TECHNICAL Q&A

PROBLEMS AND FIXES

I own a 2002 LB7 Duramax. I replaced the heads (using ARP studs), injectors, and head gaskets about 85,000 miles ago. The truck smoked at idle before all that work. It didn’t have any cracked or leaking injectors, but I also changed them with the new heads (due to a blown head gasket and a warped head).

The truck ran OK after that service work, but the smoke at idle never went away completely. The engine didn’t use much oil and the coolant level didn’t change for 80,000 miles. A few thousand miles ago I noticed I had to add a little coolant from time to time. Not much. I then drove the truck to Colorado, and on the return trip something changed, which caused coolant to blow out at highway speed. At that time, I did not notice any bubbles in the coolant surge tank at idle. I finally made it home (after 1,500 miles of frequent stopping to add coolant), and the next day I noticed the dreaded bubbles in the surge tank at idle, which got more violent and blew a couple of gallons out.

I will mention that halfway home a leak developed in the oil cooler gasket and coolant was leaking on the ground from that. I believe the pressure in the coolant system blew that out, so I left the radiator cap off to avoid a pressure buildup. I have already replaced both the oil cooler and gasket, which solved that leak.

So, now I’m faced with the decision on how to proceed in solving this problem. I really don’t know why a head gasket would have blown, and thought the ARP studs would have prevented a repeat. The new heads had the injector cups installed when I got them, so I didn’t fool with them. I hate to replace the injectors because I don’t feel like I got the life out of them I should have and I wasn’t really having any problems with them. But if I get into the heads, should I just bite the bullet and replace them again? I’m still not sure of the reason for the smoke in the exhaust. Could I have had a leaking gasket or injector cup all along?

Where do I start? Is there any way to determine which side of the engine the problem is on? Then, before I pull a head or both heads, is there a way to determine if it is the head gasket for sure or an injector cup? If money was no issue, I would just replace the heads, gaskets and injectors, or simply get a replacement engine. However, I don’t have the money right now to do that.

I’m also wondering if the smoke is related to the rings. This truck did have early injector failure that resulted in diesel fuel finding its way into the crankcase. At the time, GM couldn’t get the injectors and told me to just drive the truck that way. I avoided that as much as possible, but don’t know how many miles were put on it with diluted oil.

Everyone says the smoke indicates bad injectors, but it acted the same way after the replacement injectors as before.

Ken Z Via Email The amount of combustion pressure finding its way into the cooling system says to me that one or possibly both head gaskets have failed. A leaking injector cup typically produces a lot less overpressure in the cooling system—more info on that in a minute. As for determining which side of the engine is at fault, a Tech II scan tool could be used to look at injector fuel rate balance. An injector with a significantly higher fuel rate could indicate a weak cylinder. And you could have a cylinder compression test performed. If the head gasket leak is as bad as you say, it should be evident in a cylinder compression test. The smoke you’re seeing could be related to a low combustion pressure for one of the cylinders or it could be due to a weak/bad injector. When the ARP studs are correctly installed, the pro shops I’ve been in contact with say they should prevent a future head gasket failure. “Should” being the operative word here. However, an injector problem could result in combustion pressure spikes in that cylinder, and could stress the head gasket over time to the point of failure. Here’s what I would do: 1. Document the injector fuel rate balance for all eight injectors. 2. Document the cylinder compression pressures. This is done by substituting an air fitting for each glow plug. Look at those two sets of numbers and determine whether there is a match, such a higher-than-average fuel rate for a cylinder that matches a lower-than-average compression for that same cylinder. The vehicle computer will attempt to increase the fuel rate to a weak cylinder in an effort to make the engine run more smoothly. Read from front to rear; cylinders 2, 4, 6 and 8 are on the driver nside, while cylinders 1, 3, 5 and 7 are on the passenger side. Upon disassembly, carefully examine the area of the head gasket surrounding the weak cylinder. Otherwise, if the entire head gasket is normal and undamaged, the cooling system overpressure could be due to an injector cup leak. A chronic combustion pressure leak that leaks past a cup can, over a long period of time, result in erosion of the cup sealing surface in the aluminum head. This can result in a leak that becomes worse over time. Pull that injector cup and look for evidence in sealing surface erosion in the aluminum head where the cup seals against the head. At the same time, have the injectors tested. Each of them should have been tagged as to which cylinder it was removed from. Also, the piston crowns should show evidence of any mismatch in combustion efficiency. A weak cylinder will produce a piston crown that is darker and possibly have more accumulated

carbon deposits than the others. If you can’t link a bad injector, head gasket problem or low cylinder pressure to a piston with more carbon, you may need to pull that piston to examine the rings. There could be two or more problems working together that produce a confusing set of symptoms. That said, I believe your problem is simple in nature, which will be revealed in the steps outlined here. After 85,000 miles of service I’d replace the injectors during the upcoming service work. If money is too tight, I’d just replace any injectors that don’t meet specifications. They will all need to be tested though. I’d also replace the water pump. A cooling system overpressure problem almost always results in a water pump seal leak. Now is the time to replace the water pump.

The best engine rebuild shops near to me all reseal the injector cups when they service the aluminum Duramax cylinder heads. This service usually includes “skimming” the head deck a few thousandths to produce a fresh sealing surface. The cast iron block deck is best hand-prepped using a flat block and 600-grit wet/dry paper. Then, carefully clean the head/block decks using solvent, and place the new head gasket on the block by touching only the edges of the gasket. Fingerprints, oil, or any other residue can reduce the life of the gasket.