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Discussion Starter · #1 ·
Hello, I have a 1994 gmc k 1500, I purchased it at a local government auction, it only had 98000 miles but for some reason it dropped or burnt a valve, I purchased it for $500, paid $1400 for the shop to get heads rebuilt, and install new gaskets from head gaskets up, that was about 5000 miles ago , I have been driving it every day for about a month now while I am working on my other vehicle. It idles really rough, almost like it has a small cam to it, I run high test fuel in it(93) and I have already used one bottle of BG 44k fuel treatment, I was going to see about replacing the fuel filter ( located under drivers side cab) and possibly tune it up, it has the 4.3 engine in it, I was told it was tuned up not long before the head work was done, it has plenty of power when running, plenty of pick up, just idles like crap, thanks for the help.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Don’t buy the “it was tuned up“ line without verifying some things. Many mechanics today know nothing about how to keep the TBI engine running well.

Start with checking the fuel pressure. You’ll need a TBI pressure kit. For certain, change the fuel filter. It sits on the frame rail under the rear of the cab.

I don’t like spending other people’s money, but I’ll share my perspective if I was in your shoes.

Change the coolant temp sensor on the intake manifold. Very inexpensive and often overlooked. It’s the single most important sensor on the TBI engine at startup. It also quietly degrades over time and you won’t realize how much extra fuel you’re wasting if it’s not spot on. That grounding lead by the thermostat housing: Clean it well. That’s the computer ground reference.

Other grounds to check while you’re under there:
The other ones on the intake. Should be at least 2 more counting the battery cable.
Behind the passenger side head back to the firewall and down to the frame just above the starter.
Fender well at the battery.
Driver side core support: Headlight ground.
Frame rail at the fuel filler tube. This one‘s a little harder to get to. 13mm machine screw on top of the frame close to or under the hose. Very important to clean this well.
Since you‘re laying under there, slide back to the bumper area. Driver side rail close to the spare tire is the tail light ground. Same 13mm machine screw.

The PCV is also an inexpensive part. Pay attention to the vacuum line to the TBI. Even if it looks like it’s new, today’s aftermarket replacement is made with very poor quality rubber. I replaced mine twice in the nearly four years after I swapped in a 5.7. Very poor quality junk. I got tired of it and just used a piece of 3/8” fuel injection supply hose. Problem solved.

The throttle position sensor should be checked but they rarely go bad. An analog voltmeter measuring the resistance of the sensor across the full range of motion will show you if there are any breaks in continuity. Move the throttle plate open very slowly. Any deflection of the gauge is a sign of a break. Also, check voltage with the key on but truck off across that same range. It should read from about 0.7V to ~4.5V. Anything close to that is fine.

Resist the temptation to do anything with the computer. It’s almost never the problem. Are you familiar with how to check for trouble codes?

Check for vacuum leaks on the MAP, brake, and EGR sources. The EGR solenoid won’t have active vacuum on the EGR valve itself at idle, but get your hands on a hand held vacuum pump and check to see if the valve will open manually. Your engine will stumble badly if it opens with the pump. If it opens, I wouldn’t worry about replacing it for now.

Also, the 4.3 TBI base gasket could be bad. My ‘95’s 4.3 TBI base gasket was folded in half from the factory! An unlit propane torch or even a can of carb cleaner sprayed around the base of the throttle body can help this diagnosis. An idle increase is a solid clue.

The distributor cap and rotor should be checked, but don’t just replace them. Lots of folks only go this far, but consider that it may be the original and now it’s nearly 28 years old. Check the shaft for end play and bearing slop. If the shaft moves any appreciable amount vertically or side to side, it should be replaced. Also, if you can, get up on top of the engine and check inside the distributor. The magnetic reluctor may be damaged. A bright flashlight should be used to inspect for cracks Heavy rust or even fractures may be present. anything of that nature inside would be a trigger for me to replace it. Wires and plugs would also be on my list.

As to fuel, 93 octane is not needed. You can buy it if you wish, but it really won’t give you measurably better performance. A full on tuneup will cost you less and provide better long term reliability.
 

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Discussion Starter · #3 ·
Don’t buy the “it was tuned up“ line without verifying some things. Many mechanics today know nothing about how to keep the TBI engine running well.

Start with checking the fuel pressure. You’ll need a TBI pressure kit. For certain, change the fuel filter. It sits on the frame rail under the rear of the cab.

I don’t like spending other people’s money, but I’ll share my perspective if I was in your shoes.

Change the coolant temp sensor on the intake manifold. Very inexpensive and often overlooked. It’s the single most important sensor on the TBI engine at startup. It also quietly degrades over time and you won’t realize how much extra fuel you’re wasting if it’s not spot on. That grounding lead by the thermostat housing: Clean it well. That’s the computer ground reference.

Other grounds to check while you’re under there:
The other ones on the intake. Should be at least 2 more counting the battery cable.
Behind the passenger side head back to the firewall and down to the frame just above the starter.
Fender well at the battery.
Driver side core support: Headlight ground.
Frame rail at the fuel filler tube. This one‘s a little harder to get to. 13mm machine screw on top of the frame close to or under the hose. Very important to clean this well.
Since you‘re laying under there, slide back to the bumper area. Driver side rail close to the spare tire is the tail light ground. Same 13mm machine screw.

The PCV is also an inexpensive part. Pay attention to the vacuum line to the TBI. Even if it looks like it’s new, today’s aftermarket replacement is made with very poor quality rubber. I replaced mine twice in the nearly four years after I swapped in a 5.7. Very poor quality junk. I got tired of it and just used a piece of 3/8” fuel injection supply hose. Problem solved.

The throttle position sensor should be checked but they rarely go bad. An analog voltmeter measuring the resistance of the sensor across the full range of motion will show you if there are any breaks in continuity. Move the throttle plate open very slowly. Any deflection of the gauge is a sign of a break. Also, check voltage with the key on but truck off across that same range. It should read from about 0.7V to ~4.5V. Anything close to that is fine.

Resist the temptation to do anything with the computer. It’s almost never the problem. Are you familiar with how to check for trouble codes?

Check for vacuum leaks on the MAP, brake, and EGR sources. The EGR solenoid won’t have active vacuum on the EGR valve itself at idle, but get your hands on a hand held vacuum pump and check to see if the valve will open manually. Your engine will stumble badly if it opens with the pump. If it opens, I wouldn’t worry about replacing it for now.

Also, the 4.3 TBI base gasket could be bad. My ‘95’s 4.3 TBI base gasket was folded in half from the factory! An unlit propane torch or even a can of carb cleaner sprayed around the base of the throttle body can help this diagnosis. An idle increase is a solid clue.

The distributor cap and rotor should be checked, but don’t just replace them. Lots of folks only go this far, but consider that it may be the original and now it’s nearly 28 years old. Check the shaft for end play and bearing slop. If the shaft moves any appreciable amount vertically or side to side, it should be replaced. Also, if you can, get up on top of the engine and check inside the distributor. The magnetic reluctor may be damaged. A bright flashlight should be used to inspect for cracks Heavy rust or even fractures may be present. anything of that nature inside would be a trigger for me to replace it. Wires and plugs would also be on my list.

As to fuel, 93 octane is not needed. You can buy it if you wish, but it really won’t give you measurably better performance. A full on tuneup will cost you less and provide better long term reliability.
I will be checking all the things you mentioned, as for tune up, any ideas on good spark plugs and wires, I know gm uses ac Delco, just wanted to get an idea of what products not to get, or what works best, ( best spark plugs, wires and cap, for a 94 4.3)
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Others may have different opinions, but for my ‘94 5.7, I only ever ran these.
 
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