I have a 1988 with 351 with 100k miles. I’m trying to resolve a lack of power and poor MPG problem. The check engine light is not on and there are no diagnostic codes set. I believe the O2 sensor has never been changed. My question is can I assume the O2 sensor is good because there are no fault codes set?

 

I'd say that after 100k miles, it is time to replace the O2 sensor. They don't necessarily quit working or show an incorrect reading when they go bad, but rather, they don't switch rich to lean as fast as they should. It can hang on either side, rich or lean for longer than it is supposed to, and thus the computer thinks it is running the engine incorrectly and compensates for the O2 reading. I would also doulbe check your timing to make sure it is up to spec, as that can cause a loss of power and mileage if it is retarded.

 

I have a 91 w/ 5.8, and that sounds exactly like my truck... what's the best way to check the timing?

-ben

and BTW, if my exhaust is too free-flowing, could it screw up my O2 sensor? I have 3in running back from my stock y-pipe.

 

Well the only way is to use a timing light to sight the timing marks on the crankshaft pulley/harmonic balancer. Be sure to unplug the SPOUT connector (near the TFI ignition module) before checking the timing to disable computer control of advance while you check the timing. Exhaust flow shouldn't have any effect on the O2 sensor.

 

I have the sig unit, '94 F250 460. It has no spout connector. What is an alternate meathod for checking the timing?

Thanks

Frank

 

That's about the only way there is to check the timing. You should have a SPOUT connector somewhere on your truck. I've heard of people setting the timing with a vacuum guage, where you advance the timing until the vacuum is highest, then back off a couple degrees. That way isn't as accurate as using a timing light though.

 

On my '94 300, I have a SPOUT connector, but man I don't think it does anything. It's just a plastic cap, nothing metal to make the 'jumper' situation. Weird. Anyway, a question about this. Where's the darn timing marks? You'ld think I'd see them when the light flashes, but I don't see them, not even with a flashlight and the engine off. I imagine if I could see the timing, I'd notice the vacuum advance working, but I'm not sure.

Thanks

 

Yes, 100K is proper mileage to change the sensor if you want to get best mpg. It is certainly still working but probably "lazy".

 

surveyor, the marks are sometimes hard to see. I have a white line painted over the 10 degree mark. There is also no vacuum advance - the computer controls all advance. The spout connector jumps those wires, otherwise the truck would be lazy from no advance and the computer would store a code. If you can find the marks, rev the engine with the SPOUT connected, and you'll see the timing jump advanced.

 

Slow O2 sensor response should throw a code......

 

It will throw a code if switching is not detected. If it's slow, it should still detect it and think everything is ok.

 

Sorry, NOT TRUE!!

P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)

P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)

P0145 O2 Sensor Circuit Slow Response (Bank 1 Sensor 3)

P0153 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)

P0159 O2 Sensor Circuit Slow Response (Bank 2 Sensor 2)

P0165 O2 Sensor Circuit Slow Response (Bank 2 Sensor 3)

 

Not on an '88. Those are OBD-II codes...

 

Sorry, I stand corrected, I didn't notice the year of his truck!!!

OBD-I could possibly throw a 61 code.......

 

No problem there Bob, I didn't know that about OBD-II either until I read your post. Good to know, since I'm really not that familiar with OBD-II.