Engine temperature running at 200 to 210 after 30 to 40 minutes of driving

210 degrees isn't anything to get too excited about. That's the temperature at which Chrysler vehicles with electric radiator fans kick them in. Many GM engines run over 226 degrees, and that is considered normal.

How are you determining the coolant temperature? If you're going by the dash gauge, those are notoriously inaccurate and should only be used to notice when something is out-of-the -ordinary or unusual. Older engines used two coolant temperature sensors; one for the dash gauge and one for the Engine Computer. Only the computer's sensor is fairly accurate. Later models just used the sensor for the computer, then the instrument cluster learns the coolant temperature from the Engine Computer.

If you're reading the coolant temperature from a scanner, that is accurate, but again, 210 degrees isn't much of a concern, especially if the truck is standing still or is in slow-moving traffic. You've already replaced the most common items. Some other things to look at include a plugged radiator, both inside and with a butterfly collection on the outside. If you can get your hand in there without getting hit by the fan, feel from top to bottom and side to side. It should cooler at the bottom, and there shouldn't be any cold sections as you go across it. Also run your fingertips along the cooling fins, particularly along the bottom where dirt and road salt collect. If the fins crumble like a rotten chocolate chip cookie, they've lost their ability to give heat up to the passing air. One clue to that is the engine will actually run hotter the faster you go. Other running-hot problems will usually be less of a problem at higher speeds when natural air flow is higher.

Many people used to remove the fan shroud thinking they'd get more air flow, but that is counter-productive. That shroud forces air to be pulled through the radiator. If yours is missing, find a replacement.

Next, look for any place air can sneak through the core support without going through the radiator. That's the large sheet metal piece the radiator is bolted to. There are holes in it, but then there's always something planted in those holes to block them. Charcoal canisters, washer fluid bottles, coolant reservoir tanks, and things like that have to be in place so air can't go through there and bypass the radiator.

There's a rubber seal that sits between the top of the core support and the bottom front edge of the hood, also to block air from sneaking over the radiator. That strip is usually attached to the bottom of the hood, but sometimes it's on top of the core support. Be sure that's not missing. There's also usually a similar rubber seal along the rear edge of the hood. That's less for engine cooling, but it should also be there.

If the radiator was leaking, you'd see wet areas on it and / or on the ground, and you'd have been adding coolant periodically.

What you're describing happened years ago to my '88 Grand Caravan. Only on days the temperature was over 60 degrees, and while driving at least 60 mph, the temp gauge ran higher than normal; (high enough to get my attention). I made a 3,000-mile trip like that. At one point it got down to the mid 30s, and I could drive as fast as I wanted to without running hotter than normal.

After about six to eight months, it ran hotter all the time, regardless of outside temperature, but still only at highway speeds. I probably never would have replaced the radiator except it sprung a leak in one of the tubes, ... Then another, ... And another. I soldered up over eight pin holes before I realized I needed to give up. All those holes were in one lower corner, and all the cooling fins were a reddish-brown color of corrosion. Most weren't even attached to the tubes any more, so the heat couldn't be given up to the air flowing through them. This is why the clue is the running hot occurs at higher speeds. That's when the most heat is generated. It's also when there's the most air flow through the radiator, but the heat isn't getting to it. It stays in the coolant and goes back into the engine too hot.

Those cooling fins crumbled when I ran my finger across them. Some fell apart, and some turned to powder. I finally bit the bullet and bought a new radiator. After that, I used that van to drag a tandem-axle enclosed trailer to the nation's second-largest old car show swap meet in the middle of hot July summer days. The trailer is bigger and heavier than the van, but I never had another temperature problem.

What are you looking at that makes you think there's a problem? Are you going by the dash gauge or is there some other symptom?

Removing the thermostat can make it worse. The hot coolant doesn't stay in the radiator long enough to give up its heat. Think of the heater core which is a small radiator. There's no thermostat in that circuit to limit flow, and unlike the radiator, we want it to stay as hot as possible.

While you have the thermostat out, remove the radiator cap and watch the flow before the coolant gets too hot. If it's sailing along at a pretty good clip, you might want to check for a leaking cylinder head gasket. Here's a link to an article that shows how to do this:

https://www.2carpros.com/articles/head-gasket-blown-test

Also check out this video:

https://youtu.be/5zpNjYmmiYY

You might be better off having a mechanic do this test for you. You can usually borrow the tester from an auto parts store that rents or borrows tools, but they'll make you buy your own bottle of fluid. That's because it will become contaminated if you get any antifreeze in it. That, along with it freezing, renders it ineffective. They made the last person buy their own fluid to insure you don't get contaminated fluid.

When I do this, I suck out some of the coolant so I can see the tops of the cooling tubes inside the radiator. That makes it easier to see the flow. You should see it flowing about as fast as you'd have water flowing from a garden hose if you were trying to get a drink from it.

The next thing is to stop the engine and feel the two radiator hoses. Under normal operation, the top one will be too hot to hold onto for very long. The bottom one should be much cooler. If it is cold, there's insufficient flow. Either the radiator is plugged internally, the belt is slipping over the water pump, or, if this was a Volkswagen, the blades on the water pump's plastic impeller are corroded away.

If the lower hose is almost as hot as the top one, no heat is being transferred through the cooling tubes to the air. There's either insufficient air flow through the radiator, it's plugged externally by bugs, the fins are corroded away, or air is able to bypass the radiator, either from a missing shroud or from something else that is not as it was designed.

