QuestionQUESTION: Good Day. I have been getting a 1984 SIII 4.2 XJ back on the road after it was in storage for 11 years with PO. I have found most of your archived questions of great benefit. It had classic "no start" issue (New coolant temperature probe fixed that). Have sorted fuel supply system (now flows back to correct tank etc.) It still runs very rich though - as I live in a warm climate (seldom below 16 deg C) can you explain the operation of the thermotime enrichment system - is it possible for it to stay "on" in error? How can it be tested and can it be disable dif "stuck on"? I am thinking that it may not be needed at all for this car in its current environment. Thank you.
ANSWER: Hi Mark,
The Cold Start injector and the Thermo-time switch that grounds it, only operate while cranking. However it is possible for the cold start injector to stick in the open position as it has fuel pressure at all times. The cold start system works independent of the injection system other then using the injection system fuel supply. The easiest method to test to see if the cold start injector is open (for what ever reason) is to pinch off the fuel hose to it.
The common causes for a rich mixture in the injection system are, coolant temp sensor, high fuel pressure, failed O2 sensor, stuck open injector, failed throttle position sensor, failed air flow meter, incorrect voltage due to Alternator / regulator or a cable problem (power or ground)
If it were mine and I noted a rich mixture I would clean the plugs and start it up and run it for a short time until you note a rich mixture and shut down and remove all the plugs to see if all cylinders are rich (flat black color) If they are all rich you can rule out a stuck injector other then the cold start injector. If they are all rich, start testing. Fuel pressure first. About 32 PSI running with vacuum on the regulator and 42 PSI with vacuum removed.
Keep in mind that the coolant temp sensor has the most influence on mixture along with the airflow meter. The throttle position sensor has a lot of influence but is only a full throttle switch and should have no influence at anything below full throttle (unless something is wrong with it) the O2 sensor has only minor influence and the voltage it produces (when heated) is .5v (normal) higher then .7v is a rich signal and below .4v is a lean signal.
Howard
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QUESTION: Dear Howard,
Thank you for your advice. I did the start-stop test as suggested. This pointed to a very rich mixture on all cylinders. I checked the fuel pressure and that was fine. Cars out here never had O2 sensors so I could dispense with that as a cause. I found the throttle switch about to fall apart and replaced that. I then had the car put on a scope by the local Bosch service center (as you have indicated in many previous posts) and they had to re-set the AFM as it had been "tampered-with" by PO and they showed me how the spring had been relaxed so much that the mixture and timing had to be re-set quite drastically. I replaced the plugs with 6 new Champions as well and have replaced all fluids/oils and filters. She is now responsive, smooth and an altogether happy cat as far as the engine operation goes. Fuel mileage is now about 29 mpg (imperial) on a run I did over the past week-end. Thanks once again.
Another question if I may? I found a blanked-off vacuum pipe hanging down in the engine bay against the right firewall in line with the rear of the inlet manifold (we are RHD here). This is connected to a T-piece - one part then runs to the interior, the other leads to what I assume from diagrammes is the vacuum reserve tank for the heater system controls(tucked into a space against the right wing) There is a vacuum tapping at the rear of the inlet manifold with a pipe that runs to an (obviously aftermarket) tube joining it to a further tube that runs below the inlet manifold. I suspect that the pipe from the tank should somehow be connected to the engine vacuum and that this is where it should fit via another T-piece? As the heater seems stuck "on" and no adjustment of the heat setting makes any difference, I thought this might be the root cause and would like to check before going off at other items that may well be OK but lacking some operative vacuum "power". As the car barely has 70K miles on it and the interior is pristine, I doubt that much could be wrong in there aside from congealed lubricants on actuators/flaps and blowers. Thanks once again for your kind assistance and the time you took to answer my initial question. I appreciate your interest in helping us all get our cars in good working order.
AnswerHi Mark,
Your car sounds great. I am not sure our vacuum system is exactly the same and none of my books ever had a good vacuum diagram in them for the engine compartment.
There should be two vacuum ports in the firewall. One is for vacuum supply to the heat/AC system control doors and the other is for the heater control valve.
I drew a vacuum diagram from a 83 Series III that I felt was close to correct. I have put it on my web site for you to copy if it is of any use.
I wish I could get that kind of fuel mileage. It may be because you are on the bottom of the world and your cars are much lighter where we are pulled down hard by gravity and our Jags (tanks) only get about 20 MPG US. Here is the address to the diagram. You can also cut off the /JagVac1x1.jpg and look at my web site. (Mostly car, motorcycle and airplane stuff) Also a Tech tips section for car stuff.
http://mg-tri-jag.net/JagVac1x1.jpg
Howard
