Car won't start when hot

I know this has been covered before but I still can't find the cause as to why the car won't start when its hot. It runs fine and drives fine but if you leave it 20 mins when hot it won't start until its cold. I can't seem to find any issue for whats causing it but I'm wondering if it could be anything electrical as the wiring in the bay is not in a good state. My thinking is that its the warm up regulator but I had this apart and it looks to be fine. The injectors are okay and aren't leaking but if I pull them out and try starting the car no fuel comes out. The pressure in the fuel system drops below 3bar which could be this issue? it doesn't loose any pressure once turned off, the car has a new fuel accumulator and non return valve. My other thought is the fuel pump, I bought the car off a friend and he said he didn't have the issue before and he changed the fuel pump not long before buying it because of another issue he thought was the fuel pump. I helped him find the issue so I know he wasn't covering up the issue, I've known the car and helped him with it before he thought of selling it.

Please, if anyone can help I will be so grateful!! 

Hi,
There are four things that aid with hot starting:
(1) There is a one way valve on the end of the fuel pump, it has a ball in it, this valve must shut off otherwise the pressure in the system drops.
(2) The accumulator, this dampens oscillations in the fuel system but also holds the system pressure when shut off.
(3) The seal on the end of pressure regulator in the metering head, this stops fuel returning to the tank and therefore maintaining pressure at the metering head.
(4) The WUR hot pressure.


Testing - connect a pressure gauge:
(1) Start the car and get it up to temperature, get some one to turn the engine off and then immediately clamp the return hose off the metering head. If the pressure drops then either the one way valve or accumulator are at fault (the accumulator usually leaks fuel if its diaphragm has gone). If the pressure remains, unclamp the return line, if the pressure drops then the seal on the end of the pressure regulator is not sealing.
(2) Check for WUR pressures through cold to hot, it should be linear.
(3) Check system pressure is correct.
(4) Check that the fuel pump is delivering enough fuel over a 30 second period. Remove the injectors and run the pump by bridging the fuel pump relay.

Thank you for this!!
for the 4 main things this is what I found:
1- there was no one way valve so I purchased a universal one and put this in after the fuel pump but also after the accumulator
2- I bought a brand new one,
3- not checked
4- not checked pressure directly off the WUR

for the 4 tests:
1- the pressure when tested after the 1 way valve I installed and the fuel filter did not go down for a good hour or so and dint drop much after that.
2- I checked the pressure from the supply line to the car and the pressure was high when cold and as it got hot the pressure dropped down to below 3bar (ill find what it was)
3- not sure what the correct presure is as some places say different values
4- what is the correct amount of fuel?

Check in Haynes for the pressures and fuel quantity delivered.

When I tested the pressure it started high and dropped, from what I've seen it should increase as the temperature increases, is this correct?

Yes the control pressure managed by the WUR should start at about 1bar and rise to 3.4bar when warm. System pressure should be constant at about 5.4bar

Check the wiring to the main fuel pump. Its possible that the poor wiring (or poor battery, etc) isn't giving enough voltage, so during starting the fuel pressure just can't get up there and open the injectors.

It will start when cold because the 5th injector (which is an electrical injector and different to the mechanical ones, which vary due to fuel pressure) fires when cold, even if the voltage is quite low.

lhasadreams said

Yes the control pressure managed by the WUR should start at about 1bar and rise to 3.4bar when warm. System pressure should be constant at about 5.4bar

Mine is going the other way but I don't have the pressure gauge set up in the right place so I don't know if this could be the cause of my poor readings.

paul_c said

Check the wiring to the main fuel pump. Its possible that the poor wiring (or poor battery, etc) isn't giving enough voltage, so during starting the fuel pressure just can't get up there and open the injectors.

It will start when cold because the 5th injector (which is an electrical injector and different to the mechanical ones, which vary due to fuel pressure) fires when cold, even if the voltage is quite low.

That seems like a good idea and would fir with why it starts only when its cold!! the fuel pump was changed just before I bought it and the car was never really used so potentially the aftermarket pump is at fault? my friend still has the original one but is kept at his brothers lockup so its hard to get this back.

For now would I be right in thinking if it gets hot and doesn't start, could I open the 5th injector by putting 12v to it and seeing if it'll then start? if it does then it would indicate this being the issue!?

It could be anything in the electrical circuit (including a new pump…). There's a lot of potentially dodgy connections in that circuit. Its worth checking it all, not just assuming an aftermarket pump is faulty. Mixture needs resetting after replacing a pump, because they are never precisely the same fuel pressure output.

Yes you could manually "fire" the 5th injector to help diagnose. I used to have a simple wiring loom which would plug into it, with a switch and some battery connections.

I'm going to get the pump off my friend as its the original one and its free, this way I can rule that out before going through all the electrical connections.

Going off what you said this morning I disconnected the 5th injector because if your theory is right then it shouldn't start cold without the injector, although it is cold. The car wouldn't start at all, just turned over and over, I plugged it back in and it fired right up. I then let it warm up and shut it off for the usual 20 mins, again it wouldn't start. I then connected the negative from the thermotime switch to a switch and then connected that to ground. Effectively making the car think it was cold, I closed the switch and tried again. This time the car started but died but with a touch of throttle it starts right up and runs so I think its fairly safe to say its the pump or the wiring for the pump circuit.

Im going to get hold of the pump and see if that solves the issue, if not ill go on the hunt for some dodgy wiring. If you have any other suggestions or if I'm being stupid please let me know!!

Have you checked the accumulator?

There's a little screw on the back, if you up do it no fuel should come out, if it does then it's shot.

The accumulator job is to hold the fuel pressure to add hot starting. It's underneath the car near the fuel pump.

 

The accumulator is a brand new Bosch one so I don't think this is the issue. Definitely seems to be that the fuel pump isn't building enough pressure when starting because no fuel comes from the injectors and it starts if I open the 5th injector.

My knowledge on wiring and electronics is pretty poor, without changing the fuel pump is there a way of testing if it's the pump or the wiring that's at fault? Is it just a case of if I have 12volts the wiring is okay or do I need to test the current? If so then what am I looking for?

Seems like the old pump is going to be hard to get back from my mate so wanting to test this and of its the pump I'll just buy a good quality new one. The one that's on there now is new but doesn't seem to have a brand, although it was bought from VW heritage

You test the pump in two ways:

(1) Connect a gauge with a switch to the WUR circuit outlet on top of the metering head. With the switch open it will read WUR pressure, with it closed it will read System pressure - this needs to be about 5.4bar.

(2) Disconnect the fuel return pipe from the metering head and put it in a jar (1000cc). Locate the fuel pump relay and bridge the two large contacts on the relay plate to make the fuel pump run. Run it for 30 seconds and you should have between 750cc and 900cc of fuel delivered.

This proves that the pump is working, ie it can generate enough flow to deliver the correct amount of fuel at the correct pressure.