Cold start problems - ideas?

[GravelBen]

My '89 MX5 seems to be getting increasingly reluctant at cold starting lately - well it'll start up fine initially and then bog down and stall if I don't give it some gas to keep the revs up(or sometimes even if I do), then takes a lot of winding over before it will restart.

Not ideal conking out when the revs drop as I stop at an intersection, then I sit there holding up traffic while I try to start it again. Especially as I'm currently trying to sell the thing.

It is winter here so its breathing pretty cold air at times (frosty mornings often -5° and colder), it seems related to air temperature as its ok cold-starting when the air is warmer but it does seem to be getting worse.

I checked the ECU for error codes last night and got nothing. So apparently its not a simple sensor fault, or if it is the computer doesn't realise.

Checked the idle screw, seems to be set about right.

Also took off the air bypass valve (not sure if thats its proper name actually) to clean up and check if it was working, seems to be ok.


Any bright ideas for other likely causes?

[Ordinary Bloke]

When was it last serviced?

[GravelBen]

Ordinary Bloke said:

When was it last serviced?

Gave it an oil and filter change a few thousand Km ago.

Edited by GravelBen on Wednesday 9th July 02:23

[BB-Q]

If it has a separate temperature sensor for the ECU (many cars do) and the sensor is shorting it will only provide enough fuel for a warm engine and so will only run very reluctantly when cold (an engine requires a mixture of around 4:1 to start easily from cold). It's easy enough to check by having a look with the diagnostic software. Disconnecting it to check doesn't always work as the ECU can see infinite resistance as an implausible signal and throw a hissy fit.

[shouldbworking]

BB-Q said:

If it has a separate temperature sensor for the ECU (many cars do) and the sensor is shorting it will only provide enough fuel for a warm engine and so will only run very reluctantly when cold (an engine requires a mixture of around 4:1 to start easily from cold). It's easy enough to check by having a look with the diagnostic software. Disconnecting it to check doesn't always work as the ECU can see infinite resistance as an implausible signal and throw a hissy fit.

Most ECUs will default to the side of running excessively rich actually if they dont recognise the temp sensor input, better to use a bit more fuel than run the risk of lean + detonation.

[GravelBen]

BB-Q said:

It's easy enough to check by having a look with the diagnostic software. Disconnecting it to check doesn't always work as the ECU can see infinite resistance as an implausible signal and throw a hissy fit.

I don't have any diagnostic software to check it (and the nearest Mazda dealer is 2.5 hours away), was checking for ECU fault codes via the old OBD flashing check engine light trick. Or in this case the MX5 trick which involves plugging an LED into the diagnostic connection as the Jap versions didn't have a check engine light.