I'm almost 2 months into ownership of a 2004 Disco SE7 with 126,000-ish miles and fighting with some coolant system issues. I'd like to say that this site has been an awesome resource already to track down little issues and to read about those that others have been having. Sorry for the long-winded post.

History:
Water pump replaced by dealership before I purchased it.

Popped off overflow to throttle body heater hose while driving, lost almost all coolant, overheated (see other thread), shut engine off at around 240-ish degrees and refilled after about 30 minutes after fixing hose on side of the road. Ran fine with good temperatures for a week or so afterwards. Note: I had to use green "mix with any color" coolant along with Dexcool due to necessity. I flushed and refilled with Dexcool (it was what I had on hand) that same night.

Had so-so heat when I got it. I flushed just the heater core and heat returned to normal afterwards. Heat remains good now, even with these issues.

Symptoms:
On the highway, or with engine speeds above about 2,000 RPM, temperatures stay in the 195 - 200 degree range. When cruising at around 1500 RPM or idling, temperatures reach 220 degrees. Revving the engine or downshifting almost immediately lowers temperatures below 200 degrees.

What has been checked/noticed:
I noticed that the upper radiator hose is very firm. I would not say rock-hard, but firm. I haven't checked the lower hose. The upper hose is hot. This is with the engine running or after turning the engine off while hot (above 212 degrees).

No loss of coolant has occurred. The coolant level stays perfectly consistent. No water in oil, no white smoke (aside from that due to sub-freezing temps). Also, not boiling over or experiencing "water fall" sounds.

I checked the routing of the serpentine belt to confirm that it is correct. I also verified that the viscous clutch is good and the fan is aimed the correct direction. I also verified that the electric fan comes on at about 212* and that it is facing the correct direction.

What has not been checked:
I have not checked the lower radiator hose to see if it is rock hard or hot/cold. I also have not yet checked to see if temps on the radiator vary significantly from the top to the bottom of the radiator. I plan on buying or borrowing an IR thermometer to check this soon.


So, my thoughts are that it could be
1) a blown head gasket, though I don't think there's enough evidence to back this up.
2) a clogged radiator. Seems more likely considering that the heater core was clogged and there was possible evidence of either a stop leak product or dexcool sludge in the heater core that came out with the back flush.
3) a faulty pressure cap. It seems like the pressure on the hose would exceed the 15 psi or so that the cap should open at, but I'm not yet sure if the pressure is in the whole system or only at the inlet of the radiator (top hose).

I have a 180* thermostat that I haven't installed yet. I've bled the system per the RAVE (system cold stone, raise reservoir 12-18", open bleeder screw on the "T" and fill reservoir until no air comes out of bleeder). The heat still works great, so I'm pretty certain that the water pump is circulating sufficiently.

Suggestions? Where should I go from here? Any ideas are appreciated.

Thanks!

 

I think I would be thinking about flushing the rest of the cooling system, see if the rest of the system contains the same sludge. Particularly, backflush the radiator. Dexcool sludge can be removed by a mild acid solution (that's what commercial flushes are)

On my TR8 I flushed with a mild acid solution, then with a hosepipe.

 

The first post in this thread explains why you might be seeing temperatures drop with higher RPMs - clogged up t-stat. I think I have something similar.

 

Everything you just described in your post is what happened to us. If you get a chance check the lower portion of your rad and I bet its stone cold while the top you can barely touch. The top hose is hard due to pressure which is due to poor or no flow. We change to the 180 t-stat which barely worked and found the rad was plugged solid on the lower half. We bought a new rad, new hoses and put the 180 t-stat back in and see runs perfect now. We bought our rad from Rimmer Bros in the UK for way less than in North America.

 

Change the T-Stat.

Change the dexcool out to the green stuff.

Bleed off the air.

 

-- Awesome. I'm thinking this is what's going on. I'll check the lower portion of the rad and the lower hose later today and see if there's a big difference in temp. If so, I'll pick up a new radiator and hoses. I'll do the 180 T-stat too.

-- Would this account for the harder-than-normal upper rad hose? Like I said, I have the 180 T-stat on hand, so I can certainly change that out this weekend.


If the Radiator is plugged up, should I bother trying to flush it? I've read elsewhere that these radiators don't respond well to flushing.

Thanks for all the input!

 

Don't waste your time flushing it just get a new one, they are cheap enough. When you install the new rad tear the tanks off your old one and you might be shocked what you find inside it, I sure was and no flushing was going to get it out.

 

Thanks. Anyone have preferred sources for Radiators? I've checked Atlantic British and they are like $320. Is that a respectable price, or should I look elsewhere?

 

Check Rimmer Bros in the UK and shipping isn't that bad.

 

I got on Amazon and found one for for less Nissans I think
220 isn't all that abnormal with a stock thermostat. Try the 180 thermostat. mine runs around 195 all the time with it but would get to 215 with the old one when at an idle. Changed thermostat and it was better on the 99. The 04 had to have a new radiator due to leaking but temps are lower now. So all that said I would try the stat before I bought a radiator. You may or may not need it.