Hi all I bought a 2003 Discovery 2, a month ago for $1300. I bought the car with a cracked radiator and bad front propshaft. In that time I have replaced coolant hoses, water pump, radiator, thermostat (replaced with 180 degree), and front prop shaft.


The past two days I've been test driving it. After about 20 minutes of driving from cold the temperature will start to rise from the middle of the gauge. When I shut the car off at that point I can hear a 'hissing' noise from the area close to the water pump, not sure if that is air being sucked in past the gasket or something else. When I replaced the water pump the old gasket was cleaned off and all bolts were torqued down


I've tried bleeding the cooling system twice now and thought I got all the air out.


Bleeding procedure has been as follows with a cold engine:

1. Place coolant reservoir on top of the battery/ jack cover.

2. Open the bleeder screw at the t junction.

3. Add coolant to the reservoir and watch for bubbles/air to come out of the bleeder area. Squeeze coolant hoses to help move coolant

4. Stop once no more air is coming out.


The heat is blowing hot on test drives, so I'm not sure what else I'm missing. My brother and I think maybe the temperature sensor is bad and as it warms up the sensor reads high. I'll have a scan tool tomorrow to help diagnose.


There have been no coolant leaks I have noticed.

Any other ideas on what else could cause overheating?

Thankfully I've checked my oil throughout this and confirmed there is no evidence of a blown head gasket in the oil

 

Suggest you read the inline thermostat thread pinned at the top of the Forum. One of the benefits of the thermostat is you can run the engine with no thermostat element installed to try to troubleshoot other issues like a plugged radiator or other issues.

 

@Niohdachi Hissing is usually indicates a leak. Wrap paper towel around all the hose unions in the area of the water pump, zip tie in place and run the engine up to temp for a couple of minutes then check the paper towel.

If the paper towel is dry check the lower edges of the heads,it could be an external head gasket leak.

Any movement from the center on the temperature gauge is a huge problem, the means you are well over 220F and heading towards overheat.

 

Update:

After bleeding the system one more time, I got a tiny amount of air out of the system.

I then checked out the fan clutch as I read that could be an issue/ part of the issue. When the engine was completely cold I could move the fan. I gave it a few good pushes and on the 3rd push it spun multiple times.

Then I started the car up and monitored temperatures while idling. Once the car reached 178 F the thermostat appears to have opened as I immediately could feel the hose begin to warm up.

Then I let it warm up to 197 and attempted the 'paper' test on the fan. I folded up a piece of paper a few times then held it in the fan, after a few seconds the fan came to a stop and slowly began to spin again when I pulled the paper out. Am I correct that this is indicative of a bad fan clutch?

I'm looking into the Hayden 2991 Fan Clutch and Dorman 620 112 Fan blade to replace it.

If the fan clutch is truly that shot would it explain overheating? Local temps on my test drive the other days were about 60F-70F and I did notice the heat increase greatly when at a stop.

 

Also @Richard Gallant I did attempt your paper towel test earlier today. I did not notice any seepage from the hose connections or around the head. It sure was a chore cleaning up the head to check that though lol. Do you have a recommendation on how warm the car should be? I checked when the ECT on my scan tool read at 200F

 

@Niohdachi 200 is ok. As to your fan clutch question it sounds dead to me, even if you stop it with paper it should start spinning instantly. The Hayden/Dorman combo work well, and you will run cooler,but a bit noisier.

You can get some uv coolant dye and add it to your coolant if you think you have leaks, with a UV light that stuff glows bright green. And you can leave it in the coolant tank pretty much forever.

 

Congrats on the D2. First - I’d say you *** must *** get an ultra gauge if you want to keep this rig so you know your accurate engine temps.
https://ultra-gauge.com/ZC/index.php...roducts_id=195
Otherwise your gauge is doing nothing and you’re going to ruin the motor on the car. By the time it rises you’re starting to get to “ruin your heads / need new head gaskets territory” 110%.

Some ideas.

The hissing you describe is I think the “tell”. I think there’s a leak that’s sucking air somehow in the system.

Have you done a pressure test of your system? To -- Find the leak. If it’s hissing, there’s a leak. If air is coming in under vacuum, coolant will come out under pressure. Just be careful how much pressure you apply... You don’t want to blow something else. Read up on how much PSI to use, I can’t remember off hand.

D2’s aren’t that hard to bleed fully with the bleeder valve up top there -- but I do pressure fill my D2’s these days. It’s better. I recommend it. Using cooling system / vacuum fill & adapter from a compressor, etc. This to me doesnt necessarily sound like you have air in your system, but it could be.

One super obvious place for coolant leaks you may be missing: Under the throttle body are 2 hoses. They are the throttle body “heater” to de-ice it. That thing always leaks if it’s’ not properly maintained. You can bypass it or fix it. There’s also a gasket.

Also those small hoses can get crystallized dex-cool in them and that’s an issue.. Did you find anything like this in your cooling system? If so you may have blockages in there.. Not uncommon on old systems that had air in them (ie cracked radiator).

Fan clutch will cause overheating, yes, but I still think if you have hissing something weird is up. Without the viscous fan working properly, the engine will overheat.

 

Thanks for your perspective @nashvegas .

I agree there might be something else than the clutch fan. Later this week im going to drain and fill with 50/50 coolant. I worry that through bleeding several times with distilled water I may have lowered the boiling point of the coolant too much which may explain the hissing noise once the coolant gets hot.

This weekend I'll install the new fan clutch and have my brother around to monitor coolant temps on the scanner when we test drive.

Hoping that it is just the shot fan clutch and extremely water heavy coolant mix that explains the issue.

In a few months I think overheating might not be an issue in Wisconsin lol 😆

 

Hey all, I'm replace coolant today with 50/50 premix and the new Hayden severe duty clutch and dormant fan.

Outside temperature today is about 80f
After replacing the parts I idled the engine to 210 which took about 25 minutes.

Does this sound about right for stable idle temperature on an 80F day? @nashvegas

Ill repeat the bleed procedure tomorrow to confirm all air has been evacuated then take it for a test drive with a passenger along to monitor temps.

Notably I dont hear any hissing noises after shutting the truck down.

 

Maybe you’re ok, maybe not.

I like to see a lower temp than 210. Especially with a 180 stat and a stock cooling system in good repair. Mine runs 194 all day long every day with the stock cooling system and the 180 grey LR thermostat on the ultra gauge. Bounces up to 199 in traffic or on super hot days.

Couple other data points. May or not be helpful

-- LR has the cooling fan (electric) set to come on at 212 degrees stock.
-- I have my coolant alarm on the Ultragauge et at 215 degrees, to give you an idea... that’s when I start to wonder if there’s an issue with mine. I panic at 215. It starts beeping at me. ( 210 is very close to that.)
-- Land Rover puts the red light on in the temp gauge at 250, that’s when they think a D2 is overheating. Well, that’s why the D2 has a horrible reputation ...

In all reality, 210 in and of itself is not going to cause you any harm, in fact, back in period (03/04) I remember seeing my factory fresh D2’s run at 210 from the factory -- pretty hot for emissions reasons which you can read about here all day long. But there’s really no headroom between your 210 and 225 or so which is when -- I think -- too hot for a D2 motor. That’s when plastic stuff starts popping and you quickly get head gasket issues from failure points. Those plastics were fine in 2003, not so much in 2025 with a 25 year old motor.

What I would do. Get the ultragauge. Monitor. See if you’re stable at 210. If you are, see how it goes. If that 210 starts turning into 215/220/225, even for a moment, you’re too hot. I suspect your 210 will creep or bump up.

Frustrating, I know.