General Occasional starter

[gandl]

I have a 1.6 weekend which runs a treat ........ most of the time. Every now and again it decides not to start, I have changed the spark leads and this has helped. It appears to be a problem when the car is left for over say 12hrs without running. Much to my neighbours pleasure I have taken to turning her over late a night to ensure I can get to work the next day. Thus far always succesfully. I took her into my local dealer who diagnosed a RPM sensor fault. Would that effect starting?
When she fails it sounds like there is petrol starvation. Should I spend £100 on getting the RPM sensor or does it sound like something else?
Any help appreciated.

 

[jug]

yes the rpm sensor is commonly the cause of starting problems, but i would also expect other problems, such as cutting out at idle and poor performance. the rpm sensor is used for baseline fueling strategy, so a fault would affect fueling at all times, not just when starting. the rpm sensor is also the crankshaft position sensor on the 1.6 models, which is a critical sensor, so if the car drives fine once started i would think the problem is not the rpm sensor.


what rpm does it idle at when first started on a morning?
what rpm does it idle at when warmed up?

 

[gandl]

Start or hot about 800 rpm.

 

[jug]

i thought you'd say something like 800rpm at both cold and warm
this type of problem is very common on the 1.6.

on a cold start it should hit 1200rpm+ and slowly fall to 850rpm over the first few minutes, before settling at 850rpm.

if your idle starts off at 800rpm even when cold it shows that the engine is not adapting itself to a change in temperature and it is not adjusting the idle rpm. this will make it difficult to start because the fueling is changed for a cold start, and the idle stepper motor adjsuts to ensure the required amount of air will enter. since a cold engine is more difficult to start, if this adjustment to fueling and air is not occuring it will be hard to start the engine, especialy if the increased fueling is occuring but the increased air in not. the cooler it gets the more difficult it will be to start. that is why it gets harder to start the longer you leave it.

so what is the actual problem? its almost certainly either the idle speed control actuator or the coolant temp sensor.

first thing to do is remove the idle speed actuator (throttle body, 2 bolts) and clean it, also clean the channel it sits in on the throttle body as this needs to be clear to allow air to bypass the butterfly valve.

if that doesnt fix your problem you need to check the coolant temp sensor circuit. if your temp gauge is working you know the sensor is working, but you dont know if the ecu is getting its signal so you need to check the wiring from sensor to ecu (consult haynes manual, or ask if you need further advice)