Sorry for this but I need help with my 1992 K3500

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NorthernSwede

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First of all, sorry for this! I know this might be the wrong forum for this but you guys are some of the best people on the internet so I thought I might ask anyway.

I have this 1992 K3500 Chevrolet pickup that's getting horrible fuel mileage.. Somewhere about 7/7,5mpg.

i almost always run it in 2wd and so far the following has been changed..

o2 sensor
MAP-sensor
TPS sensor
spark plugs
sparkplug wires
distributor cap
rotor
air filter
fuel filter
ignition module
both engine temperature sensors
thermostat

Unplugged the AC compressor and changed the alternator (was bad anyway)

No noticeable change at all so far.

Hope you guys can help me since I need the truck for work..
 
Let's start with "how are you" figuring mileage? Are you using the speedo? If so, trash that idea, and go buy yourself a GPS. I have picked up about 3 or 4, less than 40 bucks apiece, on CL and the thrift stores.

These make excellent accurate speedometers and odometers.

ALSO you need to be honest and define your driving. Is this "open highway" driving, or a mix of "in town" hills, etc? How big is your right foot, figuratively speaking?
 
Original engine ? & how many miles ? Any trans issues? Is engine reaching full operating temp? (yeah I see you are from Sweden lol) Progressive loss of mileage or all at once? Thx !!
 
Just for comparison I drive a 92 shortbox 4X and get about 10 around town. Check the injectors, they get to dripping.
 
Assuming that it has a tbi454, you're not going to get much better. That gen 454 is the worst BB Chevy made. It had roughly 7.5 to 1 comp and crappy peanut port heads. With hp and torque numbers as low as a typical 350 it wasn't very efficient.

My 91 c3500 got 9.5 on flat ground at 55-60mph if driven at 70 it got 8.


Edit: make a post over at ck5.com in the garage forum. They are always helpful and really know Chevys.
 
And...........gearing? and Tires? RPM at XX mph?
 
Oups! Seems like I forgot some really basic stuff!

Regarding the ODO I actually checked it by driving a stretch of road I know and the mileage counter seems to be quite accurate

It's a 350TBI that's bone stock.
4L80E transmission that seems to be in quite good condition.

The mileage has been horrible since I got it.

Normal road tires on stock rims

Engine temp according to the gauge at least seems OK

Quite a soft foot actually, no racing machine anyways ;)

Can't tell the RPM since theres no RPM gauge...

240.000 kilometers on the ODO
 
A friend had a one ton duelie, with a 5.7/350 cubic inch in California, He had the same situation : Simply not enough engine torque. A rootes blower,a stroked short block,might help. ...After that,gear manipulation.....
 
did you change out the tbi gasket , unless you have the metal coated one this could be the problem ,it might be sucked in . also try using the middle grade fuel to see if the knock sensor is retarding timing . I have been fighting fuel millage problem with my 4.3 and just fix it by accident, it went from 20mpg to 16 for the last 8 years ,after replacing the intake gasket and major tune up with a fuel pump, the problem started with the 10% alcohol fuel ,so last week our gas station was out of reg so I put in the middle grade fuel and first trip out I noticed more throttle response and in the end went up to 24mpg first trip out. It has stayed between 23/25mpg on hwy and 19 in town I'm finally happy because this is what the blazer should be getting also my 92 suburban with a 305 pulls down 22/24 on the highway. ALSO THE STOCK GEARING FOR YOUR SET UP IS A 4.10 REAR 5.7 TBI WITH AUTO./THE STICKS HAD 3.73 GEARS.i THINK THE BEST YOU SHOULD SEE IS 14/16 MPG WITH THE O/D TRANS ON HIGHWAY
 
At what altitude are you operating at? I seem to remember Sweden varies from sealevel to near 7000ft. That will wreak havoc with mpgs.
I haven't worked on those old TBIs much, but the ones I have seen, were running pretty rich, and had lazy timing.That's a pretty heavy truck for a 350 to have to tug up and down hills.
Those early systems were pretty basic. I don't remember them having a baro-sensor either.If your plugs came out black,that would kindof point to a no-baro system.I'm not sure about GMs, but some systems checked the Map at every start-up, for Baro compensation.So if you are going up and down, it might pay to perform a restart at every 1000 ft elevation change.
I would also go through the dizzy. I think 92s are still mechanical. If it has a vacuum canister on it, I would tune on that sucker quite a bit. I would also see if the engine will like the mechanical timing curve to come in a little earlier,and faster.You can do a lot with timing, for an engine that is running a lot of steady-state load and rpm.
Make sure the engine is running as warm as you dare and that the computer's temp-sensor is switching out of cold-start mode.If it has an 02 sensor,scan it and make sure it is cycling rich/lean.
 
I had a 97 GMC Sierra - K1500, 350, AT - it was a gas pig, it did better than 7mpg but, if memory serves, it got maybe 13-15mpg.. one thing that will use up that gas - remote start...
 
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