Driving a 1991 240 NA Wagon
Stock setup, stock diff and so on

I am getting about 16-17 MPG's mixed highway and city
car idles fine and starts fine in cold weather
Car idles fine and cranks a bit (cranks 8-12 times) in warm weather or when warm

I am thinking it must be the timing or coolant temp sensor
I can't find any leaks
and the exhaust is a straight pipe to a hollowed out CAT and the resonator, the whole thing is chopped off just before the rear axel but it is sealed up (so I don't think its sucking in fresh air and confusing the O2 sensor but idk)

 

Just tested my MAF and I'm getting 38 ohms of resistance on pins 2 and 3
Add I read this should be 3-4 ohms

Does anyone know the proper way to test a MAF?

 

Update

MAF and O2 sensor test good

I got the engine to operating temp to test the O2 sensor by holding the accelerator steady at about 1,500rpm and while I was doing that it would constantly but randomly miss... Any ideas

 

when was the ignition system (plugs, cap, rotor, wires) last serviced? missing is almost always ignition, unless you have particularlly fouled and sticky injectors.

a good test for the O2 sensor, find the connector near the wiper motor at the firewall in the back of the engine compartment, unplug the single pin connector, wrap a thin piece of bare copper wire (like 3" of 1 strand of stranded wire) around the pin and reconnect it. make sure this wire sticking out doesn't touch a ground, hook it up to the + (red) probe of a digital volt meter, with the - (black) probe to ground, start the engine and let it warm up... with the meter on a DC Volts scale, you should see a voltage swinging from around 0.2 volts to around 1.0 volt and back every few seconds. at idle, the swings might be lazy, as slow as 5-10 seconds, while at higher RPMs its more like 2-3 seconds. the exact voltage isn't important, just that its going from low to high to low ... if its doing that, you've got 'lambda cycle' and things should be good.

these engines are optimized for a certain amount of back pressure, running straight pipe exhaust doesn't provide that, and certainly could be part of the problem, never mind having the exhaust tip under the passenger compartment could let toxic gasses in.

my experience with LH2.2/2.4 non-turbo automatics is around 25 MPG typical mixed, and as high as 28 MPG on long freeway road trips if you drive mellow. 17-18MPG would be the low end of serious flogging around town. BUT... are you sure your odometer is reading accurately? if its sticking and reading slow, then you're actually going more miles per reading than you think... And, we are talking US Gallons and statute miles here, right?

 

I will run that test tonight. I did a similar one and it was within spec, but I would rather test twice.

As for the exhaust, I chopped it off about a week ago. Its been in need of one for a long time and I haven't gotten around to it unfortunately. That being said, the car has always had this mileage problem, regardless of the exhaust configuration. Infant, I thought the major lack of power was due to it being clogged, some to find out it was not the case. I did however find some CAT chunks in the rear most muffler.

I did the plugs, rotor, cap and wires within 10k miles ago. I have not performed a comprehensive test on the coil however. My next step is replacing the timing belt as it needs to be done at this point and I haven't touched that since buying the car.

 

Did the O2 test and it came back exactly in speck

I also made some test drives with my phone GPS and a Garmin. My odometer looks like like it's exactly on mark

 

Have you checked your coolant temp sensor and for exhaust leaks upstream of the O2 sensor? I had a similar issue with my 240 a few years back where I was getting 15-18 mpg. I finally chased it down to exhaust leaks. Use a plastic hose as a stethoscope to pinpoint leaks. I have found a typical place for leaks is around the front exhaust port where the header attaches to the head for some reason that seems to corrode and develop leaks. I ended up replacing my header with another I had laying around that wasn't as corroded.

 

As it turns out I do have a leak at the flange between the down pope and what is left of my CAT
quite a bad leak actually

As for the temp sensor, trying to find a good way to get a tester on it, I have the numbers that I need to look for...now it’s just a matter of getting in there

 

Testing the temp sensor is best done from ECU end by back probing the connector. That tests the sensor and wiring together. If it checks out ok there, then you can assume it's good. If it doesn't, then you have to test at the sensor - which may be best done by removing it. It is hard to get to.

 

Great suggestion! What are the wire colors on the ECU connector?

 

Sealed up the exhaust leak, I’m curious to what the O2 readings will be now
currently I am resetting the fuel trim and computer memory (leaving the battery off for an hour or so)

I also tested the Temp sensor, and everything looks good there so far. I got about 2100 Ohms at about 62 degrees Fahrenheit

I will check again at some warmer temps by idling the car and taking samples of the coolant temp, then cross reference it with the chart I found.

 

According to the FAQ below, you check the resistance between pins 5 & 13 on the ECU - but given your previous post, you probably already figured that out.

https://www.brickboard.com/FAQ/700-9...eratureSensors

 

I tested through the LH Injection module (smaller black one) and that turned out to be a ground and pin#2

Ill go through he ECU today like you said. Looks like in need to back probe the connector and power the circuit. I like doing different types of tests because this is one part I would rather not change out...if I did I guess ill just take off the intake and clean the whole thing out while Im at it

 

Tested the temp sensor at cold warm and hot measuring the temp with an ir-thermometer and everything checked out 100%

 

*PLOT TWIST*

Went back and tested the MAF again
I’m pretty sure I am testing it correctly...

Back probed pin 1 and ground = 0 Ohms resistance great!

probed pins 1(Ground) and 2(i guess this is return signal) = 12.5Volts but I expected to get between 0.1-0.2V

Disconnected and checked resistance on pins 2 and 3 = 0 Ohms resistance but I expected 2.5-4Ohms


So basically the MAF is not doing a thing, its shorted...unless I'm testing incorrectly?

So for laughs, I put it in a 93' 940 Turbo and the car would not stay idling but in my car, it runs exactly the same whether its plugged in or not

 

Generally if there is a fault with the AMM you will get a code thrown. Do you have codes?

I really don't know much about AMM diagnosis rather than if the engine idles better without it then it probably is shot. I always keep a known good spare laying around for diagnosing purposes.

 

Just for laughs I tested the MAF again today using 2 different multi meters

I'm getting 3-3.2 ohms across pins 2 and 3 but I'm getting 1.3 volts rather than 2.3

I'm just going to replace it because it's so inconsistent

No CEL or codes on it, but it runs exactly the same wether or not the AMM/MAF is plugged in


I'll check back in with the results
Thanks for everyones help thus far!

 

Finally got the new MAF

well "New"

Bosch "016" used. Tested out good but no real difference yet

 

No real difference in mileage or your measurements?

 

Haven't driven enough to get accurate mileage
The readings on the meter are also almost identical, butt at least they are stable, meaning they read the same each time I measure it.