Truck has the 5.8, bone stock with 5sp and 134k. I have no CEL on which confuses me as to why it would fail by so much. Any ideas? Bad cat or O2 sensors? Idles smooth, runs good...gas mileage is awful...but that's like saying the sky is blue so...any ideas?

Cruise mode:
HC Limit: 300ppm
HC Actual: 9ppm
CO Limit: 1.6
CO Actual: 0.00

Idle Mode:
HC Limit: 300ppm
HC Actual: 383ppm
CO Limit: 1.6
CO Actual: 5.7

 

It's probably running too rich. The O2 sensor is cheap and easy to replace, so you can start with that, especially if it's original.

Check the plugs to see if they are fouled.

> "Achieving success through lowered expectations"

This is too funny, but sadly all too true.

 

Your idle is way rich, causing the cat to be overwhelmed and not be able to deal with the CO produced when that extra fuel burns up in the cat.

Cruise is fine, which would indicate a good cat imo.

No CEL showing, but have to tried to pull codes anyway?

Bad mpg could be in part due to this. Usually cruise is high too when you have a rich idle due to being stuck in cold mode or similar.

 

No light so I went out and ran codes anyway. Came back 172 and 538. I'll have to look those up...why wouldn't that throw the CEL?

 

Not sure, but I've read about "stored" codes here. Once you know what those are, you might have a better idea which way to go.

 

172 points to the O2 sensor...but which one?

 

You have one on each side, or one on the y-pipe before the cat?

 

I believe one pre cat, one post...not sure if there were two pre or not?

 

This thread discusses those same two codes:

https://www.ford-trucks.com/forums/1...and-538-a.html

 

Thanks for the link. Now I'm wondering why this would only be showing up so much at idle. Are the cruise numbers too low? I didn't hear any obvious vacuum leaks, I'll put my gauge on it to see what I'm pulling at idle. I'll also check the exhaust for any leak near the O2 bungs.

 

Your HC seems low given the 300 limit. My '98 4.0 SOHC Exploder blows 1 (one) with a 100/170 idle/cruise limit. My '01 4.3 Lexus blows 5/3 with a 47/31 limit at 15/25 mph. (Exploder is 4x4, Lexus gets the dyno)

CO for both is 0 or .01.

So while it is odd that a rich idle wouldn't cause a high speed problem, that appears to be the case here.

UNLESS you are stuck in cold mode (rich) but have a vacuum leak that leans it out at speed.....

Good hunting on this.

 

Well, I just tested the vacuum and she stays steady at about 20.5" or so at idle. Vacuum response under WOT and steady RPM seems to be fine as well so I think that would rule out a vacuum leak. Did a visual inspection of the exhaust last night and didn't appear to be any leaks near the O2 sensor. I'll have to see if I can get a fuel pressure gauge to test that, maybe a bad injector or FPR?

 

The link given in the previous post about those codes would lead me to replacing the sensor at this point. No guarantee that will work, but it could be the issue.

 

I'm thinking about changing the o2 sensor and running it on alcohol to bring it through

 

Just to follow up on this, I replaced the O2 sensor and threw in a gallon of denatured alcohol and brought it through emissions finally, passed...easily.

BEFORE
Cruise mode:
HC Limit: 300ppm
HC Actual: 9ppm
CO Limit: 1.6
CO Actual: 0.00

Idle Mode:
HC Limit: 300ppm
HC Actual: 383ppm
CO Limit: 1.6
CO Actual: 5.7

After
Cruise:
HC: 4ppm
CO: 0.0

Idle:
HC 58ppm
CO .1