Sudden loss of power / returning
 

Been touring the alps for the last couple days. Today my naturally aspirated '95 NA had a couple hiccups. Sudden drop of power and then surging forward again after a second or two. Feels like the car is rubberbanding if that makes any sense. Happened maybe 6 times today
Restarted my msdroid logging after the last occurance - it appears to go full lean despite being under load - TPS >10% and MAP showing throttle open.
I've attached a couple edited megalog files - issue occuring right at the end of each log.

Fuel tank was half full, fuel filter changed 3 years ago. Fuel pump probably still original one and currently at 250.00km. Would a fuel pump on its last leg cause these issues ?
Probably not the fuel pump relay - that would just die and cause a no start right ?

I would suspect the fuel pump is the issue. If it is the original I would replace it.

I'd blame the Alps.
The same thing happened to my car driving over the high pass into the Engadine on the way to St Moritz.
Of course it was 1990, in a late 1970's Renault 20TS with a carburetor that never wanted to function properly.
So that does not help I imagine.
Although, the mechanic said he thought bad, dirty gas had damaged the carb's operation, and cleaned it out.
Maybe they just pump dirt with the gas in the Alps even all these years later.

OK, on a more helpful note.
My 2001 NB would basically cut out, and then cut back in, at WOT and high RPM while driving on the flat, when the tank was below 1/4, somewhere just below that line near the bottom of the gauge that is less than a 1/4 and more than 1/8.
(MS3PnP, Deatschwerks 660 injectors, FM turbo, 200,000 km on OEM parts)
So not really a fuel pick up issue, as the Miata is good to nearly the last drop even on the race track.

My best guess is my pump ran too hot when it was not cooled by the fuel at low fuel levels.
I replaced the fuel pump and filter, and it is fine now.

Maybe climbing the hills your pump is not getting cooled?
Maybe bad gas really has plugged the fuel sock, or fuel filter?

Maybe the tank breather is doing weird stuff at altitude, creating a vacuum to work against? (Back to blame the Alps!)

Yeah I'm leaning towards blaming the FIRMIN fuel pump in Caldonazzo or a clogged/shifted fuel pump filter sock at the moment.

The issue didn't reappear after filling up the tank again and driving home 700km on ze Autobahn. Also beating the little NA around the alps and steep mountain roads on the regular and this has never happened before.
Spent 3 x 20min and half a tank of fuel on the Red Bull Ring Moto GP track setup in Spielberg yesterday, lots of g forces in high speed turns and again - no issues. Even with the tank sitting at 50% during the last stint. So idk.

I'll probably inspect the fuel tank and pump assembly during winter hiatus.

The relay contacts build resistance as they wear and don't power the pump as well, too. This makes heat at the relay and is tough on the pump.

Could also be CAS wiring.

That's a long drive Caldonazzo to Speilberg. Sounds like a lot of fun in a Miata.
Not to mention a few hours of track time on the Red Bull Ring.
Havin' some fun, that's for sure.

Glad it cleared up for the rest of the trip.

You mean the internal contacts inside the relay ? Had assumed as long as the car starts and runs it's not the relay. I'll also put that on the list to swap then.
It was very foggy / humid on that day, so maybe related..

Yes, the internal contacts don't always fail instantly and completely. Often the heat from resistance due to carbon build up on the contact points creates the failure. If the relays were easily serviceable without destroying them, you could open them up and take a little sandpaper to clean the contacts before reinstalling and it would renew them.

I know everyone says the Miata tank is good to the last drop and if it is not, then the owner has done something wrong.

Way back when I had the same issues. After checking everything I gave up and installed a surge tank. With aero, big sticky hoosiers, 350 BHP (LS swap), and lap times 5-10 secs (track dependent) under the Spec Miata pole time reference, I never had another issue.

Maybe some can get the stock tank to work under heavy side load conditions. I was never able to.