If you have an infrared thermometer, use that to check the radiator's temperature at all areas. It should be the same at any height from side-to-side, and every section should become cooler the further down you go.

Before you think about replacing the radiator, try back-flushing it with a garden hose. It's easier to remove one heater hose at the engine, then push water into that hose, then into the port on the engine. Leave the lower radiator hose connected so the water has to exit through the top of the radiator with the cap removed. Use a hose pinch-off pliers to block the top hose. That way the water has to flow up through the radiator to get out. When the fresh water enters the port on the engine, you'll have to hold your thumb over the end of the heater hose so the water doesn't come out there. Try to not go over about ten pounds of pressure. That's plenty to see the water gushing a good three to four inches up out of the radiator. You don't have to pinch the upper hose if the thermostat is in place.

Run water into the reservoir too to clean it out. Pull the small hose off at the radiator's neck and let it hang down. Once the reservoir is full, water will run down that hose and syphon it empty.

When you're done, drain the water out, then add antifreeze. My preference is to remove the lower radiator hose at this time to drain the system. Leave the petcock alone. There were a lot of radiators around that time period that developed a crack inside where the o-ring on the petcock seals, then you'd have a drip about once every five seconds. There's no fix for that other than to replace the radiator. That crack only occurred when the petcock as disturbed.

My recommendation when putting in the new coolant is to not waste your money on the "prediluted" stuff. Half of what you're paying for is just water. The way I did this at the dealership was to add one full gallon of antifreeze, then one gallon of water. That just about filled the system for a four-cylinder engine. Next, add a half gallon of antifreeze, then a half gallon of water. Keep adding equal parts of antifreeze and water until the radiator is full. Pop the cap on the radiator, then run the engine until the thermostat opens and coolant is circulating. Four-cylinder engines have to have the air trapped under the thermostat burped out. There's a threaded plug on the thermostat housing, a temperature switch that can be unscrewed, and usually both of those, and on newer models there will be a bleeder screw. V-6 and V-8 engines usually bleed themselves, but keep an eye on it to be sure the upper radiator hose is getting hot. Thermostats have to be hit with hot liquid to open. Hot air trapped under them won't do it. That should only have to be done once. It's when combustion air is sneaking in through a leaking cylinder head gasket that the air pools under the thermostat causing it to close, with overheating the result.

The coolant will be mixed after ten to fifteen minutes. Use a tester to check the freeze point. There's room in the reservoir to add over a half gallon of antifreeze to get the freeze point lower. After draining the system earlier, there's always a lot of water still trapped inside the engine, so you'll usually find you have to add antifreeze to the reservoir. Lower is not better. Shoot for minus 35 degrees. Beyond minus 50 degrees is misleading. The common testers look at the weight of the coolant. Antifreeze is heavier than water, so more of it falsely makes it look like the freeze point gets lower and lower. In fact, as the percentage of water continues to go down, the freeze point actually starts to go back up. Straight antifreeze turns to gel at around minus ten degrees. It's the chemical reaction between the two that makes for the lower freeze point. A second problem with too much antifreeze is that can also lead to overheating. Straight antifreeze can't hold as many BTUs of heat as water can. Water is needed in the system to carry the high volume of heat to the radiator.

Once you fill the reservoir, it's going to take at least a half dozen warm-up / cool-down cycles over a few days to draw that liquid in to mix evenly, so don't bother testing the freeze point until then.

Dandy. Did you figure out if it was plugged inside or did it have rotten cooling fins?

What you're describing is caused by spread terminals in the relays' sockets. This is real common on mid '90s GM trucks, but it can happen to any brand or model. One thing I always warned my students about was poking the terminals with voltmeter probes. They're fatter than relay or fuse terminals so they'll spread them open too much, then they make intermittent contact.

If you can get in there, the easiest way to fix them is to push a thin pick in on the outside of the terminal to squeeze it closed a little. Sometimes there isn't room to sneak the pick in, then you have to pull the terminals out, one at a time, and squeeze them tighter with a needle nose pliers.

Often when a relay or fuse is removed as part of diagnosing a problem, the terminals don't match up exactly in the same place when that part is reinserted. A light film of corrosion may have formed over time on the exposed areas, and now the two mating terminals have some of that corrosion in between the contact area. Sometimes that can be remedied by spraying them with electrical contact cleaner, then reinserting the part multiple times. The scratching action of the terminals sliding across each other can clean up a good contact spot. If there's heavy corrosion, the terminals have to be replaced.

When no better option is available, a trick that might get you home is to twist the fuse or relay terminals very slightly. This has the effect of making them fatter so they make a more solid contact. We would never "repair" a customer's car this way, but it can get you back on the road when nothing else works. This should not be considered a permanent fix. It will just spread the female terminal more. If it comes to the point you have to replace a terminal, another temporary solution is to slide a thin strip of metal into the terminal then plug in the fuse or relay so its terminal slides alongside that strip. Years ago it was somewhat common to use just such a strip, or "add-on" terminal with a wire attached to run to a new accessory. Those were slid into one of a fuse's terminals. The fuse was pushed in alongside that new terminal, then the wire supplied power for a new radio that needed a memory 12-volt circuit, or one of those new telephones that you could call with from your car. Those were called "cellular phones". A problem we ran into quite often was a leased vehicle was turned in right after that accessory had been removed by its owner, and that left that fuse socket terminal stretched and making intermittent contact. When we removed that accessory ourselves, we cut the wire instead and left that add-on terminal in place where it wouldn't cause any harm or other problem.