Automotive: temperature gauge spikes/falls repeatedly, then coolant top-off fixes problem. Odd?

Factual Questions
1

I took our 2003 Ford Taurus SES (6 cyl, DOHC) to our mechanic yesterday, presenting the symptom described in the question above. After driving the car for about 45 minutes Saturday afternoon (mostly interstate) and noticing the behavior of the gauge, I parked the car in our driveway and opened the hood. No steaming or hissing at all under the hood. The oil dipstick showed absolutely normal oil (no coolant in the oil). Exhaust was not abnormal (no steam, no “white smoke”). I let the car sit overnight – no fluid leaks onto the ground. At that point, I had assumed a problem with the thermostat, or maybe air pockets in the coolant lines.

Long story short: the mechanic performed some kind of chemical test of the coolant (akin to this). He showed me the results, and told me the car had a blown head gasket**. The fluid test revealed that exhaust gasses (as opposed to ordinary air) was getting into the coolant.

Now, OK. The mechanic proceeded to top off the coolant and tell me that the temperature gauge will stop spiking. He then recommended that the head gasket be replaced at another shop (his shop didn’t work with or disassemble blocks) for an estimated $1,500-2,000.

Because (a) the engine showed no visible/audible signs of overheating, and (b) I couldn’t detect coolant in either the oil or the exhaust, and (c) there wasn’t fluid leaking onto the ground … it occured to me that if the head gasket was indeed blown (cracked, sealed poorly, whatever), the car might have the World’s Most Fortuitous Head Gasket Issue and might actually be driveable for some time. So I asked. The mechanic told me that short trips would likely be OK for a good while. I then asked if I could safely drive the car for a few hours on the interstate (we’ve got a vacation planned soon). The mechanic said that was chancey, and didn’t recommend it. The shop manager was listening, and walked over to opine that taking the car out of town would probably be fine so long as we brought extra coolant along and paid attention to the temperature gauge – if it stated spiking again, pull over at the nearest exit and top the coolant off again, just as the mechanic had done five minutes earlier.

As I research the issue on my own, there does seem to be a kind of just-so head gasket problem involving the water jacket that can present the symptoms described above. I also read through this page which described cases where exhaust gasses in the coolant cause true overheating (signs visible upon raising the hood).

What I can’t seem to figure out on my own (excepting the shop manager’s opinion above) is just how truly risky driving the car is in this state. If I saw some of the usual signs of a messed-up head gasket, I’d feel more definite about getting the car repaired ASAP. But being that the one and only sign was a single incident of the temperature gauging spiking/dropping repeatedly during the course of a drive … I dunno.

*** - I do not know if the mechanic performed a true pressure test of the engine block, or any other test besides the coolant analysis. Calling the shop now to find out.*

2

The temperature gauge is accurate. When it spikes, so does the temperature of your engine and cooling system. Get it fixed, which probably means a complete radiator flush by somebody who knows what he’s doing, and maybe a new radiator.

3

Just got off the phone … no pressure test was performed. Now I’m wondering if it’s possible that the blown head gasket diagnosis is in error :confused:

4

The chemical tests checks for the presence of combustion byproducts in the coolant. Yours tested positive, so combustion gases are making it into the cooling system. Most likely via the head gasket but you could also have a cracked head or block.

5

Except when you have no coolant in your system. Then the gauge falls and your engine temp goes way up. This is why if your coolant is low and going past the sensor in spurts the gauge is all over the place.

6

If the diagnosis of compution gasses was done by chemical analysis rather than a running gasses check I would not be so concerned. On most cars some combustion gases migrate to the coolant. If it only happened once I would wait and see how long before it happens again. If it takes a week or less I would really recomend getting it fixed. If it takes longer you might try some alumasealer or other product designed to seal heads and you might get by with it.

7

The Taurus 3.8L was notorious for blowing head gaskets (although I thought they had worked that problem out by 2003) so it was either an easy or lazy diagnosis for your mechanic to make. Get a pressure test done. If it is a head gasket, Ford might even kick in a few bucks toward fixing it.

And, unfortunately, bad head gaskets can sometimes fail suddenly.

8

Really? How?

9

First, they have to get a visa and some shots…wait - that’s emigrate.

I too, am curious about such migration.

10

Since its an old car, and its a small leak, I would recommend that you use one of the radiator stop leak products from any store like walmart, autozone, etc. I have previously used the Bars brand - but they are all pretty much the same.

The best solution is ofcourse what the mechanic recommended, but in my experience whenever the mechanic opens up an old car like yours, he/she always finds other problems (or creates other problems) that will then need fixing too.

So I would highly recommend the stop leak stuff first - see if it fixes it or keeps the problem manageable.

11

Admittedly my experience is more with diesels than gas engines. We would send in oil and coolant samples for analysis each oil change. We had a normal range on the chart for chemicals in the coolant caused by combustion gases. If concentrations were below a certain range it did not throw a red flag. Compression pressures are higher than cooling system pressures so any minute seepage would go into the cooling system or the atmosphere. I am talking very small but detectable amounts.

12

I recommend renting a car for the vacation trip. Getting stuck in the middle of nowhere will certainly ruin a vacation.

Driving it locally where a breakdown wouldn’t cause major grief is OK. I’d start saving for a replacement car though. Driving with a blown head gasket is likely to cause problems once coolant does make it into the crankcase.

am77494’s idea of using something like Bars Leak is a good one.

13

Thanks for the input, all. Definitely interested in hearing more from anyone else who may have some perspective and/or experience with this particular issue. Starting to get the impression it’s kind of a rare problem to have.

JerrySTL, renting a car to go out of town is a good idea.