So I have a 1991 240 n/a that sends to run fine, but even after lots of tuning and repairs it still lacks power and gets poor mileage.

I assumed the coil was fine because the car runs, but I tested it anyway and things don't match up.

Primary winding = 0.00 ohms
Secondary winding = 7.98k ohms

I expected 0.6-1.0 ohms on the primary, and 6.5-9.0k ohms on secondary

So if the primary winding is bad, how does the car run at all

 

many multimeter meters don't measure low ohm resistances very well. if you're getting spark, its good.

I'd be looking at things like air leaks in the intake path between the MAF and cylinder head, bad coolant temp sensor, etc.

has the compression been tested in all 4 cylinders?

does it run rich? you'd smell a 'gassy' smell around the exhaust in this case.

how bad is the gas mileage? our experience with a 240 automatic was 25 MPG typical and max 28-29 on a LONG freeway trip in cruise control at 65MPH, thats like burning a full tank at a constant speed.

 

I will test again to be sure, but I have 2 multimeters that I use a lot to test resistors and both came up the same (although one was spotty)

It seems to run rich, in that is smells like an old carbureted car.

Ive checked for leaks using starter fluid and butane around any gap I can find, all seems to check out fine.

Compression is good

Temp sensor tests correctly (tested in the car measuring the contacts in the ECU connector)

Replaced the MAF with no significant change

Timing is good since changing the belt, as it had jumped a tooth when I purchased it (same car I've chatted with you about before Pierce)

I get 16-17 MPG mostly short trips about 10 miles

 

Check for exhaust leaks upstream of the oxygen sensor. These let air in the exhaust stream which the ECU interprets as a lean condition and adds fuel to the mixture to compensate. Common place for leaks are at the exhaust manifold where it mates to the head. Also cracks can start in the downpipe when the bracket fails. I use a piece of plastic hose as a stethoscope to pinpoint leaks.

 

ill check this as I ave had some pretty massive leaks in the past.

as for spark, with an ignition tester I get a clean blue spark across a 3/4" gap, so that far exceeds the requirements of the car.

I tested the O2 sensor using Pierce's procedures from other threads and it checks out fine in that it is oscillating between the ranges, however I would need an oscilloscope to ensure proper functionality