Uneven power through rev range

[Gilly91]

Since I've owned my V6 VS commodore it has not liked being driven hard at all. This isnt much of a problem because I drive it like a grandma in order to preserve tyres etc. However, it has an annoying problem that occurs when accelerating; it almost feels like what happens when you hit the power band on a two stroke motocross bike.

When I accelerate through the gears, the car feels gutless if I put my foot down until it gets to a certain RPM, then it feels like an extra cylinder decides to start firing. It is that noticable that it makes your body move with the sudden power hit. I recently replaced my coil package because a terminal was rusted and it was misfiring but this strange problem is still occurring. It occurs more in 2nd and 3rd gear.

The A/C condenser has been removed from the front of the car and my running temperature always reads very cold (barely off the bottom of the gauge) even though I've changed the thermostat 3 times. So I'm not sure if this problem could be due to the car running cold or if that is just a faulty temperature reading. And if I do ever drive my car even remotely hard, when I stop it gives off that burning engine smell. Not sure if this detail helps but I'm just trying to explain in as much detail as possible.


The car runs smooth and doesnt sound like it is misfiring prior to this sudden hit of power. I was just wondering what this problem is, and if it is fixable, or if it is just one of those things that happens due to the engine being so old.

Thanks for your help!

[pjdm1980]

Have you tried a code check?

[TI3VOM]

If he has the same problem as I do a code check won't show up anything, but it's best to try that first.

[Gilly91]

Yeah I tried one ages ago and it came up with "air flow sensor" and "canister purge solenoid". Not sure if I fixed those problems, but I fixed a dodgy wiring problem to the air flow sensor and havent had any engine lights since.

TI3VOM, does your problem sound the same as mine? Have you tried to fix it?

Thanks guys.

[IBLOWN]

Change your fuel filter.

[Gilly91]

I changed my spark plugs and fuel filter about 6 months ago. Spark plugs probably didnt need changing and the fuel filter was pretty clean. Only filter I havent changed is the air filter and it looks clean.

[TI3VOM]

I have no idea what it is, I replaced my fuel filter less than 3 months ago yet it was still doing it after I changed it. My spark plug leads and coils are brand new, my spark plugs are about 1 year old but and iridium but it still did it after the spark plug change as well.

Which lets me think there are maybe only things like an: Air leak somewhere, fuel pump is doing something wrong, maybe the injectors somehow.

[Gilly91]

Yeah it seems to be one of those things that could be anything. It would just be nice to drive a car that feels like it wants to be driven haha.

[Not_An_Abba_Fan]

Do you have a 2 1/2" exhaust on it?

[Gilly91]

I certainly do. Why is that?

[Not_An_Abba_Fan]

That is why you have nothing down low. You have no exhaust velocity at low RPM, the exhaust is too big. Unfortunately just about every manufacturer produces a system that sounds good but sacrifices efficiency.

It's not that detrimental, but the trade off in a 2 1/2" system is loss of torque up to about 2500 rpm. It is about back pressure, but not the loss of it, actually having too much. A system that is too big has the exhaust pulses banking up and the engine has to work harder to push them out. Once engine rpm builds up and the exhaust starts moving faster, the pulses help pull the next pulse along behind it. Until that happens, you experience the exact effect you are having. Fitting extractors will help, but having the correct size exhaust is best.

But the best size exhaust is never what people want, they just want it to sound good and a bolt on 2 1/2" cat back is the cheapest way to achieve that.

[Gilly91]

Haha oh I see well that makes sense. I do have extractors but I suppose the problem isnt that bad and sometimes its not even there.

Thanks for clearing that up for me.

[wortus]

When I looked into exhausts for my VN I was told 2 1/4" with flow through mufflers I think they were called or 2 1/2 with high flow mufflers that had the same path for the gas to flow through them as standard just bigger. I chose the later because it would not be so noisy and the car didn't loose any low down power so I think it's a combination of exhaust pipe size and choice of muffler to a point. The system I put on was a Luley cat back.

[TI3VOM]

Hmm that 2 1/2" exhaust comment doesn't help me though, as my car does it above 2500rpm and is very noticeable at three quarter throttle or around that. I do have one on my car by the way lol

What would be the symptoms of and air leak somewhere?
or a fuel pump failing??

[The1]

sounds like dirty MAF or exhaust leak, id check those first if code check doesn't show anything.

[Gilly91]

Yeah mine seems to do it at higher RPM also. And it is a very sudden hit of power when it stops. Best way to describe it would be to say it's like a cylinder is not firing and then eventually kicks in some time. My car was running on 5 cylinders before I changed the coil package and it just feels like a less severe case of when the 6th cylinder would kick in sometimes and work properly.

I did a fault code check a while back and it came up with manifold air flow sensor. Would that be a dirty filter like you said? It looks clean. I fixed some faulty wiring on the sensor ages ago and haven't had an engine light since so I assumed all was good.

[TI3VOM]

sounds like dirty MAF or exhaust leak, id check those first if code check doesn't show anything.

I cleaned my MAF less than 2 months ago so that ones ruled out. But the exhaust leak I haven't thought of, hoe would I go about checking if it was one?

[Not_An_Abba_Fan]

Getting under the car and looking and listening.

[Brett_jjj]

Ive seen the guys at a local exhaust shop going over someones exhaust system,they were listening to the exhaust joints and flanges etc through a piece of hose,so that idea must work ok and would be worth trying.

[Not_An_Abba_Fan]

It's like a stethascope. The hose makes the leak veru easy to hear. I have mentioned that method many times over the years on here.

[Brett_jjj]

Good stuff.You know it must work good when you see or hear about the pros doing it...

[TI3VOM]

Thanks for that info guys, I will do just that.

[Gilly91]

Yeah, thanks for all your help!

[TI3VOM]

Good luck Gilly I hope you find out what's going on with your car as well.

[Gilly91]

Cheers mate haha I'm not gonna lie, I'm not expecting to find anything. Thats the way it normally goes haha.