Low speed knocking, worse with a full tank of gas (1 Viewer)

[pjt087]

I just bought a '95 4x4 pickup V6 edition, drove it down from Riverside, CA to San Diego no problem. The following days I experienced some pretty serious "chug" or knocking- especially in low gear. Have not been able to pass CA Smog- labelled as a "Gross Polluter". Replaced an after market cool air intake with the original, and have been trying to troubleshoot. MIL code 25 (running lean on fuel vs air) makes sense-- the tachometer remains steady when the knocking starts and usually occurs under load (I live on a hill and starting out can be hard).

The knocking has since subsided, but the error code returned and I still failed smog again today. Any sage advice out there? Thanks!

 

[pappy]

I'm a big fan of starting with the basics. Check the timing. Tune-up. Ox sensor (those suckers do stupid things when they fail).

 

[pjt087]

I'm a big fan of starting with the basics. Check the timing. Tune-up. Ox sensor (those suckers do stupid things when they fail).

Thanks, the O2 sensor is new, but I had had a feeling that there may be a timing issue.

 

[jfwhite]

I rode in the truck a week ago, and the hesitation really felt like a fuel issue to me. What's odd is that

a). the issue is worse on a full tank, rarely an issue on a low tank
b). the truck fails smog when it's not hesitating

I know, I know, probably two separate issues. Timing may be off a hair but can't explain the terrible hesitation, and it's unlikely that would cause a smog issue. Has anyone experienced intermittent hesitation issues with a clogged cat?

 

[pappy]

When my cat plugged up it just wouldn't run. Very sluggish. Some of what you are describing sounds like fuel pump, or fuel pressure regulator. But I don't see how that could effect emissions unless running lean could be doing something goofy.

 

[pjt087]

When my cat plugged up it just wouldn't run. Very sluggish. Some of what you are describing sounds like fuel pump, or fuel pressure regulator. But I don't see how that could effect emissions unless running lean could be doing something goofy.

From what I can gather-- the truck is actually running rich despite the error code, the CO emissions are 4x the limit. My thoughts pointed to the cat as well, could be a combination of both the fuel pump and cat?

 

[vegasfj40]

Might be a problem in the evap system. Might be pulling in fuel when you have a full tank through the vapor hose. Check the charcoal canister and lines and make sure that system is right.

 

[pjt087]

Might be a problem in the evap system. Might be pulling in fuel when you have a full tank through the vapor hose. Check the charcoal canister and lines and make sure that system is right.

Thanks for the tip, the charcoal looks clean without any apparent fuel residue- good idea though.