MX-5 OC Forum

I'm having starting problems. Sometimes it is fine, other times it hunts about a few seconds then stops. On others its fine for a few seconds, although with slightly elevated revs. Then stops. 

When it stops it won't start again until you turn the ignition right off and on again. No faults are stored.

I have swapped the afm, throttle body with idle valve, air valve, pcv valve, cas, ecu temp sender and main relay.

I realise the ecu can't flag up coilpack, but I have wideband and if anything I'm there screaming 'put some fuel in' when it's failing as I can see it dropping away. It never ends up in a rich state like I imagine failed ignition would cause.

When it first started playing up it was occasional and started second go. It seemed to get worse as the weather did (6 weeks or so back now) And very progressively. Hand in hand for weeks so I just assumed ecu temp sender. I get the briefest of full rich readings before its pulling the fuel back out. Unless it is going to start propperly that is.

I have added a fuel pressure gauge and it all looks good. If anything a bit high, as I have a 255hp and there known to overwhelm the stock fpr.

Last thing I did was the cas and clean up the main relay. It seemed to go from 20 attempts to start, back to second time lucky. Like I'm close. Then the new air valve dropped water on my alternator killing it. Fitted a new alt along with my old valve and a different main relay and it's gone rubbish again. The valve swap alone did nothing to help.

I think it's related to that fuse box the main relay sits in. There is nothing in one though, I have one here in bits. Working round there seemed to make the difference though. I was quite convinced the main relay was just not seating right, So needed the ignition off and on again. Apparently not though.

I'm still working on it. I have emb so will have to start logging this and that shortly. It will do timing and fuel and airflow information. Maybe battery voltage..

Anyone know what might be tripping up my car but not producing fault codes? Its something the car needs resetting to fix.

It's starting to do my nut now. It is becoming a life changing experience. I have never failed to fix anything, and I'm mr fix it. Yet I have visions of selling it for peanuts and buying a car mazda can fix for me.

There is a fuse in the interior fuse box that powers the permanent live to the ecu (can't remember which one but it's a 10A red one).  If this is loose it won't store fault codes (and won't start).  

Hello :)

I see the one. The room fuse. It gets its juice from the btn fuse in the other fuse box. It's a 10 as you say. The only two things it does for me are ecu permanent live and thankfully my interiour lights. I know they work, but will watch them as a means to judge supply quality :)

Thank you

friendly1_uk:

The room fuse. It gets its juice from the btn fuse in the other fuse box. It's a 10 as you say. The only two things it does for me are ecu permanent live and thankfully my interiour lights

...and the lighter socket and the stereo (I think).

I don't think the lights are a reliable method as it only takes a few milliseconds of dropped voltage to clear the ECU memory, which would be completely invisible to the naked eye watching a lamp. The best method would be to use a multimeter to check the resistance between the live side of the fuse to the ECU connector (so, through the fuse and connector and down the wire) - this should be very low, a handful of ohms, maybe a few tens. Then have a wiggle of the fuses and connectors and stuff and you should see the resistance change significantly if there's a problem.

It's a pitty I don't have a data-logging multimeter. Mine just have peak hold. They won't record a minimum. My lamps are leds though so have a better chance of seeing a fault than incandescent one's. It is far from ideal though. 

I'm not sure the ecu has anything to report as supply issue's are not on the fault list.

I might be close now. I have found I can pull the main relay. Switch on the ignition. Take a piece of wire to the relays socket to do the relays job and get a high degree of reliability. In fact of the 10 or so attempts this way, only once has it not started. I was basicly getting rained on heavily and perhaps not following procedure propperly. I have however tryed two relays and tested both. Sticking in the wire I sometimes hear the icv and injectors click and see the fuel pressure flick. Other times I don't. If I don't I try again till I do.

I will get the fuse box to pieces, although there is little inside it. I can't see why, but fudging with it has real effect. 

My fuel pressure gauge seems faulty. Changing it's orientation in my hand see's the readings change massively. While I trust it's indication of pressure, I can't trust it's actual reading. My fpr makes a racket though, which I didn't expect. The gauge actually says there is pressure in the line with the car switched off, but I didn't realise the filter had a non-reverse valve within it to accomplish this.

