’87 TII, 92K miles, dropped in a rebuilt S5 about 400 miles ago. Still running the Rtek 1.8 moded S4 ECU. Original S4 CAS, ignitors, TPS, etc..

About 100 miles ago began to experience stalling after warm while moving. Tach goes to zero and won’t start immediately. But after just a few minutes engine fires right. Nedless to say, break-in trips have been short since.

Visually I c nothing obvious under the hood.

The 3 times it has happened, has been while rolling from a stop.
1st time while in 1st gear @ ~ 3/4 pedal pressure. Remaining times while shifting out of 1st.
Engine would not fire back up as it rolled to a stop in gear.

Last incident, after coming to a stop, I tried to re-start immendiatly and noticed the tach not moving.
But after about 3-4 minutes (hood still closed), hit the start again to c the tack moving and fired right up.

In the past, I've had the issue on a few instances with the tach not moving BUT ONLY while trying to start from cold. But after playing w/the the leading coil's connector and removing/re-installing the ecu's fuses, i'd mysteriously get spark and fire right up.

Just an FYI, although I don’t believe this to be the issue, I’ve removed both switches tied to the clutch pedal after dropping the S5 in. One of which was the reason for no starter at times.

 

have you checked the plugs just after one of those episodes?

what condition are you harnesses in?

check the CAS and check the coils (trailing especially) for sure. i must admit,

left field idea ... oxygen sensor???

 

just had another idea ... fuel filter or fuel pump

 

thx much for your thoughts. and to answer your Qs:

1- immediately after no. but after returning home checked the L on the front rotor and color is a light brown. Plugs installed new on the rebuild ~400 miles ago.
2- I consider the condition to be very good.
3- have spares of those which I will swap 1 @ a time. but want to have the current and original ignitors checked which means a trip to Mazda for the special tool.
4- will check
5- filter changed once about ~ 30k miles/15 years ago.

expanding #5, not sure how they would relate to sudden loss of spark...

 

tach signal comes from the trailing coil... so you're ether loosing the trailing coil, the crank signal (from the CAS) or the computer is going wonky... (or wiring between each of those of course)

 

My money is on the trailing coil. Usually the culprit.

 

While removing the trailing to inspect connections, the main relay's connection containing the wire powering the coils/injectors slipped apart. It's been a very long time since I disconnected that connector while trying to isolate same intermittent issue but starting from cold. Have not had further issues since fully connecting the connector. No more of the slight hesitation while accelerating on 2nd thru a corner.

Appreciate the feedback