My 1987 XJ-S V12 has been showing some undesirable behavoir recently.

When the engine is warm, i.e. has been run 20 minutes or so, and I shut it off, on the next restart I'll have to crank it about 5 seconds or so before it starts. Five seconds doesn't sound long, but it sure feels and sounds like it. It always starts, but it always takes longer than I think it should. In fact when it's been sitting overnight it'll start more quickly.

My mind wants to think vapor lock, but when the outside air temperature is in the low 40's fahrenheit it seems less than likely, especially since when it was in the 70's and 80's it didn't exhibit this behavior. So, either I'm on the wrong track or something has changed.

Other than the above, it runs fine and stays cool.

Guess I need to put a temp probe on the fuel rail. At what fuel rail temperature does vapor lock become a likely problem?

I suppose I need to check the thermal valve in the fuel rail to see if it's working correctly.

I suppose too it might not be getting a spark...what could cause that delay?

Thanks,

John
1987 XJ-S V12
63,000 miles

 

I agree that vapor lock doesn't seem likely under the conditions you describe.

I think the fuel temp switch operates at about 160ºF...but I'm foggy on that.

Just for the heck of it....

If your fuel temp switch is the non-electric type you can simply unplug the hose to the fuel pressure regulator.....as that's essentially what the switch does. It "opens to atmosphere" to dump the vacuum signal to the regulator.

Have you tried depressing the throttle a bit while cranking ? Although unusual for a fuel injected engine that's the method specifically mentioned in the owners manual for warm starting a V12. But, from your description this sounds condition sounds like a new development...and I reckon you haven't changed your starting procedure.

The ignition coil and module are both known to develop problems when hot but 20 minutes of driving just doesn't sound long enough for that to happen. Still, though, you could check for a bright blue spark at the plugs.

Do you get any black smoke when the car finally does start, or any other sense of over fueling/over rich mixture? If so you might consider drippy injectors. An already-warm engine would not like the extra fuel.

For that matter a skewed coolant temp sensor could be giving the ECU a "cold" signal. The ECU would respond by commanding longer injector pulse....which would result in over fueling a warm engine.

Other will chime in with more.

Something like this might be a little tricky to track down. The symptom isn't all that severe....which means the fault might be very subtle.

Cheers
DD

 

Thanks for that Doug.

I've not noticed any black smoke at anytime.

I do wonder if it's running rich just because I feel it should be getting better mileage (maybe 15 or 16 mpg) than it does (11 mpg). The last time I pulled a plug I didn't notice any remarkable carbon buildup, maybe time to try that again.

Ok...I'll admit to being fairly fuzzy on the vacuum system operation. So, if I unplug the hose to the fuel pressure regulator, what would be the expected result? Should it, so to speak, run rich, in the sense of sending more fuel through the rail thinking that it's vapor locked?

I've not tried depressing the throttle a little while starting. I'll see what happens when I do.

I can check the resistance values of the coolant temp sensor, I think they are in the engine setup guide.

What fun :-)

John
1987 XJ-S V12
63,000 miles

 

Low vacuum (or no vacuum in the case of unplugging the vac hose) = higher fuel pressure.

High vaccum (like at idle or part throttle steady cruising) = lower fuel pressure.

The theory behind the fuel temp switch is that the higher fuel pressure will overcome the vapor lock.

Excessively high fuel pressure can cause rich running, yes. Whether or not an unplugged regualtor will let pressure go high enough I can't say for sure. I wanna say "yes" but I don't know if an unplugged regulator = a totally closed regulator. A totally closed regulator would cause excessive pressure for sure as the pump can produce 74+ psi which is way outside of norm

Cheers
DD

 

FWIW- old fashioned 'vapour lock' doesn't occur on vehicles with remote electric fuel pumps. The phenomena is restricted to cases where a fuel pump on or near an engine becomes so hot the fuel inside boils and the pump loses it's prime, so to speak.

The issue was solved in the mid '60s on most cars by adding a bypass line from the pump back to the tank. The constantly recirculating fuel kept the temperatures below the vapour threshold.

Vapour lock only affects hot engine, not cold.

 

Right.

On old Jags, though, we have newer-fashioned vapor lock in the fuel rail. :-)

Or at least Jaguar was awfully worried that we'd have it as they used the fuel temp switches, in a couple different forms, for many years....and fuel coolers to boot!


Cheers
DD

 

Not sure how that can happen- unless the term 'vapour lock' is being swapped for 'percolation'?

 

John, turn the key to run and depress the gas pedal all the way down twice then turn the key to start with your foot off the pedal, it should fire right up. In the "run" position the pump primes the rail and the "pedal" will pulse the injectors which will in turn fire it right up when there is suction/compression/spark as there is a spray of fuel you just put in prior to turning it over. (I do the pedal twice for good measure) Try it and see if there is a difference.

 

I think you're on to something!

Cheers
DD

 

We have something resembling a winter storm bearing down on us, but as soon as kitty can go outside again and get warm I'll give it a try.

An easy solution is much preferred, so I'm hopeful.

Thanks!

John
1987 XJ-S V12, 63,000 miles