Hello

2006 RX-8 at 121.000km

Engine was replaced with a brand new one at 88.000km
Spark Plugs were replaced at 119.000km (NGK Iridium)
IG Coils were replaced at 120.750km (Mazda OEM)
Plug cables are NOT replaced yet.
There's no Catalytic Convertor on the exhaust.
The car has been making this explosion noise for the last several months.
It makes this sound only on cold startups. No such a sound on hot startups.
The sound seems to come from the middle of the exhaust.

By the time I noticed this explosion sound at the startup, the IG Coils were very old (7+ years). After I replaced the IG Coils, the level of the sound decreased but not completely went away. Before the IG Coils replacement, it used to make similar kind of explosion sound at high rpm's (~8.000) during driving. After the IG coils replacement, it almost never did an explosion sound at high rpm's.

I will replace the plug cables with NGK blue cables at the first occasion when I take the car to my mechanic. I don't know how old the cables are but from their looks they look old.

I was wondering what could be causing this mild explosion sound at the startup.
Here's a video. The explosion sound can be heard at the 5th second. Could it be caused by plug cables being old or something else?

 

That sounds exactly like the pops I get when revving and between shifts but I get them at much higher RPM's. I want to say its normal but I'll let others chime in.

 

Because it's a sports car, some explosion sounds coming sometimes at high rpms and between shifts is not surprising, but I don't think it can be considered as normal.

I remind you that it used to make explosion sounds very frequently at high rpm's before I replaced the IG coils and now it makes none.

 

UPDATE: I have replaced the spark plug cables and that explosion sound is still there.

It hasn't made any explosion sound when I started the engine for the first time and that made me think that the problem was solved.
But after I parked the car last night and when I started it this afternoon, it made that explosion sound again.

 

Mine occasionally does that on cold start ups ever since I gutted the cat. It persisted after coils, plugs, wires and many other changes. It bugs me a little when it happens, but it's not very common on mine, and something tells me it wouldn't happen if we had stock cats on...

 

normal

 

not "normal" but nothing to really worry about. On older rotary powered vehicles I've found a couple of different causes for this:
1. leaky fuel injector
2. Low compression

What's happening is extra fuel is being left in the combustion chamber causing a "mild explosion" on startup. Nothing to really worry about. With the mileage on your vehicle I would check the compression and the fuel injectors for any leaks.
In the past on my FC I solved(more of a bandaid) this problem by putting my fuel pump on a switch. instead of hitting my ignition switch to cut off the car I would hit the fuel pump switch. This helped with flooding also which I suspect will be next to happen.

 

I like how people always tried to compare old FC to a car like rx8 that's made 20 something yrs later.

this is normal for rx8, done.

 

i guess since i haven't experienced this or had any of those close to me experience this i can't chalk it up to being normal. I drew conclusions based on my personal experience. Maybe it is normal but if so I suppose the 2 rx-8s I have owned are anomalies.

 

Hello

I've had this problem since last May and have 2 threads about it, but they were about the symptom (mild explosion sound at the exhaust) and about the possible failure of a part which was not directly related to the problem (the starter)

The basic problem is: The engine gets stuck at around 1000rpm for a second or two in cold starts. (In the video you will see this accompanied with a terrible explosion sound at the exhaust.)

Chassis is at 123.300 kms (~77k miles) engine is at 35.000 kms (~22k miles)
There's no catalytic convertor, it was removed at 101.000 kms (~63k miles) by its previous owner because of unwanted noise complaints.
The plugs (NGK), wires (NGK) and coils (Mazda OEM) and air filter are all replaced new at around 120.000 kms (~75k miles)
The oil and oil filter were changed recently. The battery (Varta 60Ah) was changed new.

During last summer, it kept on making this cold start problem.

Early this month, I applied NVRAM and KAM resets and that seemed to solve the problem for a while:

The engine was now starting much better at cold starts but after 7-10 days, the problem returned back again.


In my previous threads, some members have commented that this problem was not something to worry about, and some others said it was happening because the cat was removed.

Upon those comments I should say that removal of the cat cannot be the cause of this problem. It can only conceal the problem. And I should remind that the car never made that problem for years until last May, and it had no cat during all that time.

And it's really something to worry about because it disturbs the people in my neighborhood, especially when I start the engine at nights.

Now I plan to

--> Clean the MAF sensor once more
--> Do NVRAM & KAM resets again
--> Check the fuel injector

to see if it will solve the problem persistently.

Do you have further suggestions about this problem?

 

I merged your threads, no point in creating an additional one.


I disagree with prior comments about it being normal though. A backfire from a catless rotary is certinaly normal when driving during decel or shifting. Not normal this immediately after starting. Considering you can see the tach needle jump slightly at the same time as the backfire, I do think something else is going on here that isn't kosher.

