Hey folks, looking for some suggestions on this issue, this is a bit long-winded but I wanted to present everything that I've learned so far (also searched several LR overheating topics).

Here are the details:

96 Disco I with 202K miles on it.

Problem:
The truck will overheat after ~10 minutes of idling, or a couple miles of driving (30-45 mph). Last time I drove it, the overflow cap literally blew off so I gently drove it home and got it in the driveway just as the temp gauge crossed over the high mark and nearly hit the 'H'.

What I've done since then:
- Replaced the cap with a new one since the old one had cracked threads and wouldn't screw down tightly, and refilled with 50/50 premixed green coolant. Since then I haven't driven it anywhere but just did some testing in the driveway. I've gone through several warm-ups and cool-downs, looking for leaks, but haven't seen any and haven't had to top off the coolant.
- I have an OBD scan/gauge that shows no fault codes, but once it's warmed up the water temp keeps climbing into the 230s F before I shut it down.
- Once I let it get up to 240F and the electric fans kicked in (this is expected I think)
- With the heater on, the air is just barely warm... until I rev it up to 1500rpm for a good 10-15 seconds. At that point I hear gurgling from the heater core and the air gets VERY VERY hot, meanwhile the water temp drops into the 200-210 range. I let off the throttle and the air starts cooling down while the water temp starts climbing again.
- With the heater off, revving the engine doesn't do anything, the water temp just keeps climbing.
- When I rev the engine after it's warmed up I can see coolant shooting up into the reservoir from the small front hose that's attached to the top of the radiator (no idea if this is normal, or if that's the water pump cranking the coolant through the system due to the rpm increase).
- If I try to spin the fan when cold it's quite stiff, once warmed up and shut down, it's less stiff but definitely not freewheeling (I think this means the fan clutch is working, didn't try the newspaper test)
- The radiator is clear of debris.
- After the last test, the water temp hit 235 and I shut it down & immediately read the top corners of the radiator with an infrared thermometer and both read ~250, the top hose from the pump was slightly cooler (thermostat, water pump, and radiator ok I think).
- No oil in the coolant, and no coolant in the oil or under the oil filler cap.
- No white exhaust, doesn't smell "sweet" (no cracks/gasket problems I think)
- With the fountain that shoots up into the overflow, I thought maybe there was still some exhaust leak, or maybe just steam/boiling coolant coming from the radiator.
- Just to be sure, I got a block tester from Napa and wasn't able to detect any exhaust gases from the overflow reservoir... this is not how the instructions say to do it, but the radiator opening is too small to seal it with the tester. (I think if there were an exhaust leak it would have made it to the overflow, not sure though).

So... the final analysis (comments please):

My heater core is jacked and I need to bypass it. If the overheating then stops, I'll look at getting a new core. Anyone think I'm on the right track, or other suggestions?

 

Bad water pump or fan belt routing is wrong.
Look in the tech section for a diagram of the correct belt routing, if it is routed wrong the water pump will turn to slow or even backwards.
If it is moving to slow then not enough coolant will pass through the heater core to give you heat, revving the engine will make it move coolant faster.
Same thing with cooling the engine.
At 30-40mph the engine rpm are low which means the water pump is turning slow.
If the impeller on the water pump is bad it too will not move enough coolant.

 

Alright, I like that logic, thanks a lot for the tips Spike, on this 402nd Disco overheating thread

The belt goes under the fan/pump pulley as it should (smooth side of the belt touches it). I did notice some white rtv/sealant gasket-type gunk around the water pump so it had definitely had some work done by the PO. Could be they replaced it with a junkyard part. I'll remove it and check it for a couple things I've read in other posts about the wp:

1. Missing/damaged fins
2. Hard to turn/bad bearings

I will work on pulling that apart hopefully this week and report my findings, thanks again!

 

I'm not familiar with the cooling system on the Disco (mine hasn't failed.....yet) but if this problem was on my old jags I would say the thermostat is stuffed. Its job is to stop circulation around the heads until the engine has had a chance to warm up. When It fails shut the temperature and pressure will rise quickly. Because water isn't circulating properly the heater won't work. Not sure if this is relevant.
P.S. In Brisbane we can remove the thermostat and throw it away as it never gets cold enough to worry about. It's worth a try, rip it out and give it a run.

 

Result of correct belt route is that fan will suck paper towel to grille, rather than blow it away. Would think over temp electric fans would come on before 230. Sounds like coolant bleeding is needed also. When checking that radiator, read the top fins, then the bottom fins, hoping the two readings will be within 10F of each other. If radiator is clogged, the bottom rows block off first, and not as much coolant goes thru there. New thermostat could help. Try a 180F one with a hole (see pix) or jiggle device at the 12:00 position to pass some of this steam past it. You can also drive with no thermostat for testing, my D1 did maybe 150F with no stat. You can bypass the heater core like this picture, from a member in San Juan.

 

Thanks for the follow-ups guys, I have some updates.

I pulled the thermostat and it is the OEM 88C with a jiggle hole at the top, cooked it in a pot on the stove and it opened between ~190-210 F which is right.

It would seem however I was measuring the radiator temp wrong. I put the tstat back and warmed up the motor today and shut it off when the gauge showed 230F. Then I pulled the fan shroud off and stuck my infrared thermometer down there and sure enough the bottom of the rad is reading 60-70F while the top is in the 90-100 range (it's about 50 outside). Obviously that's a bigger temp differential than it should be. The bottom must be completely jacked since it's just barely warmer than ambient temp.

So, new final analysis: Pull the radiator and have it reconditioned or replaced.

I'll update more as I make progress, and thanks again!

 

You are on the right track.
Let us know how much it costs to have your radiator rodded out and the name of the shop if you are pleased with their work.

 

202,000 miles.
Was the head gaskets replaced???
When? What milage??
Ours went at 138,000 miles..

 

jfall: unknown, I bought it at 198K last year, can't find any signs that the HGs are going/gone.

 

Well I couldn't find a place nearby to service the radiator, in fact I called/emailed 3 different places and got either no answer or no reply on all 3... seems it's a dying art.

So I went ahead and ordered a new (used/reconditioned) radiator from Roverlandparts.com.

While I'm waiting for that to arrive I've picked up a $48 Chevy fan clutch from autozone, and about to pull the water pump off so I can inspect it as long as I've got the clutch off. Oh and I've got the truck on ramps so I can pull the y-pipe off so I can pull the cross-member off, so I can pull the tranny oil pan off, so I can replace the $8 tranny filter... sigh.