Improving Fuel Economy for a GM 4.3 TBI

I’m just now getting to around to driving the 1990 GMC K1500 more, a little over a year after I bought it. Based on other 4.3 5-speed trucks I’ve owned, including the 1991 C1500, I know this transmission and engine combo can get good gas mileage for a full-size truck. The two tanks I had on the ’91 were were 20.3 mpg and 18.2 mpg.

So I was surprised to see 13.4 mpg for my first “tank” in this truck. I say “tank” because it was only 196 miles and 14.7 gallons, and this truck has the larger 34 gallon tank. So it wasn’t even a full tank and most of it was spent driving around my yard and property moving brush. But still, I’ve gotten better gas mileage than that in an extended cab truck with a 350 V8, the 1990 K2500.

DateMilesGallonsMPG
8/26/2019196.914.65613.4
5/2/2020371.225.54314.5
Historical Fuel Economy for 1990 GMC K1500

However, I still expected better. Even my second tank with much more road and highway driving was only 14.5 mpg. So I’m on a mission to improve the fuel economy for the truck. Here are a few things I plan on doing:

I’ll either update this post or create a new one to document any changes in gas mileage as I make these improvements.

The non-GMT400 Truck

Forgive me father, for I have strayed. I own a non-GMT400 truck. It’s a GMT800, a 2002 Chevrolet Silverado 1500HD crew-cab.

  • Year: 2002
  • Engine: 6.0 L LQ4 V8
  • 364 cubic inches
  • Horsepower: 300
  • Torque: 360 lb-ft
  • Trim: LS
  • 2-wheel drive
  • 3.73 gears with locking differential
  • 4-speed automatic (4L80E)
  • Wheels: Stock GM wheels, 8 lug, 16 inch
  • Miles when purchased: 93,264 (September 20, 2019)

I drove three hours one-way to buy it because it was in such good condition with very low miles for a 17 year-old truck. It’s still in great shape because I usually only drive it when I need to pull something heavy, like a loaded horse trailer.

I haven’t done much to it. I took off the tonneau cover and put my camper shell on it. I added a dashcam and a ScanGuageII.

I plan on adding a backup camera/stereo head unit and maybe an amp and subwoofer. The first picture in the gallery below was taken while I was driving it back from Oklahoma. The others are pictures the previous owner took for the listing.

“New” Wheels and Tires for the 1990 GMC K1500

New wheels – 17 inch stock GM wheels (2014 – ??)

When I bought the 1990 K1500, it had my least favorite GMT400 wheels on it. The tires where cracking and one had radial wires showing through the rubber.

Old wheels – 16 inch stock GM truck wheels (92-98?)

So, as usual, I checked Facebook Marketplace for 6 x 5.5 lug pattern wheels as part of my daily “what new GMT400’s are for sale out there” routine. I found some for stock GM steel wheels with mud terrain tires for $200. That seemed like a great price compared to the other listings on Facebook.

I arranged to meet the seller at his house where the wheels were the next day. When I arrived, I saw that the tires were not in good shape, but they were still better than the ones I had on the truck. Two were extremely worn on the inside edge:

You can see the tread is much more worn on the inside/left of the tire

The seller said the truck they came off of had an “alignment problem”. That truck probably had enough negative camber to make an appearance on Fast and the Furious 14.

But I decided the deal was still good at that price. They still have much more tread remaining on them than my previous set did.

The old tires were 275/65R16 and the new ones are 275/70R17. That’s a 1.1 inch increase in overall height. Couple that with the aggressiveness of the mud terrain tread and it makes the truck look better, in my opinion.

The 1992 Chevrolet C1500

  • Year: 1992
  • Engine: 5.7 liter TBI
  • 350 cubic inches
  • Horsepower: 200
  • Torque: 300 lb-ft
  • Trim: Unknown (Scottsdale or Silverado)
  • 2-wheel drive
  • Tires: Cooper Cobra GT All-Season Tire – 275/60R15
  • Wheels: Vision Soft 8, 5×5 lug, 15 inch x 8 inch
  • Miles when purchased: ~276,000 (May 19, 2019)

Picture of it the day after I bought it:

Facebook Marketplace listing:

The 1990 GMC K1500

1990 GMC K1500
  • Year: 1990
  • Engine: 4.3 liter TBI
  • 262 cubic inches
  • Horsepower: 160
  • Torque: 235 lb-ft
  • Trim: Sierra
  • 4-wheel drive
  • 5-speed manual
  • Tires: Mastercraft Courser MXT – 275/60R17
  • Wheels: Stock GM wheels (2014?), 6 lug, 17 inch
  • Miles when purchased: 223,400 (January 18, 2019)

I originally bought this truck as a “farm truck” — just something to use around my property to move firewood, dirt, etc. It was supposed to be one of those trucks you don’t care about and don’t worry about being careful with. That never works out very well for me.

