Help confirm auto diagnoses for a friend

A guy at work wanted me to look at his car the other day. It’s an early 2000’s GM car with a 3.8 liter. He said it overheats, but only at idle. My first thought was the electric cooling fans were not coming on. However, when I looked at the car I noticed it had an antifreeze jug behind the drivers seat, so he’s having to add coolant. The level in the radiator was low, so we have a leak and air in the system. I fill the radiator, and have him start the car. After a few minutes, the temp gauge pegs and the fans come on. The engine temp is only about 160 degrees F when I check with a temp gun. So I assume the temp sensor is in an air pocket, and steam, falsely reading high. I open the handy bleeder screw on the thermostat housing and bleed the system. I don’t notice any leaks as we let the engine idle. I do notice the upper radiator hose bulging. We shut the engine off, and I took the cap off the radiator and the hose shrunk back to normal. So I had him start the car with the radiator cap off. The engine is still around 170-180 degrees or so. As soon as he starts the engine, water really starts burping out the radiator filler neck. I told him I’m pretty sure he has a bad head gasket or a cracked head. The oil looks normal, by the way. Is there any chance it isn’t a head gasket failure with the coolant pouring out the filler kneck like that at idle? We lost over half a gallon in less than a minute with the engine idling…

As a non-mechanic that would be my thought. If you take the cap off and see bubbles, that’s a pretty good confirmation. Those are tough engines though and I had three of them. I nursed my diesel home 50 miles once with the cap loose to vent the pressure but yeah sure seems like its getting combustion gases. There is a test kit though to detect it so that might be the best next step.


With a failed head gasket the combustion gases generally get trapped behind the thermostat so you won’t see bubbles in the radiator. The combustion gas trapped behind the thermostat with cause the displaced coolant to flow from the radiator cap.

With a major head gasket leak if you fill the radiator, leave the cap off, then crank the engine with the coil unplugged you will see the coolant gush from the radiator nearly one foot in the air from the cylinder pressure leaking into the cooling system. The next step is a leak-down test or just inspecting the spark plugs for water.

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He said he’s had the plugs changed recently. Might be a clue as to why if it had started misfiring.

I’m just trying to think of any other reason 1/2 gallon of water would puke out of an open radiator filler with the engine idling for 30 seconds or so, other than a failed head gasket (or cracked head). I can’t think of any other reason. I admit I don’t know enough to definitively say that there is no other reason. But I don’t know of one.

The car has over 200k miles and isn’t in that great of condition. I figure it’s probably cheaper to pick up an old Buick or something with lower miles than to try and repair the gasket or drop in a used engine.

Agreed, it could be a cracked head/block if it’s been spiking like that for a while. It gets really hot in a spot, then is hit by room temp coolant.

I don’t know about a half gallon, but I’ve had cars with an air pocket surge out 3 or 4 inches of water from an open radiator cap. With some cars you can get some of that air out by putting the cap on and pumping the upper radiator hose. But I don’t think that’s the original cause of overheating.

I have had a thermostat that wouldn’t fully open. I’ve seen a 4 Chevies with water pumps where the fins had corroded away. (Ok for higher RPMs, inadequate in traffic jams.) As you originally thought, the fan thermostat might be not working properly. It comes on at a higher than normal temp. You could temporarily wire it to constantly run. The fix on a Jag was to hook a switch on the dash for a customer.

Head gaskets can fail between the combustion chamber and a coolant passage. Sometimes this can be detected. The dealer used a probe in the exhaust on my wife’s Toyota… didn’t detect any that time. If bad enough, sometimes the exhaust will smell slightly sweet.

I’m sure you took a gander into the radiator. Could it do with a flush? Like the other guys, I’d give odds on the blown gasket, if it came up suddenly.

Oh yeah, the radiator has had Dexcool in it (originally), and green coolant (more recently) and there’s been a coolant leak for quite some time, I’d say. So you probably already know what it looks like inside.

The coolant blowing out of the radiator was my concern. The overheating could be other things. But the blowing coolant makes me fear the worst.

That and the fact that he said he had a shop burp the coolant system recently. And now there’s air in it again, without an obvious coolant leak.

Ah… then I’m betting the exhaust sniffer (never caught the tech name) will find traces of antifreeze, if you can’t smell it.

Just from pix, it looks like the rear head could be hard to get off. Might be lucky and it’s the front one?

Check the lower radiator hose and see if it collapsing. Most new cars have reinforcement, but as things don’t seem to add up that is a thought. It has been many years since I saw one do that but the symptoms sound similar. Can’t draw enough fluid, thus too much pressure at the top. Clogged radiator maybe.

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He can’t do the work himself, so he’d be paying a shop. My thinking is, if it’s a blown head gasket, most shops would want to check both heads. So the engine would have to come out. In which case, he’d probably get out for not much money more in installing a used engine with less miles. Since they made a bazillion 3.8’s. But the rest of the car has 237k miles or so and isn’t immaculate by any stretch. So…you can probably pick up a similar GM car with half the miles and in better shape for under 3 grand around here. I think that’s the way I’d go. No real sense putting much money into an early 2000’s GM sedan with over 200k miles.

All of the above is assuming it does indeed have a head or head gasket issue. Which I recommended he get a shop to confirm.

