GM 350 TBI tune up?

Ok so I have a 94’ sierra with a 350 cubic inch TBI. I’ve already replaced the thermostat, spark plugs, clutch and had the oil changed. when I bought it it looked like the plug wires and distributor cap had already been replaced recently. the big question is, other than replacing the air filter (which doesn’t look like its in to bad of shape) what else should I look at doing to tune it up a bit? this is my daily driver/work/scrap hauling truck so I want to keep it running well and for a long time. there was a small metal hose that ran from my intake to the heat shield around my exhaust that got mangled pretty bad and just removed, what was it for and is it worth the expense to replace it? also I’ve seen ads for throttle body spacers, are they worth it? and would I need to make any changes to the ECU at all if I put one on or even put performance headers on it? any information you can offer would help.


You’ll fail the visual part of your smog test if you don’t replace that part you removed. It sounds like it may have been an EGR tube

Forget the throttle body spacer and the ECU changes

That engine wasn’t designed to be high performance

Here’s a few things I would do:

Change the diff fluid . . . cheap and easy to do
Brake fluid flush . . . the stuff is probably almost 20 years old
Change the fuel filter if it hasn’t been done recently

How are the brakes, steering and suspension?

So, to sum it up, I’d just be up to date on the maintenance, and forget about any improvements

Don’t try to make the truck something it isn’t. You would need a newer truck for that.

actually there is no EGR on this engine, the hose that was removed looks like it only connected to the heat shield and there is no hole in the actual manifold for exhaust to come from.

I have tuned and modified race trucks with this same engine, the only difference is we always used a carburetor instead of TBI so we never had to worry about an ECU.

I really don’t want to make this a race truck, I actually just kind of want some more low end torque for when I pull a trailer or haul a load in the bed. I didn’t even think about changing the diff fluid so thanks for that, i’ll do both of them now that I know to.

the rear suspension is getting upgraded with leveling air bags so i’m not really worried about that but the front suspension is torsion bar which I have absolutely no experience with so I don’t know really what to look for there, and does anyone know where the fuel filter should be located on this truck, it would save me a lot of time and crawling looking for it.

BTW, I live in the sticks here in Pennsylvania, so no smog tests for me :slight_smile:


Perhaps the pipe was supposed to warm up the intake?

Is the truck 4x4 ?

If you want more low end torque, it seems to me you need a different cam

There are probably aftermarket intake manifolds, but is it worth it?

If you really want to pull a trailer, an automatic trans would be my choice

The fuel filter is most likely attached to the inside of the left frame rail

That mangled hose you removed is the heat riser for the intake. That hose introduces heat from the exhaust manifold into the intake to prevent the throttle body from icing up in cold/damp weather.


it is a 4X4. it looks like warming the intake is the only reason why it would be there, but why would I want to warm up my intake? cold air is better for combustion isn’t it? I’ve thought about an aftermarket cam, I just figured some long pipe exhaust headers might help with the torque, the big question is would I have to change the ECU at all for either of them? i’d really rather not spend the $800 - $1000 to put a carb on it and then still have to pay for the cams and exhaust and possibly intake manifold. I really do prefer a standard transmission when pulling a trailer, but then again that could just be left over from being a truck driver.

Thanks Tester, that makes sense now kinda.


I do think you’ll have better driveability with TBI versus a carb

Change the PCV valve if it has one and also check the rotor under the distributor cap. If they changed the cap they probably changed the rotor but you never know. Also I like to pre-emptively change belts and hoses.

it looks like warming the intake is the only reason why it would be there, but why would I
want to warm up my intake? cold air is better for combustion isn’t it?

There should be a “thermostatically controlled flap” on the snorkel of your air cleaner housing. When the engine is cold, the flap draws warm air up from the heat riser. It’s there for improved drive-ability and emissions during warmup.

Then when the thermostatically controlled flap senses the air from the heat riser is sufficiently warm, it flips to select ambient air from the snorkel opening (for normal engine operation).

You really can not tell visually the condition of wires cap and rotor from the outside. If you remove the cap and the inside is clean and not carbonned up that is a good sign. You say you bought this truck, therefore do not know when MTC was done. I would replace rotor, cap, wires. Just what I would do.

My '95 Suburban TBI had a bad habit of needing sparkplug wires, don’t know why. It’d start running rough, new wires cleared it up. Happened twice, about every 40k or so.

And I was also surprised what a PITA it was to change wires, given how big the engine compartment was. They routed them between the block and the firewall. Not fun…

The spacers for under the throttle body don’t add torque, the velocity stack part that goes under the air cleaner will help torque. Spacing the injectors up helps torque. Swap the rocker arms from the 1.5 ratio to 1.6 roller tip models. That will introduce a bit more air without changing cam timing so HP and torque will increase. The ECU change won’t bring much except the requirement for premium gas. Add 2-4 degrees at the distributor. If it pings on the gas you buy, back it off.

Long tube headers will help torque a bunch especially with a crossover pipe or a 2 into one collector and a 3 inch single exhaust. Stainless steel is best, ceramic coated is good, painted won’t last long! Keep the cats, 'cause we all like to breathe clean air.

This is about all you can do without spending serious money.