Engine cutting out

bruce · Postby **bruce** » Fri Apr 06, 2018 8:37 pm

......!

Locutus · Postby **Locutus** » Fri Apr 06, 2018 11:35 pm

Can't see how it could be coolant sensor. At most it will run rich when cold and pull timing when hot. I would rule that out, especially if the coolant temp gauge is showing something sensible.

Spark plug leads would manifest as a misfire, at most.

Since you've already swapped out the most common root causes, I suspect the issue is wiring related or a fault in the ECU itself.

Just some ideas...

Wiring, when car won't run:
- are cam and crank sensors getting 12V power?
- is the ECU getting a signal from both cam and crank sensors while the engine is cranking? This is difficult to test without an oscilloscope.

ECU:
- take ECU cover off, under carpet in passenger footwell. Look for rust/evidence of water damage.
- when the car won't run, is the ECU getting 12V power?

siege · Postby **siege** » Sat Apr 07, 2018 10:33 am

Checked for fault codes again (initially there were none)
With my diagnostic led.

I got a repeating series: long, long, short, long, long
Which I think means P0100: MAF or air temp sensor

I reset it, ran and the car several more times with it cutting out and the code hasn't come back.

It might be a red herring though because before I replaced the MAF I tried to turn it out with the MAF unplugged. Maybe that's when the code was thrown.

I guess I can try inspecting the wiring to those two sensors but ultimately I'm probably going to take it to a workshop on Monday.

93_Clubman · Postby **93_Clubman** » Sat Apr 07, 2018 11:48 am

In terms of NB8 wiring, from the factory the loom to the coilpack is a common weakness as it has no support just before it reaches the coilpack. As a result, the wires eventually deteriorate to the point where they cause issues due to poor or no conductivity. A simple way to partially test these wires is while the vehicle is idling to move the loom wires around to exaggerate any poor conductivity if present.
Additionally, note you mention that you've checked the fuel pump wiring, but did you check the fuel pump wiring loom & connectors at the inside top of the fuel tank as this is where the so called black-wire issue manifests itself. Following details the issue:
https://mx5cartalk.com/forum/viewtopic.php?f=55&t=46425
& as pics links in my original post above are US, the following shows the fuel pump loom connector, which plugs into the inside top of the tank, which is where black-wire issue occurs.
https://mx5cartalk.com/forum/viewtopic. ... 55&t=70835

siege · Postby **siege** » Fri Apr 13, 2018 2:02 pm

I ended up having to take the car in to mx5factory who had it fixed in half a day.

Ended up being that the very first thing I replaced was faulty from brand new. The crank sensor. #seethingrage.
I've sent it back to mania for a refund.

aviper4u · Postby **aviper4u** » Fri Apr 13, 2018 2:43 pm

siege wrote:Ended up being that the very first thing I replaced was faulty from brand new.

whaa! sorry to hear though glad you got it sorted

is there no way to bench test these sensors? (yes i understand sometimes they're intermittent failures via heat soak though still....)

siege · Postby **siege** » Fri Apr 13, 2018 2:46 pm

My invoice from mx5factory says they got at the ecu pins to measure signal from the sensors. I think it requires an oscilloscope so I wasn't going to be able to do that at home.

Chucking in a known good replacement would have given the diagnosis, but for that you need a factory full of mx5s right?

bruce · Postby **bruce** » Fri Apr 13, 2018 3:19 pm

That is painful.
Was the MX5mania part brand new or 2nd hand? I guess it does not change things though.

siege · Postby **siege** » Fri Apr 13, 2018 3:46 pm

Brand new non oem part. The cam position sensor they sell is direct oem replacement for what it's worth.

The American · Postby **The American** » Fri Apr 13, 2018 6:38 pm

I started replacing sensors and ended up with a faulty non OEM sensor that I purchased from MM. now have a second hand OEM unit.

But! Why did you replace it in the first place?

Postby **NitroDann** » Fri Apr 13, 2018 6:54 pm

Because the original issue was the first one was faulty I'd guess.

I havn't had a problem with MMs sensors but you can have failures from new just like thermostats.

It has always sounded crank or cam sensor related with this problem.

Dann

Locutus · Postby **Locutus** » Fri Apr 13, 2018 8:03 pm

Agree it was always symptomatic of cam or crank signal related.

Glad it's finally sorted. Go for a good long drive through the twisties over the weekend!