Pops when backing off throttle, rough-ish sound through rev range.

[AdiR34]

Hey guys! Y'all are the masters so figured I'd check in on a few items here.

- My engine sounds ever-so-slightly more raspy than the other NA RB25s I've listened to. It's barely noticeable but definitely a slightly different sound.

- When I back off the throttle at high RPM, I will sometimes get a pop out of the exhaust, assuming this can be diagnosed as misfire.

Some history on the car:

- Had an exhaust leak in auction sheet but was claimed to be fixed by seller (bought from USA importer). 

- Car's coil packs and sparks were replaced when it was sold to me, but unsure on what the gap is and whether the coil packs are correct. 

- A prior mechanic had misdiagnosed a pilot or throwout bearing sound as LSPI/detonation. Had played with timing a bit, pops/raspy sound of engine just got worse. Got timing reset.

- Had failed emissions a few times. First time went in as is, failed. Second time, replaced cat, still failed, running rich as hell. Third time replaced o2 sensor, passed. Car still smells gassy though.

- Timing belt and valve cover gasket also replaced. 

- Gas mileage is... low. about 350-400K per tank. 

Any idea if I'm having any serious issues here or does this all sound like normal old NA RB stuff and I'm fine to drive it with the pops/slightly raspy engine?

If it'll help, I can get drop a video of the sound of the engine.

Edited June 9 by AdiR34

[Duncan]

It is hard to be specific half way across the world...but it *seems* like it might have a minor issue.

Is the idle stable and does it rev smoothly?

I would not expect any pops out of what I am assuming is a standard ECU, standard tune, standard exhaust.

You can use Nissan Data Scan to find out exactly what is happening when it runs, the cable/software is reasonably affordable if you are not planning on changing the ECU (which hopefully you are not with an NA skyline)

BTW your mileage may not be that far off, these are heavy old tech cars, and being NA might mean you use a heavy foot to keep up with traffic....

[GTSBoy]

1 hour ago, Duncan said:

BTW your mileage may not be that far off, these are heavy old tech cars, and being NA might mean you use a heavy foot to keep up with traffic....

Keeping in mind that I am currently getting flat 10 L/100km on my RB25DET, which means >>500km/tank.

I would test this car for vacuum leaks, check codes and AFM signal with NissDataScan as D says, check plugs, do a compression check while they are out.

[AdiR34]

15 hours ago, Duncan said:

Is the idle stable and does it rev smoothly?

Yes to both!

15 hours ago, Duncan said:

I would not expect any pops out of what I am assuming is a standard ECU, standard tune, standard exhaust.

Yes to standard ECU and tune, but exhaust is aftermarket. Unsure on brand.

15 hours ago, Duncan said:

You can use Nissan Data Scan to find out exactly what is happening when it runs, the cable/software is reasonably affordable if you are not planning on changing the ECU (which hopefully you are not with an NA skyline)

Actually have it sitting in at my desk haha! I have to set it up on my laptop but this is a great next step, will do.

15 hours ago, Duncan said:

being NA might mean you use a heavy foot to keep up with traffic....

So I generally try to shift before 4.5K RPM most times. I largely don't do a lot of high-rev driving, but I will take it up to red 1-2x a week max, as I know the occasional "Italian tune-up" is good for these cars. 

14 hours ago, GTSBoy said:

Keeping in mind that I am currently getting flat 10 L/100km on my RB25DET, which means >>500km/tank.

Oh wow, is this way better than what I'm getting. Does this mean there's a good chance I have an issue?

 

Thanks for all the info so far guys.

[GTSBoy]

4 hours ago, AdiR34 said:

Does this mean there's a good chance I have an issue?

Hence why I suggested some tests.

[GTSBoy]

Oh, another question comes to mind though. What sort of shitty fuel are you feeding it.

Mine, a turbo, has to be fed 98RON, so it doesn't ping itself to death. 98 in Oz is good, dense fuel with minimal oxygenated compounds. Meaning that the energy density is high, and if the O2 trim is working, you will get more MPG than with a lesser fuel.

If you're using some sort of piss poor E10 blend with some other MBTE or other oxygenated additions, that could explain using more per mile.

[soviet_merlin]

8 hours ago, AdiR34 said:

Oh wow, is this way better than what I'm getting. Does this mean there's a good chance I have an issue?

Bear in mind RBs tend to be pretty terrible for fuel economy and it heavily depends on driving style.

In my experience with a heavy-ass turbo Stagea the 10l/100km is really just ideal conditions highway driving. Start-stop in the city is killer and it ends up being 16-19l/100km easily. I keep track of it every time I get fuel and it is never pretty.

There are a few topics in the forums where people share their fuel economy if you want to get some ideas of what it can be like. I don't think it is worth that much to compare though because it depends so much on the driving.

[MBS206]

On 10/06/2025 at 11:25 AM, soviet_merlin said:

Bear in mind RBs tend to be pretty terrible for fuel economy and it heavily depends on driving style.

In my experience with a heavy-ass turbo Stagea the 10l/100km is really just ideal conditions highway driving. Start-stop in the city is killer and it ends up being 16-19l/100km easily. I keep track of it every time I get fuel and it is never pretty.

There are a few topics in the forums where people share their fuel economy if you want to get some ideas of what it can be like. I don't think it is worth that much to compare though because it depends so much on the driving.

I will agree here. My R33 GTSt when stock was doing the 13 to 14 L/100km on my daily driving duties.

Out on the open road, just cruising for hours straight, that fuel usage majorly dropped.

 

When comparing fuel usage between people, you need to be VERY wary of driving styles, as well as how their normal routes look.

As an example, we have two identical cars at work, on my daily trips, I'll regularly do 9.5L/100, however, one of our other drivers is regularly using 2L/100km more than I do. We could swap cars, and we'd each still get the same returns. A huge difference in our economy differences, is the differences in the type of drives we do, I end up more on the freeway, they get a lot more purely in suburb streets.

[GTSBoy]

Yeah, but we're talking about an NA Neo 25 here. Even with the extra weight of the AD gear, it should still be reasonable.

GTSTs and GTTs only use bulk fuel around town because they run <11:1 mixtures and the drivers can't help getting up on boost just to hear the noises.

[MBS206]

52 minutes ago, GTSBoy said:

Yeah, but we're talking about an NA Neo 25 here. Even with the extra weight of the AD gear, it should still be reasonable.

GTSTs and GTTs only use bulk fuel around town because they run <11:1 mixtures and the drivers can't help getting up on boost just to hear the noises.

Seems to be common, also depends on what "full" is too.

 

https://www.sau.com.au/forums/topic/352876-rb25de-fuel-consumption/