'cold' start difficult....like fuel starvation

[Beirut944]

Dear All
My 944 was always a great starter although i believe it is missing a oomponent somewhere as it always has to start with the gas pedal right to the floor - someone on the forum explained a while back that a cold start part was probably faulty/missing.

ANYWAY, just lat week it suddenly became very hard to start in the mornings. It was exactly like the fuel that is kept in the system from the pump right to the front of the engine was no longer there. I turn the car over for almost 20 seconds before it fires up. No problem starting when hot.

Is there a flowback valve somewhere which i should be inspecting?

Bear in mind it is hot here now similar to Arizona

thanks for your help and patience


MJ in Beirut

[edredas]

A bad DME temp switch can cause this. When I had this problem it was actually just wiring. I pulled back the boot to find the temp switch wires bare and shorting out.

I hope the helps!

[Beirut944]

Originally Posted by edredas

A bad DME temp switch can cause this. When I had this problem it was actually just wiring. I pulled back the boot to find the temp switch wires bare and shorting out.

I hope the helps!

Thanks. Where is it exactly though?

Martin

[Arizona_928]

on the head by #1.
but I'm going to say FPR.

[Arizona_928]

check for gas in vac lines connecting the damper/fpr

[mytrplseven]

FPR or the check valve in the fuel pump would be my direction of investigation.

[thomasryan]

modern fuel injection should not require you to pump or hold down the pedal when starting the motor hot or cold. old school carbs required a couple pumps or flooring it if the motor flooded but not so with efi

starting well when hot seems to imply, to me, that your check valve is functioning and the injectors are not leaking down.

the only way to properly diagnose is with a fuel pressure gauge.

you might just have a weak dme relay that isn't allowing enough current to the pump or a bad connection at the pump.

I would try a dme jumper test and listen to the pump whine and/or a pressure test.

[944 S2]

Originally Posted by AZ_porschekid

check for gas in vac lines connecting the damper/fpr

Easy check. Just pull the vacuum line off to see if there is any fuel. If there is its dumping raw fuel directly into the intake making it hard to start. That was my issue with the damper.
If you look closely you'll see a drip of gas out of the vacuum line.

[Beirut944]

Originally Posted by 944 S2

Easy check. Just pull the vacuum line off to see if there is any fuel. If there is its dumping raw fuel directly into the intake making it hard to start. That was my issue with the damper.
If you look closely you'll see a drip of gas out of the vacuum line.

I checked the tiny breather pipe and no gas. BUt im confused as really what the damper and fpr each do - and how to go about checking them

MJ

[edredas]

Even a leaking FPR shouldn't cause a no-start on a cold engine. Typically, these cause rich conditions and what you are describing is a lean condition. Cold engines like lots of fuel and must have more fuel to start.

Also, our check valves only hold residual pressure for 15-20 minutes after shut-down. So this shouldn't be your issue either.

If the DME temperature sensor is bad you will have a lean condition since the DME will know not go into cold start enrichment.

I hope the helps!

[dgcantrell]

Not necessarily. If there is TOO much fuel in the intake, pressing the pedal to allow more air in to help get it to ignite. I remember the days of carburetors and when you have one that is flooded (too much fuel in the intake and fouling the plugs) you held the throttle wide open to help get some air in there to help kind of dry things up so it would fire.

[Beirut944]

i'm reading about the DME relay. If i understand correctly, when it is switched on, if the engine is not turning, then it will not work right? It waits for the engine to turn before dddddrrrrrrrr ing into life, yes?

WHat's the quickest shade tree mechanic way of testing it? I'm sure mine is original

best

MJ

[Beirut944]

"Connect a 12 VDC power supply across terminals 85b and 87 of the relay. Using the multimeter, take a resistance reading across terminals 30 and 87b. The resistance reading should be very close to zero (actually between 0.1 and 0.3 ohms)"

I wish i could understand what this means (CLarkes). Does it mean give a 12v line to BOTH 85b and 87? ("Across"?)

