LB7 DURAMAX HEAD GASKET WOES

Dear Diesel World,

I own a 2004 GMC powered by an LB7 Duramax, now with 400Km on the odometer. I’ve owned this truck since new. The first set of injectors was replaced about 4 years ago. About 2 years ago I began to notice excessive pressure was building in the cooling system. Not driving it as much as I used to, I only got around to having the head gaskets replaced a month or 6 weeks ago. A local professional mechanic with Duramax experience I trust did the work.

Now the woes begin. It seems pressure is building in the cooling system even worse now than it had been. I can even see bubbles rise in the coolant surge tank with an idling engine, and the heater blows cold air intermittently. My mechanic believes a leaking injector cup could be the source of the combustion pressure leak into the cooling system because the head gasket replacement was very carefully performed. Do you think the injector cups could be leaking? We really need your help.

Thank you,

Wayne Wright Nanton, AB Canada

Sorry to hear about your situation. The design of the first-gen LB7 Duramax cylinder heads always concerned me because of the risky design of the stainless-steel cups that surround each injector, which are themselves inserted into and through the cylinder heads. Only a Loctite 272 sealant and clamping pressure keep combustion pressures from leaking into the cooling system. That's asking a lot from the Loctite when combustion pressures could exceed 4000-psi with temperatures of 1500 degrees F or more.

The bubbles you're seeing in the coolant surge tank might be a blessing where troubleshooting is concerned. Use a Tech II scan tool to disable one injector at a time, while watching the coolant surge tank. You should see the bubbles diminish to almost nothing when the correct injector is disabled. Once you've identified the correct cylinder, I'd reseal the injector cup for just that injector. If that doesn't solve the problem, you may need to look at that head gasket and carefully examine the head/block decks to ensure perfection in their surfaces.

Incidentally, a diesel aftermarket company called Industrial Injection (www.industrialinjection.com) has begun offering aftermarket LB7 injector cups that incorporate threaded ends that don't rely on simply clamping pressure and Loc-tite to prevent combustion pressure leaks. The small end of the custom cups are threaded, which thread into the cylinder heads (after being threaded), along with the same Loc-tite sealant, and should produce a permanent riskfree solution.