Strange noises near the end of a long freeway drive
 

I drove from Northern VA to Upstate NY for T-Day this year, about 700 miles round-trip. I didn't drive more than a couple miles at my destination, so it was essentially 700 miles of highway driving with a 3-day break in the middle.

Near the end of the drive home, I was going up a hill, when I noticed my speed was dropping a bit, and the exhaust note was starting to sound low and rumbly despite the engine revving around 4000rpm, and I started to hear a rattly-chirping sound from the transmission that I usually associate with starting from a stop when the engine is cold. When I noticed this, I shut the cruise control off, downshifted a gear, and continued driving as normal.

Since I was using the cruise control, it seems plausible that the engine was running at full-throttle trying to get the car up the hill. I would've thought the cruise control would start yelling at me, or just disengage completely like on my old 1989 Mazda 626, if it couldn't maintain speed even with sustained full-throttle. Was my expectation wrong?

Anyway, running at full throttle trying to climb the hill would explain the chirping clutch and the dropping speed, but the rumbling exhaust note is still unusual. Since the RX-8 is known as a catalyst-killer, I'm curious to know if the symptoms I described suggest that I might've damaged my ignition or exhaust systems.

Where does one start...

It would be great if you filled in your Sig, I have asked for this before...it helps those who you ask help from.

I guess much of all you ask depends on how steep the hill is/was, perhaps you were labouring your engine...current CC tend to 'hang on'.

Did you downshift from 6 to 5 or 4, or from 4 to 3?

Was your car loaded with passengers and luggage or weight, fuel, how many miles has your 8 done, when was it last serviced, when were spark plugs last renewed?

As for any 'damage' I am not certain what you think/how you believe there would be damage, apart from the 'trans rattle' sound which may not be from trans.

(Next day), I would perhaps take car for a run on flat road (no load) , open it up (revs) and see if she performs OK, Engine, Trans, etc...report back..:)

30,000 miles total. No ignition system changes, still original plugs/wires/coils. I've never seen a misfire code, nor felt a misfire while driving. Transmission was recently serviced to repair a leaking seal, which involved tilting the back of the transmission out the bottom of the car. It occurs to me this might've strained the exhaust system in some way, since for some reason manual-transmission cars don't have flex-pipes like automatics have. (I never understood the reasoning for that.)

I downshifted from 6 to 5. The car was carrying me and one passenger, plus two suitcases and a couple bags of miscellaneous stuff; nothing that two people couldn't carry all at once. I don't know how steep the hill was, but it didn't seem unusually steep, and it was on a main freeway, which in the USA tends to require that hills can only be a certain maximum steepness.

The reason I'm concerned about exhaust damage is, the rumbling sound seemed to be coming from the entire underside of the car. It stopped immediately as soon as I shut off the cruise control, downshifted, and resumed driving normally. I've never heard that sound before, except on a car with a rusted exhaust.

- - -

I'm not sure how to put all that information into my sig without taking up an offensive amount of space with every post I make, which is why I haven't done it. I have the year and trim of my car listed below my username.

Well, during my commute to work today, it happened again -- this time in third gear, on a flat road, under light acceleration. For a split-second the car slowed down like I'd tapped the brake pedal, and I heard a rattling/squeaking sound from the engine bay at the same time as the exhaust suddenly got deep and throaty -- and then it went back to normal.

I don't like this game.

MMmm...

Fuel/Gas did cross my mind...could you have some crappy petrol, sorry gas in the tank?
You still have original plugs....I would be renewing them.

Do you still have any warranty left???...

I would take it to a Dealer if you do.

My 4 Year warranty runs out tonight...so I am now on my own here...not that I have ever used it.

I know what petrol is. ;) I always buy premium gas, so unless the entire underground tank was contaminated, it shouldn't be a problem. I suppose it's possible one of the fuel injectors is malfunctioning, though; they take just as much of a beating as the sparkplugs. It's a wonder they don't run at 100% duty cycle, having to service 3 combustion chambers apiece.

The "everything but the wipers" warranty expired a month ago. I got transmission seals and a battery replaced under warranty, so I don't feel cheated. The powertrain warranty is good for another 2 years, I think, and the engine core warranty is good for another 5 years.

Yeah, I should pull the plugs and take a look at them. I'm concerned that if I take it to the shop they'll damage the plugs somehow, so I have to replace them even if they're still good, and now it's below freezing at night, so it's not the most pleasant thing to check the plugs myself. Perhaps I'll do it this weekend.

What's a good source for OE plugs? I don't want ones that just barely passed quality-control, but I'd prefer not to pay dealer markup either.

Any NGK spark plug is going to be the same quality, regardless if you buy it right from the dealer, or if you get it from Billy Bob's Spark Plug Emporium.

Just buy the same plug as you currently have, and you will be fine.
I bought mine at Autozone a month ago.

BC.

I would buy all my spark plugs there even if it was a long trip.

Yep, NGK's are NGK's, Paul from Mazmart will do you a good deal on spark plugs..

