Help/Advice? 94 YJ Occasionally Spewing Coolant

Make sure your lower radiator hose is in good shape and that the spring is in there, it may be collapsing. Also pull the hose off and make sure the inner liner of the hose hasn't separated from the outer. Both of these can cause a coolant flow restriction that can do what you describe. You didn't mention of the coolant is leaking out or not. Not likely, but you could also have an air pocket somewhere in the cooling system. Some of these are tough to get rid of. Sometimes you can bleed the system with the engine running by cracking the hose connections (be careful!) or loosening the sending unit. This can be a messy job, and please be careful with the hot coolant.

I'll be totally honest with you, this usually is a sign of compression pressure getting into the cooling system. My preferred way of checking for a cracked block, etc. is to use a cylinder leakdown tester. These can be a bit expensive, and you can substitute an air hose adapter that is available at the parts stores. The hose will thread into the spark plug hole and connect to your air compressor. When you pull the plugs, look for one that is "washed" clean. This is another indication of coolant in the combustion chamber, which normally shows up as whitish grey exhaust smoke, which you didn't imply. Before you put the air to the cylinder, make sure the piston is at TDC and have the radiator cap off. When you apply the air, you can tell where you are losing cylinder pressure. You'll hear air hissing in the tailpipe for leaky exhaust, air in the carb/throttle body for leaky intake, air in the oil filler hole or dipstick tube for leaky rings, and bubbles in the coolant will indicate and head gasket or cracked block/head.

Chris

Let it idle for a while top it off slowly at the radiator until it's full and the bottle is at the full hot line then get under and look for a slow leak coming from the bottom of the water pump... You don't actually have to top it off for this but it gives you something to do while you're waiting for it to warm up completely and you'll also know that it's cycling coolant before you get under it. The weep hole may not actually be leaking coolant yet, but my guess is it will be soon enough.

I go through so many water pumps on my crew trucks at work (two in the last month) that this has become old hat. Of course being kinda pessimistic the first thing I start worrying about is a blown head gasket... So looking for air bubbles/oil in the coolant/coolant in the oil/gasket material or crud floating around in the coolant etc. is also old hat.

Damn! Great advice. I will try this method and see what I see. Thank you!

Thanks, Simon. I'll try this, too. I appreciate the help.

Chris, that seemed to be the problem. I did what Simon suggested, ran it without the caps, and fluid was pushing out the radiator, followed by a lot of bubbles, which eventually subsided.

I recall that in my dual battery installation, I had re-routed the skinnier topmost fluid hose, and it had spewed a lot of coolant out. That's what gave it the air pocket.

Seems to be fine now.

Duh...what's "TDC"? All the way up? All the way down? Would I set the tranny in neutral and crank the fan belt to get in in position?

Sorry, TDC is Top Dead Center. The reason for this is twofold. First it will prevent the air pressure from pushing the piston down, and second, it will ensure that the valves are both closed. It has to be on the compression stroke, which I failed to mention. Hopefully you've got it sorted by getting the air out of the system, because its much less costly than the other alternative. :-/

Chris