I'm really not getting my bit done here. I just keep thinking about it but spending little time on the car. I wish I knew the ecu better to establish what it needs to start, but not to run. Once it makes it past a few seconds it's 100%

Thank you

As a general rule of thumb, a human will only notice an LED flicker of over 20ms, and a lamp flicker of more like 200ms. Flash memory will fail after approximately 0.1ms, which is far below what can be appreciated by the human eye.

The reason I mention fault codes is that, if your problem (not the ECU supply, but whatever is causing the starting problem) is being logged as a fault, it will be cleared the moment the ECU loses power, even if for an inappreciably small period of time.

Four more trys since my lost post. Each time with my bit of wire, and it's worked. I would of been lucky for it to start propperly just once with the relay in situ. Something around here is not right. I know the relay does do it's job as it powers so many things.

The fact that I get a clean start or no start till I switch off and start again is on my mind. It is re-seating that relay. When I use the wire, I don't allways get a satisfying click from the head area. If I don't I pull the wire straight out and try again, until I do get a nice click.Then I get my seemingly 100% start rate

I will try it with a poor click. Perhaps it's an injector.

A few things get there juice from this relay. The switched live to the ECU the EMB and the Wideband. The Injectors, ICV and Carbon Can. The Cam Angle Sensor and the Fuel Pump through the fuel relay normally. My car uses yet another relay right by the fuel tank to control some 4mm power cables for a bigger pump. Unloading the circuit a bit for the wideband to occupy it instead. That leaves the Coilpack and Ignitor getting there juice straight from the ignition switch instead. While the Air Flow Meter and most other sensors are straight from the ECU.

Edit: Perhaps I can test the powers continuity with one of my meters. If one belives 0v is higher than -12v I can use it the wrong way round

Worked until I tryed to start it with the ac on. Then worked again once I turned it off again. However, my ac has been taken out, I only switch it on to bring in the radiator fan. I see no connection. Days later I tryed again with the ac switched on. Again, failed to start. Still I see no physical connection except, And I'm guessing here, A bit more air and fuel as it would be expecting the ac pumps load to be there. However switching it off did nothing. It instead showed temperature related stuff again. Getting better and better as it tryed and tryed again. I left it 5 mins, then came back and it started fine. This was last night. Today I have poked at everything. No fault codes. Relay refitted not the wire. Started 2nd time but ran lumpy. It has never been lumpy but kept going before.

I have a drawings here at odds over some sort of inhibitor circuit. Location 1V of the ecu. Madraki's pics show it might be a blk/blu or a brn/wht and on an automatic this wire gets 12v during cranking, while on a manual this wire goes to both neutral switch and clutch switchs, that find ground. This seems fine, as both auto and manual ecu's are different. However, the haynes looks different. It gives no ecu pin location numbers, but has both blk/blu and brn/wht wires coming off at different points. The former being labeled as inhibitor and the brn/wht going to the neutral/clutch switchs. Separate circuits.When I concidder what it's trying to achieve I can only think its starter inhibit, but thats actually a switch on the clutch pedal sat between the ign/sw and starter solenoid, it goes nowhere near the ecu. I have no idea what this ecu inhibitor wire does, it just points off the page. Besides which, my 89 roadster will crank in gear anyway, there is no inhibit switch. Any thoughts?

Was there once a story about an a/c switch causeing running problems? I don't see how, but half remember it. I know someone has been in my center console for no apparent reason, and it was around the time the fault first appeared. It was only out my hands for exhaust work though. Nobody has any reason to of been under my console, yet they have. I have to wonder...

Hi Friendly1, if by center console you mean where your gear stick is then they woukd have probably had to go through there to unplug your wideband o2 sensor assuming that they replaced that section of your exhaust as you can't undo the o2 sensor without unplugging it

Did you swap out the relay ? You'll find that most relays start to fail rather than just stop working (as is with most electrical problems) which is why electrical problems are always dificult to find!

Have you also checked for vacuum leaks as that can often make your car not start although to be fair that would normally make the ecu over fuel and not under fuel..... If it was me I'd go back to the point where the problem first started (after your exhaust work) then the obvious would be to check that the o2 sensor is tightened up properly and more importantly connected properly with no damage to the cables.

