Pops/Bangs/Plateau's 4500RPM

[Noble]

Hey all, just bought a 97 Stagea RS Four a couple days ago. Just today, I started her up in the morning and noticed that the idle was a bit rough.

The car would idle, then drop to about 500 rpm, them go back up. I decided that letting the engine get fully warmed up before driving would be smart and after it did I noticed that under full throttle the car would get to 4500RPM, plateau, and then it felt like it "hit boost" and was able to get above that mark. Whats weird is after it "hit boost", there was an audible "pop" from I'm assuming the exhaust. Considering the car didn't have a problem when I bought it, I figured calling the previous owner would be a good idea. He told me that he had installed new spark plugs and a maf a couple months before I bought the car.

I know these RB25's love to hate their coil packs, but does what I explained sound like it could be that or maybe the wiring loom?

[GTSBoy]

More like low fuel pressure.

[Duncan]

was it cold, and is it modified at all? It's midwinter, right? cold air combined with mild mods on a standard ECU could be the problem

[Noble]

No mods at all aside from a cat back exhaust. And yeah, it was about 6°C when this happened, so pretty cold.

[Duncan]

And I guess the other question would be does it drive OK now, or when it is a little warmer. It is possible the standard ECU is just unhappy with the amount of air it got through the combination of the exhaust and being cold, the stagea ECU in particular is not very tolerant of mods.

Otherwise, it could be any of the billion usual issues with a 25 year old car that has probably been around the block once or twice. Could be fuel pump (or filter) as suggested above. Could be fouled spark plugs. Could be dying coil packs, coil pack loom or even just poorly seated coil pack connectors. Could be the CAS. Could be a boost leak. Could be the AFM.....all just a bit hard to guess from here.

That pop from the exhaust was just that it was either running very rich or misfiring (which also results in burned fuel)

[Noble]

I guess I'll just have to start at the coils and the loom. What's your recommendation on testing the coils? Since the intake goes across where they sit on top of block, is it still possible to pull each one out if I just take the cover off?

[Duncan]

Sorry it's been a long time since my stagea had the cross over inlet  But yes I'd start with the car idling and unplug each coil in turn. If the revs don't drop the coil is not firing properly. You might also find one or more coil pack look plugs loose or cracked. But that test isn't a guarantee it is not a spark problem because it might have enough power to fire at idle, but not under boost

When you don't know the car or it's service history, new spark plugs are an easy starting point too

[Noble]

Right, I could definitely look at the plugs while I'm in there. Would a foul plug have any physical features that you could look at and say, "ah yes, this is ******"?

[GTSBoy]

On 1/31/2022 at 4:12 PM, Noble said:

Would a foul plug have any physical features that you could look at and say, "ah yes, this is ******"?

Yes. It'll look like shit. That's the definition of fouled.

Take the crossover off. It only takes 10 minutes.

[Noble]

But can I can't test the coils with the crossover off, isn't that right? I thought I had read that somewhere else in these forums

[GTSBoy]

On 1/31/2022 at 4:25 PM, Noble said:

But can I can't test the coils with the crossover off, isn't that right? I thought I had read that somewhere else in these forums

Well, yes, but only really at idle, which, as Duncan said, might not prove anything. A coil can be strong enough to run well at low load and poop itself the minute you add a bit more mixture pressure into the chamber. If you want to do it under load, then the car needs to be on the dyno, and popping a coil loom plug off under load isn't a lot of fun.

If you want to pull the coils and/or the plugs, then the crossover is coming off. There's a big difference between reaching in and popping a coil loom plug off and getting the tools in there to unbolt them and pull them up and out.

[Noble]

My thought is, if at idle, the engine can't even keep up with itself, pulling the coils should give me some sort of result. It's also really the only option available to me at the moment.

[joshuaho96]

On 1/31/2022 at 12:44 AM, Noble said:

My thought is, if at idle, the engine can't even keep up with itself, pulling the coils should give me some sort of result. It's also really the only option available to me at the moment.

If you want to conduct this test without the work involved in pulling coilpacks then you can use Consult to disable individual cylinders, I use the active test section of NDS1 for this:

In theory you could use this to figure out exactly which coil is misfiring under load but at least in the case of an RB26 the coilpacks lack ignition feedback signals so the ECU cannot detect any issues on the secondary windings, so you're going to be spraying quite a lot of unburned fuel into the exhaust. A misfire is really not that hard to figure out though, your engine is going to run quite rough from the imbalance in combustion forces. I say this a lot it seems but NDS is a super useful tool for diagnosing problems with these cars. In the case of your fluctuating idle just reading the ECU logs would give you some clue as to what's going on, such as weirdness in the O2 sensor signal or MAFs. 

[Noble]

This is really good info, and again, I'm new to all of this, this is basically my project car that I'm trying to rehabilitate lol. Where can I get this software from?

[Noble]

Also, the engine in my Stagea is the S1 RB25DET