Won't start when hot

[mickdavies]

I've finished recommissioning mine but it won't start when hot.
I've replaced pretty much everything: Fuel sock, fuel pump, fuel pickup pipe, fuel accumulator, fuel filter, WUR, fuel distributor, fuel lines, idle control motor, injectors, ignition coil, rotor arm, dizzy cap, HT leads, spark plugs, vacuum lines, inlet manifold O rings.
The cold start valve works OK.
The car starts OK when cold, but if you switch the car off when hot, it won't start until it's cooled down.
I've set the timing and the CO is at about 1.2.
Spraying some Easy Start at the metering plate the car tries to start and also the "swap" of the WUR plug with the CSV works to start it, but when it's removed the car stalls, therefore it must be fuelling not ignition.
Could a faulty Lambda sensor be the cause?
Any ideas?

[82DMC12]

Have you put a K-Jetronic fuel pressure gauge on the car to monitor what's actually happening and the rest pressure drop-off? If not, everything is speculation and just tossing parts at it.

[Rich]

Have you put a K-Jetronic fuel pressure gauge on the car to monitor what's actually happening and the rest pressure drop-off? If not, everything is speculation and just tossing parts at it.

+1

First, yes, it's a fueling issue and no, your Lambda sensor is probably fine.

The hot-start failure symptoms you describe (incl good startup after engine cools enough) are fully in line with poor fuel rest pressure retention after hot shutdown. When this happens it's no fun to visit a gas station unless it's close to your house so the still-cold engine will start right up after refueling. Been there.

See this DMCH Knowledge Base article about this problem. Read it through before you start. It details the fuel rest pressure gauge test connections specified in the Workshop Manual as well as the more crude air meter flap test if you can't get a gauge on it. Rest pressure should be 48PSI just after hot engine shuts off, then remain above 25PSI for at least 10min per D:02:01 in the WSM.

It goes on to describe the 5 most likely fuel rest pressure loss issues in decreasing order of likelihood.

It's possible even a new or rebuilt part may be faulty. Assume nothing to begin with.

[mickdavies]

I'll re-check the pressures, but remember I put an extra shim in the relief valve to get the pressure from 4.8 to 5.1 bar.
I jumped the fuel pressure relay to get the pump on permanently but it wouldn't start, which suggests it's not pressure leakage?
Forgot to say new Cold Pressure Regulator.

[82DMC12]

I'll re-check the pressures, but remember I put an extra shim in the relief valve to get the pressure from 4.8 to 5.1 bar.
I jumped the fuel pressure relay to get the pump on permanently but it wouldn't start, which suggests it's not pressure leakage?
Forgot to say new Cold Pressure Regulator.

The only way to rule out fuel pressure leakage (loss of rest pressure) is to put fuel pressure gauges on and confirm it.

If that's not it, possibly the cold start valve is firing and flooding the engine.

You said the car starts and runs cold, and I assume it runs indefinitely until you turn off the key? Does it run perfectly during that time with normal lambda response, perfect CO adjustment, perfect timing with no advance at idle?

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[mickdavies]

The only way to rule out fuel pressure leakage (loss of rest pressure) is to put fuel pressure gauges on and confirm it.

If that's not it, possibly the cold start valve is firing and flooding the engine.

You said the car starts and runs cold, and I assume it runs indefinitely until you turn off the key? Does it run perfectly during that time with normal lambda response, perfect CO adjustment, perfect timing with no advance at idle?

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Yes starts and runs from cold to hot, then I can check the CO. It's not the cold start valve flooding the engine as I did the plug swap to force it to inject and that started the engine when hot (but it cut out when unplugged). Timing is set, no advance at idle. My experience with cars is that the timing can be a fair bit out and a car will run, just not as efficiently.
What I don't understand is why it will run sweetly from cold to hot and then (possibly) lose pressure when I jumped the fuel pump to run permanently? If the fuel pump could supply pressure when hot, why, after switching off, does it decide not to?!

[82DMC12]

Yes starts and runs from cold to hot, then I can check the CO. It's not the cold start valve flooding the engine as I did the plug swap to force it to inject and that started the engine when hot (but it cut out when unplugged). Timing is set, no advance at idle. My experience with cars is that the timing can be a fair bit out and a car will run, just not as efficiently.
What I don't understand is why it will run sweetly from cold to hot and then (possibly) lose pressure when I jumped the fuel pump to run permanently? If the fuel pump could supply pressure when hot, why, after switching off, does it decide not to?!

Are you trying to start the car with the RPM relay removed from its socket but the fuel pump jumped? If so, the engine will not start. The RPM relay does more than just power the fuel pump, the ignition system also relies on it.

[Ron]

It's not the cold start valve flooding the engine as I did the plug swap to force it to inject and that started the engine when hot (but it cut out when unplugged).

Are you saying that when it fails to start hot and you then do the plug swap, it will start and run, but the engine will die if you undo the swap?*

Timing is set, no advance at idle. My experience with cars is that the timing can be a fair bit out and a car will run, just not as efficiently.

Agree. But, the timing (and ALL other settings) should be done before setting the CO...

What I don't understand is why it will run sweetly from cold to hot and then (possibly) lose pressure when I jumped the fuel pump to run permanently? If the fuel pump could supply pressure when hot, why, after switching off, does it decide not to?!

It could be that, when you shut it off, the heat spreads and seizes it up until it cools (common symptom when they fail).


*I agree with those who say your best bet is to check all of your fuel pressures.

[mickdavies]

Are you trying to start the car with the RPM relay removed from its socket but the fuel pump jumped? If so, the engine will not start. The RPM relay does more than just power the fuel pump, the ignition system also relies on it.

No, only did that to make sure the system def had pressure, replaced the relay and tried immediately to start but it didn't.

[mickdavies]

Are you saying that when it fails to start hot and you then do the plug swap, it will start and run, but the engine will die if you undo the swap?*

Correct!