* Radiator Fan: The radiator fan might be malfunctioning. At lower speeds, airflow from driving might be enough to cool the engine. However, at higher speeds, the increased engine heat generation exceeds the cooling capacity without the fan's assistance. Check if the fan comes on when the engine temperature rises and if it's spinning at the correct speed.
* Radiator: The radiator itself might be clogged with debris (internal or external), restricting coolant flow and reducing its cooling effectiveness. This is especially problematic at higher speeds where the demand for cooling is greater. Inspect the radiator carefully for blockages.
* Coolant Level & Quality: Ensure the coolant level is correct and the coolant is the correct type and concentration. Incorrect coolant can reduce its effectiveness.
* Air in the Cooling System: Air pockets trapped in the cooling system can hinder efficient heat transfer. The system may need to be bled properly to remove any air.
* Failing Water Pump (despite replacement): While unlikely if you recently replaced it, a faulty water pump could still be the culprit, especially if a low-quality replacement was used. Verify the pump is indeed functioning correctly.
* Transmission Cooler: The Tarago's automatic transmission cooler is often integrated into the radiator. If this cooler is clogged or malfunctioning, it can contribute to overheating issues, particularly at higher speeds when the transmission is working harder.
* Blocked Airflow: Something might be obstructing airflow to the radiator, such as debris, a damaged shroud, or even a poorly fitting accessory.
* Head Gasket (less likely, but still possible): A failing head gasket *could* cause overheating, but the speed-dependent nature of the problem makes it less likely than the other issues listed above. A head gasket leak usually causes overheating regardless of speed. However, a very specific leak *might* only manifest under high-pressure conditions created at higher speeds.
Recommendation:
Start with the simpler and less expensive checks first:
1. Check the radiator fan operation.
2. Inspect the radiator for clogs and damage.
3. Verify correct coolant level and type.
4. Bleed the cooling system to remove any air pockets.
If these checks don't reveal the problem, a professional mechanic should diagnose the issue further. They have the tools and expertise to properly test the cooling system's pressure, flow, and identify any subtle leaks or malfunctions. Mention the speed-dependent nature of the overheating to them – this is crucial information for proper diagnosis. Don't continue driving the van at high speeds while it's overheating, as you risk significant engine damage.
