Surging Diagnosis

AlpineRaven · Post by **AlpineRaven** » Fri Sep 14, 2012 8:55 pm

El_Freddo wrote:What's your engine knock sensor look like? If it's cracked they don't do too well and can be faulty without throwing a code.

It should be on the top of the block, near the bell housing on the passenger's side, just off centre - if they haven't moved it since my EJ was produced ('91).

Cheers

Bennie

Yeah it would be near under the air inlet/inlet manifold on passengers side above passenger's cylinder.. looks like this -

you can get them off ebay for about $10-15 bucks. I wouldnt bother 2nd hand ones unless they're in good nick - I do agree with Bennie that it doesnt normally throw a code, same with O2 sensor as well.
Cheers
AP

gerzod · Post by **gerzod** » Fri Sep 14, 2012 9:00 pm

Thanks guys, i did locate the knock sensor when looking around the engine bay (kinda difficult unless you know where to look, its quite well hidden).

I did not know they were that cheap off ebay. I am skeptical of the quality of a $15 knock sensor off ebay, has anyone had any experience with them and if so can you recommend a seller? for $15 i would give it a shot.

I doubt its the O2 sensor. From memory the O2 sensor isnt used in the ECU's fuel calc algorithm for high load scenarios (i.e. full throttle), which kind of makes sense as optimising AFR's isnt super important under that condition.

AlpineRaven · Post by **AlpineRaven** » Fri Sep 14, 2012 9:05 pm

gerzod wrote:Thanks guys, i did locate the knock sensor when looking around the engine bay (kinda difficult unless you know where to look, its quite well hidden).

I did not know they were that cheap off ebay. I am skeptical of the quality of a $15 knock sensor off ebay, has anyone had any experience with them and if so can you recommend a seller? for $15 i would give it a shot.

I doubt its the O2 sensor. From memory the O2 sensor isnt used in the ECU's fuel calc algorithm for high load scenarios (i.e. full throttle), which kind of makes sense as optimising AFR's isnt super important under that condition.

I think the last time I bought mine for about $25 because I thought it was using more fuel resulted cracked knock sensor but it wasn't that.
Cheers
AP

gerzod · Post by **gerzod** » Thu Nov 08, 2012 5:25 pm

More progress on this now.
I recently took the car to an auto-elec who ran it under an engine analyser.
There were no fault codes and he mentioned that he was looking at the sensor output in real time and everything appeared "reasonable".
It was misfiring on two cyclinders and upon taking the spark plugs out they were heavily fouled despite being replaced recently (< 20k ago, and they are platinum plugs).
Upon replacing them the car was running like new but I was certain that it wasnt the plugs as i checked them (he did give me the plugs as proof and they are fouled, but I dont believe this caused the problem, in my opinion the problem caused the fouling).

Anyways after driving it for a couple of weeks i can feel the missing/surging coming back again, so i think my suspicions were correct.

Upon inspecting the old plugs they all have "corona stains" two of which were very pronounced (see case 4 over here for what I am talking about - http://www.globaldenso.com/en/products/ ... index.html). It appears that this isnt really an issue but i will investigate further.

I am going to do some more research on what could be causing the problem but for now I am completely stumped again. I just checked the ignition lead resistances and all are well within subaru specs.

If anyone has experienced similar issues or may know what it is then I'm all ears

Post by **Alex** » Thu Nov 08, 2012 5:55 pm

id be looking at the ingition relays now.

If youre not getting any codes id suspect something like the relays or injectors, fuel pump etc etc.

check the coil and ignitor.

theres still a pretty large amount of things that could be causing this.

alex

gerzod · Post by **gerzod** » Thu Nov 08, 2012 6:34 pm

Thanks for the suggestion Alex.
How can I check /test the relays?
am looking in my service manual and cant find it (maybe i am missing the section but i cant find anything on them in electrical section unless it is somewhere else obscure).

RSR 555 · Post by **RSR 555** » Sat Nov 10, 2012 3:07 am

AlpineRaven wrote:Ive been there done that - cleaned and re-fitted it - cleaning didnt make any difference, so changed to another one yes it was dirty but it did improve.. worth a shot.
Cheers
AP

I'm going to say this is where I'd be looking. These MAF sensors do not like being cleaned. I would grab one of a known car and try it.

gerzod wrote:Thanks for the suggestion Alex.
How can I check /test the relays?
am looking in my service manual and cant find it (maybe i am missing the section but i cant find anything on them in electrical section unless it is somewhere else obscure).

I'm not sure where in Vic you live but I'd try and find someone with the same model and try both the MAF and relay from it.

littlewhiteute · Post by **littlewhiteute** » Sat Nov 10, 2012 8:41 pm

I replaced the front O2 sensor in a 06 Liberty on Friday.

No codes, surging at 1/4 throttle at 2500 rpm and low power throughout the whole rpm band.

Subaru calls the front O2 sensor the A/F sensor, for good reason.

The car is now completely different, very driveable, no surging, and good torque and power.

Owner very happy.

steptoe · Post by **steptoe** » Sat Nov 10, 2012 9:12 pm

O2 sensors are a service item with an expected limited lifespan by manufacturers, in essence, they are a power supply that generates Voltage from exhaust gas, and only seem to be a replace when necessary. Many cars get away with never needing them replaced !