Hope you get it sorted as I know what it's like, I had running problems with mine and ended up buying another car so I could swap parts out to find the fault so was without mine for about 3 months!

I have been looking for another car! lol

It is the that bit that was removed yes. It's nice to hear an explaination :) There is little to achieve there as I have the harness under the car adjusted correctly, but perhaps they dived straight in. They did have to call me to be told how the downpipe came off.

I have swapped and looked over the relay and yesterday poked at everything. That caused a poor start that surprisingly kept running. I disturbed something..

If slapped me in the face that my ecu cant see the afm, the Emanage Blue is in the way.  A fault in the wires from afm to emb might leave the emb telling the cars ecu there is low airflow rather than a missing reading. This in mind, I just hooked up the lappy to log the emb. Car worked each of the 10 times I tryed it which is testament to me poking at something I believe(for today). I found something odd though. If I switch off, wait a few seconds then put the dash lights back on, the engine kicks. Every time. I have never noticed this before. A bit of fuel must be present still that a spark is catching as the cars ecu gets its act together. A situation I would not expect as the ecu never looses power completely. Another car would be very useful right now! :)

Would a vac leak leave it rich? I thought it would suck in air that the afm never saw, so never fueled.

I'm flying.. Late as usual. Hope it starts! lol

Thank you.

Going to quote this post over from another(less helpfull)site

Well this is taking far to long. I'm going to have to quit being a sparky at this rate.

It's looking like the emb or wiring between emb and afm. Probably the emb, although them very wires were moved about just before the fault first occured.

Plug in the laptop, and it seems much more likely to work. That aside, the emb has a green light that turns red when there is a fault. Having dug it out from under the carpet, the light knows. Perhaps 20% of the time the emb powers up with a flashing comms light(comms cable is connected permanently)And a green light for go. Car works. The other 80% there is no comms light, and a red light not green. This means it won't start. If at this point I use a laptop, It won't say that I can't connect so I know there is some sort of handshake. However comms are down. I can't do anything, except the afm real time display is functioning.It says about 4.9v is coming from the afm, and it's telling the cars ecu something extremely similar. Like 4.93V

Now the afm voltage should rise as the airflow gets lower. A little counter-intuative I think. With the engine not running and the emb working that 4.9/4.93 is more like 3.8V in and out. Once started and a bit of airs going through(doing 945rpm in this instance) the afm sends the emb 1.78V and the emb tells the cars ecu It was 2.11V which is actually a blag. It says 2.11 because of my bigger injectors. It tells the ecu 2.11 as thats(pretending to be) less air, so the ecu will fuel less. This is how the correction for bigger injectors is made. It's all good.

So.. Back to the red light and why. Well I dunno, but on the later version it could flash fault codes, nearly all of which say afm issue. Which I'm buzzing around all the time already. I can't believe for an instance that (both the) afm are actually sending this 4.9V to the emb. The emb must be making it up. Either because it heard nothing from the afm, or, seeing as the comms are down, the emb is screwed.

I feel a lot happyer. Just gotta give the emb it's own 12v to run it independantly of the car. Lets see if it's consistant sat all on it's own. I expect a red condition to occur through lack of active afm on each try, but the comms will remain inconsistant.Thus... new emb needed. Or a repair to mine.

And just for here... The kick is to be expected really. Odd I never noticed before.

Thank you for your time and effort. I will of course let you know :)

It's the Emanage Blue. Last night it powered up the comms light but not the status one. That was a first, so I pulled it out. It can't decide what to do even sat here just getting 12v. It's broke..

Looked inside. Wiggled everything wiggleable. Cursed all the curseable bits. Examined and probed the voltage regulator circuit. It looks a lot like it's all on one chip, with a handfull of support chips such as op-amps and the transistors that do the heavyer stuff. I was thinking the comms chip might be separate, and the alternating fault down to power regulation. It appears not. One chip that can't make it's mind up is beyond my talents. At least I know now though :)

Well... As I poked about the other night the status light changed from steady state to flashing. It was after I moved the switchs about. I was not 100% sure, but reading my old posts varifys it. After that it still did this not starting up right thing, so it didn't really matter. Thing is, it works now. 13 out of 13 trys. Another of them annoying magic touch jobs. Dare I put the cover plate back on lol