Where to start diagnosing issue

[Escaport]

Weird intermittent issue.

Car is M20 2.7 stroker with W.A.R. chip and MAP conversion. I did not perform these conversions.

Car runs very well most of the time. Occasionally it just dies. I can be cruising on the highway or just driving in a neighborhood. Power just disappears and the engine dies. I'm usually unable to start again for 5 to 10 minutes, it just cranks but doesn't fire. It never happens on short drives, usually only on drives of 45-60 minutes or more. Seems also to happen when the ambient temperature is above 85F/29C, but not when it's colder. I've owned the car for almost a year, and this has happened last summer, and now the spring about 10-15 times. Always allows for restart after the 5-10 rest though.

How do I even test for this being so intermittent? My first thought is something fuel related as there is no excess fuel smell once restarting like there might be if it was a spark issue.

[efficient]

i had something like this happen to me. it was my DME relay

[bmwman91]

Is this the convertible listed in your signature, or a coupe? Or rather, is your battery under the hood, or in the trunk (if it is even possible for a cabrio to have it back there)? I ask because there is a fusible link that is very prone to corrosion and breaking, which powers the whole DME/fuel injection system, and it can cause issues exactly like this. But, ONLY if you have the battery in the trunk.

Otherwise, as efficient mentioned, it might be the relays. You can pop the covers off of them to get a look at the contacts, although they are cheap enough that outright replacing them is probably a good idea on a 30 year old car. It could be that the ignition coil is dying too. If a winding is broken, like a small internal fracture, thermal expansion could cause it to go open circuit, and once it cools it makes enough contact to function. That's a little more rare. Lastly, get a good look at the harmonic damper on the crank and make sure that the outer toothed portion is still securely attached to the hub (sometimes the vulcanized rubber ring between/attaching the two parts starts to fail). Lastly, a similar mechanism as what I described for the coil could be happening with the crank position sensor, so you can get an Ohm meter on it and check the resistance.

Does the car just straight up die, or does it run rough / intermittent misfires before shutting down?

[Escaport]

Is this the convertible listed in your signature, or a coupe? Or rather, is your battery under the hood, or in the trunk (if it is even possible for a cabrio to have it back there)? I ask because there is a fusible link that is very prone to corrosion and breaking, which powers the whole DME/fuel injection system, and it can cause issues exactly like this. But, ONLY if you have the battery in the trunk.

Otherwise, as efficient mentioned, it might be the relays. You can pop the covers off of them to get a look at the contacts, although they are cheap enough that outright replacing them is probably a good idea on a 30 year old car. It could be that the ignition coil is dying too. If a winding is broken, like a small internal fracture, thermal expansion could cause it to go open circuit, and once it cools it makes enough contact to function. That's a little more rare. Lastly, get a good look at the harmonic damper on the crank and make sure that the outer toothed portion is still securely attached to the hub (sometimes the vulcanized rubber ring between/attaching the two parts starts to fail). Lastly, a similar mechanism as what I described for the coil could be happening with the crank position sensor, so you can get an Ohm meter on it and check the resistance.

Does the car just straight up die, or does it run rough / intermittent misfires before shutting down?

Thanks for the help guys!

It's a coupe I picked up last year, but I still have the convertible. So battery in the back. I'll check the fusible link.

Relay replacement just sounds like fairly inexpensive insurance and should be done anyway, so I'll get on that. I'll check the damper too.

It's strange. Usually no rough running or misfires. Just be cruising in 5th at 70 and it's like I just turned the key off and coast to a stop. Main thing for me is sometimes my daughter drives the car and I don't want to leave her stranded somewhere.

[bmwman91]

Yeah, that sounds a lot like the issue I had with my fusible link, and others who also did. IIRC it is ~18" up from the battery on the smaller of the two wires that come off of the positive terminal clamp, and it is either under electrical tape or some heavy shrink tubing. You will need to remove that covering to get a look at it.