Jaguar Repair: 1984 xjs jaguar, rich fuel mixture, throttle position sensor

Question
just had a engine replaced (12 cyl) runs smooth as a top. was standing by the tailpipe and black smoke was all over my pants. and when I drive, only when i step on gas to past I would notice a puff of black smoke coming from talepipe i get very bad fuel mileage, yet it does not shake or miss it runs smooth

Answer
Hi Labron,

Black smoke is an indication of a rich fuel mixture. A light puff of black smoke on a quick throttle opening is not uncommon and accounts for great quick throttle response. However, fuel injection and CATs tend to eliminate the black smoke.

As for fuel mileage. You bought the wrong car if you want any fuel mileage.

If you are getting any black smoke after warm up without a quick throttle opening, then you should be able to improve you mileage by correcting the fuel mixture.

A fuel injected car has several items that can affect mixture. The items that have the most influence on mixture are, Fuel pressure regulator, air mass meter, injection coolant temp sensor and throttle position sensor. (you should also inspect the vacuum lines to the pressure regulator/s for raw fuel, check both O-2 sensors and check both cold start injectors for dripping leakage.) Also let it isle for a good length of time and shut down and pull all the plugs and look for flat black powder carbon on the plugs. If all are flat black what ever is wrong is wrong for all cylinders but if only a few or one is flat black, then that is where the over rich mixture is at. It is possible that the ECU itself is at fault but that is very rare so everything else should be looked at first. Check the charge rate at the battery too. (13.8v to 14.5v)

Howard

Howard