The backfire itself isn't a problem, but I do think something is causing it that shouldn't be.

I'm going to stir some theories around in my head for a bit and I'll post a bit later.

 

huh?
sub'd

Weird. +1 on the not normal.

I get a chirp sound every now and then on startup.
A very distinctive chirp like a bird.

@RIWWP enough of you letting this bake on your brain for awhile .. spill it! lol

Leaking Injectors? Injectors wired wrong?

 

Well, backfires need the same basic points of fire: heat, fuel, oxygen.

With everything warmed up, backfires are reasonable since there is plenty of heat and fuel in the exhaust piping, and no oxygen (or, not enough)

At starting, there is plenty of oxygen in the exhaust piping, and we know there is general some level of excess fuel. But there isn't any heat. Any heat from the starting combustions would be up in the header, not what sounds like is back by the tailpipe.

revivo: Where does the backfire noise come from? under the car? behind the car?


I'm going to guess that it is coming from the header. It doesn't seem viable that it would be coming from the tail pipe. Just no heat back there, and plenty of oxygen between there and the place where the heat is.

So, it's also a sharp combustion. I would theorize that it is 1 or more air/fuel mixtures that simply isn't being fired by the plugs, and it dumps to the header is a fairly good AFR mix, where it's fired from the heat of a later combustion that DOES work.

Since it occurs AFTER actually starting, and I can't detect a change in engine note before or after, I don't think it's something as simple as 1 rotor not firing initially.

The slight shift in the tach needle at the same time leads me to believe that it's going to be electrical related. Anything I can think of that would trigger an ignition cut or prevent an ignition event would have caused problems before and after.

revivo: Have you cleaned the ESS? And what condition are your accessory belts in? Any frays?

 

I keep thinking it's ESS related, since the ESS governs the tach, ignition events, and fueling events. It's a point of commonality. But it has to interrupt ignition and not fuel, and on a very predictable pattern that doesn't exist with the car driving.

Whether or not it exists during hot start is an unknown at this point, as it is possible that it DOES happen, just there is not enough oxygen to fire the mixture right there, and it burns off quietly farther down the pipe.

 

Hello

Thank you for the interest in my problem.

The explosion sound seems to come from under the car, actually I'm not exactly sure where it comes from but I guess (like you guess) it should come from a place where there's enough heat. That explosion must take place somewhere near the engine.

I haven't cleaned ESS and don't know what condition the accessory belts are in. (Btw, I don't know what ESS, AFR mean and don't know about the frays, I'll look them up now)

 

I can't tell if it's the injectors leaking but it can't be the injectors wired wrong because I never replaced or disconnected the injectors as this problem happened.

Is there any video about the bird sound on startup?

 

ESS = E-shaft sensor
AFR = Air to Fuel Ratio, too much fuel or too much air and a burn won't happen

A fraying belt:


vs a good one:



I ask because I had a fraying belt on another car of mine at one point. The fraying edge would get caught in the crankshaft sensor (same as the e-shaft sensor) and distort the profile briefly, interrupting ignition and fuel.

Just a random thing that people might not even think of.

 

Thank you for the explanation and the clarifications.

In that case it 's not the frays because last time I looked at the engine and the belts, I haven't noticed any frays. The belts looked ok.

But ESS is never cleaned either. The next action will be cleaning the ESS then.

 

Yeah, it's just a magnet, and any metallic debris can get stuck to it and mess up the profile, and it's possible to be messed up in a really wierd way.

 

Clean and Reset using 20 brake pedal stomp method

 

Thank you for the comments. Now the location of the ESS is a bit uncertain though I searched pictures of it. I've taken a picture, is it the part marked with green arrow?

And how to remove it, is it removed from under the car do I have jack up the car or can it be reached from top after the air filter is removed?

 

Yup, that's it. It's more easily reached by removing the splash pan and getting at it from the bottom. It's still a bit annoying either way. You don't actually have to remove it though. Wiping it off with a rag while it's installed is viable, just run the thin edge of the rag between the sensor and the ignition trigger wheel to get the space between them.

 

Oh thanks. It looks easier from the bottom. My first job tomorrow will be cleaning it.

 

Today afternoon I have finally removed the ESS and cleaned it thoroughly. I have reached it from the bottom and taken its pictures for the record.

It had one small metal piece on top of it and many more at the bottom of it. I have cleaned them with my MAF sensor cleaner and that's how it looked after the cleaning.

After I installed it back, I've done the NVRAM reset (and KAM before it) here is how the engine started from cold before and after the ESS was cleaned.
It's quite successful.

Btw, here's how I jacked up the car.

 

The last situation: On my third cold start this evening, it again got stuck a bit around 1250rpm but this time without any explosion sound, fortunately. This shows that cleaning the ESS has definitely helped the problem but it looks like the engine keeps on hesitating on the rpm, though not so severe as before.