99 GMC Sierra UOA

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Truck has been using coolant these past 5-6 months with no obvious exterior leaks. A gallon of coolant will last me 3 months. I just top it off every couple weeks. Truck does not overheat & does not burn oil, but leaks from rear main seal. The oil did not look milky on the dipstick & neither did the oil when I drained it. Intake gasket or head gasket? Truck is a 99 Sierra with the 4.8
 
Time for a cooling system pressure test. See if the coolant ends up pooling in your crankcase, or in a cylinder.

Leave all the plugs out while you test. The offending cylinder (if there is one) will begin dumping coolant out of the hole. That will be a head gasket.

If it's only in the crankcase, then try just intake manifold.
 
Every once in awhile I’ll get a check engine light. When I scanned it, it said it was running lean on bank one.
 
Were the intake gasket issues still around in 99? My 97's 5.7 were replaced under a recall/warranty but that might not have been for a couple/few years after 97, I can't remember?
 
Wouldn't the oil be milky if the head gasket was bad? The oil I drained, which was the same oil I sent to Blackstone, was not milky at all. Was black like it always is after 5,000 miles. I've already put 400 miles on the new oil & pulled the dipstick, & it's not milky at all. Just condensation on the oil cap & filler neck. I'm gonna do what Blackstone says though, & change the oil every 2,000 miles or less till I figure it out. Still runs great & doesn't misfire, overheat, or anything. Just the high lead, sodium, & potassium levels worry me.
 
Originally Posted By: caravanmike
Could be the castec head failure?


1999 was not affected by Castech head casting failures.....But the heads could have been swapped out I guess, If it has "706" casting numbers, They have been swapped as that casting number didn't exist in 1999.
 
Originally Posted By: tech230
Wouldn't the oil be milky if the head gasket was bad? The oil I drained, which was the same oil I sent to Blackstone, was not milky at all. Was black like it always is after 5,000 miles. I've already put 400 miles on the new oil & pulled the dipstick, & it's not milky at all. Just condensation on the oil cap & filler neck. I'm gonna do what Blackstone says though, & change the oil every 2,000 miles or less till I figure it out. Still runs great & doesn't misfire, overheat, or anything. Just the high lead, sodium, & potassium levels worry me.


Not necessarily If the leak is small enough! The intake is "dry" on LS engines so it's not the intake gaskets. No oil cooler on a 4.8L.

That leaves a head gasket leak or a Busted/Cracked cylinder head.
 
Originally Posted By: clinebarger
Originally Posted By: tech230
Wouldn't the oil be milky if the head gasket was bad? The oil I drained, which was the same oil I sent to Blackstone, was not milky at all. Was black like it always is after 5,000 miles. I've already put 400 miles on the new oil & pulled the dipstick, & it's not milky at all. Just condensation on the oil cap & filler neck. I'm gonna do what Blackstone says though, & change the oil every 2,000 miles or less till I figure it out. Still runs great & doesn't misfire, overheat, or anything. Just the high lead, sodium, & potassium levels worry me.


Not necessarily If the leak is small enough! The intake is "dry" on LS engines so it's not the intake gaskets. No oil cooler on a 4.8L.

That leaves a head gasket leak or a Busted/Cracked cylinder head.


Worth it to replace head gaskets, or should I just replace the whole engine? Engine runs great but has 223,000 miles on it. Has brand new fuel pump, new a/c compressor, rear end work done, & suspension work. So this is a truck I intended to drive for another 4-5 years as I got a lot of time & money invested in it. Didn't expect the head gaskets to start going, as this truck has never been overheated or abused in the 6 years I've owned it. I've owned older Chevy & GMC trucks they never had engine problems. Only had to replace normal stuff like fuel pumps, u joints, alternators, starters & a/c compressors due to high mileage.
 
I have a 1986 k-20 GM 6.2L Diesel 4x4,that has similar problem.Ive been driving it that way for 25yrs.Its a great truck. (270,000 mi.)
 

Once the truck warms up it starts putting out a slight white fog/smoke at idle. Lots of moisture in the exhaust too, & this is after driving it home from work 15 minutes away. My trusted shop I’ve been going to for almost 15 years is gonna do a leak test on it. He said if it is the head gasket, it will be a little over $1,000 but that includes sending the heads to a machine shop. Seems reasonable.
 
Originally Posted By: tech230
Originally Posted By: clinebarger
Originally Posted By: tech230
Wouldn't the oil be milky if the head gasket was bad? The oil I drained, which was the same oil I sent to Blackstone, was not milky at all. Was black like it always is after 5,000 miles. I've already put 400 miles on the new oil & pulled the dipstick, & it's not milky at all. Just condensation on the oil cap & filler neck. I'm gonna do what Blackstone says though, & change the oil every 2,000 miles or less till I figure it out. Still runs great & doesn't misfire, overheat, or anything. Just the high lead, sodium, & potassium levels worry me.


Not necessarily If the leak is small enough! The intake is "dry" on LS engines so it's not the intake gaskets. No oil cooler on a 4.8L.

That leaves a head gasket leak or a Busted/Cracked cylinder head.


Worth it to replace head gaskets, or should I just replace the whole engine? Engine runs great but has 223,000 miles on it. Has brand new fuel pump, new a/c compressor, rear end work done, & suspension work. So this is a truck I intended to drive for another 4-5 years as I got a lot of time & money invested in it. Didn't expect the head gaskets to start going, as this truck has never been overheated or abused in the 6 years I've owned it. I've owned older Chevy & GMC trucks they never had engine problems. Only had to replace normal stuff like fuel pumps, u joints, alternators, starters & a/c compressors due to high mileage.


I would drop a good used LQ4 or LQ9 6.0L in it, I bought a wreaked 2006 3500 Express Van with 86,000 miles last month for $800 with a sweet running LQ4.
A LQ4/LQ9 will bolt right in place of a LR4 4.8L (You will have to use the LR4 flexplate on a LQ4)
The PCM will need to be tuned for the increase in cylinder air-mass....Which is only super critical at WOT/Open Loop operation. I can do the tuning for $200 if needed.

A LM7 5.3L is also a good in-between option that does not require PCM tuning (I have verified it with a wide-band), In reality.....The LR4 is tuned a little rich from the factory.
 
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