Hard start when cold

[Dirty325ix]

I am having trouble starting my M20 when it is cold after it has run for a while and has had a chance to heat up there seem to be no starting issues. I did the stomp test and it showed that i had a faulty coolant sensor.
Would that be the cause of such a hard start when it is cold or is it more likely a vacuum leak

[coldweatherblue]

yes, replace coolant temp sensor with Bosch quality part.

just replaced my coolant temp sensor with an FAE sensor and the first thing it did was throw a 1223 and fail to start cold. dumbass.

buy the bosch sensor. $20.75 at pelican.

you could definitely also have a vacuum leak though..

[Dirty325ix]

how does a bad coolant sensor affect start up? does it control the gas air mixture or sumthing.. i thought it just moves the coolant

and i did notice my temp guage needle was stuttering and wouldnt move smothly

and a funtioning stomp test is deffenetly worth the 90$ for that o2 sensor, i never thought it would work

[coldweatherblue]

bad sensor would probably overload the DME with rich fuel info leading to no start. someone smarter than me will be able to better answer that question.

coolant temp sensor sends info to the ECU/DME about engine temp which changes the fuel/air delivery to account for differences in cold/warm/hot engine conditions. afaik.

all I know for sure is that with the crap sensor my car had a good idle while warm but threw a 1223 and wouldn't start cold.. with the 260k 25 year old bosch sensor it starts fine w/ no CEL but has a lopy idle.

[jlevie]

How a bad ECT sensor would affect the engine depends on how it has failed. A shorted sensor would cause the DME to think that the engine is really hot and thus lean out the mixture. An open sensor would make the DME think that the engine is really cold and thus richen the mixture.

But the most common cause of the OP's symptoms are intake leaks, which a smoke test will find.