Engine straining. Lost power. Poor MPG

Hi to you all from the UK.

You may have heard this question a hundred times before but worded similarly. But I've trawled through the forum and I don't think its been answered correctly yet as all the answers point to something different.

Basically, about 3 weeks ago I noticed some problems with my 04 Prius. It has 72k miles on the clock. The exhaust was making a farting sound. I have since diagnosed this as a hole in the back box which I promptly replaced. But the real problem is the power.

When driving in electric mode, the car runs as normal. But when I press down on the gas just a little more to the point where the ICE kicks in, it doesn't give as much power as it used to. For example, in electric mode, cruising at 30mph and achieving 99.9mpg, I'd press the pedal down about half an inch. Where this used to kick the ICE in and show 60mpg+ (with the engine charging the battery and providing power to the wheels), it now shows about 30mpg. When I press the pedal a further half inch, it would normally show 35-45mpg, but now reads 16-25mpg. And when its fully depressed, it used to show 12-16mpg but now barely hits double figures and averages about 6.7mpg. Even on highways when I've achieved 60mph and gently ease off the pedal to maintain speed or really slowly increase, I used to achieve 60-70mpg but now I'd be lucky if it makes it to 40.

I also noticed that it produces considerably less pulling power than it used to. Almost as if the engine is really struggling, even when gently pressing the pedal and accelerating from 30mph upwards. This is having a large effect on my overall fuel consumption and my ability to not piss normal drivers off. I get so scared that I'm wasting too much fuel that I drive really conservatively now.

So in essence, I'm asking for help because at the moment, I'm barely achieving 300miles from a full tank where I used to get 450+. My driving habits have not changed at all so it HAS to be the car. Could it be linked to the damage or repair of my exhaust? I doubt it because changing the rear end has made no difference except to stop the farting noise from persisting.

Thank you all in advance.

 

Hi. Thanks for the replies.

Easy rider 2 - apologies for using the word BACK box. What I meant is muffler. And this had to be replaced regardless. As I said before, it was making noise and then on inspection I found it had holes in it.

Aaron - I had the spark plugs changed during my half year service which was approximately 4 months (4000miles) ago.

Same with the air filter.

The engine is combusting correctly. But between combustion and power transfer I have noticed there is more lag and the output has significantly decreased. I will be taking it to a garage tomorrow with hopefully some good news. Although as I am tight on funds I hope it is a DIY fix. I seldom trust mechanics with my car as I know the games they play to try rip me off.

 

P.S. the oil level is fine. Its half way between both holes on the dipstick.

Battery will be checked tomorrow but I don't think its that as I had it change 1.5 years ago

 

I haven't had the codes checked. I will be going to a garage tomorrow for that.

I have used injector cleaner as a precaution but to no avail. Didn't think it would either as the fuel is getting to the chamber. It is even revving fine. But the power is lost somewhere after that. I've just been out for a drive and even in town as soon as it switches from electric to petroleum mode, I see the figure drop to 24-29 mpg which is really bad as I normally achieve 45+.

I know you may be thinking I have a lead foot but I don't change my driving style at all and have noticed this problem very recently. Has anyone else had this problem which they have fixed? Regardless of how many different causes there could be.

 

Hi. Tyres are fine. 36psi all round

 

Data Daedalus - I havent changed anything on the car for the past 3 months except muffler. Tyres are the same. This problem started happening BEFORE I fixed the muffler.

Edthefox5 - no difference in the way the battery charges or discharges.

Goldfinger- the problem has been there since my muffler has been damaged. All this time, I though it was the damaged muffler which was affecting my mpg but this doesn't seem to be the case as the problem is still there after repair.

 

I've just checked the battery by doing the secret mode test. It was at 10.7v on a sitting mode. 9.6v in accessory mode.

But can that actually affect my mpg that bad?