A small leak in the cooling system can allow an air pocket to form, and that will cause a minor burping effect in the radiator, most easily observed as the engine warms at idle rpm and the front wheels are elevated, radiator cap removed. In fact that’s a good way to remove air pockets from the cooling system on many cars. But when I’ve seen this effect the burps occur every 15 seconds or so, and the burp severity isn’t enough to make coolant overflow the top of the radiator. So I don’t think the car simply has an external leak; more likely combustion gasses are making their way into the coolant.


Just my 2 cents here, but it seems that all too often when there is a car problem that there is a kind of knee jerk reaction to suspect the worst. Like starting with Z and working back instead of starting with A.

While the odds of a cracked head is always possible that would be my last suspect. IFa cracked head or head gasket is suspected then use a cooling system pressure tester, pressure the cooling system up, and allow it to sit for a bit to see if there is a pressure loss with no external leaks.
Some car parts store sucha s AutoZone and so on even have a pressure tester for free us with a deposit on it to assure that someone brings it back.

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I didn’t start out expecting the worst. I started out expecting the electric cooling fans not coming on.

The only thing that lead me to a head gasket problem is coolant blowing out of the radiator neck with the engine idling at a rate of probably a gallon per minute. Not the usual “bloop, bloop” you get when air is purging like George mentioned. A real gusher! I don’t really know what else would cause that other than exhaust entering the coolant. I posted to see if there was something else that might cause that.

Barky did have another explanation that sounds plausible, though. At this point he’s just driving it and adding coolant…and there’s still no visible leaks.

The lower radiator hose is not going to collapse at idle with the cap off, nor is it likely to collapse under 20 PSI of pressure while driving.

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It sounds like you have your finger on it.

I know they want to pull both… rather wise, if they actually trace it to the heads. Still, I guess it depends on how much the car is worth to fix. One head might be 1/3 the cost and I might gamble on that repair, knowing it’s a 50% chance. But that’s just me.

I had three 3800’s like I said. I put 120K on one, 350K on another, and 520K on the other one. I finally got rid of the 500K one because of stalling and unable to figure out why. All of them had no internal engine work done except for a timing chain and used no more oil than new. So they are good engines but my rule is usually to not open up an engine with a few miles on. If you pull the heads, then you’ve got to magnaflux them, then you might as well do valves. If you do valves, the higher compression can lead to more oil usage, etc, and then hoping all the work was done well. So I dunno, after investing a thousand or so, the results may be questionable. I did everything wrong on my diesel though.

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Yeah, that’s pretty definitive IMO. The 3800 has a history of head gasket trouble from my experience as the spacing is marginal. I had one fail and it was more benign as a compression leak between cylinders. Just for giggles, I pulled this up-

  1. Remove your pick up or car’s radiator’s cap. Check the coolant level, since the radiator has to be full of coolant for this test to work. If empty… add some water or coolant to bring the coolant level to full.
  2. 2

Now, get your helper to crank the engine, while you stand at a safe distance from the open radiator.
3. 3

You’ll see one of two results:

1.) The water or coolant inside the radiator will shoot up and out of the now open radiator.

2.) The coolant will not be disturbed. In other words, cranking the engine will have no effect on the level of the water or coolant in the radiator.

OK, now that the testing part is done… let’s take a look at what your results mean:

CASE 1: The coolant bubbled out or shot out from the radiator : This is bad news and this let’s you know beyond a shadow of a doubt that the head gasket on your 3.8L GM car or mini-van is blown. No further testing is required.

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Nice, thanks! I do know he’s taken it to a shop recently for overheating and they replaced the thermostat and purged the air out of the system. Kind of a chicken and egg thing there. Maybe the thermostat was bad and caused overheating and the head gasket leak. Maybe the shop wasted his money replacing a thermostat when the cause of the overheating was a breach in the head or gasket.

My remarks were not meant to irritate anyone. If they were taken that way I sincerely apologize.
The reason for my thinking is that simply because over the years I’ve seen so many poorly (or wild guessed) opinions about a problem that was claimed to be serious but in reality it was penny ante.

The most glaring example of this is the old man whose Ford truck was towed repeatedly and hit with a bill each time for whatever. The last time around the shop said the reason for spontaneously dying (and restarting fine later) was that the valve springs and rockers were all bad. The problem; a shaky fuel pump fuse.

It seems to me there are several different ways a head or head gasket can fail. My only point is that before wading into it too deep run some diagnostic tests. Again, apologies up front for any offense I caused.
Breach into the cylinder; meaning white smoke out the back and a bleached spark plug.
Breach externally; obvious leaks.
Breach into an oil passage; meaning diluted motor oil.
Breach between cylinders; meaning rough running.
Breach into the cooling system; exh gas into the coolant, sudden pressurerization of the radiator.

It also seems to me that a cooling system pressure test and/or a vacuum gauge test should show a faulty head or head gasket.

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I couldn’t agree with you more. It makes most sense to start with the cheap and easy stuff first. Properly executed diagnostics almost always pay for themselves in the long run. In this case, once the radiator geyser was mentioned is nearly definitive. Not many things can cause that and these have some history in that regard.

Certainly didn’t offend me. I’ve gone in plenty of times expecting the worst and the problem turned out to be less dramatic. I seem to expect the worst if it’s my vehicle for sure!

What you stated is my real reason for asking the question. I’m thinking head gasket (the worst), but maybe someone else knows a less dramatic explanation for the puking coolant.

I asked him about the car today, and he said it’s running fine and not overheating since I bled the air out of the system. So…time will tell.