This past winter I put in a new water pump and radiator in and everything seemed to fine until these past few hot days up here in the northeast. The hot weather has revealed something a little weird: the (early 85) runs cooler, according to the gauge, when driving slower in mild traffic than when I'm on the highway at 3000 rpm/70 mph. I thought maybe a thermostat problem and tried removing it altogether- it now takes longer to get up to the first bar but otherwise behavior is much the same. On the highway with AC on max the temp needle is running between 2nd and 3rd mark, when going up hill it creeps up to 3rd line and would maybe go past if hills were longer. As soon as I get off the highway, temp starts to come down to between 1st and 2nd mark (AC still on same setting). Any ideas? I did a quick bleed check at idle and I don't see any bubbles... both fans appear to be at high speed with AC on (as they should)... The oil pressure drops maybe a bar when temp is at 3rd mark so I think it really is running hotter. I plan to test the temp sensors and check grounds anyway but what else?

Also, the car has original cat converter with 250K miles. Could a partially clogged cat cause this? Any way to check cat? (car passed emissions 4 months ago but I don't think this proves cat is flowing properly).

 

i think it has to do with radiator efficiency. guessing that tstat is wide open but radiator still can't dump the heat. are the fins clogged with debris? or maybe something hit it and bent fins flat?

 

I think a clogged cat might raise temps, but it would also be real obviously down on power. Tell us about the radiator... cleaned used? New OEM? There are two causes of high temps; lack of air flow or lack of water flow. I'd say you've eliminated the water pump and t'stat, unless the pump is really really bad (loose impeller?), so that pretty much leaves the radiator. I'm fighting an identical problem on my Mustang and have concluded that something in a near-new radiator is blocking proper water flow. I suspect the radiator's PO put in Bar's Leak to "fix" a leaking heater core.

 

You could be running to lean at highway speeds, When was the last time you changed your fuel filter. I know it sounds weird but I have had that happen a lot. No or low fuel pressure at "speed", will cause the motor to run lean and Hot.

 

It has been quite a while since a fuel filter change... Wouldn't hurt to change it. No bars leak was ever put in the rad. I tested the temp gauge per Clark's and it seems to be working properly...

 

Personally I would vote for poor radiator efficiency. I replaced a leaky radiator with a new 'identical' aftermarket model on my old Maxima and had exactly then same issues as you. Something about higher-speed driving made it run so much hotter. I had kept the old unit and measured the liquid volumes. The new one held about 30% less coolant even though it 'looked' comparable. The radiator shop provided a better one from another supplier at no additional cost and it solved the problem.

 

Can you find out if it is speed, or load, or RPM?

I would wonder if it's a spun water pump pulley or damaged impeller (was it plastic?)

Try driving around slowly but at 3500-4500 RPM.

 

So after a few days of hot weather in the NE your car needs help?

1. Soak the front and back of the radiator and condensor in a diluted solution of Simple Green or similiar cleaner.

Rinse from the engine side OUT.

If you did it from the grill side in you push the dead fly bodies and wings deeper into the fins.

2. Put in some Red Line or Kwic Kool or ??? to drop the heat a few degrees.

3. Keep the thermostat in for best engine performance and emissions.

4. Put Redline MT90 or similiar in the transaxle.

5. It will cool down in the NE soon and the AC will not have to work as hard.

My temps go up on the highway a little. City driving is a lot of go and slow.

250K-congratulations. Normal temps for engine wear I think.

GL
John

 

Very interesting... I've been wondering if this is the problem on my Mustang. The rad is not that old, but is an aftermarket replacement. It behaves like it has more restriction to water flow i.e., smaller tubes. It has blown three heater cores in 2 yrs (and it's a 10 - 12 hour job to replace), from all appearances due to excess pressure. I guess I'll bite the bullet and replace it.

 

The radiator may just need a good cleaning. I took mine out a few weeks ago and used a pressure washer to blast LOADS of sand and gunk and who knows what out of the fins. Made a dramatic difference in temperatures.

 

How's the coolant level? Has it dropped any? Pull the spark plugs and see if any one of them looks particularly clean. That would indicate your headgasket is leaking.

At any rate, my 220K mile 83 performed similarly in the hot Caribbean climate since I had the A/C on year round.

The stock aluminum engine protection pan helps cool the engine during highway driving. If you don't have it on the car, I'd put it back on.

 

I had a similar problem with my race car last year. My car would idle fine and not raise temps, but on the track at speed and the temps would climb.
...... turned out to be a surprisingly easy (though embarassing ) fix.

Make sure the radiator fan(s) are blowing in the right direction! On my car (83 944), the wires attached to each fan are simply two spade connectors; 2 females on one side and 2 males on the other. Following some other work, the fans were accidentally wired backwards, and were blowing air towards the front of the car. At speed, the fans were fighting the air trying to push in, and a static bubble was created allowing the radiator the heat up. We finally noticed when someone, wearing shorts, was standing in front of the car, while we tried to diagnose, and he suddenly said "Why are my legs getting warm??".

 

Not the first time I've heard of this happening, but it still makes me smile every time. Too funny!

 

Thanks for the responses guys. I did pressure test the system and found no indication of HG leakage, nor does it lose coolant. The rad looks clean (and has only been in there for a few months). I still have the original fan connectors on there and they only go on one way (although the fans were upgraded to 6 blade a few years ago so I'll check). The pan is on there as it should be. I suspect the AC load has something to do with it since I did re-install AC after several years without it (luckily I left the long lines/hoses in place). I guess there could be something weird with the new rad... I was also thinking of adding a 951 oil cooler to help.

 

What brand radiator did you put in? I have the same conditions after replacing my water pump/thermostat/hoses/radiator. I used a Valeo radiator since it was the only one I could get quick locally.