So I bought a 99 Pontiac Montana (for $1200, too much considering the problems it apparently has). It had new tires, looked clean in the engine area, and seemed to run fine. So I bought it.

The low coolant light way on, but there was some, but little, in the reservoir, so I figured it was fine.

But as I took it on a 40 mile drive, I noticed that the gauge would run hot on slow traffic, but always short of the overheating area. Running it at highway speeds, the gauge would stay in the middle. Also, revving it in park would bring the gauge down.

Reading online, it sounded like it could be a low coolant problem.

So I peered into the radiator, and saw no fluid but lots of brown gunk. Looking under the oil cap, there was similar gunk. Although I don't think that was there when I first bought it (even though the oil wasn't new and had been driven at least 40 miles) and I changed it since then.

So now I'm pretty sure that either the intake manifold or head gaskets are bad. So I'm nervously going to attempt to replace them myself. I have some experience working on cars, but more experience working on my watercooled scooter, learning by doing maintenance as necessary.

Here'* the gasket set that I think I'm going with: 96 99 Pontiac Oldsmobile Chevrolet 3 4L High Performance Head Gasket Bolts Set | eBay

Is there a reasonable way to clear the gunk out of the radiator, or will I probably need a new one?

How likely is it that the engine was damaged by heat despite the gauge now being that high?

These engines are known for two things in these vehicles.....lower intake gaskets, and/or, head gaskets, especially #1 cylinder....

many times, techs will assume it is just a lower intake leak, when it is also a head gasket leak....and end up doing the job all over again, and then onto the heads.....

Doing this job in a van, is much harder than in a car, due to limited access to the back of the engine....you can get more room two ways...with vehicle in neutral(block wheels, apply ebrake), rotate engine forward and hold in place.....

You can also lower the front cradle by loosening the front bolts, to tilt it forward...

Interesting!

I started a thread about this on another forum, and have been strongly advised to leave the heads alone unless I'm sure there'* a problem with them.

To me it makes sense to replace them when I have the engine apart anyways though and then I can be sure I don't have to do it all over again.

Thanks for the advice about rotating the engine. Being able to reach the back if it definitely sounds like the most troubling part of the procedure. I didn't realize the engine would be able to rotate just by having it in neutral. Is there a risk of rotating it too much?

I am not sure of the year, but I think this vehicle has dog bones from the radiator support to the engine...remove them, I had a special tool to pull the engine forward, but you can do it manually, and place a block of wood between the firewall and engine, or just tie the engine forward.....you will only be able to pull it forward about 3-4 inches.....removing the wiper linkage helps too......

Before doing any intake job, you should always check for a head gasket problem, on these engines....

A simple test, is fill the coolant system and burp.......bring the engine to normal operating temp....let it cool a bit, and safely remove the radiator cap and attach a coolant pressure tester......pressurize overnight....next day remove the plugs, and disable spark(pull two wire connector from left side of ICM) and injectors(disconnect injector harness)...have someone crank over vehicle, if coolant comes out of one of the plug holes, usually #1(back side, first on passenger side), then you have a head gasket problem....

My $0.02: This platform has to rotate the engine forward to access the rear spark plugs (hence how easy it is to rotate). Be sure you can get to them before performing this test.

I wouldn't replace the head gaskets, unless necessary....with the top of the engine off, now is the time to do a complete tuneup....get those new plugs and wires installed while it'* easy....

Also, while you have the lower intake off, replace the thermostat, and also replace the o-ring and gasket on the dummy distributor shaft....