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Old 06-04-2013, 01:48 PM
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poor milage on 92 silverado 3/4 ton

ok guys just want to get some opinions on this as i think i know the issue but not 100% sure

i have a 92 silverado 3/4 ton with the nv4500 trans , right now its averaging about 7 mpg intown it should be getting about 12-14

it runs great no missing not down on power just really crappy milage intown
on the highway it pulls down 20-23 mpg

if im cruising at the speed limit it gets about 20mpg (80mph) if i do 75 it get a lil better , which is right around were it should be


no what ive noticed intown is the temp gauge never gets over 145-155* , so im assuming this is my issue , on the highway it stays between 200-210*

my first thought was bad coolant temp sensor but if that was the case it when im on the highway and its reading 210 the thing should be way over heated but it runs perfect

im assuming the cold temps when in town is causing my poor gas milage, it has a brand new clutch on the fan and new thermostat though im unsure of what temp the thermostat is as i didnt put it in



what would u guys lean twards as the cause of bad milage in town
would u guys think bad coolant temp sensor , bad thermostat or another issue ?

truck has no stored or current trouble codes

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Old 06-04-2013, 05:37 PM
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I would start by adressing the low coolant temp. It really doesnt make sense that it is allways low , like 145 or so in town as you said. Maybe you didnt drive it long enough for it to fully warm up.
Need to get to the bottom of the coolant temp thing.The sender for the coolant gauge is not the same one as the sensor for the EFI system, at least not on your truck.It is on some of the V6 mini vans GM makes though.
Diagnose any coolant problem by using the datastream PID for the coolant temp.Not the dash gauge.
Then I would look at the datastream and check the long and short term trims. Trims generally vary between -5% to +5% as a norm.
If you are seeing 10+ or higher , look at the 02 sensors. Chances are they are aged and not working as they should.
Oxygen sensors are a maintenance Item just like a spark plug. Lots of guys dont know that.If you are guessing about an 02 sensor being a problem ...just change it.
Also If the trims are staying kinda heavy in the - side like -10 or better, read the plugs. if you have some black ones chances are the injectors are leaking.Lay them out in order and look at them all on an individual basis.

Remember upstream sensors control fuel trim and supply A/F ratio feedback, down stream sensors are ONLY to check Cat health.
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Old 06-04-2013, 05:45 PM
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I would start by adressing the low coolant temp. It really doesnt make sense that it is allways low , like 145 or so in town as you said. Maybe you didnt drive it long enough for it to fully warm up.
Need to get to the bottom of the coolant temp thing.The sender for the coolant gauge is not the same one as the sensor for the EFI system, at least not on your truck.It is on some of the V6 mini vans GM makes though.
Diagnose any coolant problem by using the datastream PID for the coolant temp.Not the dash gauge.
Then I would look at the datastream and check the long and short term trims. Trims generally vary between -5% to +5% as a norm.
If you are seeing 10+ or higher , look at the 02 sensors. Chances are they are aged and not working as they should.
Oxygen sensors are a maintenance Item just like a spark plug. Lots of guys dont know that.If you are guessing about an 02 sensor being a problem ...just change it.
Also If the trims are staying kinda heavy in the - side like -10 or better, read the plugs. if you have some black ones chances are the injectors are leaking.Lay them out in order and look at them all on an individual basis.

Remember upstream sensors control fuel trim and supply A/F ratio feedback, down stream sensors are ONLY to check Cat health.
cant check datastream as i dont have a scanner that will do that, as far as not driving long enough for it to warm up ive driven 4 miles to pick my gf up from work and let the truck idle for 20 mins in the parking lot , come out and its still not upto temp. the only time it gets upto temp is when im on the highway or going 55+ on a few of the roads we have around here in town

its a tbi truck i have seafoamed the engine and poured half a can in the tank, injector spray pattern is good with no drips or dribbles

