Where to look first - poor starting and poor economy

[tristancliffe]

Hello.

My little MX5 Mk2.5 1.8 Sport has a couple of annoying 'features', and I'm hoping some of you guys that know the cars inside-out can point in the direction to look first.

1. The car is sometimes hard to start, and cranks for 6 or 7 seconds. Doesn't sound like much, but given that every other 'modern' car (including my previous Mk1) starts in less than a second, it is pretty poor. Sometimes it starts quickly (rare), and other times it sounds like it's not going to start. The battery charge makes no difference (it will do it after lots of short journeys, after long journeys, when hot, when cold, when I've charged the battery manually). It doesn't feel like a poor connection or earth - the engine spins over fairly quickly.

2. I rarely acheive more than 27mpg. Most of the time is in the 25 and 26 region. Recently I had a 23.5!!! I do not think it's me driving too 'hard', and nor am I labouring the engine. It seems to make little difference whether I drive fast or normally. My previous Mk1 (1600) would do about 30 everywhere, and 35 on motorway journeys. I was expecting a slight drop in economy with a bigger engine, but not that much. I'm wondering if there is a sensor that fails (without a warning light or code) that can cause this, or if there is a 'chip' or 'remap' that is available that causes this (presumably fitted (if it is that) under the false promise of more power!)...

Would like to hear some ideas of likely causes, given that it isn't basic stuff (oil, filters, plugs, leads). I do have a slightly blowy exhaust (exit of the rear box), but that won't be the cause of either problem.

Thanks in advance.

[vdubbin]

Maybe the battery itself is on the way out, have you tried with another battery?

I wonder if there's extra load on the alternator that causes extra effort from the engine?

Edited by vdubbin on Wednesday 30th May 12:47

[MX-5 Lazza]

Sounds more like it's struggling to get the right fuelling. I'd suspect a sensor somewhere playing up. Not sure if Mk2.5 uses the same engine temp sensor on the back of the block for ecu temperature readings as earlier cars do but this would be likely if it does.
O2 sensor failing could cause it to run rich which would explain the mpg but not the poor starting.
MAF is a possible too but they don't fail very often and again I'm not sure it would explain the poor starting.

[The Tea Boy]

Id start with the basics, spark plugs and ht leads and possibly even try a new coil pack, my mk2.5 sport doesnt always like starting but after 2-5 tries it normally starts I just put it down to immobilser etc.
As per mpg, I average 28, 11 on a trackday and 34 on a v long motorway run.

Matt