I really enjoy having at least one vehicle with a manual transmission, and this is the first one I’ve had with (working) four-wheel drive. Add the mud terrain tires and 17 inch steel wheels and it really started growing on me. I also like the fact that the 4.3 V6 paired with the 34 gallon gas tank a long bed comes with will get some pretty impressive range on a tank of gas. So I’ll probably be driving this quite a bit more than I planned on. And the more I drive it, the more I will want to fix it up.

I’ve named it “Magic Truck #9” after my favorite beer and since it is the 9th truck I’ve owned.

Here’s what the truck looked like when I bought it. Not much has changed except the wheels/tires. For now.

Removing Paint from Vehicle Windows

I bought a camper shell for the 1990 Chevrolet K2500 and it’s great except for one thing: the previous owner painted the windows black on the inside. This looks cool (as much as a shell can) from the outside and is perfect for sleeping in. It looks like the darkest tint you can get and is nearly pitch black when you spend the night in it. However, you can’t see at all when trying to back up. So I needed to remove the paint.

The best tool I found for the job is a razor blade scraper, like this one:

Soon I will have it back to clear windows so I can see when I back up and not run into anything. If you’re looking for a scraper, you can get one on Amazon: Stanley 28-500 Razor Blade Scraper with 5 Blades.

GM TBI Engine Idles High

If you’ve been to this site before, you might be familiar with my 1991 Chevrolet C1500 project truck. Long story short, I needed to swap the V6 in the truck for another engine (it had a rod knock). I got another 4.3L TBI V6 from a salvage yard. I recently got to the point of starting the new engine up, and it would idle at 2,000 RPM for 30 seconds or so and then drop to 1200 RPM. Both of these speeds are too high for idle.

I reviewed my copy of the Haynes manual for this truck (you should buy one here if you haven’t, they’re extremely helpful: 1988 – 1998 Chevrolet and GMC Pick-up Repair Manual). Here’s a list of troubleshooting items for a high idle:

  • Sticking throttle linkage
  • Idle speed incorrectly adjusted
  • Valve clearances incorrectly adjusted

I’m surprised they didn’t include “vacuum leak” as a possible cause. That’s essentially what mine ended up being, except it wasn’t located at a vacuum tube or port. I found my solution here: [SOLVED] extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI, with this post:

Hi NID FTL

Two things come to mind very quickly.

1. Leaky throttle body base gasket.Very comon on this engine.I don’t know if you serviced this when you pulled the intake.

2.Misadjusted or bent throttle linkage.You don’t know what was done before you got to it.

If these ck ok there are other more complicated issues you would need a scan tool to ck.

IAC count, EGR percent,TPS etc.They all have an effect on idle speed and may not set codes in an OBD 1 system.Don’t forget a leaking pcv valve (Too obvious).Also a bad coolant temp sensor or connecter.Try wiggle test on 2 wire connecter near thermostat housing with it running.You may get lucky.Harness has been known to go bad at sensor connecter.

Good luck,Cardoc.

I popped my throttle body off and the gasket was in terrible condition with multiple sections missing, as you can see in the picture above. I still had the gasket from the previous engine and throttle body, so I threw it on there to see what would happen. Boom, perfect idle! So if your GM TBI engine has a very high idle, check your throttle body gasket. It might be letting in too much air and be causing your issue.

Chevrolet C/K 1500 Centurion Conversion

Mentions of production numbers

About Centurion

  • Southern Comfort buys Centurion in 2006
  • Here’s a little info on Chevy Centurions… Centurion is the leader in automotive conversions and is well known for their crew cab conversion trucks featuring the most popular Chevy 1500 using 100% GM parts that are warrantied through your GM dealer. They are known for building extremely rare models that few have heard about… or ever seen! The 1997-1998 half ton models are hard to come by and if you do they are still worth a pretty penny! The 97-98 1500 crewcab/short bed is extremely rare so if you own one then you have a needle in the haystack! Centurion Conversions had a very limited production number of these trucks. Aprox every 4 months one was picked off the assembly line and sent to Centurion. After the conversion most seemed to have been purchased by GM employees. The 4wd models were fewer and started just under $40k at $39,975. All GM dealership brochures in the 90’s only offered a standard & extended cab in the 1500 series trucks… however a limited amount of 2500 & 3500 crew cab longbed W/T (work trucks) were available by special order only. Centurion Conversions were bought out by Southern Comfort Conversions 1/24/06… link