Sorry, but this is a process of elimination, renew your plugs first and see how she goes.
Check your Air Filter element ( I guess still the original?...I would be renewing this also).

They definitely aren't all the same. Mass-produced items fall within a range of tolerances, and the highest-paying wholesalers get the units closest to the ideal spec. However, some of those wholesalers slap a big markup for retail sale (such as dealerships), while others only add a small markup. The trick is finding out which wholesalers have the best units with the smallest markup, vs. the wholesalers with the cheaper, wider-tolerance units but a larger markup.

Sorry mate but that is just is not the case...

The 'product' (NGK) is definitely all the same.

Of course there are different NGK models and heat ranges.

I have dealt with NGK for decades and I can tell you now they do not sell off seconds or those that are not within specs...It just does not happen.

The reason OEM NGK's are so much more expensive is ..
They are boxed in printed ''Mazda'' Spark Plug boxes and go through in some cases two more levels of distribution, more middle men wanting their cut/mark-up.

Like most service parts it is how you buy (except OEM), the quantities you purchase.

I would buy in cartons of 250 x 5, so much better price than a box of 10 or a blister card of 4.
All the same part.

You seem to have confused the NGK spark plug company with Kellogg's Cocoa Krispies that they sell to Walmart, or King Soopers.

But, since you know everything, you go hunting for that magical NGK spark plug, that performs better than everyone else's.

BC.

Have you never bought the same product from a specialty shop and from some place like Amazon, and noticed a difference in the quality of the two items? Factories don't throw away products that aren't quite good enough for their favorite customers, they just sell them at a lower price to discount retailers.

And no, I don't know everything, but I do know that not all mass-produced units are exactly the same quality. So I ask again: what is a trustworthy source for sparkplugs, since this engine uses different sparkplugs than every other engine ever made, and it's unusually sensitive to misfires?

Billy Bob's Spark Plug Emporium.

BC.

NHK DOES NOT MAKE SECOND OR CHEAP PLUGS FOR ROTARIES....PERIOD!

If you have seen their process of manufacture/testing, like Denso and ,most other brands of cars (99% I think are made in Japan) because their technology (machines) and quality control are far superior, I think Bosch even make in Japan by Denso or NGK ( I cant remember, does not matter.)..

I gave you a contact to purchase....MAZMART.

Anyplace that sells NGK...or 'a' online Mazda Dealer Here.
Mazda Parts - Online Mazda Parts

From a Mazda Dealer you will pay $180 online for 4 NGK plugs.
N3Y1-18-110A (RE9B Trailing) Blue dot.x 2
N3Y8-18-110A (RE7C Leading) White dot. X 2
Optional (hotter Leading Spark Plug for those who don't rev engine much)
N3Y9-18-110A (RE6C Leading) Orange dot x 2

sounds like a bad cat if the car slows down going uphill. You also hear a rattling, so guts of cat are loose and you may be hearing them rattle.

Check the CAT.

Sorry Ash, I missed the link before.

It hasn't happened in a few days. If it does it again I'll swing by the dealer and let them investigate, since a failed catalyst would be a warranty issue.

Hmm. Still hasn't happened again, but I had to let the car idle for a couple minutes after a cold start Sunday morning, and after a couple minutes it started to idle pretty rough -- not low, but rough. No obvious problems once I pulled out of the parking space and drove away.

There were a few rattly CAT issues back in early days on 2009 S2 Manuals in the USA, I think there were a few in a batch that were defective, from memory these S2 CATS showed up very quickly and at a low mileage/use.

For the first time the RX-8 CATS used on S2's for North America were also used on Australian S2's.
N3Y6-20-55X CAT S2 Manual.

I would still do Spark Plugs, and then is doing it take to Dealer and tell them you have an issue with Exhaust

Do your Spark Plugs ASAP.

Yeah. First I need to clean my air filter and airflow sensor, though, just to make sure they're not "helping" cause the problem. I dropped in a reusable filter when I got the car, and never thought about it again for three years.

EDIT: Nope, not the air filter. Barely has any dirt on it at all. I rotated it 180 degrees to expose the cleaner sides of the pleats to incoming air, and I rinsed the airflow sensor with electronics cleaner just in case there was a bit of oil on it. Looks like I'm back to suspecting the ignition system as the cause.

ANOTHER EDIT: Cleaning the airflow sensor did not destroy it, which is a good thing. I was bemused to discover one of the screw-holes was stripped from the factory, though; nothing holding it in at all. I'll have to fill the screw-hole with epoxy and re-tap the hole to restore the screw-threads. How tedious.

YET ANOTHER EDIT: The car feels a little more spry with the clean airflow sensor, which isn't too surprising since I've been using an oiled filter for the past three years. It could be the unseasonably warm weather, though. It always did like temperatures in the 50s-60s. Out of curiosity, are the dual oil coolers constantly active, or do they have a thermostat to close them off when the engine is cold? It would be a terrible waste of energy if my engine is dumping heat through the oil coolers when I'm cruising down the highway in sub-freezing weather.