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The New Project: 1991 C1500


1991 Chevrolet C1500, the day I bought it

  • Year: 1991
  • Engine: 4.3 liter TBI
  • 262 cubic inches
  • Horsepower: 160
  • Torque: 235 lb-ft
  • Trim: ? (Mystery trim)
  • 2-wheel drive
  • Tires:
  • Miles when purchased: ~140k (maybe? speedometer is off by a lot)
  • Current miles: ~140k

It has been just over a year since I bought the 1990 Chevrolet K2500, so obviously I needed more to work on, right? Not really, but I couldn't pass this one up. I've been on the hunt for a very cheap short-wheel-base to fix up. I had a 1990 short-wheel-base Chevy C1500 in high school and it was one of those vehicles you say you should have never sold. This new one is a 1991 with a V6 and 5-speed manual transmission just like my old one. I paid $1,200 for it.

It had a rod knock and extremely low oil pressure at running temps when I got it, so I started tearing down the engine for a rebuild. I'll update this page after that is complete.

GM OBDI Trouble Codes


OBDI trouble codes for 1988 - 1990 GM vehicles.

  • 13 - Oxygen sensor voltage stays between 0.35 and 0.55 volts for 60 seconds.
  • 14 - Coolant temperature sensor signal indicates a temperature of over 275° F for 2 seconds.
  • 15 - Coolant temperature sensor signal indicates a temperature colder than -27° F for 30 seconds after the engine has been running for at least 30 seconds.
  • 21 - TPS voltage was above 2.5 volts for 8 seconds when the MAP sensor signal showed manifold vacuum to be 15 pounds or more.
  • 22 - TPS voltage was under 0.2 volts for 2 seconds when the engine was running.
  • 23 - Inlet air temperature sensor signal showed an air temperature below -22° F after the engine has been running for 5 minutes.
  • 24 - Vehicle Speed Sensor (VSS) Circuit .
  • 25 - Inlet air temperature sensor signal showed an air temperature over 302° F after the engine has been running for 5 minutes.
  • 32 - EGR open command did not change MAP sensor signal. Normal EGR flow should cause slight decrease in manifold vacuum which would change MAP sensor signal.
  • 33 - MAP sensor signal voltage was too high (low vacuum) for 5 seconds when throttle opening was under 4%.
  • 34 - MAP sensor signal voltage was too low (high vacuum) when engine speed was under 1200 RPM or the engine speed was over 1200 RPM with a throttle position angle above 21%.
  • 35 - Idle Air Control (IAC) System .
  • 42 - The EST signal did not change when the ECM applied bypass voltage to the ignition module.
  • 43 - The ECM did not detect a knock signal during near wide open throttle operation with coolant temperature above 194° F or the knock signal was present for 5 seconds or more during normal engine operation. Electronic Spark Control (ESC) Circuit .
  • 44 - Oxygen sensor voltage was under 0.2 volts for 50 seconds of closed loop operation. (Lean Exhaust Indicated)
  • 45 - Oxygen sensor voltage was over 0.7 volts for 30 seconds of closed loop operation with a throttle angle between 2% and 20%.(Rich Exhaust Indicated)
  • 51 - PROM error. (MEM-CAL)
  • 52 - CALPAK error.Fuel CALPAK Missing .
  • 53 - System Over Voltage .
  • 54 - Fuel pump voltage was not present at fuel pump sense line for 2 seconds after the ECM has sent the fuel pump on command.(Low Voltage)
  • 55 - ECM error. Replace ECM.Faulty ECM

Installing Headliner Insulation in a Truck


It's getting hot here in July and my 1990 Chevy truck's AC hasn't been converted to R134a refrigerant yet. And it's out of refrigerant. So I'm noticing all the ways in which the interior gets heated up. The other day I was sitting in the sun waiting for someone and I realized heat was radiating off the inside of the truck's roof. The front part was cool to the touch because it is double-layered metal, but the back part was another story. It was nearly too hot to touch and it felt like it was pumping out heat into the interior.

The interior headliner area before I added the insulation

That's what it looked like before I added the insulation. I used a roll of insulation I found at Home Depot. It was $10 for a 2 ft x 10 ft roll. I cut it down to 18 inches x 45 inches to fit the area I wanted to insulate. I then used 3M spray-on headliner adhesive to make it stick to the metal.

Insulating material and adhesive that I used

The interior headliner area after I added the insulation

I wish I would have measured the temperature before and after. It's a drastic improvement. I left the truck sitting in a parking lot in similar conditions for about the same amount of time as the day I noticed the heat. The interior insulation was barely warm to the touch when I came back out.

The stock headliner would have accomplished this insulation, but it's made of pressed cardboard-like material and had fallen apart over the years. I plan on adding an aftermarket headliner from LMC at some point, but this insulation will just add to the affectiveness of the headliner when I get it.

New Seats in the 1990 Chevy Truck


New seats from a salvage vehicle for the 1990 Chevy Truck

I finally got some "new" seats. These came out of a salvaged 1994 Chevy truck, but they were originally from a 1990 model. Either year would have fit. It took me about 6 months of watching and waiting to find some decent condition, blue seats for the truck.

Before installing the seats, I thoroughly cleaned them with Shout Auto Dirt and Stain Remover (you can get it on Amazon here). They came out very clean and smelling fresh.

Here are the old seats for comparison:

Old seats out of the 1990 Chevy Truck

I would say it was a worthwhile upgrade :).

New Exhaust on the 1990 Chevy Truck


1990 Chevy Truck Custom Y-Pipe

I finally took the plunge and got the exhaust replaced on my 1990 Chevy truck. I went with a simple one-in, one-out magnaflow muffler with a turned down outlet pipe. It sounds great, but the star of the show is the custom Y-pipe I asked my exhaust guy to make. It looks like the one pictured above. I'll take a picture of the actual exhaust when my garage drops below 98 degrees F.

After reading in numerous places that the stock Y pipe on these trucks is very restrictive, I wanted to see if I could squeeze some performance and efficiency out of getting mine replaced. My exhaust also had holes from rusting throughout it, so it was time. Reading on, you'll find this:

GM intentionally made that Y pipe restrictive to increase back pressure and most have paid someone to spread a myth that back pressure in the exhaust is GOOD. Its NOT. Air velocity is good not back pressure. The reason for the back pressure was so that the EGR smog system would work better and the increased pressure would build up a lot of heat to keep the o2 sensor hot. A good free flowing exhaust does typically need a 3 wire heated o2 sensor conversion to maintain its temperature. To fix 90% of the exhaust issues, I recommend a Flowmaster Y250300 collector from your favorite vendor and replace that section of Y pipe where the 2 pipes merge. Dramatic improvement over that stock GM design.

Here's what the stock Y-pipe looks like:

Stock Chevy truck Y-pipe, 88-98

I showed the exhaust guy a picture of that Flowmaster collector/Y-pipe and he said "I can make a pipe like that, just not as pretty". So I said go for it. And boy is it awesome. The truck feels like it has more torque off the line, seems to have more power on the highway, and my gas mileage increased 17.5% (13.1 mpg to 15.4 mpg).

The whole system rang in at $196. Not cheap, but it was needed and I'll recoup some of that (or all of it, eventually) with better gas mileage. Thanks to David Jones at Extreme Muffler for building and installing the new exhaust.