Q: I was just informed the coolant leak in my GMC Yukon is due to a leaking intake manifold gasket. Apparently this is a common problem. I was shocked at the price quoted for the repair and am not sure I understand why there would be coolant in my intake manifold in the first place. Any insight into this issue?

A: Leaking intake manifold gaskets are a fairly common problem on several GM V-6 and V-8 engine types built during the past two decades. In a V-type engine, coolant, incoming air and fuel, and exhaust are individually directed to or between the cylinder heads, and leakage of any of these can occur either internally or externally.

A failed intake manifold or cylinder head gasket can leak air into the engine, which causes rough running, or crankcase vapors into the intake, which causes excessive oil consumption and fouled spark plugs. This kind of failure can also leak coolant externally - you would notice coolant loss and a sweet odor - or internally, creating milky oil that leads to engine failure. It could also leak exhaust, externally or elsewhere. Coolant leaking into the crankcase, or the internal engine cavity, is by far the worst of the bunch, as the diluted oil can't provide proper lubrication to vital engine parts.

There are a variety of causes and theories of failure. Engines that employ plastic intake manifolds are subject to manifold cracking and gasket sealing problems. Aluminum intake manifolds bolted to cast-iron cylinder heads have issues as well, as the two metals expand and contract at differing rates. Your particular engine uses fewer bolts securing the intake to head junction than in the past, possibly exacerbating sealing issues. Some theorize controversial orange Dex-Cool coolant is the culprit, as the composition rubber intake gasket deteriorates where it is exposed to the coolant. GM released an improved intake manifold gasket design, and aftermarket/OEM gasket manufacturers such as Fel-Pro have improved design gaskets, but repeat problems can still occur.

You didn't mention where the coolant was leaking to. If it's internal, with milky engine oil, the repair needs to be made immediately. An external coolant leak might be Band-Aided for a while with use of a leak stop product such as AlumAseal or Bar's Leaks, but keep a close eye on engine oil in case the leakage migrates.

Renewing the intake gaskets requires removal of the intake manifold, distributor and air conditioning compressor, and it's a fairly big job. I'd also insist on employing a highly experienced technician that uses the latest gasket design available and uses meticulous, correct practices for cleaning sealing surfaces, cleaning and sealing bolts and threads, and fastener torque/sequence. On the V-6 engines, it's recommended that the fastening bolts also be replaced.

A final thought: Regardless of the coolant type used or brand of vehicle, it's a really good idea to flush and renew engine coolant every three or four years, shortcutting the maintenance schedule, which is often not indicated, or antifreeze lifespan claim. Old coolant becomes acidic and its corrosion inhibiters degrade, leading to many problems that are expensive and easily avoidable.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.