Engine shutting off after 40 miles of constant highway speed

[damianm.config]

1. My model of MX-5 is: 1991 Eunos Roadster 1.6L
2. I’m based near: St.Albans
3. I’m looking for technical help or recommendations on: Engine shutting off after 40 miles of highway cruising.

Hey, I’m a new and proud owner of my first ever vehicle a 1991 Roadster but on the day of purchase, approximately 40 miles of highway cruising with overtaking but keeping to the speed limit, the engine shut off completely, pulled over to the hard shoulder and tried to start it back up. The engine chugged while trying to start it up again but would die down slowly again with very little power if left in gear. (Important: coolant temp was at running temp/half way) We tried getting help from the seller as he owned a flatbed, but then after 1-3 minutes of leaving the car alone it fired right back up, rushed off the hard shoulder and got to the near stop off area. Confused, we said we will keep on pushing. Again, same problem at roughly 20 miles from the first break down. Cooled down the car for again 1-3 minutes, fires right back up. Then we finally decided to get a flatbed tow from the seller and some money back.

Now, I may work at a garage but I’m still young and need experience (I’m 19), I have been told that the ignition coil pack is failing when the engine heats up to running temp from the seller as it is a common failure with it’s side effects being losing spark, which makes sense for what happened. However I have also been told it could be the Temperature sensor for the engine or even a intake temperature sensor? How do I diagnose this problem for either of the possibilities.

Also note: I do have a minor oil leak from the rear cam cover, could the interference of oil on the coil pack/ cables/ sensor cause problems at running temps?

Let me know what you think.

[Maya]

I can’t offer any technical advice but that sounds quite frightening. Once you get the problem fixed, I think you will love driving the NA. It’s a really nice car to drive. Good luck.

[ast]

The IAT is part of the air flow meter (AFM). I know some people have taken this apart to attempt repairs, but its basically non-serviceable. I purchased remanufactured AFMS from US sellers, which is an alternative to a hideously expensive new part or a risky scrapyard part.

But there are many possibilities. I had a 93 that would just randomly cutout during driving. The solution was a faulty fuel injection relay in the main engine bay fuse box. A previous previous owner had fitted a relay from a Ford Probe, which is basically the same, but what he put in was in a shocking state. The next owner had all kinds of running issues. with MX-5 specialists telling and billing him for a £500 cam angle sensor, which of course never fixed it. We diagnosed the issue accidently, by knocking the relay, and then getting the engine to start.

Hopefully you are getting a big discount from the seller, otherwise, why are you trying to fix it.

Fault codes are very limited on these early cars. Someone was selling a b ox of tricks that could “read” codes and I think put the engine into various test modes.

Otherwise, buy this:

Diagnostic Connector manual / book, Mazda MX-5 mk1 & mk2

The box of tricks I think is no longer made

https://mx5tech.co.uk/diagnostic-tool

An oil leak back there can cause electrical issues, especially around the connectors. When the coil packs go bad, its usually one of the coils, leading to a lumpy engine. Sometimes you can see arcing damage caused by the ignition leads. Some of these imports came from Japan with good quality aftermarket items, using the same bits as Racing Beat’s leads, but the design can result in arcing damage.

Does the engine look undisturbed, ie stock, or has someone been modifying, or worse, done something like put a turbo on it, then put it back to standard. Aftermarket mods like this result in cut wires with bullet connectors/ scotchlocks in various places. They might be fine, but it just brings in another possible failure point.

Many of these imports also have now quite old aftermarket immobilizers that can lead to odd behaviour. In the previous car, to eliminate the immobiliser being a a cause, I stripped it out, to put the wiring back to stanard, as much as possible. In my other import, the 20 year old immobiliser just died one day.

[damianm.config]

Thank you for the reply, the engine is as stock as it can get. It has some new shiny bolts for the cam cover which I suspect could be an issue if work has been done under it, But even then the engine runs and sounds healthy with some minor hydraulic lifter tick. Along side it there is a new shiny vacuum pipe going to the right side of the cam cover. Some questionable defects would be the vacuum lines, although have a clamp on them, are easy to remove with the clamps. And on the left side of the cam cover, nearest to intake manifold, the plastic connector going into the cover is fairly loose and feel like I can take it out by pulling hard enough. Overall, no suspected mods seem to have been added in the past as either the nuts and bolts are rusty, and all the wiring looks in good condition.

AFM has a high chance of being part of the problem, as one person which I met at GoJapan! Brands hatch told me that the IAT could be faulty, will check by bridging the diagnosis box connectors. Additionally, rusty bolts, most likely hasn’t been touched since it came from factory.

I haven’t checked the relay for the fuel injectors, hasn’t come across my mind really as it runs perfectly fine unless slight burbles from the exhaust could be a sign? It does have an aftermarket rear silencer so could be just that but I’m no expert yet. Will check that asap.

This import has no immobilizer in it, I have checked what type of immobilizers there are but none seem to be present with obvious signs.

Yeah, I got a free towing and £150 back, paid full price at first cause it was cheap (£2150) and honestly I did not know where to lower the price at, has new pads and discs + the mods which were not listed and everything was good. Until it shut off on me haha.

[ast]

The cam cover bolts and breather pipe are chrome plated, so tend to always look “new”. The pipe from the cam cover into the inlet manifold is for the PCV valve, a one way valve. It can be just pulled out. The rubbers can harden up over the years.

PCV Valve, MX5 Mk1/2/2.5

When disconnecting the AFM, DO NOT undo the two screws; you’ll break the connector. Instead prise the spring clip out.

The oil leak is probably a leaky CAS O-ring, which costs pennies. Replace that and the cam cover gasket at the same time (use genuine Mazda parts here, not pattern, the pattern gaskets don’t last long).