It seems to be in american english (no offence to the yanks who helped us in the war). Can anyone tell me in pain english what to do? I have electronics skills and often repair stuff. Have multimeter and am ready to go.
thanks

MJ

[dgcantrell]

When you turn on the key, it energizes one of the two separate relays that live inside the DME relay module. This turns on the DME computer located in the passenger footwell. When the DME comes on, you will hear the injectors click once and the fuel pump will come on for a second or two and then go back off. This is the DME turning on the power to the fuel pump via the second relay in the module. The DME will not turn the fuel pump back on until it sees input from the reference sensors. Once it sees that the engine is turning, it will turn the fuel pump back on to allow the engine to receive fuel. Should the engine stop turning for any reason (say, in case of an accident) it will turn off the fuel pump for safety reason.

Since your car does run, I don't think it's the relay at all. It could be the coolant temp switch as mentioned above. But an easy check of the fuel pressure regulators by pulling the vacuum lines off them and seeing if there is ANY fuel making it past the diaphragms in them that could be causing the hard start issue is where I would start. There should be ZERO indication of fuel on the vacuum side of the regulators. If there is, that is the one that is causing the issue.

[Beirut944]

Originally Posted by dgcantrell

When you turn on the key, it energizes one of the two separate relays that live inside the DME relay module. This turns on the DME computer located in the passenger footwell. When the DME comes on, you will hear the injectors click once and the fuel pump will come on for a second or two and then go back off. This is the DME turning on the power to the fuel pump via the second relay in the module. The DME will not turn the fuel pump back on until it sees input from the reference sensors. Once it sees that the engine is turning, it will turn the fuel pump back on to allow the engine to receive fuel. Should the engine stop turning for any reason (say, in case of an accident) it will turn off the fuel pump for safety reason.

Since your car does run, I don't think it's the relay at all. It could be the coolant temp switch as mentioned above. But an easy check of the fuel pressure regulators by pulling the vacuum lines off them and seeing if there is ANY fuel making it past the diaphragms in them that could be causing the hard start issue is where I would start. There should be ZERO indication of fuel on the vacuum side of the regulators. If there is, that is the one that is causing the issue.

I found two switches which looked like coolant temp switches, one at the front by pot no 1 and the second one at the back. Any advice on checking them would be appreciated. i looked at the cables/connectors but they looked ok

It's wortt doing the check with multi meter though on the DME relay (the one in the fuse box), no?


MJ

[dgcantrell]

Have you checked for fuel leakage on the vacuum line side of the pressure regulators?

The one at the front of the engine is the DME temp sensor. Not sure of the readings you should get though.. I don't have that info in front of me.

[Beirut944]

Originally Posted by dgcantrell

Have you checked for fuel leakage on the vacuum line side of the pressure regulators?

The one at the front of the engine is the DME temp sensor. Not sure of the readings you should get though.. I don't have that info in front of me.

there are two FPRs right?

[dgcantrell]

Yes, sort of. One is a damper and the other is the regulator. One at each end of the fuel rail. The one at the rear is the pressure regulator. The one at the front is the damper. Leakage on either one will dump extra fuel into the intake via the vacuum line.

[Beirut944]

Originally Posted by dgcantrell

Yes, sort of. One is a damper and the other is the regulator. One at each end of the fuel rail. The one at the rear is the pressure regulator. The one at the front is the damper. Leakage on either one will dump extra fuel into the intake via the vacuum line.

thanks - is that the only way of checking them?

M

[dgcantrell]

That's one way and the quickest way. They're pretty simple inside so not many ways for it to fail. The most common is a diaphragm breach. Fire it up and let it run for a few seconds to build pressure. Shut it down and immediately pull the vacuum lines off the backs and see if any fuel will drip. Even the smallest amount will indicate a diaphragm rupture.