I'm interested in knowing the outcome of your inquiries here and elsewhere. I'm also starting to lose power in my high rpm range. I'v recently replaced my OEM cats with Kinetix high-flow. I'v gained a huge amount of low end torque and fuel economy but still lack over 4k, not much push or pull.

 

My neighbors dad which is a mechanic of 30 years just recently retired from Carmax, is going to take a look at it on Sunday. I mentioned to him the issue and he is hinting at a bad Throttle Position Sensor or Catalytic Converter. He thinks that since I have done the ECU reset several times since I bought the vehicle its possible I am not allowing it time to show thats why Advance probably couldn't detect the code. Cause its not there yet. I mentioned a faint rattle which is why he also hints at the cats. Thinking there is a clog or damaged cat. He can't say for sure until he looks at it.

He said keep driving it normal. Don't get on it to much and dont reset ECU to see if a code will appear from here till then. That way he can scan it and see if something comes up.

Its not affecting my daily driving. Its just not normal for a vehicle with this type of HP not to produce it when demanded.

 

Rattle could be just a loose cat converter heat shield. Pretty commom for the spot welds to break causing a raspy rattle from the area around your feet.


I think you said this before, but there is no check engine light on correct? But the light does appear when you turn the key to on and don't start the car

 

I have a feeling it's your throttle body that has begun to fail or the camshaft sensors. stick to NGKs

VQ's are not bosch happy.

 

Funny you say that cause the way that I saw the TB work the Throttle Sensor is attached. Its not something you can buy separately and is sold as a complete assembly. If I were to say the sputtering I feel and reference it to a butterfly in the TB sputtering, it would almost replicate the idea.

How could I test this without putting the vehicle under load. Would it sputter the same if I was to rev open throttle in the drive way vs putting load on the engine?

 

ima jus put this out there but isn't wat op is experiencing just the nature of autos?

 

My auto pulls hard to redline, it also shows consistant hp increase on the dyno to 6000 rpm and then levels off. So no, its not "just the nature of autos". Good luck and post the outcome.

 

Just seeing this. This sounds like bad Cat's. The G has a pre-cat and post-cat on each bank, they're both in the same pipe though (replaced as a whole).

Have a pressure test done. They remove the O2 sensors and install a pressure gauge and compare readings before and after the Pre-Cat portion.
This will tell you if the precats are plugging up.

If it's actually SPUTTERING though, it could be something else, But if it's just bogging down and unable to get any steam rolling, I'd put money on the Cat's being bad.

Nissan pre-cats are complete trash.

 

You can check 2 things related with a fairly basic code reader if you have one:

1. Check fuel trims. LTFT is LONG TERM fuel trim, STFT is SHORT TERM fuel trim. Neither should be above +/- 25%

2. Check engine load. This is a % reading on your scanner. When idling a warm engine it should be below 40% with the heat, defrost, A/C, etc all off. If it isn't, those cats are likely plugged. If it's at, or almost at 100% while lightly cruising low speeds, it's almost certainly cats.

The only 100% verification is to do the pressure test though, or just replace them with a decent HFC, which will give you a nice increase in HP output anyways. But the second option isn't a cheap one. Better to know for sure.

Unless the car is bucking, mis-firing, or very obviously sputtering (losing power, then back, then gone rapidly, almost like it's starving for fuel) then stop looking at sensors, Throttle body, spark plugs, and so on, you're wasting your time and money.

If your precats are plugged enough, you might experience a flutter/backfire event, this is because the engine literally cannot push the air out of the exhaust fast enough, leaving high pressure gas in the combustion chamber during the intake valve opening when it want's to 'recharge', this causes excess pressure in the intake side, which will make noise, fluttering, etc. The cat's have to be pretty bad to start doing that normally, and normally only happens at fairly high speed and high engine speed (pulling hard on the highway) when the engine is trying to move a ton of air through it.

 

Update:

Guys I might have found the issue. As TunerMax was stating, Im 60% sure the Cat is the issue. Did a 3 step reset of ECU, TB, Pedal etc because I noticed my RPM's at idle in drive were at 1k - 1100. in park I see the RPM's in the 1100 - 1200 range. Not normal the idle is to high for it not going anywhere even with AC on. Took it for a test drive again after the resets and the RPMs are normal and ofcourse Same symptoms. Light Rattle at higher RPM's and no power in WOT.


I get home park it, leave it on and give a few good pushes on the gas to see if I hear anything out of the normal. If its the transmission, I don't believe I would hear anything in park. So what do I hear?

Sounds like a rock or two rattling around at idle and a little more when you give it gas. You have to get out and put your ear under the car to actually hear this. In WOT its more distinct.

Just to add insult to injury, filled the tank up yesterday and I am a little bit above half and really haven't gone anywhere but work and back home with a few test drives in between.

What do you folks think? My conclusion is sound or am I still on the fence between fuel, spark, and transmission?

 

It still could be a loose heat shield that you are hearing.

Cat's don't really "go bad". They break apart maybe, but it wouldn't cause a power lose at higher RPM's. I've had personal experience with trying to remove cats on an old car and not doing a good job. The broken cat rattled for a few days, but there was no power loss or misfire.

If the cat clogged up, it would be glowing cherry red and you would have no power down low as well. Car would feel as if it was being restricted and wouldn't rev free.


I have a broken heat shield on my G, and the symptons you describe sound just like my car. When i had a purposely broken cat, it rattled around like hell at all times.

 

I respect this guy, he knows his stuff, but in this case you're incorrect. An engine moves the most air at higher RPM, under load, on the highway. If the car is not free revving with no load sitting still then the issue is excessively major.

They are not completely blocked, they're just getting there.. They'll break apart soon if they haven't already, and when they do, they'll have no where to go but into the bottleneck right after, or into the Post-Cat, which will effectively plug the exhaust up, expescially if the honecomb turns sideways which is common, air cannot pass through it.
That may have already happened and that's the noise he hears.

I will literally put money on this being the cause based on what I've read here so far about what hte car is doing.

 

I think you are right.

I was reading this as a misfire issue at higher RPM's, but after rereading the initial posts, this is not what he claimed at all.

So now i could suspect a cat issue as well.

 

Would you expect the car to throw a code though if the cats are going bad? Thats the part that stumps me. No code or warning just symptoms.

 

In the most basic form:

The ECU knows that the Pre-Cat's aren't working when it doesn't see much difference in air flow/oxygen level before and after the Pre-cat.

If the engine is seeing a difference in air flow between the 2 sensors, it will think everything is fine. If the precat is broken apart and the broken stuff is sitting on the 2nd sensor, then the 2nd sensor won't see much air flow because it's blocked. This would be considered normal to the ECU.

The 1st sensor should read a bit higher air flow than the 2nd sensor.

So in short- no, just because you don't have a code, doesn't mean there isn't something wrong with it.