85 XJS HE warm starting problem

(bruce) #1

I have a problem, replaced ignition amplifier with new unit and moved it to just in front of the windscreen washer container and it runs much cooler. After driving for half an hour or so the car won’t start until it cools down a bit. ( ECU did not seem to trigger correctly ) I have replaced the CT Sensor and AV sensor, Suspecting a fault with CTS or wiring I checked voltage at CTS and AVS connectors with ignition turned on, I am getting 5 Volts at AVS but only just under 4 Volts at CTS. I am sure they should be both 5 volts. would the low voltage at the CTS sensor be causing the problem ? Hope someone can help
Cheers, Bruce

(John) #2

Check your fuel pressure after shutdown. You might have a vapour lock problem if the pressure in the fuel rail doesn’t hold up after shutdown.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #3

How? You moved it from a gigantic air-cooled heat sink. If you didn’t provide an electric fan and a heat sink where you put it, chances are it runs hotter.

(bruce) #4

Thank you Kerbert for your input, I have put a large heat sink on top of the unit .Have you any clues re. voltage at CTS connector ? as the same problem occurred before I moved the unit.

(bruce) #5

Thank you for reply, I have replaced the fuel pump and both regulators, I am sure it is electrical

(John) #6

That doesn’t rule out vapor lock as you might have a leak somewhere in the line or leaky injector.
Only way to test it is to hook up the pressure gauge.

(Aristides Balanos) #7

Still, if it’s inside the engine compartment, all that the heat-sink will do is transfer the engine heat to the Amplifier.
You should either leave it where it was or take it outside the engine and ad a cooling fan.

It’s quite possible that your no start is caused by a overheated faulty Amp.

(Paul M. Novak) #8

There is a lot to the fuel system other than the fuel pump and pressure regulators. The only way to positively rule out vapor lock is to install a fuel pressure gauge in the engine bay and observe fuel pressure when the problem occurs.

Have your tested your thermal vacuum valve on the fuel rail?

BTW, from your description so far I suspect that you have a vapor lock issue. If I were you I would also return you ignition amplifier to its original location.


(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #9

Obviously the first thing to check is the voltage at the ECU in the trunk. If the voltage is low there as well, I think that would indicate an ECU fault. If the full 5V shows up there, it’s probably a corroded connector in the wiring somewhere.