Have an intermittent problem with my 2002 Mk2.5 SVT which is proving very hard to diagnose… Most of the time it drives absolutely fine, but occasionally it’ll just lose power completely – accelerator down, but revs drop to idle – then power will kick back in momentarily before dropping out again, and so on. Sometimes this goes on for a while and the gaps between power off and on are longer – sometimes it’s more of a rapid/jerky on/off, and will completely stall a few seconds later. Usually it goes back to normal after leaving it sit for a while, but every time it’s slightly different (will give more details later), and there seems to be no pattern as to what’s causing it.
Have taken it in several times now to a trusted mechanic who specialises in MX5’s, and they’ve tried all sorts of things – new fuel pump, new fuel regulator, bypassed the fuel pump cut-off circuit, checked wiring, replaced throttle position sensor (as one time it registered an error code for this), and each time I get the car back, all seems fine - then a few days later will start cutting out again. Am on the verge of giving up… But it’s otherwise a good car, and I can’t help feeling there’s probably just one inexpensive part, or one bit of dodgy wiring, that needs sorting – but tracking down exactly where the problem lies, is the problem!
Some more info on the symptoms: The first time this happened was actually the odd one out. All the subsequent times it’s been after a longish drive, but this one was only about a minute down the road after the car had been sitting all day – also, this is the only time it wouldn’t restart at all and had to be towed. After this initial breakdown, fuel pump and fuel regulator were changed, wiring checked, and all seemed fine - then about a week later, towards the end of about an hours drive, I felt the power cut out very briefly around 5,000 rpm – then a few minutes later happened again at lower revs – and kept happening more frequently, and for longer each time (tho never actually stalling) until by the end of the journey home I was mostly coasting at about 10mph, getting power back momentarily, then coasting a bit more – was not popular with the drivers behind!! After that, back to the mechanic, they did some sort of bypass procedure on the fuel pump cutoff circuit – am a bit sketchy on the details here, but apparently this is something that can go wrong on Mk2.5’s and it’s no real issue to bypass it, so they tried that. But a week later, about an hour away from home (and after several trouble-free journeys earlier in the week also), it started cutting out again – jerky on/off power for maybe 10s before stalling completely, and this time engine warning light came on too (this was when it logged an error code for the throttle position sensor). Unlike the first time this happened, I was able to restart the engine immediately, but power was still cutting in and out when revving it, so I left it sitting for about an hour – then tried again, and it seemed to be back to normal – was able to drive on with no problems! After this, throttle position sensor was changed, and they stripped back and resealed all the wiring around that area to make sure there were no dodgy connections, but all seemed fine. Did a few more journeys after that with no problem, then today, same thing, power started cutting in and out for about 10s, then stalled. This time, I had less patience and only let it sit for about 5 mins – but after this I was then able to drive home completely fine. And no warning light this time.
Obviously this is not good though… It’s clearly going to keep happening. So far I’ve been lucky and have always been able to pull over somewhere safe – but I don’t don’t want power cutting out (and if it stalls, losing power steering and brakes too) when I’m on the motorway, or driving fast on twisty roads.
I am wondering if it’s something to do with temperature? Except for the first time (when it had been sitting all day), it’s always happened after a longish drive. Temp gauge doesn’t indicate any overheating though, and while it’s definitely on the hot side when I lift the bonnet it’s nothing that I’d say was out of the ordinary after a spirited drive. But it does seem to go back to normal after letting it sit for a bit – whereas if I restart immediately, the cutting in and out is still there.
Apologies for the very long post… figured I may as well give as much detail as possible in case anything provides a clue. Any suggestions that could help save this car would be very much appreciated!
edit: I’ve seen cam position sensor mentioned as a possible heat-related cause - that’s the one at the top rear left of the VVT engine, right? Maybe worth swapping this?