92 K2500 - terrible fuel economy
 

Hi folks,


I've been trying to bring back to life this K2500 I got for my kid. It's destined to be his when he turns sixteen.


When I got it, it was running terribly. Very rich (visible black smoke at idle), would stall whenever you'd let off the gas or when you'd put it in reverse, and would have trouble holding speed up hills. It was shaking like a cylinder or two was not running. Also it would shift very harshly, and the 2-3 shift was like being kicked by a mule.


I pulled codes (spacing what they were now) but I ended up replacing the MAP sensor, the throttle position sensor, and the ignition module under the distributor cap. I also replaced the fuel pump and filter.


These made it idle correctly and cured the stalling problem, as well as making it shift nice and smooth. It also gave it considerably more power, although it's still not what I think it should be.


Now, the SES light is still on. It's still throwing code 42 (though, I have not disconnected the battery to see if the code goes away and stays away).


The two big symptoms now I'm trying to get rid of are really poor fuel economy (8.7 mpg unloaded, doing an average of 60 mph on the highway) and two noticeable flat spots in the powerband, at around 1000 RPM and maybe 1500 RPM.


Does anyone have suggestions for what I can do to solve these? I nearly ran out of gas the other day because I didn't realize it was getting such terrible fuel economy. Seriously, with a 35 gallon tank I can only go about 250 miles right now.

I have a 2002 Silverado 2500HD 6.0L runs perfect but its a freaking gas PIG 8-9 mpg at 65, when I'm towing a 5000lb trailer 6 mpg. If you want a big horse you need allot of food.

Fair point. But, while I know the 350 TBI isn't very good on fuel, it should be better than this -- especially unloaded. I've looked at data on Fuelly for this exact same truck (and surrounding years), and mine should be getting 3-4 MPG better.

Seriously, no one has any ideas at all?

leaking TBI injector?
O2 sensors staying in open loop?
Hows the compression, bad valves would make for poor fuel economy and loss of power...
Does the oil smell of gasoline?

I'll make a list and look into the first three. But I can tell you that yes, the oil does smell of gas.


I thought I had solved the rich running problem by replacing the MAP sensor -- before I replaced it, I could see black smoke coming from the tailpipe, and that's no longer happening -- but it seems it's still running rich.

I'd check RockAuto and see what TBI injectors run, given the age of the vehicle I bet they are pretty tired

That makes sense ... they're about $50 each, a bargain compared to diesel injectors. I'll put them on my list.


Now this is what I was looking for! Thank you.


Oh, and hey -- how many O2 sensors are there on this truck? My Tahoe had four but I seem to remember hearing that the older trucks only had one set (upstream or downstream, can't remember which).

Mine is a '90 5.7. Only one O2 sensor. Also, assuming yours has OBD1 like mine, I think you can clear the codes with a jumper on the reader port...but I don't remember off hand which two pins. You can also just pull the fuse marked ECM. Code 42 says you have a timing problem, so timing stays at base timing. That would definitely kill mileage and power. Pretty sure you can find OBD1 Code descriptions somewhere. Try here...[http://www.straighttalkautomotive.co.../GM_OBD1.html] Check the temperature sensor. Mine was having starting and running issues and I pulled the connector, sprayed contact cleaner on the connector contacts, added a SMALL bit of contact grease and all is well. You should get MUCH better mileage than you're getting for a 5.7. Air filter, no blockage of the CAT, etc. A rich mixture can destroy a cat if left too long. Just some thoughts.

Maybe clogged cats?