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XJS GAS MILEAGE/FUEL PROBLEM

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J

John Von Vleck

Freshman Member
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Hello--Could use some info

I have owned a 1987 XJS V12 for 2 years--Was
getting 15-17MPG--NOW I AM ONLY GETTING 8MPG!!
COULD A SIMPLE(MAJOR)TUNEUP SOLVE THIS PROBLEM??
It is not running bad, though it could use more
power. WHAT SHOULD I CHECK?? Your ideas would
be greatly appreciated.
 
Hi John, welcome to our humble forum!

Certainly a good tune up would be a good place to start. Then, with new plugs and a good tune up, drive around a day or two and then pull the plugs. Look to see if any or all of them are very black and sooty. If so, you are running too rich and need to investigate your carbs (if I remember, doesn't your car have 4 Zenith carbs, or is it fuel injected?) Be sure to chage your fuel filter while your at it. If just some plugs are sooty and others look ok (light brown deposits), investigate the carbs that feed the offending plugs. Also, check around your carbs, fuel lines, pump, etc. to make sure you are not leaking any gas.
Basil

Just some suggestions.
 
John, A fuel injected V12, 87 like yours should get an average of 15 MPG as your original economy yeilded. You are probably experiencing a need for a tune up or a fuel injection problem. First change out your spark plugs, wire set and possibly your distributor cap and rotor. If that does not improve the mileage then look for a fuel injection fault. In order of priority, that means the coolanat temperature sensor or high fuel rail pressure (over 40 psi) The latter can be caused by a faulty pressure regulator or blockage of the fuel return line. Let me know how this turns out for I will have other recomendations if your mileage does not return to normal. Jack
 
John,
Are you the guy I bought the Toyota speed sensor from? (I'm terrible with names)

Basil
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by John Von Vleck:
Hello, Basil

NO you DID NOT get a toyota speed sensor
from me...You did give advice on my XJS
V12.

John V.V.<hr></blockquote>

Oh, ok, nevermind then. I only asked because I recently met a guy locally who had an XJS and invited him to the forum - couldn't remember his name and thought it might have been you.

Basil
 
Take a look at the vacuum hoses that provide vacuum to the fuel regulators. The fuel pressure in the rail, especially on the left hand regulator which dumps excess pressure, are vacuum operated.

See how clean the air filters are.

Disconnect the battery and pull both of the fuel system relays. Temporarilly reconnect the battery. If the fuel boost pump comes on with both relays removed, the O2 sensors, one or both, are sooted up or are worn out. With this type of failure you will default to very rich.

Michael J.
 
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