I have one of those Series III XJ6 cars that requires I put the gear selector in Reverse and then turn the ignition to start for a few seconds to "prime" the ECU/fuel system before I put it back to Park to start it. After doing so, the engine starts with a lot less cranks.

I was recently reading about the common grounding point for the fuel system at the back of the water rail on the right side of the engine.



and was wondering if it made any sense to connect this bolt directly to the negative terminal of the battery via a long ground strap rather than relying on the grounding path through the engine block to the chassis and back to the battery. Could there be any downside to doing this?

Is there another known fix for reducing cranking time other than the gear selector trick?

 

I see no downside to grounding those wires as you've described.

Along the same lines check for voltage drop to the ECU when cranking. The ECU really likes full voltage. Long story short I fixed a "extended cranking" problem on my XJ6 by simply replacing the battery....after pulling my hair out pursuing other causes.

There's a anti-drainback valve on the air bleed fixture in the boot. It may have failed. They're NLA from Jaguar. An aftermarket valve could surely be plumbed-in somewhere. Might help but, OTOH, a healthy fuel pump will pressurize the rail in just 2-3 seconds. A fuel pressure test would be helpful here.

Cheers
DD

 

Have you tried simply leaving the ignition key in the on position for the same few seconds to allow the fuel rail to prime before turning it to the start position?

I've done that on all three of my V12 VDP's and simply use the time to put on my seat belt so it's "part of the routine".

 

E21, I put these on my bmw's and jags. On the jag I'd try to plumb it in at the front near the fuel rail supply line since it's pretty easy to work with. That said, check the new check valve buy blowing in against the valve (by mouth) for several seconds and see if there is any leak down, they can be iffy. That's why I chose the link with two... Also on my E24 bmw's I removed the main pump under the car completely, swapped the lift pump in the tank for a universal hi pressure pump from amazon using the original hanger, this got rid of the quarter tank blues these cars tend to have. I wish my jag had one tank...

 

My XJR has one tank but it’s not as sexy as having two of these or switching from one tank to the other and watching the gauge go from 1/4 tank to full or telling people “it’s a V12 and each bank has its own tank.”

 

The 6 cylinder cars didn't have the key-on priming feature. That's a V12 thing :-)

The method mentioned....turning the key to start with the gearshift in R or D.....is a clumsy but effective substitute

Cheers
DD

 

Precisely. I am the only one who performs this clumsy maneuver. The other drivers in the house hold just get frustrated when it doesn't start quickly -- like modern cars.

I should have mentioned that I have chased the fuel pressure problem extensively to where I replaced the fuel pump with a modern Japanese one and added my own check valve along with replacing all the rubber fuel lines, adding inline filters etc. It never really addressed to long crank times issue. The only thing that did was the above mentioned key to start/gearshift in R method.

Re: the extra grounding strap. I did notice the grounding bolt at the back of the water rail. I see that there are 4 thicker black wires and one thinner black wire grounded to it and all these black wires are part of what looks like the fuel injection harness. There is also a tiny ground strap that goes from the bolt to a nut on the intake (presumably to better ground it to the head).

Five black wires is an interesting number. There are six injectors and a cold start one as well. Makes me think they must be combining wires somewhere in that harness to get just 5. I know the engine runs perfectly smooth once driving so I assume there is no grounding issue with the injectors.

My problems seem to more ECU/voltage related.

I also wonder about the ECU/main relay itself. Mine clearly works but it looks old. Have they made any improvements in relay technology in the last 40 years or the standard 30A Bosch relay is the same as it ever was? I think the main relay gets its signal right from the ignition key so if there is any delay in powering up the ECU, it would be because the relay is really slow or the overall voltage in the system is really low while cranking. I do have a year old battery in the car. Changing the battery did not really make a difference in the long cranking issue for my car.

btw -- since that Bavarian brand was mentioned...my 40+ year old e21 still starts up like clock work. Never ever had an issue with it. Wonderful car but...it never turns heads like a Series III does.

 

The five ground wires at the water rail are not for the injectors per se. They ground several components, one of them being the ECU. The ECU in turn grounds the injectors

The diagram below will illustrate. Look at the bottom for those ground wires




If you wanted to do some digging check the voltage at the ECU terminal 10, which is the main power feed to the ECU....supplied by the main relay...which itself is grounded at the water rail

Cheers
DD

 

Now that I see this diagram, that grounding point is much more significant than I thought. It seems the ECU, main relay and some of the sensors all ground at that one spot and it is right there on top of the engine. I will make sure to add a dedicated grounding strap to it that goes directly to the battery negative terminal. If nothing else, doing so might add a little voltage as seen by the ECU if this improves the ground reference.

 

Roger Bywater of AJ6 Engineering here in the UK mentions, in a long article about the EFI system for the XK engine that the cold start injector fires for about 2 seconds, the thermotime switch sending a voltage to it. This shortens the crank time for a cold start, but the engine will fire without it, but with a slightly longer cranking time. Fuel enrichment is also provided automatically by the six injectors firing for longer times when the engine is cold.

 

I would say the XJ6 starts two or even three times faster when the engine is cold - no complaints here. When it is warm, it seems like it cranks way too much before ignition. It's almost like ti takes a while for the injectors for engage during warm starting. I read once that all the injectors fire at the same point in time. Not sure if this is true or what event (timing I imagine) would trigger the injectors during the cranking.

 

I've never owned one or worked on one that I would call a "fast starter" when warm. They always seem to take 3-5 seconds of cranking ("one thousand one, one thousand two, one thousand three.....)

I don't recall anyone ever identifying an actual repairable fault. I certainly haven't

An anomaly I've noticed on these cars is that you can crank for a couple seconds, turn the key off, then try restarting. The engine will start with not much more than a tickle of the key ! Try it !

Cheers
DD

 

I put an inline liquid filled fuel pressure guage at the fuel rail on my '86 xj6, I see it every time I raise the bonnet. I know if it has fuel pressure or how long it's holding it just at a glance. Eliminates the tendency to assume some things. Did the same on my '88 bmw, no issues either exe that they tend to last 8 to 10 years before they need replacing (cheap).

 

On the Jaguar L-Jetronic installation, all injectors fire together twice per engine revolution. This is also mentioned by Roger Bywater in his XK EFI article.