Back story:

Found this 1987 Chevy G20 van conversion (well, my wife spotted it) for sale. Wouldn't pass smog, had a bad miss (and the rumble in question). The miss felt to me like it still had good compression and the engine seemed to be pulling strong considering it was running on 7 and had 180k miles. I figured the rumble was because of the miss. I went with my gut instinct and made a lowball offer which was accepted.

Well I did a tune up (plugs, cap, rotor, 02 sensor, thermostat) and put the spark plug wire back on that had fallen off (heh) and now it pass smog with below average emissions, and runs pretty good. Unfortunately the rumble is still there.

The rumble seems to shake the whole van, and is even (not a miss). It makes everything rattle, and it sort of reverberates with a droning quality. It only does it while stopped and in drive/reverse. As soon as you start going, it goes away and everything is smooth as butter. The transmission shifts fine (although sooner than I'd like, not sure if that's adjustable) and according to the receipts that came with it, the trans was rebuilt a couple years back, for big $$$. I thought it might be engine mounts, but I did the "step on the brake and gun it in drive and reverse while watching the engine" thing and it hardly moves. The engine is a 350 with TBI. I also wouldn't mind hearing any cheap ideas for getting a little more power out of it.. It's rated at pretty low HP for a 350. Maybe a distributor with a better advance curve? Has to be smog legal though.

Anyone have any ideas what it might be?

BTW, the wheels are crap (oval holes for multiple bolt patterns, and they're starting to rust), so if anyone has some 5 x 5" bolt pattern wheels for sale cheap, I'm interested).

I also wouldn't mind hearing any cheap ideas for getting a little more power out of it.. It's rated at pretty low HP for a 350. Maybe a distributor with a better advance curve? Has to be smog legal though.

wild ass guesses:

maybe the idle is just set too low so when you engage the trans and load the converter it pulls the motor down too much. Can you depress the gas just a little and the rumble go away (put in drive, hold brake, apply another 200rpm with the pedal).

Or, maybe there is a vacuum line that runs to the trans somewhere that somehow has a leak, and it is just enough that - again - at idle in gear the engine runs rough with the small leak but once you are moving up the rpm it starts working okay even with the leak.

WAGs for sure, but who knows...

edit - check the line to the brake booster too. in other words, does the rumble go away if you take your foot off the brake and let it roll? I had a leak in my vacuum booster on an old mustang and you put the brakes on - holy mole - major vacuum leak and the car acted really bad. Just hit me right after I replied...

Hmm, the idle seems fast enough. The repair manual says the idle is computer controlled and not adjustable. I'll try the brake idea, but I doubt it, since I step on the brake before going into drive, but I don't notice any change until it goes into gear.

Man, this engine is only rated at 185 hp, 285 torque.. and this beast weighs 4500 lbs. I bet the 8.2:1 compression ratio has a lot to do with that low number.

Possibly the IAC (idle air control) valve. It's a conical valve mounted on the side of the throttle body that helps control the idle. If it's dirty or malfunctioning it can cause a poor idle. It can be tested and reset but I don't remember the procedure. Maybe Google it? I sold my Haynes manual with my GMC Jimmy.

Did you have any problems timing it?

Rumble, in gear at idle, OK even as you stated the mounts look good I would still give them a real good look, If there original they just may be so flat theres no real isolation going on, however since it goes away at higher rpm The engine still may have something close to the frame or what ever hitting at that point. I think those things are unlikely because it does it in both D and R. The obstruction would have to be centered but its hard to say because it does take a lot of HP to just get a big van to move forward. Surely the trans mount is another place to look. I guess the crank balancer as well. I would also look at the accessory brackets etc as they can get loose and cause ugly vibrations.
I would brake it down, is it an engine firing imbalance if so you can raise the idle a bit even tho its computer controlled. The computer is looking for an area withing around 50 rpm. If you go to high the light may come on a set a code 35 which is a idle control related code, which make séance. The idle screw should have a soft cap and a small torque screw. The throttle body gaskets will leak quite often and is a snap to change. I normally just run a real slow stream of water from a hose around the throttle body, you will here it get sucked in. I did it this way for 15 years and never damaged any engine so don't sweat that part. One way you can get the idle up is to unplug the A/C compressor and turn on the A/C switch. The computer will raise the idle just a tad expecting a larger load.

