Author Topic: Head gasket troubleshooting  (Read 174 times)

Mergyver

  • Enthusiast
  • *
  • Posts: 66
  • W116 Enthusiast
  • Location: NJ
Head gasket troubleshooting
« on: 17 August 2019, 11:14 AM »
Looking for advice from the brain trust.

I have a 1979 300SD 102,000 miles, om617 engine.

Noticed a coolant leak from the top hose where it attaches to the radiator. The odd thing is the engine was cold. It had not been run in about 10 hours. I squeezed the hose and it was hard (under pressure). I took off the catch tank cap and a lot of pressure was released. I started thinking I had a bad head gasket.

I ran a combustion gas test on the tank and it came up positive. I did a pressure test on the coolant system while the engine was hot. The pressure leaked down from 10 PSI to 5 PSI over the course of a half hour. I am still thinking head gasket.

Now it starts to get odd. I ran a compression test (while the engine was cold with all injectors out) and came up with 302,305,300,301,303. All appear good. So I am thinking the gasket is fine,

The car does not overheat ( temp gauge stays a hair above 174). Changed the oil and did not see any sign of coolant. No signs of oil in the cooling system.

I recently replaced the thermostat. I found several intake/exhaust manifold nuts were loose, I recently tightened those. I rebuilt the turbo.

So what do you guys think. Could the compression test be invalid because the engine was cold? Could something else be pressurizing the coolant system? Maybe a crack in the head that opens when hot and closes when cold? Should I pull the head? The car is a keeper/daily driver, if I have a head issue I would like to solve it before I damage the engine any more.

As always I appreciate all thoughts.
1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased

Mergyver

  • Enthusiast
  • *
  • Posts: 66
  • W116 Enthusiast
  • Location: NJ
Re: Head gasket troubleshooting
« Reply #1 on: 18 August 2019, 11:02 AM »
Ok, so I brought the engine up to operating temperature and performed another compression test. The numbers were; 305,307,304,303,305. Basically 2-4 psi above the cold test result. According to the Mercedes manual I have a healthy engine as it relates to compression.

So with good compression how can I be getting combustion gases and pressure in the cooling system?

1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased

Alec300SD

  • Classic
  • **
  • Posts: 137
  • W116 Enthusiast
  • Location: San Mateo, CA
Re: Head gasket troubleshooting
« Reply #2 on: 18 August 2019, 12:43 PM »
So with good compression how can I be getting combustion gases and pressure in the cooling system?
My bet is a small breach in one or more of the fire rings on the head gasket.
The cast iron heads and blocks rarely crack.

The head bolts can loosen over time.
Before replacing the head gasket, try retorquing the head bolts (in the FSM bolt sequence) to see if the compression leak goes away.
« Last Edit: 18 August 2019, 12:50 PM by Alec300SD »
79 W116 300SD 'Stormcloud' new as of 10/22/2017
78 W116 300SD 'Desert Rose' new as of 01/26/2014
83 W126 300SD 'Rena 2.0' with engine from 82 W126 300SD Rena' (SOLD and crushed 11-20-2017)

Mergyver

  • Enthusiast
  • *
  • Posts: 66
  • W116 Enthusiast
  • Location: NJ
Re: Head gasket troubleshooting
« Reply #3 on: 18 August 2019, 08:24 PM »
Thanks for the tip Alec, I will take a look.
1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased