Oldsmobile/Buick Repair: Gas Mileage on 1995 Olds, mass air sensor, head gasket problem

Question
I have a 1995 Olds LSS (not S/C) with 232,000 miles.  The motor runs like a top, except for a faint crank bearing rattle.  A friend (who is a BMW mechanic of over 26 years) and I recently replaced the following:  upper intake plenum, mass air sensor, throttle position sensor, camshaft position sensor, coolant temp sensor and the o-rings on the fuel injectors.  Before that, I replaced the radiator and waterpump, so I know the coolant system is externally sealed.

My problem is that the gas mileage has dropped to around 11 mpg.  Before all the work I listed above, it was getting near 20 mpg. (When the camshaft sensor went out, I was forced to do the work.)  I have started the engine cold and it runs very smoothly with no unusual fumes or smoke in the exhaust.  It has very good power and runs and shifts very smoothly.  It also has a new fuel filter and there has never been an odor of raw fuel inside or outside the car.  There is no puddling occurring under the car.

Before the new radiator/water pump, the engine did overheat a couple of times, but I thought that if the head gasket got damaged, I would see white smoke out of the exhaust.  I was getting a significant amount of white smoke prior to replacing the intake plenum, due to coolant getting into the intake, but that has ceased.

The engine light was on before the sensor replacements but has since gone off.

Where is all my gas going?  

Answer
Hello Kenneth,
First, check the oil, and make sure it isn't beind diluted with gas.

If it is running that well, and you are not overheating or loosing coolant, I wouldn't even consider a head gasket problem.
A fuel pressure regulator diaphragm ruptured would allow raw gas to be sucked directly into the intake manifold, where it would just burn and pass normally, and if it didn't leak too much, may not even trigger a rich exhaust code.
A bad coolant temp sensor could keep the engine out of closed loop, and run rich. Check this link.

http://www.chevytalk.org/threads/showflat.php?Cat=0&Number=1685353&an=0&page=0#P...

If you have trouble getting to it, follow this link, and sign up. it is free, and you will love the Chevytalk site.

http://www.chevytalk.org/fusionbb/index.php?referral/23610/

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Van