Thanks, have found the torque app to be really useful and as you suggest have it running while I'm driving. The fuel pressure after start is always steady in the high 40s from what I have observed and the engine runs really nicely once started which is consistent with the steady fuel pressure. It only has the problem at start. Touch wood...

I note the fuel pump relay is the same as a number of other relays in the car and is a Jag part number like the others which suggests it is original to the car. I expect it spends its life closed the whole time the engine is running otherwise the pump would not work. Compare that to the horn relay, which appears to spend it's time open as the horn is an infrequently used item.
If I were to swap them that would allow the relay to be proven/eliminated as a cause, if used for a few weeks while swapped. Currently the problem happens every couple of days.
Are there any cautions to be observed swapping the relays? Should the battery be disconnected while removing/replacing?
Thanks.

 

There is no need to disconnect the battery whilst swapping relays.

 

Excellent thanks. I had read recently that when disconnecting the headlights the battery must be disconnected so wondered if there were other situations which warranted it.
Anyway will swap the relays and see if that has any effect. Whether it proves or eliminates the relay it's a step forward either way.

 

Well it's been 2 weeks since swapping relays between the horn and fuel pump and so far (touch wood...) the fuel pressure has behaved itself.
The problem was always at startup, so since the swap it's been used daily, I guess averaging 2 or 3 starts per day at least and there have been no issues. I've also used the horn a few times to check which has been good too but I'm not in the habit of using it frequently!
If I were to be scientific about it I'd swap them back and see if the problem returns but since at the moment it seems not to be broken I'm not inclined to muck about with it! Plenty of other little jobs to get through before I start looking for trouble.
Anyway hope this helps someone sometime.

 

Sometimes just the act of removing and re-inserting the relay scrapes away oxidation or corrosion that was causing the problem. One thing I also found out was that some of the relays in the fuse box have an additional general lug. So they’ll be two terminals to operate the coil in the relay and then at least two term rules for the contacts. But some systems have five terminals so that you have an output on one contact when the relay is energized or it energizes the other contact when the relay is powered. Don’t make the same mistake I made which was replacing a five terminal relay with a four terminal relay and then wondering why my lights weren’t working on one side

 

Consider the general usage and duty cycle of a particular relay, too.

For nearly all drivers, the horn is rarely used. And then when used, it's only very briefly. In most cases, the horn relay can be considered a nearly new spare. (This does not apply to Florida, where many drivers can't see over the steering wheel and listen for the honk from behind to let them know when the traffic light turns green.)

Compare that to the fuel pump relay and a few other similar applications. When the engine is on, the relay is energized. If nothing else, this constant usage will cause them to wear out more quickly. Heat will build up, too. Some heat is normal, but additional heat will be generated if arcing occurs at the contacts. This extra heat can lead to an early demise.

I fully agree not to mess with the new(ish) fuel pump relay if all is good now. But I would suggest looking at the old fuel pump relay, presently in the horn position. See if any of the prongs show evidence of discoloration from excess heat. If so, the relay sockets at the fuel pump location may also be damaged. They might just be lightly oxidized, and replacing that relay may have been enough to clean them up.

But if the heat was excessive, the spring temper of the sockets may have been affected. The sockets won't grip the relay prongs as tightly as designed. You can test this with a thin strip of metal the same width and thickness as the relay prongs. Test each socket individually. If only one is loose, you may not realize this if checked all at once using a relay as your test piece. Loose sockets are not an uncommon problem and can usually be fixed by squeezing them a little from the backside with needlenose pliers.

 

Thanks! I made sure the part numbers were the same, otherwise wouldn't have been brave enough to swap them!
Good point on the corrosion. I think as a general rule down here in New Zealand we don't have the corrosion problems other areas do with mild temperatures and no salt on the roads, that said my car did have the standard S type boot leak so there was a good moisture supply which is now sorted out.
Thanks for the pointers.

 

My diagnosis hadn't gone any further than swapped=good! I think your approach to checking the old pump relay and the socket (if heat is evident) makes sense. I'll take a look and report back if there are any useful findings.
Thanks for the help.

 

I don't know what a relay costs in NZ, but if it is not excessive (here in the States they cost $7-10), I would toss the old fuel pump relay and install a new part in the horn relay socket; it would be a shame to really need the horn and find that it did not work...

 

On my 03 I've had three relays go bad. And the last one had me 100% convinced it was a fuel supply problem because it acted exactly like I had no / low fuel pressure. And even measuring FP on the rail.. my pressure was all over the place.

In the end... it was in fact a relay.

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