Check the temp sensor on the left bank, near the thermostat housing. Make sure it is connected, and the wires are not broken as well. Also, on the crossover pipe at the rear of the engine is a small diameter vacuum hose. It runs to a metal pipe that you can't see, and from there to the trunk. It usually has a small reservoir in line back by the battery, and it should connect to the ECU in the boot. I would bet on the temp sensor though. They used to fail a lot.... If the circuit is open, the ECU thinks it is 40 below zero and adjusts the fueling to suit.
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Jesse Bogan
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'93 XJ6
'89 XJS coupe V12
'87 Audi Turbo Quattro Wagon
'67 Chevy Corvair coupe
Uh ohh 64 Chrysler newport hardtop wagon