QuestionDear William Brady and/or Howard M. Fitzcharles
I would be most grateful if you could review and answer a question for me regarding a 1987 XJ12.
When started and allowed to idle the first 20 to 40 seconds is uneventful and the engine runs just fine and sits at 750 RPM as should be the case according to the emissions sticker. About 40 seconds into the engines operation the solenoid air valve (right side air filter housing) kicks in increasing the engines RPM to an unacceptable 1200 RPM. In addition to this high idle a slight oscillation begins to occur, first varying 50 or 60 RPM and gradually getting worse to the point where the engine idle fluctuates between high and low points of 300 RPM to 1400 RPM respectively.
At 300 RPM the engine has difficulty running, sounding like it is on its last stroke before stalling out and than all of a sudden springing back to life shooting right up to 1400 RPM where once again, after residing there for less than a fraction of a second, falls back to 300 rpm. This cycle continues until I decide to shift the vehicle into drive or reverse.
When shifted into drive or reverse the oscillatory idle still exists, however only to a very negligible extent. When shifted into gear, engine idle oscillates between 700 and 800 RPM. My guess is that this is because the problem causing the oscillation; becomes damped out when a load is placed on the engine.
I should also mention that the oscillation does not follow through when accelerating or driving the car. The vehicle accelerates and drives flawlessly. Idle problem returns gradually when stopped at a light or intersection for any appreciable amount of time.
In the parked state of the vehicle, the oscillation completely disappears as throttle is increased causing rise in engine rpm. Relieving the throttle and allowing the engine to return to idle is when the oscillation will again reappear as such.
I have checked all the following and have deemed them to be proper working condition.
• Auxiliary air valve (replaced with used one that works)
• Solenoid air valve at air filter housing
• Both left and right over run valves.
• Dump valve
• Distributor vacuum capsule
• Ignition timing properly set according to emission sticker (18 degrees BTDC @ 3000 RPM)
• Left and right throttle plates have proper clearance according to emission sticker. (0.0025 inches)
• No vacuum leaks were detected; search was thoroughly conducted and is on going to no avail.
• Throttle position sensor reads .62 volts at closed position and around 4.90 volts at full open position.
• Throttle position switch read continuity when throttle pulley triggers it
• Left and right O2 sensors bench tested using home mechanic method of connecting voltmeter and reading voltage when heated with a butane torch. Both sensors read 0.90 volts when flame tip applied and 0 volts when flame tip was removed.
• Coolant temperature sensor read correct resistance through out temperature range of operation. (There existed a small discrepancy of what Jaguar Service Manual suggested. At 100 degrees Celsius book value of 190 ohms, measured value of 240 ohms, discrepancy followed through on measured resistance from 100 to 18 degrees Celsius)
• Air temperature sensor
• Fuel Pressure read approximately 45 PSI at idle and maintained this pressure as engine was revved.
I did notice one peculiar result when disconnecting and plugging the computers MAP sensor at the manifold cross over pipe that connects left and right intake manifolds. (Exhaust fumes became incredibly noxious with the smell of unburned gasoline fumes, accompanied by this was a rougher idle that no longer oscillated up and down as described.)
Weather this is just assigning blame to a component as a result of an unrelated cause and effect or if it is in fact the sub system at fault; I am unsure. Only wanted to state as a simple observation I made as to provide the most information possible, in ascertaining the true cause.
Any advice to possible avenues I may have not investigated or possible culprits to this elusive problem would be greatly appreciated.
A most sincere thank you for taking the time to read my post, it is appreciated.
Regards Arjun Sinha
AnswerHi Arjun,
Your elaborate symptoms don't tell me what is wrong but if I were faced with all that, I would use some basic facts to help find such a problem. If you don't have a scope and CO equipment, do this.
Warm the engine up first, then put a volt meter on an O2 sensor while it is in it's high oscillations and read what the 02 voltage is. Check both. A .5v reading would be a normal reading if the mixture were correct. High readings are a rich mixture and low readings would be a lean mixture.
If you have any gasoline engine at idle there are only a few things that can make it go faster. Lean out the mixture, advance the timing and / or allow air to enter the intake manifold.
When it is in the high oscillating mode start pinching off vacuum lines and block off the Aux air valve. Don't forget the brake booster. An engine can't go from 300 RPM to 1400 RPM without additional air.
Also, your injection coolant temp sensor is a major influence on mixture at all speeds.
If your 02 voltage indicates lean, try disconnecting them when it is in the high oscillation mode. If they indicate lean, try spraying something like WD 40 into the intakes when it is in the high oscillating mode.
When the engine is warmed up and at idle pinch off the vacuum to the fuel pressure regulator. If the RPM increases the fuel mixture is too lean and if it slows down it is too rich. (unlikely)
Let me know,
Howard
