I've got a 1990 Mazda MPV 4cyl. 5spd., used as a work truck that has a peculiar problem.
The coolant temp. goes to 3/4, and sometimes almost at red zone, on the highway, doesn't matter if you're doing 50mph or 80mph. All other times, it is perfectly at half point, in blistering heat in stop and go traffic or idling for 1/2 hour.
No loss of coolant, no overheating, drives really well. Gets almost 22mpg loaded with 600lbs of construction supplies and equipment.
any ideas?
runs hot only on highway
- Thread starter berge
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Turn the heater on and see if it takes a nose dive back to normal. Also consider installing a real aftermarket guage. There's just too many reasons for this to happen at highway and not at all during stop and go.
Feel your lower radiator hose, is it soft and mushy? It could be colapsing at high speed and restricting flow.
Another possibility. Do you lose coolant when it gets hot on the highway?
Some cars have air deflectors mounted up under the radiator that divert air towards the condenser/radiator at speed. If this is missing your car might not be recieving the "proper" amount of airflow through there.
Most likely the water pump. Impellers are probably shot.
Overheating at highway speeds but not at idle is almost always a water pump problem.
Overheating at idle but not at highway speeds is almost always an airflow problem--usually a bad fan (or fan control circuit).
The rad is partially plugged or the stat is stuck partially open.
Alright so change thermostat, water pump, and radiator. That should do it. ![[Big Grin]](images/icons/grin.gif)
Those are really good tips: The collapsing lower hose, bad impeller blades on the pump.
A loose drive belt fot the water pump is a possible problem.
I can assume you checked the condition of the radiator, right?
In a nutshell:
cooling related problems.
Check all the related components: water pump, upper and lower rad hose, thermostat, electric fan, fluid coupled fan clutch, clogged radiator, etc.
When is the last time it has received fresh coolant? If not recently it could have scaled up and might have a restricted flow due to scale buildup. Did this start problem start suddenly or gradually? Suddenly might indicate a thermostat issue. Either way I think that if you start with simple things like changing the thermostat and radiator cap after flushing the coolant system(possibly with a cleaner to clean some of the scale out if its there), then check on the more expensive things afterwards. A new thermostat and radiator cap, some radiator flush, fresh coolant, and distilled water don't really cost much if you can do the work yourself. As others have said hoses would be something to look at as well if they are original you might as well change those as they are pretty cheap too. Good luck
Having just gone through this same thing with the wife's truck ('88 Nissan), all the above are good ideas. My wife's also did it whenever the AC was on. The water pump/fan with clutch were replaced, and it still did it. Thermostat and both radiator hoses were then replaced, and it doesn't do it anymore. So I don't know if it was hose or thermostat related. I doubt thermostat, because I had changed it with no effect once when it started doing it. I'm betting it was the lower radiator hose collapsing and the system not being able to keep up with cooling demands.
Dave
so, what do I get RE: prize?
*grin*
quote:How was the thermostat failing and still letting enough flow through to keep cool in stop and go traffic? I suppose at those low engine speeds and lower coolant flow, it wasn't enough of a restriction?
Originally posted by berge: The coolant temp. goes to 3/4, and sometimes almost at red zone, on the highway, doesn't matter if you're doing 50mph or 80mph. All other times, it is perfectly at half point, in blistering heat in stop and go traffic or idling for 1/2 hour.
quote:I adapted around the problem by keeping high revs(lots of coolant flow) and low load (less heat) in city driving. on the highway, though, with the aerodynamics of a barn, there was no way to compensate other than driving in 3rd or 4th gear if the temperatures got dangerously high. it is quite nice not to be driving around in the middle of summer with the heater on full blast though.
Originally posted by Tosh:quote:How was the thermostat failing and still letting enough flow through to keep cool in stop and go traffic? I suppose at those low engine speeds and lower coolant flow, it wasn't enough of a restriction?
Originally posted by berge: The coolant temp. goes to 3/4, and sometimes almost at red zone, on the highway, doesn't matter if you're doing 50mph or 80mph. All other times, it is perfectly at half point, in blistering heat in stop and go traffic or idling for 1/2 hour.
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