Fault Finding: Occasional Non-Start

[Cameron 16]

Since replacing the tachymetric relay my car has been 99% good as gold, but occasionally it still struggles to start. It's only done it for a few seconds before but today it completely flat-out refused for a good 5 or 6 10 second long cranks on the starter, then randomly burst back into life. I had my "emergency starting kit" in the glovebox from my tachy relay woes, so connected that up but it still didn't work. It only ever seems to do it when I've driven it a couple of minutes from stone cold then switched it off, or when it's been warm but cooled down for a good 30 mins or so.

 

I'm kind of at a loss as to what it could be to be honest as it runs absolutely perfectly once it's started, even on very long drives. Engine sensors seem out of the question as it would only make it over / under-fuel, and while wiring could be a strong possibility I'm a bit puzzled as to why it only does it some of the time and never while driving along; I'm even more puzzled why hot-wiring the tachy relay didn't appear to work.

 

For a bit of extra info: it cold starts straight away and is a bit lumpy for the first 5 seconds or so (but they've always seemed to be?) until it clears out and idles, albeit quite high at around 1500rpm for the first couple of minutes, then once it starts warming up the idle drops back down to normal levels; when I drive around it's absolutely fine, pulls strong and clean, doesn't hesitate or stall and gets good mpg. The issue happens in 2 scenarios: 1 - I drive round the corner to fill up having started it from stone cold, it sometimes will take from a few extra cranks to infinity; 2 - I've driven to the shops having got it nice and warm then am away from it for 30-40 mins, same issue.

 

So.. any ideas what it could be? I don't know much about the wiring to the tachy relay but I've heard it can be problematic - could this be the cause?

[Tom Fenton 1,548]

Does the fuel pump run when it won't start? They are quite reliable but usually have not been changed since new so are 20 odd years old.

[Cameron 16]

Nope, no fuel pump. However, when I hot-wired the tachy relay the fuel pump ran but it still didn't go, or at least not for a good few long cranks!

[Tom Fenton 1,548]

Duff pump? Also check earth wiring for the pump which is next to the NSR tail light.

[Cameron 16]

I'm sceptical tbh as surely if it was a duff pump or wiring it would happen a lot more often and also when I drive it? It's worth ruling out though so I'll check it!

[Anthony 1,003]

Next time it fails to start, look at the tacho and see if it "flutters" slightly when cranking.

 

Generally speaking, no fluttering tacho means no spark (and no injection or sometimes fuel pump either)

[Cameron 16]

Thanks I'll make sure I remember that next time! Does the tachy relay also control the power to the coils? It's been a while since I last poked around with my multimeter so can't remember.

[Anthony 1,003]

No, it's the other way around - the tachymetry relay is energised by the pulse from the coil.

 

(it should also energise from the starter solenoid wire being live, but this doesn't always seem to work)

 

Coil and ignition amp are powered direct from switched 12v ignition on 8v Jetronic.

[welshpug 1,666]

coil is ignition switched, rpm signal comes from the coil to the tachymetric relay and ecu, so no spark = no fuel.

[Cameron 16]

Aha I see! So if the wiring in the engine bay is at all dodgy it could cause it to not get the signal.. that would make a lot of sense if you could see the state of some of the wiring under there!

 

Does the relay use the signal from the coil to keep it latched on while the engine is running, and the starter signal to activate it in the first place, something like that? If that's the case it could be that the starter signal wire is buggered as it seems to run perfectly once it's going.

[Anthony 1,003]

Basically yes - the coil pulse keeps it latched when running.

 

The relay will initially latch with either the starter solenoid wire going live, or when the coil starts pulsing.

[Cameron 16]

Ok so it did it again yesterday, thankfully(?) it did it for long enough for me to check a few things before it eventually started. The fuel pump runs when it's cranking and the rev counter is completely flatlining, so I suspect it's an ignition issue rather than fuelling as I'd first thought.

[Cameron 16]

I don't want to jinx it or anything, but I may have fixed it.

 

Changed the ignition module for a new one and it's started on the button since.