Internal engine vibrations don't normally just happen at idle, IE piston crank type problems even unbalanced sets. That happens higher up the rpm chain.

If the engine run good/smooth at idle in park and seems to be firing balanced, you can normally listen to the tail pipe tone to determine this dynamically. Its likely your hitting something with something thats not insulated via mounts. Improper mount installation/binding can also cause a harsh feel. Theres always the torque converter but god nose testing that is just not done without replacement. I spent 20 years as a General motors tune-up and drive-ability specialist dealing with this kind of thing everyday. The truth is being there is often the only way to ascertain the issue. Good luck, I'm sure you'll lick the problem.

Sorry for the scattered placement of suggestions it just comes out that way

Thanks for the advice. I think the IAC is ok, as I'm pretty sure it is idling at the correct speed. I've never timed it, I think the computer controls that too. I think it may be time to get it up on a lift and take a look at things while it's running in drive. I've got lots of plans for this van, but running/driving issues come first.

if its an auto it could be a problem with the flex plate or the torque converter , not sure on that particular vehicle but i have seen boxes do some strange things in that particular area and given the box was done up its not impossible even right up to the crank spigot bush/bearing being gone

anyhow good luck hope this helps

I believe your timing is adjustable. The advance timing is controlled by the ECU but the base timing is adjustable. On my nearly identical GMC Jimmy with a 350 and TBI I had to disconnect a single wire that came out of the bulkhead on the drivers side to disable the ECU's control of the distributor while timing it. If you don't disconnect that wire you'll never get a steady reading, the timing mark will jump all over the place. Of course disconnecting that wire set a MIL code, so afterwards you'll have to clear the code (disco the battery for 30 seconds).

Base timing is adjustable, the method is to look in the passenger side of the engine bay likely from the front hood not the doghouse. You will see a single connector, black, about 1- 1/2 inch long, simply disconnect it, start the engine and you can check the timing. I think its ether TDC or a few before.
If it was smog tested that is on of the tests required, doesn't mean it was done but since its simple it likely was. Once you disconnect the wire the service light will come on and set a code, just disconnect the battery for 10 seconds and it will clear once reconnected. My long experience is the IAC motors are replaced often but rarely ever a fix. Theres always another part after which was the problem.
Having said this I don't think you'll find you problem here.

I had a 91 Suburban that had that same TBI 350 in it. Pretty simple system just doesn't make as much HP as the newer Vortec injection. You might take a look at the fan clutch. They are thermostatically controlled and when they go bad they lock causing a rumble like you describe.

Ok, here's an update:

The van sounds completely normal on the outside and underneath, while in drive. The sound seems to only happen inside.

Timing is spot on.

Engine mounts and trans mounts look good, are not collapsed, and the rubber is still rubbery.

Nothing is contacting the body that's rigidly attached to the engine (including the exhaust, which is rubber isolated the whole way down)

The fan clutch seems fine.

The idle seems fast enough, but I don't know how to put a tach on the HEI coil.

The IAC is working fine (idle goes up with AC on).

If I put the brake on and add throttle, the rumble gets worse before it gets better. It goes away at an estimated 1200 RPM.

The harmonic balancer rubber has some cracks in it, but I don't see any evidence of the outer ring rotating in relation to the inner. I would expect a harmonic balancer problem to be least noticable at idle and unrelated to being in drive.

The upper fan shroud has to come off to see the timing marks. This allows the upper radiator hose to sag down into the path of the fan.