Help...Engine turns over but won't fire up

[MADzaMIKE]

My 2002 2.5 NB has me pulling my hair out...which is becoming a problem, as I don't have much left to start with. Story so far...

Car was stood, uncovered for about 8 weeks in November/December without being used. On trying to start the car battery was flat...no problem, off with the battery, out with the charger, all should be good!!! Well...in theory!!! Car would only turn over and wouldn't fire up at all. This has never happened before (always started first turn of the key) so checked all fuses, fuel, any loose connections, sprayed around plugs etc with WD40...still no joy, car turns over but won't fire up.

Tired over several days and nothing, didn't have time to investigate further so put it's cover on and left for another couple of weeks. Had a nice sunny (but blinking cold) day so took cover off and tried again after charging the battery again...couldn't believe it, she fired straight up. I left it running for 20 minutes or so and decided to take hwer out for a run, but was worried she may not start again, so I switched of the engine, went to start her again but to my horror...she wouldn't, and hasn't started since.

I have tried several things after looking at forums. Firstly i cleared out the garage (no small feat) and pushed the car, single handed (and with one or two minor 'cuss' words), up my sloping drive, and into the garage. I have stripped out the passenger side and wasn't surprised to find very wet insulating panels in the footwell underneath the carpet...aha, thought I, this is the problem, a wet ECU. I dried everything out overnight with an electric heater, but couldn't get behind the plate to the ECU, so put a dehumidifier in the car for 2 days to see if this helped. It didn't, so I purchased nut removal tools and eventually got the plate off. On inspection, the ECU doesn't look like it has any signs of water ingress, and no damp at all. Darn it...I though I had got it solved.

I don't think the transponder in the key is the problem, as I have both keys, and car won't start from either. I now suspect it could be the immobiliser, after reading up on it, apparently the immobiliser symbol on the instrument cluster, should light up when switch is in acc or on position, and go out once car is started...it doesn't light up at all. I have tried the double lock procedure and it does light up to indicate that the procedure has worked and it is double locked, so the bulb is definitely working.

I now have a car that won't start and am missing her like mad

If anyone can help me to sort this, I would be eternally greatful, is there anyway I can test if it is the immobiliser, or the ECU, I am aware that they are paired with one another, and the keys, and that I can't just replace one or the other.

Yours hopefully...

Poorly MX5 owner

[Mazda Mender]

Damp can give issues to a ECU.....but have you tried a OBDII reader yet before we condemn the immobilizer or ECU...it could be a faulty sensor like the cam or crack sensor at fault...
M-m

[seagryhill]

I had a similar problem recently. The engine would fire but not idle and there was no response to throttle. The only way to deal with it is to work through the symptoms in a logical sequence. As M-m suggests, always start by looking for fault codes. If you have a MK2.5 you can buy an OBD bluetooth connector (for not much money) that will drive freeware on a smartphone or tablet. In my case my car is older so it only had the diagnostic block so you need an LED. If you have codes - fix them (but make sure to clear first and then retest otherwise you could be seeing old problems). If you don't have codes - move on to the next bit.

In my case I wasn't sure if it was fuel or ignition. Looking back it was all about what I saw on the plug leads. I put a timing light on each lead and I could clearly see a spark on all 4 cylinders but they were very erratic. That pointed to the fact that something was wrong in the timing system. A process of elimination eventually tracked it down to a failed crank angle sensor. Once that was changed it ran like a top. It's worth noting that the crank sensor is exposed at the front of the engine near the pulley and so gets all sorts of stuff thrown at it. In may case I suspect a leaking steering rack (that's since been replaced) had coated the sensor and caused it to foul up. Removing the power steering belt makes changing it a bit easier.

As an aside I first tried changing the camshaft angle sensor - because that is really easy to do. A new unit was used and in fact, made the problem worse. With a new aftermarket CAS in place the car would not fire at all. With the old unit in place it would at least fire a bit. Even after I replaced the crank sensor, putting in the new cam sensor stopped the engine firing at all. I guess looking back, the fact that a failed new unit changed the problem told me I was on the right track.

If it's not ignition then changing FP relays and the fuel pump are not too difficult providing you're a contortionist with very small, very strong fingers.

These problems can be very annoying but you just have to stick with it and work the problem. Let us know how you get on.