Why does my gas mileage suck?

Yes. The ECT sensor is installed in the metal tube that feeds your heater hoses on the passenger side of the intake near the #2 runner. It sends the ecu a voltage signal that is proportional with the coolant temp in the metal tube.

The gauge sending unit screws into the intake manifold on the driver's side near the #5 runner. If you're still using the factory gauge/sender - you really should get an aftermarket gauge. Especially with all you've done - the factory gauges are notoriously inaccurate -- get an high quality VDO or Autometer temp gauge, and oil pressure gauge so you know what's really going on with the motor.

TPS, replace it. The TPS can cause an over fuel condition.

mjbtv said:

TPS, replace it. The TPS can cause an over fuel condition.

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I would test the TPS before performing an ad hoc replacement.

It sounds like a Probst manual would be a wise investment at this point.

For the TPS, one can check for VRef (5 volts) and ground. Then connect the VOM (VOM is better for this, but a DMM sorta works) connect the red lead to signal return and black lead to the ground wire on the TPS. with the key on, car off, open the throttle very slowly with your hand - look for a nice consistant sweep in the TPS (which correlates to a constant, even and linear upward trend in the voltage reading on your meter. the range would be ~.9 - 4.5 volts. if there are breaks or the base setting is too high, go from there.

JRichker has posted the ECT and ACT thermistor values on here before - search his posts for the values. you can check the sensors at various temps to check their calibration. sometimes just cleaning the sensors helps a lot (VERY true on SN95's).
Troubleshooting for other sensor could be done with a search for more info - I could go on for many posts but you all will thank me for not doing so (as will my fat fingers).
Good luck.

Have you pulled the codes? If those are bad, you almost always get a code on them.

Lynx331 said:

I am getting roughly 10 mpg, if that. 8 gallons got me 88 miles.

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88/8=11. so your on the plus side of roughly 10 mpg.

TheUser said:

Doesn't a vacuum leak create a rich condition? The metered air is escaping, while the fuel is not, therefore there is enough fuel for the metered air, but since some of the air is escaping, there is not enough air for the fuel = rich

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No. Nothing escapes. We are talking about a vacuum, not pressure. A vacuum leak allows EXTRA air into the system causing a LEAN condition. A vacuum draws/pulls, not blows.

Try pulling a vacuum line loose such as the big one to the brake booster. You will find it sucks, not blows. If you create a vacuum leak, you will notice RPM climb shortly. If an engine did not create vacuum, air would only enter due to barometric pressure such as it does during WOT. Imagine trying to inject fuel into a positively pressured intake system. It would fly out the air inlet, not into the cylinder.

"Imagine trying to inject fuel into a positively pressured intake system. It would fly out the air inlet, not into the cylinder."

Perhaps you can elaborate then on how all those cars with boost (superchargers/turbochargers) are able to get so much more fuel into the cylinder to create those even higher than naturally aspirated levels of HP/torque. They have the positively pressurized intake systems you speak of -- and I haven't seen any of their fuel flying out of the air inlet.

Forced induction. Proper term for boost. That is a horse of a different color from what I was describing. I was refering to a naturally aspirated vehicle that depends on vacuum/barometric pressure for air feed. Forced induction RAMS an excessive amount of air into the cylinder to over pressurize the cylinder. Some "boosted" engines suffer from a lack of vacuum that is much needed to operate certain accessories such as the brake booster. As the power adder forces air in one direction constantly, the fuel is caught in the flow of air and forced into the cylinder along with the air.

A vaccum leak in a forced induction vehicle will cause a different set of problems than the same leak in a N/A engine. Which are we talking about here: F/I or N/A?

I was just trying to help. Maybe I'll just sit silently on the side then if my answers seem to bother others. Just trying to use my experience as a professional mechanic to help.

Just trying to point out that you got the dynamics just a little off when you said fuel would fly out of the air inlet if the manifold were positively pressurized. That's simply not true. Fuel will flow from the area of higher pressure to the area of lower pressure. Period.

And I disagree that boosted (also a 'proper' term for systems that aren't naturally aspirated) systems are a horse of a different color from the dynamic you were describing. About the only difference between a boosted system and a non-boosted system is you're pushing the air into the cylinder with a bit more pressure differential -- so you get more flow, which means more air/fuel in the cylinder and hence more power. Naturally aspirated systems function almost exactly the same as boosted systems do - it's just that you only have the pressure of the atmosphere doing the pushing. It's simple pressure diffential that causes any fluid to flow from the higher pressure area to the lower pressure area.

When the piston moves down the cylinder you've got an increasing volume. Take a fixed amount of any gas - and expand the volume that it occupies, and the pressure has to drop. Since the pressure drops below atmospheric -- atmospheric pressure PUSHES air into the cylinder. All boost does is PUSH that gas, or air, into the cylinder with a greater pressure differential.

With a throttled opening, the throttle plate limits flow. And that's how pressures lower than atmospheric occur in the intake - the pistons drop in the cylinder with the intake valves open, but the throttle plate blocks the flow of air into the manifold - voila, a partial vacuum is created. Some times these are (mistakenly) referred to as vacuum(s) -- a vacuum is the absence of any air/gas in the volume in question. What occurs in the intake is simply a reduction of pressure -- creating the potential for air to flow from higher pressure (upstream of the throttle plate at atmospheric) to lower pressure (into the intake and cylinder).

As long as the absolute fuel pressure is greater than the absolute pressure in the port, fuel will flow into the space, and be drawn into the cylinder along with the air flow. You can get a bit of reversion -- flow of air out of the cylinder through the intake valve -- if you have the valve timing to cause it -- lots of overlap. The exhaust valve is closing while the piston is coming up and intake is opening. The pressures are such that the exiting exhaust can be at a higher pressure than the air in the port. When that happens the exhaust pressure can limit or even reverse the flow (briefly - very small fractions of a second at high rpm). That's what makes for lopey cams.

Forced induction or naturally aspirated - a vacuum leak (if vacuum is created in the boosted set up) is a vacuum leak. Of course, once under boost, you can't have a vacuum leak. Because there's no partial vacuum. Leakage from a pressurized intake tract simply reverses the direction of flow - from into the manifold to out of the manifold.

Vacuum pumps to power accessories have been around for ages. Diesels have had them for eons - diesels have no throttle and hence pressure in the intake tract stays close to atmospheric - no partial vacuum. And, of course, boosted diesels can run higher than atmospheric pressures in the intake tract.

Yeah we know - we go down a rabbit trail sometimes - the nature of public forums.

Don't know what else to tell you -- except I just filled up mine, and on an around town very easily driven tank I got 20.8 mpg. Usually gets 25-27 on the highway - that's with mild H/C/I and 3.73 gears.