3800 overheat

I have a '99 Intrigue with 3800 motor that is overheating.
First time my daughter was on her way back to school. Overheat, got it towed in. They couldn't find anything so they replaced the thermostat
and no more Deathcool.
I drove it around and it seemed fine.
Then before my daughter could get home from college to pick it up, my girlfriend took it and it overheated on her. She let it cool and drove it home. Took it to the shop. Nothing wrong.
So... I took it out. Took it to the shopping center, let the fan cycle. It would idle for an hour, no problem. Took it on the highway for about 15 minutes at high speed. (Yeah, really high speed) The gauge went up about 2 needle widths, but no problem.
My daughter was supposed to take it back to school but just in case, I took it out and went for a 1 hour or so drive. Not at high speed, just on the highway. It worked fine for about 50 minutes. At times the gauge would climb slightly, about 2 or so needle widths, then come down.
Then it shot up. In about a period of 2 minutes it crept up to pin at full hot. I pulled over. No steam, but the gauge was as far as it could go. What was really bizarre was that the hoses were soft and not overly hot. In fact, I shut the engine off and expected a boil-over. Nothing. About 5 minutes later I popped the cap and there was fluid. It wasn't low. I even felt the water pump on the engine block, and it was hot, but no so hot I couldn't touch it. And again, no boil-out from reducing the pressure, and the fluids were ok.
So, I'd say it was just a gauge or sensor. Except that when I got back in the car there was an ADDITIONAL red light on at the temp gauge. I thought sensor/gauge. But when I tried to drive the vehicle it would get to about 3000rpm as I was pulling on the highway and then 'shut off'. The ignition would die.
I had it towed in.
And of course now it's working perfectly. Sitting there idling and cycling the fans, for an hour or longer. And even driving around town it works fine.
It only seems to be triggered by SUSTAINED highway driving.
I'm about to try to get a scan tool and leave it hooked up and take it back out on the highway. Would that tell me anything more than I know (or don't know) already?
Any ideas?
Mike
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Mike Y wrote:

Drop the coolant and use a good flush mix on the system. Sounds like maybe the radiator has a flow problem. I would also check the pumps flow rate. Refill with coolant and make sure you get all the air out.
--
Steve W.
Near Cooperstown, New York
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That was actually similar to my first idea. I thought air purge or blockage. But the hoses didn't pressurize to be hard, and it didn't 'spurt' when I popped the cap. If it was blockage, wouldn't the radiator boil over and the motor be too hot to touch for quite some time?
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Mike,
The hoses not being hard is an indication of a leak. Under normal operation there should be pressure in the system when the engine is at normal operation temperature.Check the cap.
Al
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Mike Y wrote:

It sure sounds like a sensor. I forgot which is which on that engine - one for the gauge and one for the ecu. It also sounds like the ecu commanded the egr to open up wide creating the rpm and stall problems. On the other hand, if it was overheating the egr would open up anyways.
Sensors are reasonalby in expensive. You could replace one or both or borrow / rent an infrared temp sensor and check it if you can get it to overheat again.
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I think this is the best info yet. Especially the tie-in to the EGR and stalling.
I have some DS sensors on an octopus. I may place one of them on the engine. (It's a cable that plugs into my laptop USB and has 8 temp sensors on it to measure temperature at up to 8 places at one time.)
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I disagree. Sensors do not cause an engine to spew coolant. The engine is geniunely overheating.
--

-Mike-
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Hi Mike
Perhaps there was a misunderstanding. The engine never spewed coolant.
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Sorry Mike. I began to wonder if I had misunderstood something. Sorry for that.
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Could be one of the hoses is collapsing and blocking the flow a bit. Or the inside of a hose is delaminating causing a partial blockage at high rpm
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wrote in message

the
I considered that, but it still comes down to no boilover. And the radiator isn't loosing fluid.
Also, it didn't overheat when I did the 15 minute high speed run, only at a sustained 65mph cruise.
Anything that blocks flow would mean the heat is trapped in the engine, and would look for sudden release if I popped the cap. I carefully loosened the cap after 5 minutes. No hiss or pop. The gauge at this point still was pegged full hot and the little light on. I could also put my fingers on the water pump area of the engine. It was hot, but not so hot to burn me or even make me move my fingers away quickly.
Mike
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I might have misunderstood something - I thought you said it was losing coolant.
--

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Two possibilities: At a sustained 65 it is running hot. You slow down, the engine revs come down, the hose opens up, coolant flows, engine cools, no boil over.
If, in fact, it is not truly running hot but the sensor is saying so, it is a bad sensor or gauge. My Regal in its final days only knew two positions. All the way down or all the way up. I drove it for 20,000 miles running full hot according to the gauge.
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I believe that this is the engine that has the plastic plenum failure problem.
This may have nothing to do with your problem, but has that already been dealt with in your car??
They usually fail quickly and catastrophically, leaving little room for doubt.
Since your daughter is using this as a school car, you might want to consider whether this plenum needs to be replaced. If the failure (eventually) occurs while she is on the road or is away at school, it could be an unpleasant experience.
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HLS wrote:

I agree on checking out the plenum "fix & reseal" recall. Have a GM dealer check the recall applicability and history on your specific VIN. IIRC, that recall was pretty well finished up in 2003. No clue as to whether it would be handled as a freebie on an eight-year old vehicle -- definitely worth getting it done though if it's applicable on that engine. The "unpleasant experience" HLS speaks of would cost more to fix than the wholesale value of the car.
Meanwhile, make sure the "Check Engine" light is working and dump all the codes. Then reset the ECM (clear all the codes), drive the car till it overheats again and recheck for a code. (Cheaper than throwing parts at it.) Check and see if you can read all the engine codes from a blinking light or from the radio/clock dial before spending money on a scanner. Some parts stores will dump codes for free.
And, just for grins -- check and see if someone has contaminated the cooling system by adding the wrong type of anti-freeze. Between 1996 and 2000 GM was transitioning from green to red anti-freeze -- important to not contaminate one with the other.
hth
-- pj
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checking out the plenum "fix &

We never got a recall on this problem. GM said that if it happens within the warranty period, they pay. If not, you pay.
Cost to replace plenum, including labor, here is about $350.
Plenum is about $250, labor about $100. If you go to the f***ing dealer, it could easily be twice that much or more.
But, the key word here is "daughter". You cant let her go out with a dying GM product.
Fix it, buy her a better car, or get her a bike..
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