Engine dies immediately under heavy braking...
 

Hey there folks,

Took my Miata to the track for the first time today.
The idle and timing were reset in the morning (800rpm and 12 degrees).

Had a strange thing happen during my 3rd session... as soon as I stepped on the brakes the engine immediately died.

The clutch was still engaged.
Over half a tank of gas in the tank.
Reviewing the video shows that the car immediately died when I got onto the brakes.

Of course, this was the one time I decided to see just how much it would take to get the ABS to actuate, i.e. I was braking much harder then I had before.

The car started up just fine. Ran well for the rest of the day. Didn't die out on me the one other time I hammered the pedal.

If I didn't know any better, I'd think there was an inertia kill-switch on the fuel pump or something. This was the hardest I'd ever braked in this car, and the effect was immediate.

Searches on the net came back empty handed. It's not an issue with the car dying when moving at speed with the clutch in... nor do it seem that it would be an idle control valve issue.

The effect was immediate. There wasn't any stumble or anything. Just the tach starting to slow and then ZERO and coasting. Oh. And a locked tire at one of the passenger corners.

Looking for ideas from you folks. I know better than to cross post this over at M.Net...

Thanks in advance,
Joe

PS. Forgot to add, it's a '94... and it looks like I need to check fuses and relays under the hood tomorrow in the daylight.

PPS. I use a single key in the ignition while at the track... so that would/should eliminate any issues with the ignition switch from swinging keys/etc.

I seriously doubt you're braking any harder than others on slicks and cobalts, so I doubt it's some inertia switch, unless a previous owner had a shitty alarm, but it'd probably start beeping or something. And it should be deactivated once the car is on.

Have you verified the check valve in the booster hose is good?

There is definitely no inertial kill switch in the factory design of any NA or NB Miata.

This, to me, smells more like an intermittent wiring connection somewhere. Not that any of us will be of any help at all diagnosing it :D

I'll check the check valve... thanks!
The previous owners installed a few wonky aftermarket things (keyless entry, random buttons which are no longer connected) but other than the occasional door locks not working, things have been fairly trouble-free.

I read where other vehicles had this issue... something about ECU blackouts if the voltage dropped too low when the ABS kicked in... has anyone heard of this in a Miata?

Guess I'll check my voltage as well...

Did the engine start running again once you released the brakes?

No. I had to turn the key to get it to start.
I've only had the ABS actuate a handful of times (two or three) on the street...

A tire locked up... pumped the pedal... and when I got onto the gas pedal realized the car had stalled.

Here's a clip of the incident.


35 laps that day... and this only happened once.

I'd check the brake booster check valve too.
I've a friend that has the idle dip really low under braking, checked every fuse and electrical connection and nothing. He's about to check the booster valve too.

should be rally easy to do

Thanks folks.
I haven't had the Miata nearly long enough to know the ins and outs of it yet... and I haven't seen anything like this in the last year of forum-browsing.

Look at how fast the RPMs drop. The drive wheel(s) locked up, thereby stalling the engine. RPMs never drop that fast, it must have been from the rear locking up. After you locked up and stalled, maybe in the heat of the moment you put the car in neutral, which would explain the motor not just restarting when you released the brakes. I can't tell in the video if you went to neutral when the tires locked up, but that would explain why the motor didn't restart itself.

Also, it seems like this car locked up the tires way too hard and long for an ABS car. It was a solid second of lock-up, much more than I have experienced with any other ABS car.

I checked the rear video camera... only one tire locked up.
I'm assuming the wheel locked since the ABS didn't work after the car stalled... but that's just a guess on my part.

The car wasn't in neutral... and rather then attempt to bump start it, I used the key to get running again.

Didn't want to lose any precious momentum in the event I needed to coast off the track surface.

A buddy has suggested I check the main relay as well as the CAS and ECU plugs for corrosion, especially since the check engine light did not come on when the car stalled.
Nor did the ABS light come on prior to the tire locking up.

Hopefully one of these things will lead me to a smoking gun.... never had to chase down electrical gremlins before, hopefully this one will show itself easily.

I'm pretty sure Dustin nailed it on the head. Rear locked up, stalled engine, the sound of the locking rear wheel proved that. That's the only way RPMs could of dropped that fast. Not good for the engine I'd imagine either.

Perhpas, but not so sure. If there is a short that shuts off the main relay, wouldn't the tach die? It also looks like the tach dies (and, presumably, ABS also dies) and THEN you hear the locked brakes.

Also, if it wasn't a shorted electrical system, as soon as the brakes were let off and the rear wheels could roll again, the engine should have been alive again, right?

Finding intermittent shorts stinks. Good luck.

I think he was saying he went clutch in when the brake locked up, and he hit the throttle before releasing the clutch and noticed the car had stalled, and didn't want to bump start so used the key.

If OP can verify this it will help with troubleshooting.

Keith

Keith,

The clutch was out when the tire locked up.

The tach/car died first, after reviewing the video. No lights on the dashboard either.

As soon as I hit the brake pedal the tach drops... and then the tire locked up.

I can't heal/toe anymore... so I brake first, then downshift... no clutch involved during the initial braking (which is when this happened).

Got it, the extra information will help with diagnosing the problem... looks like a strange one! Sounding more like an electrical problem now, when you restarted, did you cycle the key to key off, key on, start... or did you go directly from key on to start?

Keith

Off-on-start and continued driving.

Thanks for the input!