1991 xjs v12 high idle speed.

[dave01]

when fully warmed up idle speed is 1000rpm in neutral or park, when in drive or reverse it drops to 800 with foot on brake. this fault just suddenly happened, the car was fine yesterday.any ideas?

 

[Exotexs]

hi, what do you mean by "fine yesterday"? did it stay at 1,000 rpm when in Drive or pressing brake pedal? the transmission places a load on the engine when in Drive or Reverse, dropping some rpm's as a result, so the drop seems normal, my 6 cylinder does it too, also when I place the foot on the brake, the booster places a small load dropping rpm's too. When I turn the A/C to ON, it also drops rpm's due to the compressor clutch grabbing. a drop of 200 rpm's seems normal unless the engine starts coughing or stalling.

 

[Michael J.]

Look for a vacuum leak!

There are several vacuum lines that connect to front of L/H intake manifold and to rear of R/H intake Manifold.

There are two vacuum lines connected to the L/H Throttle body and a vacuum switch on the l/h water rail at the front that could be loose.

There is a Y shaped vacuum line on top of the fuel rails that also connects to the PCV valve. Check those hoses for condition.

It can also be the Extra Air Valve on the rear of the L/H water rail where the idle speed is set. Two hoses connect to it. one from under the L/H air filter housing and one is a short elbow to the back of the L/H intake. Of the two, horizontal hose is easier to remove. Spray some WD-40 in the opening. The valve has been known to stick.

There is a vacuum line to the transmission modulator valve. Check where it connects on the R/h rear of the transmission. It is possible that a rock was kicked up and knocked the hose loose.

On the R/H Throttle body is an unused vacuum port. Check the rubber cap for proper seal.

A small idle speed increase sounds like a vacuum leak to me.

 

[Exotexs]

he's not experiencing an INCREASE in Idle Speed, but a DECREASE which sounds normal to me when you load the engine from Neutral to Drive.

 

[Michael J.]

Perhaps, but the idle is too high in park for the V-12. It is about right in drive at a stop.

 

[John S Farrington]

I would recommend treating the symmptoms in this case as I dont think this is fatal. Adjust the idle speed down to 800 or 900 RPM via the idle speed adjustment screw on the rear left hand side of the motor. It is a hex screw in the idle speed control off of the left intake manifold. The reduced idle speed should cause a lesser drag on the motor when in drive or other loads from the sub systems.

 

[dave01]

maybe I should have said that sometimes the idle speed in p or n is 800 othertimes 1000 also what should idle speed be when engine is started from cold?

 

[Michael J.]

Owner's manual says idle should be 850. I don't think that 1000 is too high if starting from cold and gradually dropping as the engine warms up. I still suggest spraying some WD-40 in the Extra Air Valve. I have experienced a similar problem that appeared to be the valve sticking.

Since you describe this as having a sudden onset, can we assume that previously, when starting from cold idle was about 800?

Could still be vacuum leak, but only if the idle stays high all the time even when warmed up.

 

[dave01]

when starting from cold rpm is around 1150