Really Head Gasket?

[furser]

I am the new owner of a 1998 Deville. Runs great xpt on freeway - after about 30 minutes or going up hill, it overheats dramatically. I never see steam, etc. from tail pipe when this happens. Dealer used a "sniffer" to check for exhaust gasses in coolant - nothing. Dealer cannot find anything else wrong so he says it is probably a small leak in the head gasket. Seems like a weak diagnosis for me to trash the entire car. Any ideas?

[jadcock]

The proper procedure to diagnose a bad head gasket is to pressure check each cylinder. Until they do that, everything is just conjecture.

[zonie77]

What has actually been done beside the "sniffer" test?

I'm familiar with Sevilles but I think the cooling system is similar.

Purge line clear?

Flush radiator and change coolant? (remove radiator to clean including front side of fins)

Verify cooling fans working properly?

[furser]

Thanks to both of you for your insights. Will a leak show up on the pressure test even if the problem only occurs when the engine is at highway speed?

[furser]

Originally Posted by zonie77

What has actually been done beside the "sniffer" test?

I'm familiar with Sevilles but I think the cooling system is similar.

Purge line clear?

Flush radiator and change coolant? (remove radiator to clean including front side of fins)

Verify cooling fans working properly?

Just replaced the radiator, water pump, thermostat and did the dealer cooling system flush. To no avail.

[Ranger]

Originally Posted by furser

Thanks to both of you for your insights. Will a leak show up on the pressure test even if the problem only occurs when the engine is at highway speed?

Yes. It will produce bubbles in the surge tank but, check the purge line like Zonie said, and don't forget to check the cap to be sure it is holding pressure.

[jadcock]

Originally Posted by furser

Thanks to both of you for your insights. Will a leak show up on the pressure test even if the problem only occurs when the engine is at highway speed?

furser, a head gasket leak is a head gasket leak, regardless of under what conditions it may or may not produce an overheat situation. If there is a leak, a cylinder pressure test will find it.

Another thing to check is the water pump belt tensioner. The water pump is driven by a small belt off the left bank exhaust camshaft (this is the front bank, as the engine sits in the car). There is a small spring-loaded tensioner pulley to provide proper tension on the water pump drive belt. Sometimes, this tensioner can lose lubrication and "stick" in place, not providing enough tension on the belt, and allowing it to slip under certain high-load or high-speed situations.

[Guyz1996deville]

Doesn't make any sense, maybe try making sure all of your passages for you coolant are flowing. I would even say try to change your thermostat.

[furser]

Thanks, I did replace the thermostat but I will check the rest of the passages. The dealer just did their "super coolant system flush" is that a good indication that things are in order there?

[furser]

just did a new cap. I will check the purge line and - hey jadcock, that sounds like a reasonable thing to check. Thanks all of you.

[Tommy Deville]

I have been wondering if my head gasket is leaking as well....lately i have noticed a lot more "steam" coming from my exaust but for the most part it goes away when the car is warmed up.....and it had been more noticeable when the weather has been cold below 45 degress..the other day i noticed my temp guage slighty off center, usually once its warmed up it never moves from dead center..than I got a check coolant message and I needed almost 3/4 of a gallon of coolant. I have no idea were the coolant went. I did a WOT recently im wondering if that has somthing to do with it. btw car has 99k on it
and at idle lately not the last 3 or 4 days i have noticed that its idleing a little rough almost like a slight surge not really rough, but i noiced it..almost like a slight miss.... only in park. I put a tank of super in and it seems to have gone away.
my only concern is that all the concidences seem to be mounting pointing to a possible head gasket

[zonie77]

There was a guy about 6mos ago who did cyl pressure test and never got bubbles. Eventually found out it was head gaskets. That was when Bb0b was still on the board.

I havn't run the cyl pressure on a Caddy but the first one I got involved with (my brother's) initially passed compression test fine. After about 3 mos of working on overheating problem it showed about 30lbs low on 2 cyls (I only use first compression stroke for diags).

I suspect that in the very early stages of failure there can be some tension on the gasket that seals up to some level of pressure. Eventually the flaw gets to the point of constant leakage. Soooo, If it holds to 500psi a pressure test at 100psi won't show anything. Driving on a level road may put (just hypothesizing) 400psi on it, going uphill may put 900 on it. As you keep driving it the the gasket deteriorates more and soon can't hold the 100psi for the test. Then a cyl pressure test will show bubbles.

The dealer saying a combustion byproduct test was clear makes me scratch my head. You've done a lot to the cooling system, not much left there. I think I'd have the test run again, preferably somewhere like a radiator shop. It will be a lot cheaper there and they do them more often. One other point on the byproduct test, the car has to have been driven (and at a point the gasket leaked) with that coolant for the test to show a problem. If they did it soon after changing coolant it will show good.