This is a great site. I have read a lot that has helped me get my $500 Ford back on the road after sitting for 10 years. Thanks for everyone's posts.


I have run into one little issue that I can't seem to resolve. In metro Atlanta, I have to pass an emissions test before I can get a tag. I failed the first one and just failed the second. The EEC test showed code 173, which I read is running rich and could be the O2 sensor. I disconnected the battery for 30 minutes, installed the new O2 sensor, failed the second time. What other things could cause 173?


Thanks

 

1994 F150 XLT 5.0

 

Did you already perform a tune up?

I would be inclined to check both the ECT and ACT sensors. You should check both when cold and then when up to operating temp. The resistance for each sensor should be close to each other when cold and the same when hot. If either are not close to each other in value when cold and warm, then they should be replaced. They seldom throw a code unless the KOEO or KOER sees them cold when those tests are performed. When those tests are performed, engine should be at operating temp.

I would check those 2 sensors since they can have a negative impact on the engine and emissions, and they may never throw a code for either.

Also, when I mentioned tune up, when was the last time the air filter was changed?

Just a couple of ideas.

 

I completely disagree with that in bold. Those sensors are variable resistors. The resistance varies based on the temperature, that's how the computer derives the feedback signals.

Warm the truck up then run the Key On Engine Off tests and stored (Continuous Memory) code display. If nothing is flagged as out of specification then it is HIGHLY advised to measure the sensors using voltage feedback or simple resistance checks.

I would also throw in measuring the MAP sensor and inspect the Fuel Pressure Regulator for signs of a leaking diaphragm, a very common failure mode. Cycle the key On/Off a few times, then pull the vacuum hose off the FPR. Look for signs of fuel leaking out the vacuum port by sight and smell. Replace if necessary.

Some good reference material here, be patient it may take a bit to load: http://oldfuelinjection.packrad.net/

 

I'll check both of those.


Air filter is new, along with plugs, wires, cap, rotor, fuel filter, tanks, fuel pumps, and now O2 sensor.


thanks

 

Take both an ECT and ACT sensor and lay them on the table. Each sensor will read within each other after a short while. I just happen to have one of each lying inside the house, next to the computer. The ACT read 31.40 k and the ECT read 31.30. Both are Motorcraft sensors. The exception to this, would be the IAT sensor which was implemented into the airbox on MAF equipped trucks and cars. The incoming air is cooler than the coolant, so the IAT will have a higher resistance versus the ECT. The IAT is the latter version of the ACT.

There may be a slight difference in the resistance values once warmed up, as coolant will hold heat. When the sensors heat up, the resistance starts to drop, but as the sensor's cool off, the resistance of the sensor increases. But the ACT sensor should read close to the ECT sensor when taking a cold, non-operating temp and then an operating temp. If the IAT sensor is being checked, then it would have a higher resistance, since its located in the air box and not originally in the intake manifold.

Hope that cleared the confusion, as I believe the OP has a MAF 5.0.

 

Yes, that makes more sense....one sensor should read close to the other. I was taking your original comment to mean each sensor should have the same
resistance cold vs. warm....

 

Thanks a lot. I'll check all this tomorrow. Hopefully, it will stop raining here.


It's getting there.......

 

That's a really nice looking $500 truck. I hope you figure out what's going on.

For pulling codes, get an Innova 3145. They're $30 and they make reading codes much easier and more accurate. People trying to read codes with a paperclip misread them ALL the time and end up chasing a problem they don't have. Plus the book that comes with it is a trove of useful information.

Does this truck have mass air? If it is, there will be a sensor nearer the intake side of the air tube(s). That sensor can get dirty and throw off A/F ratios.

 

Thanks. It's cleaned up pretty well. It does have a mass air flow sensor. I've cleaned it, but I guess it could be bad. I may take your advice if that thing is only $30. This is how I found the thing.

 

Sorry to bump an old thread, but how'd this work out for you OP? Hunting a code 173 gremlin myself.