up a lot of issues.

 

That groove in the T-belt is not good it means something was/is wearing on the back side of that belt. If you think about your engine at low speeds the belt has more slack than at high speeds it could be a clearance issue where it only rubbs at a certain RPM range. It could also be a failed belt guide or debris lodged on there somewhere.If that belt has visible wear on the back side do not wait a year to fix it, do it now to avoid a new engine.

[Chulew]

No code for two weeks and best gas mileage I've ever seen at ~28 just by putting the hose back on. I'll do the belt soon.

 

What does everyone do as far as the water pump with timing job? I know to do the pulleys and tensioner every time but how often does the wafer pump go in these? If it does go, do I have to replace tensioner again?