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#4610523 - 12/21/17 06:48 PM 99 GMC Sierra UOA
tech230 Offline


Registered: 11/22/12
Posts: 37
Loc: FL, US
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

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#4610527 - 12/21/17 06:53 PM Re: 99 GMC Sierra UOA [Re: tech230]
tech230 Offline


Registered: 11/22/12
Posts: 37
Loc: FL, US
Some gunk on the oil cap & filler neck.


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#4610529 - 12/21/17 06:56 PM Re: 99 GMC Sierra UOA [Re: tech230]
DoubleWasp Offline


Registered: 05/21/12
Posts: 5227
Loc: Fort Lauderdale, FL
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.
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#4610534 - 12/21/17 07:00 PM Re: 99 GMC Sierra UOA [Re: tech230]
tech230 Offline


Registered: 11/22/12
Posts: 37
Loc: FL, US
Every once in awhile I’ll get a check engine light. When I scanned it, it said it was running lean on bank one.

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#4610549 - 12/21/17 07:15 PM Re: 99 GMC Sierra UOA [Re: tech230]
caravanmike Offline


Registered: 05/09/09
Posts: 1071
Loc: ohio
Could be the castec head failure?
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#4610583 - 12/21/17 07:46 PM Re: 99 GMC Sierra UOA [Re: tech230]
irv Offline


Registered: 10/08/06
Posts: 934
Loc: Oshawa, Ont. Canada
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?
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#4610656 - 12/21/17 08:57 PM Re: 99 GMC Sierra UOA [Re: tech230]
tech230 Offline


Registered: 11/22/12
Posts: 37
Loc: FL, US
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.

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#4610679 - 12/21/17 09:31 PM Re: 99 GMC Sierra UOA [Re: caravanmike]
clinebarger Offline


Registered: 12/19/13
Posts: 1739
Loc: Fort Worth, Texas
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.
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#4610689 - 12/21/17 09:44 PM Re: 99 GMC Sierra UOA [Re: tech230]
clinebarger Offline


Registered: 12/19/13
Posts: 1739
Loc: Fort Worth, Texas
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.
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#4610740 - 12/21/17 10:49 PM Re: 99 GMC Sierra UOA [Re: tech230]
Chris142 Offline


Registered: 06/05/03
Posts: 16514
Loc: Deplorable in apple valley, ca
The 4.8 has a dry intake. A common problem was the heads cracking on the valve spring side
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#4610754 - 12/21/17 11:09 PM Re: 99 GMC Sierra UOA [Re: clinebarger]
tech230 Offline


Registered: 11/22/12
Posts: 37
Loc: FL, US
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.

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#4610761 - 12/21/17 11:23 PM Re: 99 GMC Sierra UOA [Re: tech230]
rubberchicken Offline


Registered: 07/07/08
Posts: 399
Loc: Maryland USA
You might consider GM Cooling tabs to slow down the coolant loss. The pictures to me clearly indicate coolant- sort of a reddish hue.

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#4610787 - 12/22/17 12:01 AM Re: 99 GMC Sierra UOA [Re: tech230]
slowrench Offline


Registered: 11/14/15
Posts: 24
Loc: california
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.)

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#4610845 - 12/22/17 03:24 AM Re: 99 GMC Sierra UOA [Re: tech230]
DoubleWasp Offline


Registered: 05/21/12
Posts: 5227
Loc: Fort Lauderdale, FL
Replace the engine. 4.8 motors are cheap as rocks.
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68 Charger R/T / