Porsche Enthusiasts Club Forum

hi,any ideas why my 928 s4 hunts when cold but is ok once its warmed up,cheers ian

you need to be looking at what controls your idle when the engine is cold.
Your fuel delivery and ignition demands are controlled by the ECU depending on what all the sensors are telling it.
When cold your cold start mechanism will increase fuel supply (probably by a seperate cold start injector.)

I would test the engine coolant temperature sensor first ,( probably somewhere on the inlet manifold or engine block) . Try disconnecting the electrical connector to the cold start injector . if there is an improvement it would indicate that your cold start injector is not being shut off by the engine temperature sensor .(It normally only injects extra fuel for 15-20 seconds or so when the engine is stone cold as measured by the water temp sensor)

You could also try disconnecting the electrical connectors to one of the injectors and start and run on 3x cylinders when cold . If there is any improvement it would also indicate over-fueling when cold ie temp sensor/ cold start injector

Oh welcome by the way

Hi
What year is your 928 ? I am thinking from what you say that it is prbably fitted with cat/O2 sensor ?

How many miles on the car ?

You mentioned it's an S4, in which case there's no cold start injector!

so what is there instead then ? would be helpful to offer your advice and knowledge because you clearly are familiar with the model .




Ian I don't know the S4 specifically as I had the 79 model but the Bosch fuel injection and engine mangement system is pretty much the same throughout the period in the principles of operation.


If there isn't a cold start injector then your ECU will use the temperature sensor and o2 sensor to increase the injector pulses to give the engine more fuel when starting from cold.
You should still check the temp sensor and also the o2 sensor . The rest of my advice still applies. There are other components that could have an effect also like the AFM for example but without testing its just guesswork really.
Hope that is helpful anyway

hi,thanks for the welcome,its a 1989 s4 with 75000 miles

Hi peanut, I didn't intend to be deliberately inscrutable in offering (minimal) advice!

Fact is I read this thread originally on my mobile stuck in a hotel, so it was only feasible to send a very short response, and your suggestions revolved round the cold start injector of the earlier Bosch mechanical system so I wanted to emphasise that point. Now back home with luxury of laptop for extended reply.

Nothing I can add to your very helpful additional post, other than to say that if there is a problem with the Temp II sensor then it would show up as running problems throughout the temperature range. The Temp II sensor (as you look at engine from front of car, it's on top of left side of coolant bridge on left side with blue connector) can be tested very easily by pulling off this connector, and measuring the resistance between each terminal and a suitable ground point for cold and warm engine (i.e. don't measure between the terminals - the sensor contains two identical thermistors for the injection and ignition brains). However testing this way won't pick up a fault in wiring between the Temp II sensor and the LH and EZK ecus.

Suggest original poster gets hold of a "Spanner" diagnostic computer from JDSPorsche, and to see what faults emerge from a thorough test of the LH, EZK and various actuators. It may not definitively identify the problem but would at very least eliminate many of the possible causes without expense of needless replacement.

Oh that sounds like a lovely low mileage S4.

Does it have cats ? Look under the car oppostite the driver's seat, and you'll see the first silencer boxes (or cats). Cats will always be rusty, and have stubby wide fins on them, non-cat cars have a relaively shiny looking silencer box.

The we can help find the problem. Whre are you ?

just checked and it has no cats,im live in castle bromwich birmingham.

check vacuum lines and idle control valve???

You need to check that the ilde CO is correctly adjusted when engine is hot. 1-1.5%

Then see how it behaves when cold. Then we can advise you further.

op hasn't been back to the forums for over 2x weeks

hi jdsporshce,how do i check and adjust the idle co ?