COLD START PROBLEM
- Thread starter rogero
- Start date
alwigley
New member
Will have a shot for you...... there are two potential issues I can think of: 1. Hidden under the intake manifold is a cold start valve known as the idle air control valve (or similar) which I believe only operates on cold starts - this sould be giving issues and failing open, resulting in a weak mixture. When you say it won't start, will she turn over and try to go at any point, or not even try to start. 2. There is a temperature sensor for the DME mounted on the intake side of the engine towards the front which can fail - they fail under different conditions. Try disconnecting this sensor and see if she will start - I think the S2 has a get home mode on the DME which allows for total failure of input from the sensor, but if it's giving dodgy output, then it could cause a no start. Generally it sounds like the mixture is not rich enough, so somehow the DME must be thinking the engine is warm........ I have an odd idea that the air flow meter also has some sort of temperature sensor built in which could also be a fault. Best bet is to get out the trusty multi-meter and follow the testing procedures from the Porsche workshop manual, which has a great section on DME diagnosis....... just consulted it, and yes there is a temperature sensor on the AFM, and a short on this sensor will cause a leaner mixture. How well does she run when warm - little hesitant perhaps ? Do the spark plugs indicate a weak mixture after a decent run ? Sorry for the rambling thoughts, but hope this gives you some pointers ! Cheers, Alan
peanut
Active member
Cold start fuel injector not working Engine temperature sensor faulty ! Sticky injectors Faulty FPR (low Fuel pressure) Faulty Auxillary air valve Low fuel level in tank (fuel pump not fully pressurizing system) When it runs from cold does it blow blue smoke ? or sound lumpy and rough until it warms up?
robwright
New member
The Idle Control Valve ICV is exactly that. It operates under the control of the ECU (DME) when the engine is idling. It serves to keep engine revs constant under varying loads i.e. PAS, aircion, heter blower or demister. It does this by opening and closing to meter varying amounts of air into the intake manifold to schieve the set RPM point. It is a closed loop system. Cold start mixture is controlled directly by the DME at the injectors.
No, it starts as though nothing was ever amiss,runs perfectly, no stuttering or running on 3 until it clears and no smoke. But while trying to start it doesnt even try to run,just suddenly bursts into life. However a squirt of fuel and it fires up easily. We did have the valve under the inlet manifold attended to last year as it kept stalling on warm re start until it was back up to temp. Hope this gives you some useful data
doesnt even cough when trying to start,suddenly springs into life as though nothing was ever amiss,no misfire, smoke and ticks over immediatley. no running problems and the car has normally started first time for three years now. we did have the valve under the manifold cleaned last year as it kept stalling after a warm re start,
peanut
Active member
I think we can Rule out the DME relay being faulty otherwise the car wouldn’t start and run perfectly when given additional fuel. (Any doubts ? simply substitute a 3x way bypass cable for the DME relay should prove it.) The engine is clearly receiving too little fuel on cold starting,( as demonstrated by perfect starting and running when additional fuel or octane boost is applied.) Edited :- it would seem that the S2 does not have a seperate cold start injector so I stand corrected . Thank you Bertelli for correcting me [
] I would test the engine temperature sensor (thermo-time switch) first. You could also test for excess fuel pressure by removing one electrical connector from one of the fuel injectors . If it starts you have excess fuel pressure and the FPR valve is faulty. Let us know the result of these tests and we’ll take the next step of testing the fuel delivery system.
] I would test the engine temperature sensor (thermo-time switch) first. You could also test for excess fuel pressure by removing one electrical connector from one of the fuel injectors . If it starts you have excess fuel pressure and the FPR valve is faulty. Let us know the result of these tests and we’ll take the next step of testing the fuel delivery system.
bertelli_1
New member
The S2 - or any other 944 for that matter - has NOT got a cold start injector. On that basis we can discount it as being the problem! eta: are all the vacuum lines intact, particularly the ones connected to the fuel rail?
robwright
New member
I have to agree with you bertelli. Where is this mythical cold stat injector fitted? It is not fitted to my injector rail. I have had it to bits along with the manifold and it does not exist. The DME only has one temp input, a signal and return from it's temp sensor. Check the diagrams.
peanut
Active member
what would you say this is ? The thing with an electrical supply from the injector harness ? I've always assumed it was the cold start injector. The Bosch manual states that the Motronic 2.1 system has one ? Early 944's I know have a cold start mechanism which the ECU controls and gives a temporary boost to the fuel injectors when cold. If the S2 has the same system I stand corrected and will amend my previous post but the cold start system is still a likely culpret for a lack of fuel when starting cold. One thing we can probably rule out is the vacuum lines I should have thought. [
] If there was anything wrong with the vacuum system it would effect the engine cold or hot, starting or running.[] There are no vacuum lines with a direct connecion to the fuel rail otherwise the vacum lines would fill up with fuel . Vacuum lines attach to both the fuel damper and fuel pressure regulator but they simply maintain an inlet pressure on the valves . Worth checking that fuel is not leaking into the the vacuum system at these two points by pulling the vacuum line connectors off.
robwright
New member
That is the Idle Stabilisation Valve (ISV) it is fed from the ECU. Its input should be connected to the intake system prior to the throttle body and it's output to the the intake manifold. The ECU controls how much it opens and thus how much air it meters into the intake manifold to regulate idle speed in response to engine load. As I understand it cold start is enabled by making the injectors rich in response to engine temperature. In fact I can see this on my AFR (air fuel ratio meter). On start up the engine is very very rich and it slowly gets leaner as the engine warms up over a couple of minutes until normal idle mixture is reached.
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