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.

[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.

[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.

[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.

[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.

[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.

[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.

[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.