i can pull the wideband o2 sensor out of my turbo camaro and install that into the truck and see what afrs are doing though
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Old 06-04-2013, 06:07 PM
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I wouldnt be switchin 02 sensors around , lotsa fooling around and it probably wont lead to any concrete evidence of what the problem is.
You need to look at the data stream, and check the plugs. You need to start with a good basic diagnostic routine first, to try and gather some good evidence on what is happening.
Like I said the coolant issue needs to be adressed first, and you need to check the temp with an IR thermometer or a live datastream from the coolant temp sensor from the EFI system. see if it jives with the IR thermometer and/or the temp gauge. Then you need to figure out why its doing that, if the CTS for the fuel system says 190 when the dash gauge says 140, then verify it with the IR themometer.one or the other is wrong,.
As it stands you dont know if the gauge is wrong, or if its right or if the EFI system sees the right temp through its sensor, or if the coolant is actually too cool too long.
You gotta do some diggin.
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Old 06-04-2013, 06:22 PM
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Originally Posted by LATECH View Post
I wouldnt be switchin 02 sensors around , lotsa fooling around and it probably wont lead to any concrete evidence of what the problem is.
You need to look at the data stream, and check the plugs. You need to start with a good basic diagnostic routine first, to try and gather some good evidence on what is happening.
Like I said the coolant issue needs to be adressed first, and you need to check the temp with an IR thermometer or a live datastream from the coolant temp sensor from the EFI system. see if it jives with the IR thermometer and/or the temp gauge. Then you need to figure out why its doing that, if the CTS for the fuel system says 190 when the dash gauge says 140, then verify it with the IR themometer.one or the other is wrong,.
As it stands you dont know if the gauge is wrong, or if its right or if the EFI system sees the right temp through its sensor, or if the coolant is actually too cool too long.
You gotta do some diggin.
not something i can do myself as i dont have anything capable or reading the datastream and im sure as hell not paying anyone to do it

i do have a good sensor in the iroc the use the same coolant temp sensor would take me 2 mins to swap out , if i put the one from the truck in the iroc i can verify that one on the ecm as i have a standalone ecm i can hook my laptop to in the iroc

will it take more time then just taking it to a shop in town and letting them hook it up sure will but the shops in town are crooks and they rip[ everyone off dont trust not one of them ( my boss paid for 2 valve cover gaskets and intake gasket on his daughter sebring , he then sold the car to a friend front valve cover was leaking so he replaced them himself, turns out the shop charged a few hundred dollars to my boss and only changed out the front valve cover gasket, the rear gasket and intake gaskets were never touched., and to top it off the used a ton of silicone on the new rubber style gasket on the front valve cover which made it leak again thats just one example of the kind of things shops in this town do, its amazing they are still open and ppl keep going back to them)

i can borrow an ir thermometer from a friend so thats not an issue

the wideband 02 i have i could just weld in a psare bung for this will allow me to see actualy afrs under the different operating conditions and see if its running rich or lean
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Old 06-04-2013, 08:24 PM
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I'm afraid with out a scanner your just guessing. I bought a new one for 300.00 years ago a must have Also the manuals and a use cheap ones on e bay or the local auto part stores have them you can use for free, Auto zone Checker.

Help here also FullsizeChevy.com - Chevy Trucks
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Old 06-05-2013, 12:51 PM
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Originally Posted by Project89 View Post
ok guys just want to get some opinions on this as i think i know the issue but not 100% sure

i have a 92 silverado 3/4 ton with the nv4500 trans , right now its averaging about 7 mpg intown it should be getting about 12-14

it runs great no missing not down on power just really crappy milage intown
on the highway it pulls down 20-23 mpg

if im cruising at the speed limit it gets about 20mpg (80mph) if i do 75 it get a lil better , which is right around were it should be


no what ive noticed intown is the temp gauge never gets over 145-155* , so im assuming this is my issue , on the highway it stays between 200-210*

my first thought was bad coolant temp sensor but if that was the case it when im on the highway and its reading 210 the thing should be way over heated but it runs perfect

im assuming the cold temps when in town is causing my poor gas milage, it has a brand new clutch on the fan and new thermostat though im unsure of what temp the thermostat is as i didnt put it in



what would u guys lean twards as the cause of bad milage in town
would u guys think bad coolant temp sensor , bad thermostat or another issue ?

truck has no stored or current trouble codes
If the coolant temp is really running in the 145 to 155 range, the TBI is kicking into cold start mode and adding "choke" fuel. This is continuous at any engine temp under 176 F and will not throw a code. This needs to be checked out muy pronto as the excess fuel will wash the upper lube from the cylinders and excess wear of bores, pistons, and rings will happen quickly.

