BigSwede
Roll Me Over
Reged: 06/12/00
Posts: 3702
Loc: Minnetonka, Minnesota
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On the way back across country, I had lots of time to watch the temp gauge. It was interesting, the faster I drove, the hotter it ran, almost regardless of rpm. At 70+ mph in hot ambient temps, the gauge would creep up near the red, while in stop-and-go driving in 90+ degrees it was below half on the gauge.
In fact, when my direction changed and I went from a headwind to tailwind, the temp would actually drop some! This seems counterintuitive, but as I said I had lots of time to observe and confirm this behavior.
So I came up with a theory...my old 99 Trooper had a thin metal extension from the radiator skid plate rearward, sort of bridging the gap between the radiator and engine, underneath. This extension is also under the remote oil filter location, and so is a little annoying when doing oil changes. I asked an Isuzu mechanic about why it is there and he said something vague about airflow. Well, my 93 does not have this skid plate extension...not sure if it ever did, but either way there is a gap there.
My theory is that at highway speed, air is cramming under the vehicle and forcing its way up between the engine and radiator, stalling out airflow through the radiator.
Does this make sense? This is the only scenario I can think of to explain what is happening...if the radiator was clogged it should have problems in low-speed more than high-speed, I would think. The water pump is new, and the symptoms were the same before and after it was replaced. The fan clutch couldn't cause these symptoms.
I guess the easiest thing to do is get the extension piece from a boneyard and see what happens, but if you guys have any other ideas, I'm all ears. 
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Steve Carlson - 95 Trooper LS expo rig
Serenity now!
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Amigo-2k
Rock Warrior
Reged: 01/19/01
Posts: 952
Loc: Madison, WI
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plugged radiator. If it is still the oem one 12 years is a long time for an orginal rad.
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-Ryan
2000 Amigo V6 HT AT 2wd
FAQ 98-04 Isuzu Rodeo, Rodeo Sport, Amigo, Trooper
About the Owner
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BigSwede
Roll Me Over
Reged: 06/12/00
Posts: 3702
Loc: Minnetonka, Minnesota
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I hear ya, but how can a plugged radiator cool just fine in stop-and-go traffic, but not at higher speeds? I would think it would be the other way around...
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Steve Carlson - 95 Trooper LS expo rig
Serenity now!
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Anonymous
Unregistered
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This is kind of interesting. I drive 25 miles each for work and I always watch my temp gauge too. My skid plate between the radiator and engine is missing also but I have never had a problem with the temp at higher speeds. I was told it was to guard the oil filter. No idea for sure though. I would pick the extension piece up if I where you. I looked for one the last time I was at the salvage yard but didn't see one. With my old engine I could drive all day and never have a problem. But if I let it sit idle for a few minutes the temp would climb up and then I would have problems for the whole time I drove it until I let it sit for a few hours. With my new engine and water pump I haven't had a problem at all except out 4X4ing with the A/C on. Every so often I would have to turn it off and I could watch the temp drop back down to about 1/3 on the gauge.
I would think that with the troopers being a brick if you are driving into the wind there is much more resistance and your engine is having to work a lot harder.
I'm interested to hear some of the other replies.
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pplotz
Rock Warrior
Reged: 01/21/04
Posts: 906
Loc: Denver, CO
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Thermostat might not be opening all the way. Also, does it still have a fan shroud? Maybe the fan clutch is slipping (if it has one)
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JLEMOND
Body Damage is Cool
Reged: 09/21/03
Posts: 1214
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Hey steve very commmon hi speed cause the eng to run hot if your rad is the orig, drain down the coolant untill you can see the top of the core tubes and take a close look at them and see if you can see any buildup of salt looking particles on the inside edges of the tubes , after many years and prob, the same antifreeze you will get this build up and it gets worse farther down in the tubes , i use an old 2 ft + flat dip stick and insert it in one of the tubes if it doesn,t fall all the way to the bottom by itself you have corrossion buildup and it is almost impossiable to remove having it boiled out only cleans the corrosion, ,and the under pan is just what you described it is to force the air flow to go under the truck, if it is missing and the fan is working corectly it can make the fan tend to draw air up from the bottom instead of straight thru the rad, much the same way the shroud and the upper hood seal does, check the rad before going to deep Jerry
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Tom_G
Wheeler
Reged: 04/02/04
Posts: 137
Loc: Tillamook, OR, USA
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I assume that your 93 RS has the same 3.2L SOHC engine as does my 95 S model.
My temperature gauge practically never moves, regardless of temp, load, wind direction or A/C on/off. I just drove the vehicle from PA to Moab to coastal Oregon pulling a >3100lb trailer--the temp gauge barely rose pulling over the rockies west of Denver or driving straight into 40mph winds in 4th gear at 70mph/4K rpm with the A/C on for an entire tank of gas.
I do have the shield you refer to, but would be amazed if the lack of that shield would cause wild swings in coolant temp--it might be exactly that, but I'd be surprised.
Definitely a strange set of symptoms. If it's an automatic, perhaps the tranny fluid is getting overheated at highway speeds and some of that heat is transferring from the tranny cooler to the radiator. Or maybe you've got some sort of fan/water pump problem at higher RPMs.
In any case, that shield should be cheap even if you have to order it from a dealer. A junkyard shield should be less than $10, I'd think.
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1995 Trooper S
5 Speed
Bone Stock, for now...
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It's better to regret something you did
Than something you didn't do
--Michael Balzary
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Anonymous
Unregistered
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-3rd vote for restricted radiator.
-heat shield? we don't need no stinkin' heat shield.
-RSs only had the DOHC
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barak
Body Damage is Cool
Reged: 01/04/05
Posts: 1528
Loc: Airdrie, Alberta......Canada
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I'd plunk down the 110 and get a brand new radiator  I got one for when I put my 3.4 , I figured why try and use an old rotting rad with a bigger engine , and end up having problems.At least then you know your rad is working and doing its job for a few years with no problems 
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1996 Jeep Grand Cherokee 5.2 with mods.Hughes Air gap intake , 52MM TB , ford injectors, 2 3/4" lift, Doug Thorley headers , HID headlights with LED , HID fog lights , glow gauges , a few others
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tinytrooper
Getting the Wheeling Fever
Reged: 10/28/03
Posts: 97
Loc: tn
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When I did my 2nd 3.4 swap I replaced the factory rad for a rad out of a 91 2.6 p/u which is a three core instead of the trooper two core...price the p/u three core and see if its close to the same price MO RAD FOR THE MONEY...
4TH VOTE FOR RAD...
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