Idle issues ,after car is at operating temp

(Vince) #1

After arriving at destination and turning vehicle off ,hard to restart ,
It’s push throttle down about a third . After starting idle seems to hunt , then die if more throttle is not applied. Any ideas ?

(Paul M. Novak) #2

What year is your XJ-S and what engine does it have?


(Vince) #3

1987 xjsc v 12 . Automatic 53000 miles .

(Steve) #4

Classic symptoms of injector that is leaking post shut down and the pressure in the fuel system is lost, allowing evaporation of fuel.

You’ll probably have to invest in a fuel pressure gauge.

Could also be the fuel check valve, and on your car it should be external to the fuel pump, so, it should be easy to check.

(Paul M. Novak) #5

Does this problem occur only on hot days after a spirited run? Or any day, even cool ones, after a short run? Does your air conditioning work properly?

If this happens all the time regardless of temperature I suspect that Steve is correct about leaking fuel injectors after shutdown resulting in too much fuel in tge cylinders to restart the engine. If it just happens on hot days then it might be vaporization of fuel in the fuel rail due to heat soak creating a “vapor lock” loss of fuel pressure.

Checking fuel pressure at the fuel rail is a good place to start. Another thing to check is your Thermal Vacuum Valve on the left side of the fuel rail to see if it is working. It is suppose to increase fuel rail fuel pressure to prevent “vapor lock” when the fuel rail gets hot, but if it has failed then it won’t.

Fuel returning from the engine to the fuel tank goes through a fuel cooler on the left side of the engine. The fuel is cooled by the air conditioning system. If your air conditioning system is inoperative then the fuel is not being cooled as it returns to the fuel tank. The is not a problem on a cool day, but on a hot day with a hot engine lack of fuel cooling will contribute to a higher fuel rail temperature making vapor lock more likely.


(Vince) #6

Will have to round up pressure gauge. Air does work but generally do not use it . Mostly go for drives early in morning , but last two problem drives were mid day and very warm . Maybe heat related . Will try air on this weekend . Thanks for info.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #7

'87 V12? I’d expect the check valve to be internal to the pump. I’d also expect it to be pretty reliable – but it’s not expensive to ADD a check valve in the line after the pump, which might confirm if this is the problem.

(Steve) #8

Apologies for the mistake. My knowledge on the earlier cars is only from what I’ve read here and apparently I don’t know my 'abc’s
I have a 6.0L with the two submerged pumps (one for low speed driving the second one kicking in at above 2850 rpm) and thought that only this set-up has the check valve integral to the pump assembly…

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #9

Most if not all XJ-S’s have the check valve internal to the pump, I think. Early XJ6’s had external check valves (screwed into the pump) and they were a notorious source of trouble.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #10

Note that, pre-6.0, the check valve was internal to an external pump! Later cars it was internal to a submerged pump, a greater level of grief to service.

(Steve) #11

I think this was the reason for my confusion. Thanks for clarifying