Hard starting issue and fuel pump prime issue

Hello everyone its been a while since I last posted, I have been daily driving my XJ6 and it drives great! however I have had this long start issue since I got the car, and no matter if it’s cold or hot, it will take at least 5 seconds of cranking for the car to start when cold and I have to try a couple times when its hot. I have done some trouble shooting on and off for years because its only and annoyance as the car will always start eventually. I believe that I have isolated it to a fuel issue. When I force the pump to prime (which has its own issue I will talk about later) the engine will start almost immediately, no matter if it’s hot or cold. Additionally, if I shut the car off when cold or hot and try to restart it, it will start almost immediately.

The issue I suspect I am having is lack of fuel pressure in the rail when starting after sitting for an hour or so. for one reason or another. I suspect that it is either the one way valve in the trunk and/or the fuel pump is not actually being told to run when cranking the engine, only when there is enough airflow through the AFM to trigger the pump relay.

Why I think it might be the pump not running on the crank is I am unable to get the pump to prime when the key is set to START in Drive,Reverse,2,1. I can only hear it prime if I disconnect the white and red starter wire from the starter relay and turn the key to START in Park or Neutral. I am under the impression from the Fuel pump circuit checklist guide that the pump should run when in Drive, reverse, etc. but it does not.

List of new parts that have been installed and made no difference to this problem:
6x new injectors
new fuel pressure regulator
new coil and pertronix distributor
new/used fuel pump diode pack (made no difference)
cleaned throttle body and checked clearance
new fuel pump
new fuel filter
checked AFM with spare (no difference)
new cold start injector

Anyways sorry for the long post! I have been trying to diagnose this issue for ever and the car runs so well now that I find it very annoying to have to try cranking the car multiple times before it starts.

If the lever is not in P or N the starter is disabled by the neutral safety switch, but the pump should run in the start position bypassing the safety devices. If not, there is a wire from the starter relay to the fuel pump relay (foggy memory). Maybe that’s fallen off.

May not solve a somewhat typical takes-a-few-cranks-to-start issue (my S1 starts immediately, S3 almost never did). But the pump should definitely run during cranking no matter what the AFM does.

I like that you daily it. Good for you!

1 Like

Thanks i’ll do some research on that. I just performed a fuel pressure test and believe that I have found an issue. first of all the idle fuel pressure sits at 30-31 psi, also when I shut the car off fuel rail pressure drops immediately to 0 which I assume it shouldn’t. I think I have a faulty one way valve. ill report back with more info after I finish tests.

At the starter relay you should see two white/yellow wires: One from the ignition switch to the relay and one from the relay to the diode pack. Make sure the one to the diode pack is intact, making a good connection, and giving the diode pack 12v when the starter is engaged

Should hold pressure for awhile. Worth investigating.

But a healthy fuel pump will repressurize the fuel rail in just a couple seconds IF the pump is operating as you crank the engine


I agree I tried some local parts stores for a generic one way valve but no luck so I’ll just order one

thankfully watching the gauge while cranking, it repressurized the lines in about 2 seconds, so I know the pump is working while cranking. Which is kinda strange, considering it doesn’t prime in gear. I’ll install a new or generic one way valve and see if that helps much.

What is the pressure supposed to be? 31 psi seems quite low.I would expect 45-50? I’m thinking it’s the FPR. It’s leaking back to the return line, constantly. Thus, the pump gets to some of the pressure, but the additional 15 psi is bled off by an internal regulator leak. The low pressure while cranking isn’t enough, generally. And when you turn off the ignition, the remaining pressure quickly bleeds off to the return line. The one way valve in the fuel supply line wouldn’t be indicated as a sole cause if the injector pressure is so low.

Good idea on GP to replace the check valve. But if fuel pressure comes up in just 2 seconds but 5 seconds of cranking is needed to start the engine, well…there might be something else afoot.

TBH my XJ6 was never an instant starter nor have been any others I’ve driven or worked on. Five seconds seems a bit much but finding a correctable fault might be difficult, as you are discovering

Decades ago my XJ6 was suffering from extended cranking. Long story short, a new battery fixed the problem. Presumably the voltage drop while cranking was having an ill effect on the fuel injection system. Still, though, never an instant starter.

Curiously, I discovered (on mine and others) that cranking the engine for a few spins, releasing the key, and then cranking again would produce an instant start…much faster than just holding the key to start. I never knew why this happens but was able to confirm that it was not fuel pressure related.


36.25 PSI, which will vary a bit while driving. But the spec is 36.25 PSI while cranking. So…there’s something to look at, although it’s stated that a new regulator has been fitted.


Oops, missed that. I am surprised the spec pressure is at 36.5. I was going by what I remembered from my old XJ40’s and my Polaris RZRs. Still low at 31, but :person_shrugging:. So I guess the check valve WOULD be the next thing to look at. Is it integral to the pump?

from some research other seem to be getting about 28-30 psi at idle so I think that’s normal. when i take the vacuum line off of the regulator the pressure shoots up to around 38 PSI.

Not integral. It’s seaparate; attaches to the air bleed valve in the boot. NLA from Jaguar but I’m sure a generic inline check valve could be fitted into the supply hose somewhere along the line

Part number CAC2027 in the upper right corner of this illustration:



Good to know!

The CAC2027 is indeed a check valve but the Bosch fuel pups normally have also a check valve integrated in them.
On my car I gutted the check valve from the CAC2027, it was noisy, and added an external check valve.

As for the priming issues, I added a 12cec delay relay parallel to the ECU pump circuit and has helped a lot and very easy to do.
Like this I turn the ignition On and just wait for a few seconds until I sart the engine and it starts right away.

