XJS HE rough starting

Hoping someone can give me a clue... for some months my 84 XJS V12 HE had been starting with a very rough idle and frequent stalling (requiring some pumping on the accelerator pedal to keep it running) when the car was left sitting a couple of days; it would smooth out after 2 or 3 minutes as it warmed up, but I really gave the starter (and my right leg) a workout doing all those restarts. Once it smoothed out, it was fine and ran and started strong the rest of the day, and would usually start OK (albeit just a bit weakly) the next day. Left overnight at the shop several times, the car started OK for the mechanic (or so he said).

I just returned from an extended trip and could not get past the initial rough idle when I tried to use the car today. Starter cranks fine and it will fire weakly when the key is turned, but it stumbles very badly and seems to be hitting on only a cylinder or two, necessitating rapid pumping of the accelerator pedal... it will immediately stall if the pumping is slowed or ceased. I was not able to get it to smooth out, my foot and leg gave out from pumping, and I had to take a break to recharge the battery at one point, but the engine never did get beyond the coughing and stumbling stage.

Fuel pump? Fuel filter? Ignition? Some sensor or other gone bad?

Anybody else run into this problem?

WayneC

Reply to
WayneC
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I can't speak specifically to a V-12 but your description is a symptom of an inoperative cold start circuit. All engines need a richer air / fuel ratio when started cold. When cold, fuel injected engines are given extra fuel for their first few seconds to 60 seconds (approx.) of operation depending upon the engine's temperature at the start. This problem will get much worse when the winter arrives.

Basically, a generic cold start circuit consists of a temperature sensor, a relay that is ON for a longer time as the temperature is lower and an electric fuel valve that adds the fuel to the engine's intake manifold. One of these parts is either bad or disconnected.

Your pumping the accelerator tricks the fuel injection system to add some fuel because it "thinks" you want to accelerate the engine, but it's no substitute for the real thing.

Reply to
T.G. Lambach

I appreciate the reply.

Reply to
WayneC

Reply to
Pamela and Chris

Hi Wayne,

I don't know what all you've done in search of a solution for this starting demon, but I think that we talked about this car in the past.

If you have -

Checked the starting circuit and found the thermoswitch, relay, and valve to be correct.

Inspected the distributor mechanical advance for proper operation and checked the timing.

Verified that both fuel regulators are working correctly.

Cleaned the harness ground by the radiator, checked pin 1 at the 6cu computer, then checked the resistance from the harness connector, pin

1, to the negative battery clamp.

Have pulled hair out, spent lots of cash, and pissed many a night away at the local tavern.

Then.....

Perhaps your 6cu computer is to blame. These early computers could not supply the energy needed to fire the injectors, so an additional circuit was added to boost the energy. It is quite possible that the additional curcuit works when it gets around to it. I would personally open the computer and inspect for cracked solder joints and resolder if needed.

Got to go!!!!

Cheers, and good luck!

Blake

Reply to
DieInterim

I know this is a little late, but I had the very same problem.

Replaced the 6cu with a 16cu from a scrapper and the car ran perfectly again.

I did have a slight advantage though, I was running the XJS and a V12 saloon and could swap the parts until I found what was wrong!

Best regards,

Reply to
Gerald Harper

Hello,

Agreed. The 16cu *is* a *real* modern computer (all digital) compared to the half modern half stone age 6cu.

Cheers,

Blake

Reply to
DieInterim

Just out of curiosity, do you happen to know which of those would have been used in a '91 (build date 10/90) XJ-S?

Thanks...

Reply to
Philip Bailey

Hello,

Here goes (To the best of my knowledge):

6cu 1980 - 86 16cu 1985- 90 26cu 1990- 93 36cu 1993- end

I would guess yours is a 26cu, however, it is possible that a 16cu got stuffed in there.

Best Regards,

Blake Dodson

Reply to
DieInterim

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