454 is actually quite common for the 3/4 - 1-ton chassis, which is what a G-30 is. Quick breakdown --
G10 (1/2-ton / 1500 equivalent) - generally got six-cylinder and 350 V-8.
G20 (3/4-ton / 2500 equivalent) - generally got 350 gas / 6.2 diesel V-8.
G30 (1-ton / 3500 equivalent) - generally got 454 gas / 6.2 diesel V-8. Also could be had with 19.5-inch rim vs 16-inch on the G10/G20.
The TBI units are pretty reliable. I've never seen one fail (not saying they can't, but it's not that common). Usually the only trouble you'll have with these engines are ignition control modules (sit inside distributor and cheapie replacements are known to fail). Fuel pumps can fail, another uncommon issue. If and when that happens, don't put anything but Delco or Delphi back in it -- you WILL regret it.
As to the noise, it could be anything -- noisy lifter, bad alternator bearing, bad water pump bearing, the list goes on and on. And some noises will telegraph from the source, making it more difficult to diagnose. If this one is equipped with gauges, internal engine problems will often be obvious in oil pressure readings -- Most will call for 40-60 at 2000 rpm, minimum of 10 at idle. I like to see 20 at idle, most 4.3s I've seen would hold 40 at idle. And there will be a difference between readings for cold and warm engines, usually 10-15 psi difference.
A quick way to determine if the noise is in the engine / transmission as opposed to the accessory drive is to remove the serpentine belt with the engine off and start it back up again without the belt. A minute or two running without the water pump won't hurt it, and if the noise goes away, it's something in the accessory drive. If the noise remains, it's something in the engine or trans, but still could be minor. Keep reading.
A fairly easy (but potentially dangerous) trick to hunt such noises down is to get a flat-blade screwdriver and with the engine running, touch it to various areas around the engine, putting the butt (plastic) end to your ear (folding your lobe over the ear canal helps to eliminate ambient noise and focus on what you're listening for). Be careful -- too much or too little pressure will cause the contact point to slip and potentially injure you.
Make sure it is long enough to do this without risking getting hair or clothing caught in the accessory drive. The closer you get to the source of the noise, the louder it will be through the screwdriver. A small-diameter piece of metal or plastic pipe works too. When you find the source of the noise, it's going to be pretty loud through your 'stethoscope'.
Listen around the water pump housing, alternator housing, power steering pump housing, idler pulley / tensioner mount (tensioner and idler pulley ARE common, especially with cheap replacements), and various points on the face of the block should be accessible with a little care. A bearing knock should be clearly audible from the oil pan or the side / face of the block.
If you hear it around the rocker cover or intake manifold, it is likely a noisy / sticky lifter, fairly minor. Some Marvel Mystery Oil or similar additive like Sea-Foam will usually shut this up. In extreme cases, you may have to adjust valve lash or replace lifters.
Listening around the accessory drive will be tricky in a van because space is limited. But you can also remove the serpentine belt (engine not running of course), and spin each component to test for drag, wiggling the pulley to check for excessive end play in the bearings.
Timing chain tensioners have been known to go bad and cause the chain to slap against the timing cover and / or block, depending on engine design.
Exhaust leaks at the manifold/pipe joint have been known to mimic a knock as well.
One thing that has been known to happen is an intermittent knock that only appears with the transmission in or out of gear. This is usually a cracked or warped flex plate smacking against the block, and that requires transmission removal to fix. It sounds a bit like an engine knock. Again, the the screwdriver / pipe trick will help here.
A 1991 will also likely have a 700-R4 transmission. People either love or hate these transmissions. They do not have a standard kick-down cable like a TH350. Their shifts are controlled by a cable-operated pressure control valve, referred to as a Throttle Valve (TV). If this cable is misadjusted or worn, it can cause serious transmission damage, as the throttle valve adjustment must be spot-on. You can tell by looking at the shift indicator. A 3-speed (700 or 350 will have P RND21. A 4-speed (700R4) will have P RNOD21 or P RND321. I'm not sure a 91 would have anything BUT a 700-R4.
One other thing that's been known to happen with some engines that are never driven above a certain speed or RPM is carbon build-up, which has been known to cause a knocking sound if it occurs on the piston face. In this area, carbon build-up will make a knocking noise as the carbon is caught between the valves / combustion chamber and piston. Feeding Sea-Foam through a vacuum port could correct this, but some extreme cases require drastic measures.
One trick I've been taught is to fill a 2-liter soda bottle with water, and with the engine running at operating temperature, dribble small doses of water from the 2-liter bottle down the throttle body / carburetor (with a helper holding the throttle open enough to keep the engine from stalling. I know, it sounds crazy, but it works.
Just be careful not to throw too much water through it at once, (give it a second or two between doses to recover) and make sure your helper keeps the engine running -- it will NOT like it, for obvious reasons. Too much will cause hydrolocking -- if this occurs, pull the spark plugs and spin it over to blow the water out. Then reinstall the plugs and see if the noise is gone, if not, try again, more carefully this time.
The science here is that the cooler water enters the hot combustion chamber and turns to steam, which helps to clean the valves and pistons. The carbon is, in turn, blown out the exhaust.
If fuel or oil additives become necessary, Sea-Foam is some pretty good stuff in my experience. Like any other product, there will be naysayers. I used it in the fuel and the oil on a 240k mile Olds and the old girl dropped operating temperature 15 degrees and fuel economy went from 17-28 highway (no overdrive). Just read the directions and let it work for you.
Fuel economy is all in how you drive it. I've seen 37 mpg with a 3800 Buick, and I've seen 8-9 mpg with a loaded tractor-trailer (usually these beasts get around 5-6). I've found for best results, set your cruise around 58-63 on interstates. People may throw dirty looks and one-finger salutes for 5 under the limit, but I laugh at them. They're not buying my gas, and that's what the passing lane is for.
Go easy on the throttle and you'd be surprised what you get. You'll get your best mileage at your most constant throttle / engine speed. The less speeding up and slowing down, the better. Remember, Grandma and Grandpa managed to get 15 mpg with their 455 Buicks and Oldsmobiles.
A good tune-up, clean injectors, and ethanol-free gas will work wonders, believe me. And don't be afraid to upgrade plugs, wires, filters, etc. over stock. Back when I drove like a maniac, I still got 28-32 city with a 5-speed / 2.2 Cavalier, and 36 highway. K&N air filter, 8 mm wires, upgraded plugs. That car got 36 highway averaging 75 mph.
Hopefully you've found a good egg that just needs a little high-mileage maintenance. Hope all this helps!
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