Open the bleed screw when the truck is cold and sitting level. If coolant is not right to the top you have air in the system. You can add a small enough amount of coolant to get it to the top right through that hole, then replace the screw.

Cold in this case means first thing in the morning stone cold.

Drive it after this and check again the next morning. If after a few days you no longer need to add coolant then you've bled it fully and correctly. If you never get to that point you have a leak letting air in.

 

I read on other threads about running the heater full blast while engine is at operating temps. And, if the air coming out isn’t hot (as normal heater temps) then that would indicate trapped air. Is that the tell tale sign? And, am I correct that trapped air would not allow the Scan Gauge to read actual engine temps?

Would any of these symptoms indicate anything else? Clogged radiator? Stuck TSat ?

 

@Frank Gordella that would be a huge air pocket or clogged heater and would make no difference to the scan gauge reading. Disco's run climate control which means coolant always flows through the heater.

 

Ok fellas. I’ll keep y’all updated. Thanks for all the input.

 

I did the fill and bleed this morning on stone cold engine as described above. Started truck and let get to normal operating temp. No over heating, all seems good. When I give the upper radiator hose a little squeeze they feel empty. Not normal, right?

 

Also, when driving, the temps were very erratic. Went from 212 down to 174 then back up to 198 within a very short period. Has anyone ever done an exorcism on a D2? It could be possessed.
One more item - the slushy water noise behind the dash is much more pronounced now. That’s more air, right?

 

So you found air under the bleed screw and topped it up?... Then you drove and have louder waterfall noise behind the passenger side dash? Was that sound immediate or did it build up? Did you ever do a pressure test?

Let it get cold again and see how much air is under the bleed screw again... If so, top up and drive more... This sounds like bigger problems than a bleed will fix though... However it could be a massive air bubble, assuming your pressure test showed good.

The erratic temps you describe sounds like an air bubble around the temp sensor.

 

Yes, sound was louder than before, but it didn’t increase. I have not pressure tested the system yet. Does it matter if it’s warm or cold when I do the pressure test?
The coolant level did drop a bit after the drive but not significantly. Maybe an inch below the fill line. I’ll top off in the morning when it’s duly cold again.

 

@Frank Gordella an inch is a lot, less than a quarter is not. I am going to suggest 2 things.

1. remove your expansion tank and attached hoses carefully
2. Lift the tank up how high depends on your hoses
3. Brace with some 2 x 4 chunks
4. remove the cap
5. Crack the bleeder screw wait until only fluid comes out -you may have to top up the tank if you have a lot of air in there
6. Tighten the bleeder and top up the tank, pressure cap back on loose (1/2 a turn) you do not want any pressure at this point. Put the tank back in place
7. Start the disco and let it get up to temp
8. Shutdown
9. Go back and do step 1 to 5 -this is why you do not want the cap on tight
10. Now tighten everything up you should be basically air free at this point
11. After the engine has fully cooled top up the expansion tank if required

2 Add an overflow capture bottle some rubber hose an any small plastic bottle will do, this will tell you if you are getting an over - pressure situation. Which is generally a head gasket problem, although there are a few other causes.


My overflow catch bottle

 

FYI, air bleeding or trapped air is an almost non-existent problem with the inline mod. The thermostat relocation eliminates most of the issues with trapped air.