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April 2010 Techline

Willie Worthy | Writer
Posted April 1, 2010

Your Tech Questions Answered!

TBI 350 Into Old Blazer Swap
Q. I have a 1977 Chevy Blazer and a 1995 Chevy Silverado, both with 350s. I want to swap the '95 engine with the TBI into the Blazer. Can you give me a list of things I'll need or need to do, wiring harness, motor mounts, fuel pumps, etc.?
Jon Tomlinson

A. The good news is that the engines use the same motor mounting system. Considering that the "new" engine is still some 14 years old, it would be a good idea to buy new rubber mounts.

Now the hard work comes into play. You're going to need the complete wiring harness from the Blazer, not forgetting the wiring that goes to the "Check Engine" light and the oxygen sensor, along with the ECM unit (computer). Be sure you mark every wire's "goes to" location. Don't forget the MAP sensor (manifold absolute pressure), which reads manifold pressure, or engine load. It should be mounted within 12 inches of the throttle body, above the vacuum source. Also be sure to grab the Electronic Spark Control, which feeds the distributor key data.

You're also going to need the 02 sensor, which will need to be installed into a welded-in bung on the exhaust manifold. Probably would be a good idea to replace the old one with a new one. You're also going to need what is called a vehicle speed sensor, installed in line between the transmission and the speedometer.

Gathering up the wiring harness can be a real pain, so maybe you want to spring for an aftermarket wiring harness such as the one Painless Wiring offers ( It's pretty cool in that all the wiring is pre-labeled, and some great instructions come with it. If you go with the factory wiring harness, I suggest you somehow access the wiring schematics for both trucks. It will make things a lot easier when you know where things go.

Now you have to deal with the fuel system. GM fuel injection uses an in-tank electric fuel pump. However, to make things easier, you can mount a special pump on the outside of the frame as close to the fuel tank as possible and as low as possible. You will want to add a fuel filter in front of the pump as the internal rotor clearances are quite tight, and being that it spins at about 3,500 rpm, it only takes a small particle of crud to damage it. Arizona TPI ( ) has what you need. You may be able to adapt the fuel sender and pump assembly from the Blazer into your truck's tank. Oh, and don't forget that you will need a fuel return line from the throttle body back to the fuel tank. You will have to find a way to get the returned fuel back into the tank if you can't use the Blazer's fuel pump unit. Usually, you can solder a fitting to the pick up/sender plate.

Okay, what else did I forget? Probably a lot of things, like transferring the throttle-body linkage over to the truck, as they are a bit different. I only kind of covered the basics here, so be prepared for a bit of work.

If you decide that you don't want to go to all of that work of making the TBI work, you can swap the Blazer's engine over to a carburetor. Between 1987 and 1995, Chevy went to a new-style intake manifold where the two center bolts on each side are not angled. You can take the manifold off the '77 truck engine, have these holes machined to the proper angle, and use it and the truck's carburetor-or you can buy a correct aftermarket manifold that definitely will improve the engine's performance. You also will need to swap over to the older distributor or go to an aftermarket unit.

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