Diesel World - - Q & A -

I own an early 2001 LB7 Du­ra­max that has been in the shop re­cently be­cause of a cool­ing sys­tem overpressure prob­lem. Of course, this re­sulted in re­plac­ing a wa­ter pump (seal leak), ther­mo­stat(s), ra­di­a­tor, surge tank pres­sure cap, and all of the coolant hoses. All these were likely needed, but didn’t solve the prob­lem. Now I’m deal­ing with a head gas­ket re­place­ment project. I want to do this right. What do I need to know to pre­vent this from hap­pen­ing in the fu­ture? As you might guess, I keep the same truck for as long as I can keep it on the road. I bought this 2001 brand-new.

Wayne Ste­wart Cal­gary, Al­berta

While head gas­kets can and do fail in any diesel en­gine, the orig­i­nal LB7 Du­ra­max had an­other prob­lem in the form of an in­jec­tor cup for each cylin­der. The cups were “wet,” mean­ing they were made from stain­less steel and pen­e­trated the cylin­der head through the wa­ter jacket. This meant, in part, that the cups needed to pre­vent com­bus­tion pres­sures from find­ing their way into the cool­ing sys­tem. So, cups can be a cause of cool­ing sys­tem overpressure—like head gas­kets if the head-deck end of seal is bro­ken.

When the LLY ar­rived in the 2004.5 model year, Isuzu ob­vi­ously had thought it out a lit­tle more be­cause the in­jec­tors were po­si­tioned dif­fer­ently and they had aban­doned the cup de­sign. That was good for the fu­ture of the Du­ra­max, but sev­eral hun­dred thou­sand own­ers were left with the LB7 in­jec­tor cup prob­lem. It’s thought that re­plac­ing a set of in­jec­tors presents a pos­si­bil­ity for dis­turb­ing the cup seals, re­sult­ing in com­bus­tion pres­sure find­ing its way into the cool­ing sys­tem—which can re­sult in wa­ter pump seal fail­ure, head gas­ket re­place­ment (think­ing that’s the prob­lem), and all the rest.

Some years ago we spec­u­lated that a newly de­signed LB7 cup in­cor­po­rat­ing a threaded small end that screwed into the cylin­der head should and would solve the prob­lem of cup seal leaks. As it turns out, In­dus­trial In­jec­tion (In­dus­tri­al­in­jec­ is now of­fer­ing just such a prod­uct. It’s a bit pricey, but we wouldn’t do an LB7 head gas­ket job with­out these new In­dus­trial in­jec­tor cups. They should pro­vide a per­ma­nent so­lu­tion to a cup seal leak prob­lem.

Oth­er­wise, be sure to buy only the most re­cent it­er­a­tion of head gas­kets. GM has pro­duced an ev­ere­volv­ing se­ries of head gas­kets that are im­proved over what the en­gines left the DMAX fa­cil­ity with.

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