Hot start problem

Guest · May 9, 2006

I am getting the following problem, any ideas?
When the engine is hot it has developed trouble restarting if it's been sitting for between 10 and 25 minutes.
It turns over Ok but seems like there is no petrol, running on 2 - 3 - then 4 cylinders just for a few seconds and then goes fine.
Then all is Ok and the car drives completely fine until next stopped.
Any times longer than 25 mins or shorter than 10 and it restarts fine straight away with no problem.
I seem to remember from years ago this could be something called heat sink, I think where the engine heat vaporises the fuel in the carbs?
Is this a modern version?
Any ideas what could be the cause please or what should I be looking for?
Car is a 1992 944 S2.
Thanks

944Turbo · May 9, 2006

I guess it could be fuel related, Im not very familiar with the s2 but vaguely remember something about fuel draining back causing awkward starting. No doubt someone more clued up will reply in a bit [&:]
Tony

Guest · May 9, 2006

Now you say that I realise it is worse when the car is parked facing uphill.

Indi9xx · May 10, 2006

Fuel pump check valve.

If the fuel pump check valve is faulty, when the engine is hot and not running, the fuel will boil in the fuel rail.. So when you try to start it, the fuel will be vapour and much of the incomming fuel will also turn into vapour.

Have your fuel pressure checked. It should be 3.8 bar with the engine stopped and the fuel pump running (jumper the DME relay)

When the fuel pump is shut off, the fuel pressure should hold for 10 to 20 mins.. if it nosedives right away, you need a new fuel pump check valve.

os951 · May 11, 2006

Sometimes a bad FPR causes a problem like you described

Guest · May 12, 2006

Thanks for that Jon
Silly question maybe but where is the fuel pump check valve and is it an easy DIY job to replace or best left to the pro's?

Indi9xx · May 12, 2006

The fuel pump check valve is located in the neck of the fuel pump.

The fuel pump is located under the fuel tank in an 86 onwards 944, in a tin cover, held on by the main tank retaining strap.

So to change it, you need to remove the main strap (it may break due to rust), then remove the tin fuel pump cover, disconnect the high pressure side of the fuel line, undo the fuel pump check valve from the neck of the fuel pump, fit the new one and then replacing everything is the reverse of removal.

Not a big job, probably about an hour or two.

Guest · May 13, 2006

Thanks again Jon.
Think I will give it a go if the main strap looks sound enough!

peanut · Apr 4, 2010

Hi welcome to the forum.
Sorry to hear you are having hot start problems. Its a pretty common problem with fuel injected cars like the 944.

Basically if you have substituted the DME relay with no change then its likely to be a fuel delivery problem. When you switch off with a hot engine the system is designed to maintain fuel pressure in the system to prevent fuel evaporation or pressure loss.

I would check your fuel pressure is being maintained .Attach a pressure gauge to the fuel rail Run the engine up to temperature then switch off .Monitor the guage. if pressure is being lost then clamp the return hose of the FPR valve on the fuel rail. If that doesn't stop the pressure loss then clamp the hose at the fuel pump. If you still can't stop the leak then chances are you have leaky injectors.

I got a cheap fuel pressure gauge off ebay for £10 and attached it with a length of garden hose

Oneother thing to check is the engine temperature sensor which controls the cold start injector .

I'm sure you'll get plenty more advice tomorrow. Good luck and keep us informed of progress.

shaynan944 · Apr 4, 2010

Hi, thanks for the reply.

I'm not sure if its a fuel issue as there is no spark while its hot? Could that be due to either the oil temp sensor or speed sender failing?

peanut · Apr 5, 2010

sorry I didn't get back to you. Yes it could be the crank speed sensor . If the ECU doesn't see 'engine turning' or 'engine running' from the crank speed sensor at over 200rpm then it doesn't ground the DME relay or the ignition system, to prevent fires in the events of accidents.
If when you crank you see a slight jump of the tachometer then the sensor is working . Its very slight movement (about 1mph)

Its unlikely to be the crank speed sensor or the car wouldn't run when cold .
I think its most likely to be one of two sensors giving the ECU the wrong information when hot. Either the engine temperature sensor or the Auxillary Air valve . Also check that the AFM is free to move when the engine is hot.