Rough running 92, oxygen and knock sensor codes.

[spleen_merchant]

Hi,

I have an issue with a rough running svx. Lumpy, rough idle and hesitant, uneven acceleration. Exhaust note is a bit
"fluttery" (intermediately poppy and uneven) throughout the rev range but worse below 1500k. The codes it's throwing are for left hand oxygen sensor and left hand knock sensor.
Engine Temp appears too low on temperature dial (ie never goes above a quarter).
The MAF sensor appears to be a Nissan unit with a modified connector which might be a problem, not sure. Apparently they're used with modified ECUs (mine isn't).

What I've fixed/tried: -
Cleaned Throttle body with throttle body cleaner.
Replaced dodgy Throttle Position Sensor. Adjusted new one to -0.5v
Replaced Fuel Pressure Regulator as it appeared to be leaking.
Replaced fuel filter - made a big difference with the hesitation
Replaced Air Filter
Replaced a cracked air hose to the PCV
Cleaned MAF with electronics cleaner
Cleaned all connections in engine bay with electronics cleaner
Believe spark plugs were replaced a few thousand k's ago by previous owner. but I have a new set and that's my next thing to do.
Had 2 mechanics look for vacuum leaks.

Not sure what to try next, I'm guessing the codes are there from the sensors doing there job detecting the rough running rather than being faulty in themselves. Any ideas?

[SoCal LS-L]

Do the plugs, so you can inspect the coils. Some go bad, some just get soaked in oil from bad valve cover gaskets causing a bad connection and misfire. I would also do the thermostat since its so easy, they may have installed a too low temp thermostat, which is bad news on a fuel injected car if it cant stay in closed-loop. Ive seen people do that to hide bad head-gaskets too

You should check up on the online service manual I linked to in my signature, you can download it, or just look, and study the flow-charts, they are helpful in suggesting causes of symptoms in our unique control systems.

Have you been able to listen to your fuel injectors with a long screwdriver or other tool to see if they sound similar? How many miles on the car?

[spleen_merchant]

Hmm, it did have the valve cover gaskets replaced 10 000km before I got it, maybe while these were leaking oil got into the coils? (The car has a pretty solid recorded service history with all belts and such replaced). Engine is externally clean and has been since I got it, and oil and water look ok. (I purchased at 185000km has 200000 km now). Not familiar with the listening to injectors technique, where should I hold the screwdriver? I have gone through and pulled the injector leads while running without a noticeable difference in effect between each one though.

Thanks for your help.

[spleen_merchant]

Some additional info: -

Although the temp gauge stays low, heater works fine (too fine, it destroyed my windscreen last week in -7C weather).
While re-checking coolant today I noticed the inner seal of the radiator cap had corroded meaning it was probably bleeding radiator fluid back into the overflow tank constantly, not sure if this is could confuse the temp sensor and cause issues? no codes for temp sensor though (replacing with new cap this afternoon, a mid 90's impreza one, hope it's compatible).
Also, the exhaust is non-stock from at least the cat back, has 2 separate mufflers.

[SoCal LS-L]

Originally Posted by spleen_merchant

Some additional info: -

Although the temp gauge stays low, heater works fine (too fine, it destroyed my windscreen last week in -7C weather).
While re-checking coolant today I noticed the inner seal of the radiator cap had corroded meaning it was probably bleeding radiator fluid back into the overflow tank constantly, not sure if this is could confuse the temp sensor and cause issues? no codes for temp sensor though (replacing with new cap this afternoon, a mid 90's impreza one, hope it's compatible).
Also, the exhaust is non-stock from at least the cat back, has 2 separate mufflers.

The heater blows warm air long before normal operating temp, which is in the high hundred range. The loss of coolant back into the reservoir shouldnt have a big effect on the operating temp unless it was bad enough to cause an overheat, which your issue seems the opposite. The temp sensor will NOT kick a code unless there is a circuit fault.... there is no redundancy to notify the ECU of a bad temp sensor reading, the ECU depends on a correct reading to adjust the operating parameters into closed-loop. Since your car apparently does not reach the required temp (going by your sender reading, which could be bad but not usually), id do the thermostat. Or get an infrared thermometer and aim it at the thermostat housing or upper hose and see what it reads... you need to verify the car is heating up enough to go into closed-loop, normal operating temperature..... otherwise your ECU is basically flying blind using only preset fuel and timing maps... which dont work well over long periods of time and can foul up your engine and components.

[spleen_merchant]

hmm good info. I picked up a thermostat and gasket this arv, I'll do that and the plugs on the weekend and go from there.

[spleen_merchant]

Ok,
Problem is fixed (for the most part). Changing the spark plugs gave some improvement, though they didn't look too bad, the coil boots are starting to get brittle but no visible cracks.
On closer inspection, the left hand knock sensor throwing the code was visibly cracked right through the casing. I replaced this with a brand new sensor, however, as I could not get the old sensor detached safely I have attached the new one above the main ground lead temporarily.
No more CEL (oxygen sensor codes went away on their own), Idling and running relatively smoothly and way more power, definitely out of limp mode.
However, still occasionally goes back to idle surging after coming to a stop, turning AC "on" remedies this as it pushes the RPM up.
Still planning to replace thermostat and flush the coolant system, I suspect I may have a small coolant leak somewhere as a faint burning coolant smell can be detected after a descent run.

[Huskymaniac]

Did you follow my temperature saga? It was reading low and was just the gauge temp sender unit. It should be replaced with a Subaru one for ensured accuracy. It is cheaper than a thermostat and flush so, unlike me, you might want to try that first. If you do replace the tstat and flush, I would add the Subaru coolant additive. A lot of people are against it but we have one of the best dealerships and service departments up here and they add it to every car. I trust them totally.

[spleen_merchant]

Yeah, I'm going to replace that and the coolant temp sensor with the thermostat change.
It stalled on me yesterday going into gear, and is going back into surging idle when it warms up, other than that, general driving is much better than it has been.
Debating whether to order oxygen sensors with the coolant sensors/senders too as every time I make a parts order from the US it costs me about $50 postage.