Diagnosis needed: Car stalling on start up

[:Dunky]

Guys and Gals

Need some help diagnosing a problem with my 10AE. Car has had intermittent problems on start up for a couple of weeks. Car will turn over then idle very roughly, feels like its missing. If i let the car rest a while the problem disappears.

Now the car refuses to start. The starter motor is doing its job and ticking over, however the car barely starts, and if it does fires once then dies. Feels like the engine is choking. At first i thought it might be a spark issue, but now thinking it may be the fuel pump.

Any tips on things to check, simple mechanical things to do to see if i can diagnose the problem?

Cheers!

[93_Clubman]

Fuel, air & heat (combustion triangle) - see Rotary's excellent posts:

Have you checked for spark?

That way you know for 100% certainty that you have an electrical/Ignition fault.

If there is spark then its almost certainly fuelling related unless the plugs are foul or spark is not regular.

Can check spark a couple of ways.

By putting a new spark plug on one of the leads and making sure its metal (side or base) is earthing, anywhere there is a good earth connection(usually an exposed bolt attached to the chassis or the inlet manifold).

If your unsure and have a multi-meter then check it, otherwise you could damage something.

The other way is if you have an old ignition lead with end exposed, that will work without a spark plug but you must keep a 1cm gap between it and earth.

Then just turn the ignition and watch or listen for loud snapping noises.

Just need to be certain there’s no fuel leaks or spilt fuel anywhere as it may ignite it, so if there a strong fuel smell then don’t do it.

Or without checking for spark, you can just get a can of Starting Fluid, from any Autoshop, and spray a good amount in the throttle body, with it on full throttle, then try and kick it over.

If there is any kick or it starts briefly, it has spark and no fuel, no kick at all, will most likely mean no spark (so electric or ignition issue).

Another easy way to check if it's fuel related is to turn the engine over a few times & then immediately remove a spark plug to see if it's wet from fuel.

Air might seem to obvious, but it's worth checking there are no major restrictions, especially if the car is a recent purchase or has been worked on recently. Just after I bought my old NA6 I found an old cloth had been sucked into the airbox snorkle.

[Okibi]

Check the connection to your air flow meter and that the hose hasn't popped off your intake runner/cam cover.

[:Dunky]

Update on problem!

Upon closer inspection of spark plugs found number 2 was soaked in petrol, with a mild accumulation of soggy carbon crap on the tip. Lots of fuel in the cylinder which doesn't seem to be burning off.

Ideas so far is that the fuel pump may be stuffed and pumping too much fuel through;
injector number 2 may be dead or malfunctioning;

even once the spark plug was cleaned the car refuses to crank properly.

Also there is a strange hum now eminating from somewhere, cant pinpoint where. I would say it was a fuel pump hum, but its only audible in the engine bay.

Car has been taken to local hospital and admitted. Will let ppl know the outcome.

[93_Clubman]

did you check for spark?

failed/ failing coilpack on nb8a is a reasonably common problem.

visitors allowed?

[:Dunky]

Update!

Problem solved. Plugs deemed shite. Number 3 lead showing very high resistance. Replaced leads and plugs and car is running smooth again, still not clear if anything else is wrong, but the car is driving again.

[93_Clubman]

Bruce will be disappointed - he missed the chance to say 'plug leads' & be right.

Deteriorated plug leads are even more common problem on MX5s, but usually not to the point of precluding the car from starting. Usually they cause intermittent or more frequent poor running & get replaced as a result.

[:Dunky]

So seems i spoke to soon.

Since bringing the car home from mr mechanic its played up several times. Most noticeably the car wouldn't start the other night after i drove it home (5 minute drive from my office). Car finally fired and turned over but was lumpy as crap, undriveable. Came home 2 hours later and it started fine, running absolutely perfectly. Its been lumpy on a few start ups, literally 2 or 3 since i have had it back.

Taken it back to the mechanic and they cant fault it. Only suspicion is that a coil pack is dieing, however from Mazda each coilpack is close to $800 which isn't worth it unless i specifically know that it the cause of the problem. Mania has coilpacks listed at 510, however im not sure if that's each or per pair.

Any suggestions or input?

[Uncle Arthur]

I've suggested it before and I'll do so again - check the (green) engine temp sensor at the back of the block. When cold it triggers the "choke". When it is playing up it reads the same voltage (or lack thereof) as if the car was cold and dumps in too much fuel. When it isn't plugged in the car won't even start.

Replacing mine fixed all sorts of intermittent problems.

[Insurgence]

Hi,

I've had coil pack and fuel pump issues before.

If its an intermittent problem then I would say likely to be coil pack.

lumpy would indicate that one set of coil packs is playing up. next time it goes lumpy, pull out the leads from one set of coil packs and see if it has any effect. either you pull the ones from the non-functioning coil pack or from the functioning one.

if you start it and its lumpy there is also a trick to "convince" the non-working coil pack to work:
pull out one lead from the non-functioning coil pack (by now you will have worked out which one doesnt work), then slowly plug it back - it'll start arcing and magically work. hey. worked for me anyway. you'll need to do this every time it doesn't function. use at own risk