Bogie
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Old 06-05-2013, 04:20 PM
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swapped the coolant temp sensor between the truck and the iroc , sensor is good so truck is never getting up over 155* in town , going to pull the thermostat and see what temp thermo is in it . if thats not the issue im going to put a piece of cardboard infront of the radiator and see what happens
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Old 06-07-2013, 01:42 PM
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its got a 195* stat in it so thats shouoldnt be the issue but im going to replace it anyways just incase its opening early or getting stuck open
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Old 06-07-2013, 02:34 PM
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20-23mpg from a 3/4 ton truck on the highway from a TBI 350 stick truck. Same truck with even a Vortec 350 wont get close to that.

Fill tank, hit trip reset, drive, refill with fuel, take number of miles and divide by number of gallons it took to refill.

I had a 1992 TBI 350 reg cab shortbox 2wd, 700r4 3.08 gears and would get about 450km from a 100 liter tank.,
1997 Vortec 350 reg cab 2wd, 4l60e trans, 3.08 gears and would get about 575km from the same 100 liter tank this is all around driving, with a best of 723kms all highway which is still only 18.3mpg from teh smallest/lightest 1/2 ton truck you can buy..
My example illustrates teh differnce in milkeage between a Vortec 350 and a TBI 350 in teh saem exact tryck platform/gearing. And none of my numbers approach 20mpg.
You must have a serious odometer issue, or tiresized mismatch. Your numbers are very optimistic. JMO.

If your narrowband O2 sensors are woking, your a/f ratio should be 14.6:1, unless you are at WOT or during other circumstances of open loop such as when warming up. I'd change the O2 sensor, they arent expensive, and there is only 1 or 2 IIRC.

peace
Hog
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Old 06-07-2013, 03:52 PM
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20-23mpg from a 3/4 ton truck on the highway from a TBI 350 stick truck. Same truck with even a Vortec 350 wont get close to that.

Fill tank, hit trip reset, drive, refill with fuel, take number of miles and divide by number of gallons it took to refill.

I had a 1992 TBI 350 reg cab shortbox 2wd, 700r4 3.08 gears and would get about 450km from a 100 liter tank.,
1997 Vortec 350 reg cab 2wd, 4l60e trans, 3.08 gears and would get about 575km from the same 100 liter tank this is all around driving, with a best of 723kms all highway which is still only 18.3mpg from teh smallest/lightest 1/2 ton truck you can buy..
My example illustrates teh differnce in milkeage between a Vortec 350 and a TBI 350 in teh saem exact tryck platform/gearing. And none of my numbers approach 20mpg.
You must have a serious odometer issue, or tiresized mismatch. Your numbers are very optimistic. JMO.

If your narrowband O2 sensors are woking, your a/f ratio should be 14.6:1, unless you are at WOT or during other circumstances of open loop such as when warming up. I'd change the O2 sensor, they arent expensive, and there is only 1 or 2 IIRC.

peace
Hog
actually odometer matches perfectly to actual miles driven on the highway.

my routine every 2 weeks is the same , fill up the truck in town drive the 58 miles to walmart do my grocery shopping and then fill up at the gas station in that town ( another 2-3 miles so usually about 61-62 miles before i fill back up ), and calculate milage its always averages 20-22

i average 10- 11$'s in fuel at 3.56 a gallon but ive had some trips that were 9 and change and some that were 12 and change

so well go in the middle of my average 3 time's 3.56 a gallon is 10.68 @ 61 miles this is 20.33 mpg

i baby this truck it rarley sees 3k rpms , thats about were i shift it when pulling out on the highway on ramp , in town im shifting sub 2k rpms


i bought dual widebands for my turbo camaro so i could monitor both banks of the motor , and really only need to monitor one side so i may install the wideband in the truck and feed the ecm a simulated narrow band signal from the widebands output , which will be more accurate then even a brand new narrow band sensor. and will also let me keep an eye on how the engine is running afr wise in real time
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