Symptoms: 160 k miles overall. Gas mileage sucks. IAC, EGR, O2 sensor, plugs, exhaust pipes all coated in heavy soot. Truck also ran like crap. I've replaced: IACV, MAP sensor, EGR valve, plugs, wires, pcv, distributor cap, distributor rotor, manifold ambient sensor, coolant temp sensor and coolant temp sender. I cleaned the O2 sensor. New vacuum lines. Reset computer. Sigh. After all that, truck runs better, but still has issues with excessive soot, hesitation when I pull out (just like with the old lady), that gets worse if going up hill or a/c compressor is spinning. Also, if any part of the venthilation system is on, the fan "stops" blowing when truck is under load, then goes back to "normal". Gas mileage a little better but still only about 110 miles to a tankfull (optional 22 gal tank). No check engine light ever (yep, bulb is good) unless I unhook something. Key-flipping throws out a "lean" code. It does seem to get worse when engine warms up. When I unhook the battery (reset computer) she runs like I just left the show room until the computer relearns everything then right back to crap. Unless any of you have a better idea, I have a few things left to try. O2 sensor (new) which may explain problems upon warm-up and lean code if it's sending bad signal, TPS, or distributor. If none of that works, maybe speed sensor. This thing takes more of my money than the woman does! Help?

 

What is the actual MPG? Miles per tank doesn't really tell us anything.

 

Between 5-6 MPG. Sucks up the gas.

 

Wow, 110 miles to a 22 gallon tanks sucks. I doubt it's your speed sensor. O2 sensor would make more sense - soot could be from too much gas.

 

Yeah O2 sensor would be a good place to start.

 

If after all this I still have problems, what do you think about the distributor? Should I try to sync it? Just replace it? Can an OBD be used, or must I use the DRB? I really like this engine... if it wasn't for the fact that it sure costs a lot of money when you have issues like this. Heck, maybe I should just pull it out and put an old-school Hemi in there!
Anyway, new O@ sensor and TPS going in on Friday (payday). I'll let you know if it works.

 

The mileage and lean code are key. I would suspect a leaking/cracked gasket causing the lean condition (you can try the ether trick to confirm), pull your cat and see if it is clogged (you didn't mention any driveability problems). Replace coolant temp unit if your having issues during warm up (EDIT: nevermind reread post). That is astounding that it is running at 25%. Color of oil? Is your EGR system blocked in the crossovers?

 

Ok, I'm on to something, I think. Installed a new TPS. That took care of alot of the running issues, but still bad 5-6mpg mileage. Next thing this weekend is O2 sensor (when I can afford it). However, the plot thickens. Factory header goes down to the Y and then previous owner put a splitter, 2 cats, and 2 cherry bombs. The truck sounds good, but now it's bucking when I let off the gas at speed, and idles down to shut-off when I stop any where. I got out of the truck, and put my hand on the passenger side tail pipe and it blew my hand out of the way. I put my hand on the drivers side, and it was like a whisper. I think I have a plugged-up cat, causing excessive back-pressure, leading to other bad things? And if so, I'm considering taking the aftermarket exhaust off and going back to a single cat/muffler/tail pipe. What do you guys think???

 

A splitter, 2 cats, 2 cherry bombs..wth, man?? Please do get rid of that ASAP lol and put on anything better than what they did haha

 

And now that I think about it..that setup must have been expensive and pointless. A splitter, 2 cats, and 2 glasspacks, I'm guessing? Yeah, splitters and glasspacks are cheap, but I'm assuming they went with high flow cats, which I know can be expensive. And if added up..that set up cost over half, almost all of what my setup cost and thats just pipe, muffler, tips. Which if I was you, I'd take all that off..scrap the cats (buy a new ONE if you gotta pass emissions, of course), buy a Thrush Welded cause theyre cheap and sound good and run single or duals off that..whatever you prefer.