Chevrolet C1500 Cost

An ongoing list of maintenance and non-fuel costs for the 1991 Chevrolet C1500.

| Date        | Miles  | Item/Work                      | Cost     |
|-------------|--------|--------------------------------|----------|
| 2017-08-05  | 140000 | Bought truck                   | $1200.00 |
|-------------|--------|--------------------------------|----------|
| Total       |        |                                | $1200.00 |
|-------------|--------|--------------------------------|----------|

Maintenance:

| Date        | Miles  | Item/Work                      | Cost     |
|-------------|--------|--------------------------------|----------|
| 2017-08-07  | 140000 | Oil filter                     | $4.11    |
| 2017-08-07  | 140000 | Oil                            | $13.05   |
| 2017-08-09  | 140000 | Hose for coolant overflow tank | $10.40   |
| 2017-08-09  | 140000 | coolant overflow tank          | $16.34   |
| 2017-08-09  | 140000 | valve cover breather plug      | $7.62    |
| 2017-08-10  | 140000 | hose clamps for overflow hose  | $5.14    |
| 2017-08-13  | 140000 | Throttle position sensor       | $38.14   |
| 2017-08-13  | 140000 | Heavy duty oil stabilizer      | $14.16   |
| 2017-11-18  | 140000 | Engine (Salvage Yard)          | $492.75  |
| 2017-12-15  | 140000 | Spark plugs                    | $16.28   |
| 2017-12-15  | 140000 | Spark plug wires               | $25.06   |
| 2017-12-15  | 140000 | Exhaust manifold gaskets       | $14.16   |
| 2017-12-15  | 140000 | Air filter                     | $6.53    |
| 2017-12-15  | 140000 | Thermostat                     | $8.71    |
| 2017-12-15  | 140000 | Thermostat housing gasket      | $3.26    |
| 2017-12-15  | 140000 | Distributor cap and rotor kit  | $23.97   |
| 2018-01-27  | 140000 | TPS pigtail                    | $16.34   |
| 2018-01-31  | 140000 | Vacuum hose                    | $6.53    |
| 2018-01-31  | 140000 | Belt tensioner                 | $54.49   |
| 2018-02-02  | 140000 | Manual transmission fluid      | $18.07   |
| 2018-02-03  | 140000 | Serpentine Belt                | $33.23   |
| 2018-02-07  | 140000 | Coolant temp sensor            | $19.61   |
| 2018-02-08  | 140000 | Seat to floor bolts            | $9.35    |
| 2018-02-08  | 140000 | Wiper blades                   | $21.78   |
| 2018-02-08  | 140000 | Muffler                        | $40.32   |
| 2018-02-08  | 146000 | Temp sending unit and pigtail  | $39.85   |
| 2018-02-11  | 146000 | Exhaust (muffler back)         | $50.00   |
| 2018-02-12  | 146000 | Tires, Cooper Cobra 235/70/15  | $450.64  |
| 2018-02-20  | 146000 | Mount and balance tires        | $54.50   |
| 2018-02-20  | 146000 | Lug nuts                       | $35.96   |
| 2018-02-20  | 146000 | Grille                         | $96.98   |
| 2018-03-09  | 146000 | Passenger side drip seal       | $28.70   |
| 2018-03-14  | ------ | Knock and oxygen sensors       | $46.31   |
| 2018-03-28  | 147000 | Front corner lenses            | $19.75   |
| 2018-05-19  | 147000 | Transmission rebuilt, clutch,  | $1400.00 |
|             |        | pressure plate, clutch master  |          |
|             |        | cylinder, clutch fluid line    |          |
|-------------|--------|--------------------------------|----------|
| Total       |        |                                | $3142.09 |
|-------------|--------|--------------------------------|----------|

“Unnecessary” items:

| Date        | Miles  | Item/Work                      | Cost     |
|-------------|--------|--------------------------------|----------|
| 2017-09-10  | 140000 | New front seats (Craigslist)   | $200.00  |
|-------------|--------|--------------------------------|----------|
| Total       |        |                                | $200.00  |
|-------------|--------|--------------------------------|----------|
|-------------|--------|--------------------------------|----------|
| Grand Total |        |                                | $4542.09 |
|-------------|--------|--------------------------------|----------|