rough start up breakthrough for dk and spaniel

[supercharged spaniel]

following on from the previous thread...'rough start up'

me and dan knight have had the same dodgy miss fire on start up.  ive had it for about 18 month now and dan knight around a year.  we have been comunicating for weeks in a bid to solve it and i have prviously changed so much stuff in a bid to find out what it was, including...dizzy cap, rotor arm,lambda, plugs, fuel filter, timing checked manifold off, isv cleaned, throttle body cleaned, the list rerally does go on....

anyways i had previously changed the blue temp sensor but it would appear that this is the problem and we have duff sensors?  checking the resistance whilst on the phone to dk last night before starting the car up after work (been left outside cold for approx 9 hours) the resistance was intermittant and would not hold a reading...start the car and its rough as..

only way i could think of checking mine was to do the following:

in the morning when it would do it every time, take the blue temp sensor off, place in a cup of boiling water leave for 2 mins and refit.  if the miss is gone then the sensor is obviously not reading right or it at least shows it only gives a reading when warm.  The problem only lasts for about 30 secs to a minute so then car runs fine. so its obviously when it gets warm it starts giving a reading.

anyway..refit it and boom! starts no problems!  cant believe its something so simple.  fingers crossed when i get a new sensor it actually play along and fix it...but so far its a break through!

[PeteG40]

you seen the guide for testing it in the FAQ? resistances at certain temps.

[supercharged spaniel]

yep..but dan just text saying with another new sensor in...his still did it this morning?  what else controls this sensor?  what would influence the sensor?  surely they cant all be duff ha ha

[supercharged spaniel]

its got to be along the lines of this sensor because why would the car run fine with it warmed up and refitted? 

[DKnight]

well it is 100% the blue temp sender that is causing the problem.

But now the knock on problem is, the sensor is getting a wrong reading on start up, its as if the car is too cold from start?

if i fire car up with cold sensor, it idles like shit.

turn it off.

stick a different sensor in a cup of hot water, fire up, runs fine.

so what is causing the sensor to get too low a reading? or high, however it works...

im going to check what reading im getting off a cold sensor, any ideas?

[PeteG40]

the readings are in the faq section i think.

It could be overfueling for a number of reasons i suppose... the hot blue temp just cuts the fuel (thinking its hot) and the problem appears cured.

[supercharged spaniel]

its a good point pete.  so what else would make it over fuel?  what are these 'number' of things? 

[supercharged spaniel]

i feel we are close to a solution but need more input as im lost for what to check next..

[PeteG40]

a number of things.... er

fuell pressure regulator
injectors
chip
lambda (although not till it gets warm as doesn't work till hot)
timing
worn valves/seats (blow through)

off top of me head

[DKnight]

fuell pressure regulator - tested it all fine
injectors - tested all fine
chip - tested all fine
lambda (although not till it gets warm as doesn't work till hot) - tested all fine
timing - timing could be wrong but if the timing is out it only exagerates the existing problem
worn valves/seats (blow through) - mine are new

im sure its the blue temp sender still

[PeteG40]

well mine works fine so you can swap with mine if you like mate....

as for fpr - you could do with a pressure gauge hooked up
injectors - mine are refurbed so could try them.
valves - well check compression to make sure that's still tippety top.  Andy strange's head wore after 1500 miles or something similar

[hayesey]

another option is a dodgy wiring loom.  checking that involves a long session with a multimeter checking the pins on various sensor connectors back to the ecu connector, there is at least one thread I've written in the FAQ sections about doing this.  Plus checking the earth crimp inside the loom.  Although to me, it doesn't make much sense that it would only happen when cold and everytime it's cold.  Unless it was a break in a wire which fixed when some heat went into it to soften the wire insulation, it seems pretty unlikely though!

Has either of you had a VAGCOM on it yet to watch sensor values when the problem happens?

[supercharged spaniel]

hmmm..but heat getting into the wire is the wire and therfore it wouldnt matter what temperature the sensor is if you know what i mean?  but with a warm sensor and starting the car from cold there is no issue?

[supercharged spaniel]

is it possible that the plug end is at fault?  not on the sensor but on the loom?  would it be easy enough to resoulder a new one on?  if this could not hold a good reading for resistance?  im just guessing now?

[hayesey]

yes but warming the blue temp sensor could just be a red herring.  Yes doing that will cause a reduction in amount of fuel the ECU puts in but it could be something else that's causing the ECU to overfuel which you are then compensating for by reducing fuelling manually by warming the blue temp up. 

I think the best solution might be to "borrow" a working G40 for a day and keep swapping things over till you find the issue.  Or use VAGCOM as I already said to watch for weird sensor values when the problem happens.