Hey guys I hate to add another thread about overheating discos but im stumped about mine.. it doesnt always overheat, some times after setting still in traffic to long and some times after driving awhile. Both of my fans are on switches in the cab, one on a relay one manual, and I have even removed my thermostat completely but today it overheated after driving her about 30 miles home.. I dont know where to start? The fans work so check that .. radiator? hoses? sensor... please help.. i love my truck and prefer not to blow a head gasket. thank you

 

I had overheat on my 97 if idling too long, with Ac on. Found the following: (1) viscous fan clutch was wimpy, replaced with $50 new one per the "chevy conversion" elsewhere on this forum. (2) Radiator was clogged, had shop flush and rod it out ($65) - (3) Went to 180 high flow thermostat from Summitt Racing. Still had problems at idle, but only with AC on. Found electric fans had revesed polarity from PO. They were blowing out the front. This "subtracted" airflow from the viscous fan, which does not re-engage until faceplate gets to 170 degrees. It also overheated AC condenser, driving high side pressure to 400+ PSI. With all this corrected, mine cruises at 174 - 180 degrees at 60 mph, and no longer goes to 231 at idle after a short test run on the highway. Having a scanner that reads coolant temp data is a real help (because factory gauge is "normal" from 130 to 240 - don't move much). With the radiator cold, you could open the top "bung" plug and use a flashlight to look at some of the tubes. If you see calcium, it is time to flush, or take to shop. I tried a salvage yard radiator, turned out the oil cooler was blocked, that is why that Disco was in the salvage yard...

I never had problems at highway speed, so you may have some sort of restriction (inside radiator, or leaves between radiator and condenser, or just plenty of mud).

Of course if you have coolant loss or white exhaust smoke it might be wise to check for head gasket leak.

 

Do NOT remove your thermostat...it doesn't help the engine warm up properly and WILL cause issues of overheating.

Lot's of things to check at the basic level before throwing parts at it or misdiagnosing the problem which may be as simple as excess air in the system.

Is there debris between the condenser and radiator?
When the engine is at operating temp, is the top radiator hose hot and pressurized?
Is radiator fan clutch operating properly?
Have you had to add any coolant lately?
What is the condition of your hoses?
How old is your radiator? If it's the original; you most likely need to get it rodded out or replaced.
Do you see the coolant swirling around in the radiator with the engine running when the filler hole at the top is open?
How old is your water pump?

 

A quick way to evaluate your viscous fan clutch: A cold engine, the fan should show resistance to spin, stiff, etc. When you crank up (cold engine) the fan should "roar" because the silicone inside has "set" to one side, and the clutch engages. With my replacement, this roar was enough air flow to spin the electric fans, even though the AC was off. The roar stops shortly as the silicone is spread out and warms up slightly in the clutch. When engine gets to "normal" temp range, stop engine and clutch should offer a little more spin. But one that free wheels several revolutions is low on silicone, usually greasy on the front of it. At idle, when warm, you can use a rolled up magazine and gently touch the moving blades. They should not stop. Do not touch with fingers. The clutch from a Chevy Express van 4.3 liter without air is a good fit, described elesewhere in forums. About $50.

 

Check the tech section for info on the new fan clutch.
I'm betting your radiator is clogged.

 

They should really both be on relays, unless you've run heavier gauge wire to a heavy duty switch.
Or do you mean one has a manual switch and one has a thermostatically controlled switch?

Is it only overheating with the AC on? You don't need the AC fans if you're not running the AC. If you have to run the AC fans to prevent overheating without the AC on then you have issues unrelated to the AC.
Clogged radiator internally is the most likely culprit on a car that old, if it's the original radiator.

x2 on no removing your thermostat

 

my ac doesnt work so no not with the ac running.. It has a temperature gauge in the cab and it has never been warmer that 180 degrees but still overheats.. not always mind you just sometimes.. yes they are both on toggles but one is triggered by a relay one is not but I run them both on all the time when its as hot out as it is here.. mid 80s to low 90s.. I will replace with a new thermostat but im not sure where to check then.. water pump or radiator. thanks to all for the advice.. maybe knowing the temp will make the diagnosis different.. thanks

 

I do add coolant but only after a spill over from the overheating.. the radiator fill tank has plumbers tape around it so im assuming this has been a problem in the past.. no radiator cap to check for moving fluid or calcium build up

 

Right there is your problem, the coolant reservoir needs to be replaced.

There is a plug on the top left corner of the radiator that if you remove it will let you see directly into the radiator.
The coolant reservoir has the radiator cap on it.

 

Is your coolant expansion tank black? If so, it is a old tank and quite a known issue to burst at the seam (like a belt around the middle). Replace with the updated part ASAP. This could be the reason you're overheating as well...the cooling system cannot build any pressure as it's bleeding out of the expansion tank and therefore the coolant is boiling at a much lower temperature.