I just had a friend replace the water pump and immediately after picking it up it overheated on the highway. He thinks it's the radiator but I'm puzzled why it would not be water pump related. It does not overheat at idle. There is sufficient coolant. I let it run for at least an hour yesterday with the rad. cap off. I noticed air bubbles coming up and not a lot of circulation at idle but some.

Last summer I did experience temps on the high side during highway driving out west. This van has no overdrive, and I soon recognized that of I kept below 65 it ran much cooler, but I find it hard to believe it's engineered that way. A radiator guy I called on the trip said the stock radiators are junk, but mine has been replaced once prior to that trip due to front end damage in an accident.

I've read threads here on highway overheating and the causes can be many things.

1. water pump cross wired and impeller turning wrong direction
2. radiator cap
3. collapsing hoses
4. bad temp sensor(s)
5. thermostat
6. radiator
7. transmission overheating
8. head gasket

Just wondering what's the best way to diagnose this. My friend is sure that it's wired correctly. He didn't replace the thermostat. Any suggestions much appreciated.

 

1. Check radiator cap. Seal should be soft and pliable.
2. Coolant flow looking through radiator cap is not a good measure of flow as most coolant will flow through tubes and not across top.
3. Test/replace thermostat.
4. Water pump crosswired ? Check belt routing diagram under hood and vrify it is correct.
5. Burp air out of system. Elevate front right side, fill with coolant and let it run. Squeeze upper hose, help get all air out of system.
6. Has radiator been flushed lately?
7. Is it actually overheating or is gauge reading overtemp? Sensor is just that - it provides info to PCM and gauge, does not control temp.

 

replace, (or temporarily remove) t-stat.

use a better quality Stant super-stat for about $10-15.
avoid the cheapo $5 parts. you get what you pay for.
also avoid Fail-Safe. expensive, poor quality, usually fail in closed position, which is exactly what they claim to not do. there's far too many problems reported on these.

check your radiator for dirt, mud, grass seeds, etc blocking air flow. back spray with water hose to clear stubborn debris.

key question - did it overheat before you replaced water pump?

 

thermostat is where i would start, they cost like 5 bucks. i have had one before where it would still open but it would open to late than what it should and caused the temp to rise. you could always just take it out and try driving it and see if it overheats, if it doesnt id say its the t-stat

 

probably, but not always. if the sending unit and gauge is working properly, and with NO t-stat- it holds about 140-150 degrees, the problem is most likely t-stat. on the other hand, if it holds about 180-190, then the radiator is not cooling properly.

 

Thanks for all the suggestions guys. @dhvaughan, it has never overheated like this before. The timing of this coincides exactly with the pump replacement. Right after picking up the van from the water pump replacement, my son said it overheated into the red on the highway and a warning light came on (I hope it didn't damage the engine). He let it cool then took back roads home. Like I said, I ran it at idle in the driveway this week and it maintained proper temp. However just as soon as I took it for a spin in our neighborhood, the temp gauge rose rapidly and I brought it home and shut it down.

If it's a bad thermostat, could it run in normal range at idle indefinitely?

Could another part have failed and immediately killed the water pump? And so even with the new pump, it's still overheating? I ask this because it seems to be the only possibility that this isn't water pump related, because I didn't notice any overheating prior to having the pump replaced. Or is it likely that the failed water pump damaged another part of the cooling system?

 

i don't think you can really kill a water pump. i think you should check the thermostat.

 

So you mean they just fail with age. My van was leaking coolant from below the pump for a few minutes while cooling after shut down. I'll get a Stant super stat as dhvaughan recommends.

Doe the alternator need to come off this 3.9L to replace the thermostat? It looks difficult to access.

 

i guess you could say they fail with age. usually gasket fails, a seal goes or the bearing just get sloppy from age but if you look at what a pump is there is nothing that should be easily damaged.

 

...or burp system.......