As Doug says, William - the pump should run in ‘crank’ whether or not in gear. This is not ‘priming’ - it is just the way the pump should work.

It is very important that the co-connected white/yellowsare powered in ‘crank’. As Doug says, one is connected to, and gets power from, the ign key, the other activates the fuel relay through the diode pack. But also; this white yellow, when powered, also activates the CSI and tells the ECU that you are cranking - increases fueling…

All this may influence starting; while the engine cranks it draws air - which may operate the AFM flap reed switch, which then activates the pump relay through the diode pack to run the pump. In gear, as the engine is not turning; the flap does not move - no pump. Incidentally, when hot or recently cranked - the engine spins faster and draws more air though the AFM…

The rapid leakdown points indeed to the one-way valve, but the running pump will, as observed, repressurize the rail - with minimal delay in starting. Not the main cause of delayed starting…

xj6 85 Sov Europe (UKNZ)

The quest for faster starting has resulted in many interesting inventions over the decades. I once divined a priming circuit that operated the fuel pump for xxx-seconds whenever the driver’s door was opened. I was a bit proud of myself…but I don’t recall that it made starting faster. I ended up removing the system at some point. This was over 20 years ago so I can’t remember all the details.

The only thing that ever helped was replacing the battery !

The V12 fuel pump circuit is entirely different…which might confuse matters in this conversation.

In my experience, seeking instant starting in these cars is a lot like trying to achieve that perfectly (and consistantly) smooth idle: a fellow can spend a ton of time and money and, as often as not, show little or nothing for his work. Sometimes a clearly indentifiable fault cannot be found. Other times a guy stumbles across a fix accidentally, in the course of trying many different things.


It should be possible.

I started my XJ after it sat for a week.

My Mercedes has automatic starting so I‘m used to immediately let go of the key. I didn’t think much. After waiting for the pumps to get to pressure I did the Mercedes routine.

The engine was running. I didn’t hear it cranking, it just fired. it often starts that quick. My S3 never did that, even though theoretically it should’ve been faster being injected.

How about a shot as the key is turned to the first position?

Which is how the V12s, with their ECU-controlled fuel pump, are configured.

Anyhow, if you have (or feel you have) a problem with extended cranking then low fuel pressure is the low-hanging fruit which you can, and should, address. If doing so gives the improvement you’re looking for then you can celebrate.

If no joy then you can try any number of other things. Maybe you’ll get an improvement, maybe not. Like chasing-down idle-quality oddities it can be a test of a fellow’s grit and determnation…and depth-of-pockets :slight_smile:

Why some cars start so quickly, and others don’t, has long been a mystery to me. My V12 will start instantly …and I mean instantly… after sitting overnight. But, as is common, it takes a bit of cranking to start when hot. My previous V12, in an XJS, was the exact opposite. Go figure! I’m sure there’s an explanation for these things if we have the patience to dig deep enough.

I’ve long wondered how age (decades of age in the case of our 80s Jags) might slightly degrade electronics and thus contribute to extended-cranking experiences. IOW, not a single fault but a stack-up of slightly degraded circuits, transistors, solder joints, etc. ? I dunno; just musing; but it seems like a plausible explanation. IF that can be identified as a cause then what’s the fix? Fereting such things out, and fixing them, is certainly beyond my skill level. And, frankly, if it’s just a matter of reducing cranking time from 5 seconds to 2½ seconds, beyond enthusiasm level as well :slight_smile:


1 Like

Funny. I have experienced the same thing…

On the 6 the pump circuit is designed to only run in ‘crank’, David - and in ‘run’ if the engine is running…

Even if the non-return valve is working properly, rail pressure will likely drop over time due to normal injector dripping - or of course other leaks. There is very little fuel involved - so the pump will raise fuel pressure very quickly…

If the non-return valve is not working; briefly ‘crank’, then pause and again crank - the pressure would instantly drop, and the process is basically waste of time. However, to complicate matters; the CSI only sprays in ‘crank’ for a limited time - and only once unless time given for reset. And the ECU, sensing ‘crank’, will adjust fueling during cranking…

Adding the influence of settings, the combination of idle settings, air temp, the AAV and general engine condition; there is plenty of variants to cause a wide, though still ‘normal’, variation of time required for actual catching…

In theory; the same might happen on the V12; if the non-return fails open - a pause between the 2 second pump ‘run’ and cranking for effect will drop pressure. But having 12 cylinders the V12 is prone to quicker starting anyway - and the pump is equally able to quickly raise fuel pressure…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Ok small update, I have installed a one-way valve and it doesn’t appear to make all the much difference. I decided to take a look at my AFM to bleed adjustment and found that it was cranked all the way down, so i backed that off 3 turns as that’s what the engine seemed to like the most. I have to do more testing but it appears that this has made a small improvement in cranking time. While not instant it seems to start at in an acceptable amount of time. however, it still needs a lot of cranking when it’s hot.

This is the EXACT symptom i have when hot. If i crank it and don’t let go it will start but only after 6 or more seconds of cranking. but if I crank for 1 or 2 seconds then release and try again it will fire the first turn of the starter. Very strange behavior and to me now thinking about it points to a weak battery or some part of the ignition or fuel/ECU system not getting enough power. I wonder if I were to put a new and larger battery in the car if that would make a significant difference. Or if there like Doug said is some form of degradation in the ECU’s components. At this point the entire fuel and ignition system is brand spanking new with less than 3k miles on it. I have made sure there are no vacuum leaks and replaced the entire vacuum system with silicon vacuum lines. I might consider grabbing a spare ECU from a junkyard car to see if it makes any difference.

Obviously, this is somewhat of an obsession of mine, which has turned from an annoyance to almost a fun challenge to find the issue. Especially since it seems that other people have the same issue with no discernable fault found…yet.