Subaru EJ204 NA - Hesitation under load, causes?
Discussion
I recently bought a 2010 Subaru Impreza which is powered by the NA version of the their EJ boxer engine, coded as EJ204. It's official output is 150hp or thereabouts and the car has done 80k miles and for the avoidance of doubt, here's it looks like.
While accelerating normally it tends to hesitate and I'm trying to narrow down likely causes. It does it whether the aircon is on or off, whether it's wet or dry and whether the engine hot or cold. I've used regular and super unleaded and that doesn't make a difference either. I can drive through it by accelerating hard whereupon it isn't really noticeable but given where I live (congested) and the proliferation of cameras that isn't always practical.
This was the third car I test drove and this one displayed this behaviour the least, suggesting that a) this engine is susceptible to this fault and b) that it gets worse. The 2nd car I drove was terrible and could barely make it to 70mph.
I've read the codes and there are none, and there is no EML light on either. I've had the airbox out and checked the throttle body and that's all good. Some people have reported issues with EGR valves sticking and camshaft sensors but I assume that would throw a code? Timing I will assume is good as prior to me buying it the cambelt was changed at Bell & Colvill. Plus on a recent 200 mile run it returned 35mpg suggesting that once at cruise, everything is working fine.
I appreciate it's hard to diagnose running faults via the internet, but with all that, does anybody have any experience of/suggestions for solving something like this? I'm beginning to think it's either a lack of fuel under acceleration (weak injectors) or a lack of spark (coil packs breaking down) but without any codes it's a gamble knowing what to investigate next.
While accelerating normally it tends to hesitate and I'm trying to narrow down likely causes. It does it whether the aircon is on or off, whether it's wet or dry and whether the engine hot or cold. I've used regular and super unleaded and that doesn't make a difference either. I can drive through it by accelerating hard whereupon it isn't really noticeable but given where I live (congested) and the proliferation of cameras that isn't always practical.
This was the third car I test drove and this one displayed this behaviour the least, suggesting that a) this engine is susceptible to this fault and b) that it gets worse. The 2nd car I drove was terrible and could barely make it to 70mph.
I've read the codes and there are none, and there is no EML light on either. I've had the airbox out and checked the throttle body and that's all good. Some people have reported issues with EGR valves sticking and camshaft sensors but I assume that would throw a code? Timing I will assume is good as prior to me buying it the cambelt was changed at Bell & Colvill. Plus on a recent 200 mile run it returned 35mpg suggesting that once at cruise, everything is working fine.
I appreciate it's hard to diagnose running faults via the internet, but with all that, does anybody have any experience of/suggestions for solving something like this? I'm beginning to think it's either a lack of fuel under acceleration (weak injectors) or a lack of spark (coil packs breaking down) but without any codes it's a gamble knowing what to investigate next.
Thank you, it's a good suggestion and one I'd considered doing, but for two reasons.
1. It had four new spark plugs less than 1,500 miles ago at a Subaru main dealer
2. It's a pig of a job due to the engine config and last time I did it (on an EJ255 engine) it took a few hours.
I'd like to think that the local Subaru dealer knows what they are doing when installing spark plugs and the receipt shows the correct items (22401AA720) for this engine, but equally there's a chance there could be an issue.
1. It had four new spark plugs less than 1,500 miles ago at a Subaru main dealer
2. It's a pig of a job due to the engine config and last time I did it (on an EJ255 engine) it took a few hours.
I'd like to think that the local Subaru dealer knows what they are doing when installing spark plugs and the receipt shows the correct items (22401AA720) for this engine, but equally there's a chance there could be an issue.
Thank you everyone for the helpful suggestions.
I went in and checked all the spark plugs yesterday, which confirmed that the were the right plugs, they looked new (so the dealer had changed them) and they were all fine. These are the two from the driver's side bank.
I was right to be reluctant to do it though as it's a frustratingly fiddly job, given the tight access against the chassis rail and the need to unclip your ratchets before you can remove the spark plug.
I also can confirm the air filter (genuine Subaru) is new and the air intake is complete with no splits so all good there.
Once done I took it for a drive and the symptoms persist so it must be something else unfortunately.
I went in and checked all the spark plugs yesterday, which confirmed that the were the right plugs, they looked new (so the dealer had changed them) and they were all fine. These are the two from the driver's side bank.
I was right to be reluctant to do it though as it's a frustratingly fiddly job, given the tight access against the chassis rail and the need to unclip your ratchets before you can remove the spark plug.
I also can confirm the air filter (genuine Subaru) is new and the air intake is complete with no splits so all good there.
Once done I took it for a drive and the symptoms persist so it must be something else unfortunately.
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