Engine hesitation

[Rumbledriftspin]

Hi All.

I own a 2006 Legacy RE 2 litre NA petrol estate which has done about 120k miles. I think it has the EJ204 engine and is rated at about 165bhp.

It's generally in good order and drives well but suffers from an intermittent but frequent issue where when driving the engine suffers from hesitation/flat spots when accelerating through 2500-3500 RPM. The engine revs smoothly when stationary.

I've read elsewhere online that this is a common issue for this engine. I've cleaned the MAF sensor and had the pre cat lambda sensor replaced which was faulty but the problem persists. Adding Wynn's lambda & cat cleaner seemed to solve the problem temporarily but with another tank of untreated petrol the issue has returned. I've also had the car checked by Subaru who couldn't identify a fault and said the car had had all the necessary software updates.

Could anyone offer any advice please? Should I replace the post cat lambda too?

Many thanks in advance,

Paul.

[vxr2010]

cleaning a bad maf won’t solve a problem , more of a temporary resolution , you can change one thing at a time till you hit the problem , what about borrowing a maf from some one or are they cheap on your car ? the fuel pump could be failing , you can get a pressure gauge on ebay for not much , and put it in a fuel line in the engine bay , im not sure what your fuel pressure is but it should be steady and not flickering , i had a not too different issue a while ago , so car would hesitate around 2500 to 3000 and not boosting , if i accelerated slowly it was a bit better , but if i booted it it did not like it , my plugs were fairly fresh but had been removed at some point to check for wear and looked fine , so i put new plugs in any way and it ran fine as it should , it could have been the spark plug seal who knows but issue resolved , so if it was me , plugs first , check fuel pump then look at the possible maf issue , did who ever you took it to code read it ? if it is plugs it won’t show as any thing it’s just a really random issue

[Rumbledriftspin]

Thanks for your reply.

The spark plugs were changed during a service earlier this year.

I hadn't thought the fuel pump could be faulty.

I'll probably change the sensors first - I'd guess they're cheaper and easier to fit.

Thanks again,

Paul.

[Cambs_Stuart]

If fuel treatments helped then as well as the pump the injectors may need an ultrasonic clean. Companies like injector tune offer a fast turnaround.
Does your car have a fuel filter?

[Rumbledriftspin]

Thanks for your reply.

No, I checked with Subaru today and the car doesn't have a renewable fuel filter - it's a permanent one apparently.

I did also use some Wynn's injector cleaner additive in the previous tank of fuel and the hesitation has gone - at least for the time being.

Also, the car has no stored fault codes.

I'm hopeful that with a little more TLC the engine hesitation issue will be cured and the car will run sweetly all the time.

It's my first Subaru and I'm really enjoying it - it's fun to drive and handles well yet is also practical.

All the best,

Paul.

[ttrjs]

I've the same EJ204 in an Impreza. And I've been having the same problems. Tried a plug change as that was due anyhow but no change there.

Cleaned the MAF up and it seemed to work temporarily so decided to change the sensor. Bought a Hitachi branded replacement and installed, along with an ECU reset via disconnecting the battery. Weirdly the problem was still there but at a different spot in the RPM range. I was advised to get a genuine Subaru part and try again so I bit the bullet ( they're over £250 from Subaru ) and got the genuine thing and did another ECU reset.

Success! It drives how it always did, smooth pull all the way through the range and none of the hesitation and jerkiness that manifested itself before.

So my advice, if you try the MAF, is to make sure you get a proper Subaru part, nothing pattern part wise.

[Rumbledriftspin]

Thanks for your reply.

The engine hesitation issue is extremely intermittent in my car. It'll be running fine then I'll fill up with the same super unleaded I always use and it'll run rough for a while then sort itself out.

Since starting this thread the secondary air pump valve fault code has come on and stayed on so I'll be getting this fixed then continuing to try to resolve the hesitation issue.

It's just a shame Subaru genuine parts are so expensive and there are often no patent part alternatives - it's no wonder cars get scrapped.

[Koolerking]

Morning.

