Starting problems - backfire out of intake
 

Hello all,

1991 MX-5 here with MS1 V3.0 standalone (RX-8 injectors, DIY mini SC)

Long term problem with my DIY MS - in cold weather (particularly below 5C) the engine is prone to backfiring out of the intake (say 25% of the time) once or twice during cranking and very inconsistant starting. I have tuned the cranking pw in all areas (down to as low as -7C) so that there is just enough fuel to start, sometimes it will start as fast as stock, sometimes the engine will "kickback" (almost stopping the starter) and sometimes the engine will take a good 5 seconds of cranking for it to finally chug into life.

I managed to capture a log of the intake backfire and it showed that the rpm spiked to around 2700 rpm at the point of backfire. It seems that there is something wrong with my MS as I have played with the cranking pw for a good 3 months now trying all different things.

It had all above problems before the SC and RX-8 injectors went in (just running MS as replacement for stock ECU)

I have changed the cranking timing from as high as 18 degrees down to 5 with no difference.

I have completed the Joe Perez improved spark output mod which did stop it backfiring out of the intake when the MS was powering up (keys first in ignition "on" position)

I never had this problem in summer - I would say anything above 15C ambient and the problem goes away completely.

Could you post a data log with the RPM spike? You may be able to cure the RPM spike with some minor hardware mods.

I have attached the log, it happens almost straight away, you cant miss it!


Attachment 20045

Try increasing the "hold ignition" value - this will hold off on firing the ignition for a couple revolutions.

Thanks for the response, this sounds like a potential solution!

Any idea why it is happening though?

My hold ignition value is zero at the moment, what does a value of 1 mean, 1 second?

It's crank pulses, not seconds. There's generally more noise when cranking than any other time.

You're gonna blow the welds!

Blow the welds?

I tried the "hold ignition" which caused the engine to turn over a bit longer than normal before starting, it still backfired out the intake though :vash:

What was this hardware mod option? I dont mind getting my hands dirty.......

bump anyone?