I was recently given a 1995 F-150 4.9L six cylinder with Mazda 5 speed manual. 196,000 miles. Check engine light comes on after the engine has warmed up and I approach highway speed.

What's strange to me is that if I turn the truck off and then restart it the check engine light is not lit and does not come on until I accelerate to highway speed again. I'm under the impression that once the code is set the light will stay on until it is cleared??

Pulled codes from the computer with the engine off. Shows KOEO code 522 (not in park or neutral) and CM code 172 (HEGO sensor circuit indicates system lean right side).

Things I've done:

Checked the fuel pressure and it looks OK at 45psi. Checked vacuum lines etc. and see no leaks. Installed a new O2 sensor. Cleared codes. Light still comes on. Truck starts and seem to run fine.

Any suggestions are welcome!

 

Did you ever get it figured out? I'm having fits with code 172 myself. So far I've changed the O2 sensor THREE times. Changed ALL the crappy plastic vacuum lines (switched to rubber), NEW egr vacuum solenoid, NEW egr, NEW egr tube, NEW plennum gasket, NEW PCV valve, NEW vacuum canister (coffee can), NEW carbon vapor canister, NEW purge valve, NEW cap - rotor - plugs - wires, NEW fuel filter. And probably a couple other things that I can't remember.

What else could it be? I'm ready to jack up the radiator cap and slide a new truck under it but with my luck the radiator cap is probably junk too.

 

Ha I get it. That is quite funny.

 

OK first we need to know a few things. How many sensors do you have? 1,2,or ,3. If you have 3 then its saying the right bank is the problem. If only a front an rear its the rear (post cat).
Next its saying the sensor is not switching anymore. The front should switch rich/lean. The rear should stay balanced. If its the rear you have a problem with the circuit, cat, or secondary air system.
If its the right we need to look at how the sensor works. It measures only air. It doesn't care about fuel or exhaust. Meaning if you get a misfire or partial burn all the extra air that didn't burn is going past the sensor and calling it lean. If you have a left and right you can eliminate anything that would effect the left side ( ie manifold leak ). If you only have one then we aren't that lucky. So now its down to you are either getting so much extra air that the sensor won't switch, you are getting not enough fuel, you have a valve issue, you have an exhaust leak at the manifold, the MAF or map system isn't working properly, the EGR is stuck open, there is a intake or head gasket leak, poor timing, I'm sure I'm missing a few things but you get the idea. I would start by performing a full set if vacuum tests. Then compression and spark to make sure all cylinders are firing. Check for vacuum and gasket leaks with some starter fluid or WD40. Read the spark plugs. But first I would verify what the sensor is reading. If you don't have a reader you can do it manually with an analog volt meter. Digital will be all over the place.

 

Ok, I have just one sensor. My truck is a '95 F150 w/5.0L automatic. How do I test the O2? Running? Or just key on? What are the volt specs for this?
I've taken it to the exhaust shop and I have NO leaks in exhaust (I had already thought about too much air).
Could it be a dirty MAF? Is there volt test for this?

 

Did you ever get it figured out?

No, not to my satisfaction anyway. I have an inline 6 and not an 8 with only one O2 sensor, but anyway.

I taped off the throttle body bores and using a cigar, blew smoke into the engine using the power brake booster hose. I found a bad throttle body gasket, leaky egr shaft seal, a small leak at the connection to the pcv valve and minor leaks on each end of the throttle butterfly shaft.

No way to fix the throttle shaft leaks as there are no seals just a bearing on each end.

Unfortunately the code 172 and light continued as well as the 10 mpg fuel burn. The weather has cooled off and the light has now decided not to come on. So is it fixed? I suspect NO.

I'm leaning towards a poor/bad wire connection from the O2 sensor to the computer and plan on checking the voltages when I get some time.

My canister purge valve occasionally opens at idle giving me a high idle too. I'm convinced that is a wire/connection problem too. The throttle position sensor returns to it's exact voltage when I check it at the sensor. Even tried a new tps and still have the problem.

Give the smoke trick a shot and see what you find.

 

ok, I'll try the smoke. Why not, it seems like cigars are a lot cheaper than just throwing parts at it like I've been doing.

Thanks for the input, I appreciate it.

 

Even with the light on my truck runs great, no hesitation, no miss, no high idle. Truck runs very smooth at start-up and normal operating temp. Mpg isn't great but it's not terrible either.
It's just annoying as hell, if the light comes on - I WANT TO KNOW WHY!

 

Did you repair the gasket? The purge valve is an indicator that the lean condition is real. When the valve opens it dumps fuel vapors making the mixture rich. If that is making it idle up that means it is running to lean normally.

 

The best way to find out for sure is to test all the readings. It will really help to be able to access the fuel trim but you need a reader for that. The O2 sensor should switch between 0 and 5 volts constantly while in closed loop.