Late to the party, just signed up to ask another question and spotted this. I had the same on my 2005 NA legacy. Hesitation, took to get codes read and it was a possible spark plug. Cleared codes, drove fine for another 5 miles, spotted car for an hour, restarted and the engine light was on, took it back and it was a co2 sensor, post cat. In the end due to time (I was on hols) sensor wasn’t fitted, but a full dose of decent German made injector cleaner and using decent super unleaded from non supermarket stations cleared it, and it hasn’t come back (this was sept last year). Apparently co2 sensors only have around 60k lifespan, and mines at 55k.

[cornershop]

When you cleaned the MAF, did you spray the ‘bulb’ on the side (assuming same design as EZ30).

If so, you need to turn it upside down and direct the spray onto the thin wire you can see inside the plastic housing.

[Rumbledriftspin]

Thanks for your replies.

I've replaced the MAF sensor with another used one (OEM I think) and the problem persists.

The car's in the garage today to have the post cat lambda sensor replaced for a new OEM Subaru one.

Fingers crossed...

[Rumbledriftspin]

Unfortunately the issue has persisted with the new lambda sensor fitted.

The engine hesitation only occurs when the car is being driven - not when revving the engine whilst parked.

Could this actually be due to a faulty engine ECU? From an online search it looks like this has been found to be the culprit by other Legacy owners experiencing the hesitation issue. To me it seems similar to when you've disconnected the battery on some cars and the engine runs a bit lumpy until you've driven the car for a few miles to allow the ECU to relearn the driving cycle parameters.

I'm not really sure what else it could be. If it was a faulty throttle potentiometer or dirty/damaged injectors I'd guess the hesitation would be apparent through the whole rev range and also when revving the car when parked.

[NMNeil]

Engine grounding on those was a known issue, so much so that they make kits to fix it.
https://www.youtube.com/watch?v=AS6IjyXH7Bc
Might be worth checking the grounding points for corrosion.

[Rumbledriftspin]

Thanks for that - I'd never have guessed that poor grounding could cause the engine hesitation issue.

I've looked under the engine bay and noticed that the offside grounding strap (which connects the cylinder head with the chassis) is broken. Can this be replaced simply by using a length of electrical wire between the clamps instead of the woven strap?

Also, can anyone advise me on the locations of the other grounding points please? Aside from the two grounding straps I've noticed that the battery negative is grounded behind the battery box on the bodywork near the nearside suspension tower. I'd guess that there are many other grounding points.

Many thanks in advance.

[NMNeil]

You would need something that won't stress fracture due to the vibration, regular wire will work if it's got plenty of fine strands rather than just a few.
The engine ground may be the culprit so any wire will be better than none until you're sure that's the problem.
Can't help on the grounding points I'm afraid.

[Screenwash]

I’ve got a 2020 Forester with the 2.5 n/a engine and CVT gearbox. I intermittently get a hesitation, almost a stutter, on light throttle, eg when accelerating to go up a gentle hill.

Main dealer serviced every 8000 miles and they can’t find anything wrong. One mechanic suggested it was a foible of the gearbox and that if I prod the throttle more the problem will go away. Sounds like rubbish to me.

But no fault codes showing, so no problem…

[cornershop]

Have a look in the manual:

https://jdmfsm.info/Auto/Japan/Subaru/?fbclid=IwAR...

[Rumbledriftspin]

Quick update.

When I disconnect the battery the engine runs sweetly for a good while (50 miles plus) but after a few days' use the engine hesitation issue returns.

I've connected the broken offside grounding strap and cleaned corrosion from the main battery negative grounding point and the engine hesitation seems to be less abrupt but it's still there.

Would it be worth having a car electrician check out and clean the earth grounding points and perhaps fit a grounding kit (I'd guess a cheaper fix)?

Or could this actually be an ECU problem after all or something else?

Many thanks.

[vxr2010]

To improve the earth connection , just put a lead from the neg of the battery to a good earthing point on the engine , if it’s a known issue I would use two leads , there are two 02 sensors have you done both ? Code reader worth a try , a battery getting to the end of its life ?

[Rumbledriftspin]

Yes, had both O2 sensors replaced and the battery is less than 6 months old.

I replaced the MAF sensor with another OEM used one too.

I've got the car booked in to a local mechanic soon to specifically assess and clean the earth groundings and investigate other potential sources of the engine hesitation issue.

Fingers crossed.