My 92, 4WD with an A4LD has the same type of symptoms. 12.5 MPG around town and 13.25 on the highway. It has 262,000 miles. Power is pathetic; to travel on the freeway it takes about 3/4 throttle and if you come to a hill it will want to downshift to 2nd at 60 mph.
It runs like it's pulling a heavy trailer. The parking brake does release.
When it was new to me in '94 and had only 54,000 miles it got about 20 mpg on the highway and 15± around town, with good pep.
The engine was replaced at about 185K after a rod broke. New head gaskets were installed 12,000 miles ago due to coolant in #6. Cylinders, piston crowns and valves look great. The valves were brushed clean of those pesky gasoline deposits, so much for buying brand name fuel fuel (none of them clean like a wire brush).
The CEL is dark but functional and no codes come from the OBD-1.
We replaced the fuel pressure regulator because it couldn't hold pressure when the pump went off. This improved performance, including a high RPM misfire, but not to a satisfactory standard. No effect on fuel economy.
While under extreme load the fuel pressure goes up to to 40 PSI in 3rd gear at 4,000 RPMs and holds on a long steep hill. It does not drop when put into overdrive. At idle its about 35 PSI. So fuel is delivered.
The injectors are clean at the screen and at the tips. They hold fuel pressure of 75 PSI with a gauge connected to the return line.
The original oxygen sensor was replaced a few years ago, with no change in performance. The cat flows freely and we have never failed a California smog check.
The Throttle Position sensor, Mass Air Flow sensor and Crankshaft Position sensor all have normal outputs.
Air Charge Temperature sensor was replaced, its cheap and there does not seem to be a reliable test.
Replacing the plugs and wires smoothed the idle a bit, no appreciable change in performance.
A Ford dealer looked at it, replaced the filters and MAF and then said, "it's an old car."
An independent mechanic looked at it and concluded, "it was beyond his expertise." I like a mechanic with integrity.
Engine vacuum is good. This rules out mechanical timing trouble.
Under hood temperatures seem a little hot. Thermostat and fan clutch are new. The radiator is original. If you run hard on the freeway on a hot day with the AC on, then park in the shade and idle for 15 minutes it will overflow the coolant recovery bottle, which I keep at a minimal level. The dash board gauge runs between the N and O in normal; it rises to the high side of R when the coolant boils out.
Can this be a timing issue? The SAW was removed between the ECU and the EDIS module causing the EDIS to run in default mode with 10° static timing. The car runs about the same. When the SAW is replaced, the RPMs do tick up by about 50.
The timing light shows the base timing is on the money. However, when the engine is revved in neutral, the mark flits around a bit but you do not see a smooth advance. At 2,000 RPMs it seems like you should observe 20-30° of advance. I've seen claims it will advance to 59.5°.
I've read the sad tomes of those who have spent hundreds, if not thousands, on trial and error repairs by wholesale replacement of these highly reliable parts.
It could be a wiring harness problem; there is lots of wire to all the sensors. The pins and sockets in the connectors look brand new when opened.
How can it be determined if the EDIS or the ECM is the source of the trouble?
