mx5cartalk.com

Hey guys, new to the forum.
I drive a NB8A, And i've got this persistant problem.
Occasionally my car will have this issue with acceleration, it will start to judder and decelerate all over the place.
The cause is unknown, it seemed most common coming back from mountain runs, but now its happening after running for about half an hour.
A few guys from the local garage have given me these possibilities:
1. Clutch judder
2. bad coil packs
3. bad spark plugs / leads
4. something in the fuel system, ( fuel pump, filter, regulator)

I was hoping to get a few more opinions on that, and see if anyone else has had this problem and what their miracle cure was.
Thanks

Confucious say miracle cure needs miracle diagnosis. Local garage clearly not able to walk on water.

if when happening when hot. then most likely you coils. the end where the lead attach to them become brittle and the spark directly to the engine instead of through the leads and plugs. pretty common mx5 issue.

but i would also check the leads.

Bad spark plugs / leads is common.

Thanks for the input guys
Its worth mentioning that "the local garage" is a collaboration of guys who just help each other with car stuff.
but I took it to an actual garage for them to find it, but alas, nothing.
I'm hoping to have another guy have a look at it, and he believes 97% he'll find the problem.

97%? He's not that confident...

I generally try to start with the cheapest things to replace first.
I had the same problem with my NA8, turns out it was the Fuel Injection Relay contacts starting to burn out.
A good indication of this is if you turn your key to ignition, and cant hear the fuel pump going, its a good possibility its the relay
Rang Mazda parts Newcastle, and they wanted $165 and it will have to ship from Japan.
Then rang MX5 Mania, ~$35 genuine Mazda one, and they had it on the shelf.

Cheers,
Ken.

If it's while the car is moving, unikely to be clutch.
Shuddering clutch you'll usually notice when moving from a standstill.

Coils are a possibility, NB8A unfortunately tend to have somewhat of a habit with it.

Spark plugs, just replace them regardless. They're like $4 each. I do them annually at least.

Leads,try and borrow a set, since they tend to be a rip off to buy.

Fuel filter should be done every 80,000km I think? worth replacing anyway, $50 or so from Mazda, cheaper elsewhere.

Pumps on mx5s tend to be working, or not, I've never seen one kind-of work. So i;d doubt it for now.

Last option is o2 sensor, mine crapped out a while back and made the car shudder etc. It would run too rich (the sensor giving the ECU the wrong info) and then foul the plugs and drop a cylinder.

I had a similar issue on my NA8.

We replaced the O2 sensor, leads, plugs and coilpack to no avail.

I searched and high and low and found a similar issue on Miata.net.

I replaced the temp sensor on the back of the head and that sorted my issue and car ran perfectly afterwards. That was a cheap part so not sure if it differs in NB8A but worth a shot before you throw more expensive parts at it.

I've replaced the spark plugs and coil packs to no avail today.
It's booked in with the problem hunters next week, I'll pass on what they say.
Thanks for the input guys, but from what I understand the NA fixes aren't usually a problem with NB's?
Thanks all

Jdown79 wrote:I've replaced the spark plugs and coil packs to no avail today.
It's booked in with the problem hunters next week, I'll pass on what they say.
Thanks for the input guys, but from what I understand the NA fixes aren't usually a problem with NB's?
Thanks all

They can be, afterall they are the same engine bar a few minor changes made over the years.


What did the old spark plugs look like when they came out? that is usually a good way to get an idea of how the engine has been running.

Were they white and crusty? black and sooty? wet?

Well strangly all three states you just described. wet around where the spark lead goes on (oil it seemed), black and sooty around the porcelain, and white and crusty on the tip

Jdown79 wrote:Well strangly all three states you just described. wet around where the spark lead goes on (oil it seemed), black and sooty around the porcelain, and white and crusty on the tip

Wet where the lead is would just be an oil leak from the rocker cover, pretty common. Can cause some misfiring if there's a lot of oil, but usually not that much of an issue.

Black soot around the porcelain is probably just dirt mixed with some of the oil that's leaked.


The main part when you "read" plugs is the tip, the bit inside the engine, since this can tell you some things about how the engine is running.

A white crusty end usually suggests an engine running lean (too little fuel) or that the plug is too hot (I think I get the hot and cold plugs mixed up)


Most MX5's run a little rich from the factory, so usually the plugs come out black and sooty (indicating a rich mixture, a bit too much fuel)

Wet and black means VERY rich (usually so much that the plug won't fire)



Basically, with a white crusty plug (all 4 the same?) i'd be making 100% sure I have the correct plugs for the car (note: NA6 and NB use different plugs)
If the plugs are correct i'd be looking into reasons for it being too lean, blocked fuel filter, stuck injector, etc.


Since you've already changed the plugs (presumably with the right ones) i'd drive the car for a few days and then pull them out and have another look at the ends, that should be more accurate to gauge since you'll have started with a clean plug.

Guys, Could a dying CAS be the culprit?
I'd definitely change out the coolant sensor at back of engine. It costs like $22.

I'll go about checking the plugs tomorrow morning, get back to you on that one.
On the oil/rocker cover seal, I actually replaced that not two weeks ago. Should there be any oil at all getting in the spark chamber?