Subaru EJ204 NA - Hesitation under load, causes?

Subaru EJ204 NA - Hesitation under load, causes?

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James76G

Original Poster:

360 posts

190 months

Wednesday 10th July
quotequote all
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.

stevieturbo

17,470 posts

253 months

Wednesday 10th July
quotequote all
Codes are not diagnosing faults.

Pull the plugs and look at them is a free first step.

James76G

Original Poster:

360 posts

190 months

Thursday 11th July
quotequote all
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.

GreenV8S

30,418 posts

290 months

Thursday 11th July
quotequote all
I expect it will be either an ignition or fuel problem.

If you can see the lambda signals and if it's a fuelling issue, you might be able to spot it going lean (or perhaps rich) as the problem develops before it turns into a misfire. (Once you have a misfire the lambda signal becomes useless.)

Problems relating to weak sparks can often be spotted by putting a passive strobe light in series with one of the HT leads, positioned where the driver can see it.

stevieturbo

17,470 posts

253 months

Thursday 11th July
quotequote all
James76G said:
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.
You assume it had new plugs. If you did not see them install it, you cannot be sure. A receipt just means they charged for it.

And it's easy, done it hundreds of times.

GreenV8S

30,418 posts

290 months

Thursday 11th July
quotequote all
stevieturbo said:
James76G said:
2. It's a pig of a job
If you did not see them install it, you cannot be sure.
It being a pig of a job seems like a strong incentive to skip it, and it may even have been skipped more than once.

stevieturbo

17,470 posts

253 months

Thursday 11th July
quotequote all
GreenV8S said:
It being a pig of a job seems like a strong incentive to skip it, and it may even have been skipped more than once.
It's really not that difficult

But yes, dealers especially are fond of skipping work.

But it goes back to the same thing. In lieu of having proper diagnostic tools etc etc, or firing the parts cannon.....a simple visual on the plugs is easy and free.

texaxile

3,382 posts

156 months

Thursday 11th July
quotequote all
I've done the plugs on an EJ205 countless times, it's really a little bit more time consuming than an inline 4 but not that much of a challenge. With the EJ205, once the coil packs are off and the plugs out (3/8 drive is best), replacing them is easy as they tend to "self locate" as you aim the plug toward the hole.

Have you checked the MAF?. It might also be a coil pack showing early signs of failure, a split intake hose or something of that nature. Very possibly one of those annoying issues that isn't immediately obvious but can be a relatively cheap fix depending upon the approach.

Start with the simple and easy stuff first, plugs check, seal around the air filter, check everything's tight and connected (I've found intake hoses not tightened after a service before). I had a BMW which exhibited the same symptoms, it transpired the intake hose feeding the injection system was split badly, fouling the emissions and it caused a massive drop in power in the already underpowered 1.8 engine.

Next step is to make sure the fuel pressure is ok, injectors working, then move on to the more complex diagnostics. It might be worth asking on one or two of the Subaru FB groups or even Scoobynet, as they're still populated by a few enthusiasts.

jeremyb

27 posts

145 months

Friday 12th July
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I'd vote for bad plugs or coil-packs, or maybe the dealership strained a connector when changing the plugs.

If that doesn't cure it - how about a left-field idea ?? Are you sure it's misfire-related ?
I have a 2019 Outback (3.5/6 cyl US market version) that 'hesitated' around 2500RPM when cold. I was convinced it was mapping or transmission-related, but there's a known problem with soft transmission mounts causing movement.
I fitted a rubber 'anti-wobble' insert in the trans mount, from Anderson Design in Colorado, that smoothed things right out.

https://awdadventure.com/collections/2008-2011-sub...

If you want to get one, and have trouble ordering from the US, send me a DM.


julianm

1,580 posts

207 months

Friday 12th July
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I had the same engine in an Outback & the owners forum was helpful. Had a quick look & found this - https://www.subaruoutback.org/threads/hesitation-p... - basically provides good earthing to the engine manifold. Seems a bit odd but might be worth checking existing earth leads. Good luck!

James76G

Original Poster:

360 posts

190 months

Tuesday 27th August
quotequote all
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.

stevieturbo

17,470 posts

253 months

Tuesday 27th August
quotequote all
valve/seat recession is fairly common on the older turbo cars.

No idea on n/a, never work on any. Don't think the hatch models were affected by that though anyway