1999 merc 3.0l blown head gasket
 

thought that i had a manifold leak and tried changing the gasket. I found the head gasket is blown it cylinder 1 on the manifold side. I scraped the gasket and placed a metal ruler across the head and found no warping. I am getting a new gaskt but want to make sure this doesnt happen again. Any ideas on why this may have blown. I also need to change the thermostat because the temp stays around 100 so tstat is stuck open. is there a connection?

Re: 1999 merc 3.0l blown head gasket
 

I found the info below interesting. I had a 3.0L searay and had the thermostat stick open once too. They seem to have a very long travel and mine bent when open. Did you already pull the t-stat? Are you original owner? What I'm saying is could the previous owner have an overheat issue and pulled the t-stat? One overheat and cost you a head gasket.


Diagnosing Gasket Failures

Figuring out why a head gasket failed is the first step to preventing repeat failures. Sometimes the cause is obvious and sometimes it is not.
One of the causes of overheating, for example, is a dirty cooling system. As little as 1/8 inch of calcium deposits in a radiator may reduce cooling efficiency by 40%. It is also important to use the correct type of coolant and the proper mixture (50/50 is usually recommended for year-round driving).

"High temperature" replacement head gaskets are often a good solution for applications where a head gasket has failed due to detonation. These gaskets can provide additional sealing strength through the use of stainless steel fire rings and may include special beading and facings, often graphite. The gaskets will also lower compression radio slightly because they are slightly thicker than a stock gasket.

A head gasket that failed because of overheating or a hot spot will be crushed and measurably thinner in the damaged area when checked with a micrometer. By comparison, a gasket that has failed due to detonation or preignition will usually have cracked armor around the combustion chamber which leads to burn-through.

The corresponding surface areas on both the head and engine deck where the gasket failed should be inspected for damage (erosion, pitting or cracks) as well as flatness. If either surface is damaged or is not flat, the head and/or engine block must be resurfaced otherwise the new head gasket may not seal properly.

While the head is off the engine, check the flatness of both the head and block. Use a straight edge and feeler gauges to check all critical areas, especially those between the cylinders. Flatness specifications vary depending on the application, but on most pushrod engines with cast iron heads, up to .003˝ (0.076 mm) out-of-flat lengthwise in V6 heads, .004˝ (0.102 mm) in four cylinder or V8 heads, and .006˝ (0.152 mm) in straight six cylinder heads is considered acceptable. Aluminum heads, on the other hand, should have no more than .002˝ (.05 mm) out-of-flat in any direction.

Re: 1999 merc 3.0l blown head gasket
 

I just purchased the 1999 Mariah Shabah 182 about a month ago and performed an initial compression check and had all 4 cylinders at 150psi. The distributer was loose so i had the timing set and locked down the distributer. The motor now idled and ran fine up i heard the "ehxaust leak". I have not pulled the thermostat yet but will this afternoon.

Re: 1999 merc 3.0l blown head gasket
 

The loose distributor could be the cause. The timing being off would cause pre-ignition or detonation and make a small weak spot in the gasket, which will fail over time.
If I remember, there's a special procedure to set the timing with the Thunderbolt IV ignition. Make sure you have it set per your manual. (My manual is up with the boat or I could check it)

Re: 1999 merc 3.0l blown head gasket
 

Are u sure that's a Thunderbolt IV ? If so, per the manuals, here's the procedure

Thunderbolt IV Ignition
Timing

1. Connect timing light (91-99379 or similar) to No.
1 spark plug. Connect power supply leads on
light to 12 volt battery. Refer to “Specifications”
for cylinder numbering and location.

2. Connect tachometer to engine.

3. Start engine and run at normal idle speed.

4. Aim timing light at timing tab, located on timing
gear cover and crankshaft torsional damper.
IMPORTANT: GM engine timing marks (on tab)
are in 2-degree increments. MCM and MIE LH engines
will have “A” (Advance) mark to the left of
“0.” Timing must be set on the “A” side of “0”
(Top Dead Center).

5. Adjust timing by loosening distributor clamp and
rotating distributor body as required until timing
mark on damper or pulley lines up with the mark
on tab specified in “Specifications.” Tighten
clamp and recheck location of timing mark.

6. Stop engine and remove timing light.

If its Thunderbolt V then there is a different procedure per the manual:

Timing and Idle Adjustment Procedures For Thunderbolt V Ignition

Setting Base Ignition Timing

1. Connect timing light to number 1 spark plug.
Connect timing light power supply leads (if
applicable) to 12 volt source.

2. Connect a shop tachometer to engine.

3. Using a jumper wire, connect the ignition system
timing lead “13” (PUR/WHT wire) to a good engine
ground (–). This locks the ignition module
into the “Base Timing Mode”.

4. Start engine and run at normal idle speed. Allow
engine to reach normal operating temperature.

5. Aim timing light at timing tab, located on the timing
gear cover and crankshaft torsional damper.

6. Adjust timing using the conventional method.
IMPORTANT: Be sure to disconnect the jumper
wire from the ignition system test terminal before
attempting to resume normal operations. If the
jumper wire is left in place, the ignition module
will operate in the “Base Timing Mode”. This
means that the additional timing advance features
would not function.

7. Make sure that the distributor has been tightened.
Remove the jumper wire from the timing
terminal.

8. Stop engine and remove timing light.

Adjusting Idle Mixture
The procedure for adjusting carburetor idle mixture
can be found in the appropriate engine service manual.

This procedure also requires that the ignition module
be locked in the “Base Timing Mode”.

IMPORTANT: In order to properly set idle mixture,
the ignition module MUST BE locked in the “Base
Timing Mode”. This is necessary because of the
“Idle Speed Control” feature that exists in the
ignition module. See information on the previous
pages about this feature.

1. Using a jumper wire, connect the ignition system
timing lead “13” (PUR/WHT wire) to a good engine
ground (–). This locks the ignition module
into the “Base Timing Mode”.

2. Adjust idle mixture following the procedure in the
appropriate engine service manual.

3. Remove the jumper wire from the timing terminal.

Adjusting Engine Idle Speed

This procedure should be done with boat in the water,
drive unit in neutral and engine at normal operating
temperature. Refer to the Operation and Maintenance
Manual for the correct idle speed.

1. Disconnect the throttle cable from carburetor.
IMPORTANT: In order to properly set idle speed,
the ignition module MUST BE locked in the “Base
Timing Mode”. This is necessary because of the
“Idle Speed Control” feature that exists in the
ignition module. See information on the previous
pages about this feature.

2. Connect a shop tachometer to engine.

3. Using a jumper wire, connect the ignition system
timing lead “13” (PUR/WHT wire) to a good engine
ground (–). This locks the ignition module
into the “Base Timing Mode”.

4. Start engine and allow it to reach normal operating
temperature.

5. Adjust idle speed to recommended RPM.

6. Stop engine. Readjust cable barrel and reinstall
the throttle.

IMPORTANT: Be sure to disconnect the jumper
wire from the ignition system test terminal before
attempting to resume normal operations. If the
jumper wire is left in place, the ignition module
will operate in the “Base Timing Mode”. This
means that the additional timing advance features
would not be functioning.

7. Remove the jumper wire from the timing terminal.

Re: 1999 merc 3.0l blown head gasket
 

I think you're right Mikey, It's the V not the IV. Like I said, my manual is up north with the boat so I can't be sure when they switched. At least silver04roush will have the correct procedure in either case. Well done.