If you need to remove the coil packs, there is a third bolt in the middle at the back of the engine, a pain to get at. If its missing, someone has replaced the coil pack at some time. You can get a 10mm ratchet ring spanner on it, in between the coils. It has quite a long head.

Reading the codes involves getting a LED and a resistor, and poking the ends into the appropriate points on the diagnostic point, and with the ignition on, counting the flashes. The £7 book from Autolinkuk has everything you need. Miata.net has instructions if you want to go full DIY.

[damianm.config]

Just to keep this up to date.

I have checked the diagnostics port and have gotten two codes. 1 - Ignition Pulse and 2 - NE Signal - Crank Angle Sensor (Cam Angle Sensor). I have checked the documents for past repairs, cam cover seal has been replaced before purchase as well as some new coil leads, meaning either the spark plugs are failing or the coil pack. Regarding the symptoms, most likely the coil pack, but will check up on the plugs.

Crank sensor, have ordered a new one (Used/refurbished whatever) with a new O ring, and a timing light to get the angle to 10 degrees BTDC. Will make another reply to myself regarding resolving the issue. Fingers crossed it doesn’t shut down on the motorway like it did before.

[Roadster_Robbie]

I’d suspect the crank angle sensor from those codes. I reckon that you’ve cracked it.

[Scottishfiver]

If required, you dont have to buy a whole Coil Pack. You will note they come “twinned” into a common bolt on chassis, and you can obtain twin packs cheap off Ebay etc and cobble up your own, 30 minute job, saves a few hard earned wedge. Had a Mk1 for 15 years and did it twice…and Im a mechanical numpty. Im unsure if your is 3 or 4 pin though…..

My Mk1 was a ‘93 1840cc. I just accepted I was running around in an old car well past it’s sell by date, and thus I made up a wee get me home “kit” of likely culprits inc: A coil pack, just a couple of leads & plugs, fuse kit, a few bulbs, digital tyre , a can of pressure inflator, jump leads (dubious idea if battery is gel) Rad Weld….etc. Just as well Hiroshima’s electrics / looms were unsurpassed quality in the day.

It all went into a shoe box which I plonked into the deep bit in the boot..Hardly used it mind,….but…

Almost everything original still worked as new when I sold it. If you ever need to replace the battery, go Panasonic or summat, but steer clear of Westo. You’d be best curing the leaky cam cover….oil has been known to rot out the heat hoses that go into the bulkhead. Best use a Gates gasket or OEM.

Still miss the old fella….we had some adventures!

[damianm.config]

Yup.

However, just replaced my coil pack and the difference is quite impressive. Checked the spark plugs, Piston 2 and 3 needed some strength to take them out which was worrying in case they snapped, they all looked good however, no document of when they were changed which is bad. But the 4th Piston plug was showing they were old. Will get them replaced very soon now hopefully get a discount from my garage to save some pennies, torqued them down to the correct spec and it’s running nicer than before.

CAS, hopefully when I get that replaced and timed correctly, it will leave me with no headache

[damianm.config]

Yeah, I did get told I can buy them separately. However, I did not. Why? I don’t know.

But hey, the car runs way better now., and has a lovely new bracket for the packs. Just need the CAS to be replaced and timed, should be next weekend when I have free time, as I am only working till 12:30 on Saturday’s. Then will make a journey of 40 miles (Same distance when it first broke down) of pushing the car and cruising on the motorway to see if the issue is gone.

Then, road trips shall commence if all is good

[damianm.config]

Uhh yeah.

Replaced both CAS and Coil pack now the car runs way better. (Still need to do a little road trip to see if it has been fixed)

BUT

Only just now have I noticed after setting the CAS to 14 degrees BTDC (I wanted to gain some power as I am running 99 octane). As rev up the car or go to a stop when the revs come down they drop below the second mark before 1k rpm. I have been told that a way to fix it is by adjusting a screw on the MAF to adjust the mixture, but doing so I assume I have to adjust the idle speed too no?

Let me know what you think about this.

[Roadster_Robbie]

I’d try the base idle adjustment using the screw on the throttle body. Do a search for Base idle, I’m sure that you’ll get a good result.

[damianm.config]

Should of mentioned this, the car is running fairly rich still. Old spark plugs were pretty black, has made it run a little bit less rich but you can still pretty obviously smell it, whole spark system is new, so there’s only two candidates that could be making this happen.

Will adjust the idle tomorrow morning when I get to work a bit earlier, will have the car running at operating temps and will have my tools ready.

Also typo, I didn’t mean “MAF” but AFM.

[Roadster_Robbie]

The place where the screw would be on the AFM should be blanked off. If it’s not been tampered with in the past, I’d leave that well alone.

[ChrisH-0]

I had this problem last year, seems worse at speed and with hotter days. Then it got much worse, In the end swapped, coil And HT leads, no joy, reader said Cam sensor, Sadly it did not solve the problem, as an aftermarket sensor was fitted and made it even worse, Once a OE Cam and crank sensor installed (touch wood) no more problems Hope you get sorted, there seems to be a spate of this problem popping up atm

[damianm.config]

Yeah, after replacing the CAS it did help but just needed adjusting the idle as Robbie said. Has helped loads (thank you Robbie)

But. Today got this chugging sensation which got stronger and and the pauses in-between were longer, felt like what a cammed V8 sounds like. I had to clutch in, and when I released it was fine. Which has got me wondering, clutch mechanism? The pedal does have a deadzone before I start to feel resistance and it’s actually doing something, 1 to 2 inches maybe. It works perfectly fine but just at low idle speed in traffic it starts to struggle sometimes.