Hello! Have had my Phoenix Red exterior, black interior, black top 99 XK8 convertible since mid-February of this year. Very low-mileage (22.5K miles then, 24.7K miles now) car owned by a collector in Dallas, TX; looks and drives like a new car! Fixed some of the typical electrical issues, new battery in early March, Duracell brand. Car has been reliable until the recent May/June heat-wave here in D/FW. The issue is this: The car will start as usual at about 6:30 am in the morning when I go to work, but if I drive it more than twenty minutes or so with the A/C on at about noon or after, I can't get it to start unless I open the hood to let it cool! I can put the key in the cylinder and turn it, but the car is completely DEAD!! The steering wheel won't move to its stowed position, the windows won't move, doors won't shut and lock; also the red lights in the oil gauge and battery/volt gauge are on, and sometimes the red light by the shifter will blink. No warning messages when running, all "gauges" read normal, no overheating, oil pressure good; the only way I can get the car to start is to open the hood, open the trunk (no trunk lights either!), do a hard reset, disconnect the negative battery cable, and let it sit for at least an hour! When I can restart it, battery/volt gauge stays right at 13V, all other gauges "normal"! Tested battery myself; 12.6V with car off, 13.5V with car on and A/C, radio, headlights, and any accessory I can think of turned on, 14.2V with car running and all accessories turned off. I'm going to take the car to the Ford dealer where I work at in the mornings to see if they can diagnose the problem! Hoping it's just a bum battery; if that's the problem, I'm going back to where I bought it and exchange it for an AGM battery! Thanks for any help you guys can offer!

 

None of the three centre gauges read proper values ..they are fake gauges ..you could change the temp sensor in the top radiator hose ..cheap if you are sure its temperature.

 

Yeah, I know about the "gauges"! There's no sign the car is actually overheating, but I'm thinking something underhood is getting too hot and shutting-down all power until things cool-off! I've checked all fuses and relays underhood, and all look good, battery checks-out, both cables are tight and in good shape, so I'm stumped! I left the negative cable off overnight, hooked it back up this morning, lo and behold, trunk lights came on, car started right up! Let it run until it got to warmed-up temp (halfway on the temperature gauge), shut it off, then tried to start, and it wouldn't start! Dead again! Went back to the battery, checked all connections (took about ten minutes), and as I was checking the trunk lights came on! At that point I tried to start it, and it fired right up! I've got the car on the battery tender now, and I'm not going to try anything else until tomorrow morning when I'll see if it will start!

 

If the temperature sensor thinks its hotter than it is then it wont incrase the fuel for starting ie Choke in the old money.

 

Will the ECM shut-off all power if the coolant temp sensor sends an engine overheat signal to the ECM? That's the only thing (besides the battery or alternator) I can think of now, because when this happens there is NO ignition, no power, no warnings, and car won't crank!

 

Its not an overheat signal its a signal saying its hot enough not to need fuel enrichment so it does not start ........may be,,,

find me a wall I need to bang my head on it .

 

When this happens immediately check battery voltage. If lights in trunk are going off then it has to be related to battery. No sensors will completely shut down the electrical system to the point of no power to basic things like lights. My guess is battery has a cell that is faulty.

 

Ding, ding, ding!!!!! You're the winner! Lucky for me I work for a company inside a Ford dealership! Trickle-charged the cat overnight, started fine this morning, but when I got there I tried to re-start it and it was dead again! They diagnosed it as a dead cell in the battery; replacement is an AGM battery, so this one should last longer than the 3 months the previous battery lasted! Took the dead battery to the place where I bought it; they refunded me 100%. but not without a slight fight.....their testing equipment showed a fully-charged battery (12.6V) with no dead cell! I told them I trusted the Ford tech's diagnosis; maybe my car is allergic to their battery! Anyway, all solved! Thanks for everybody's help!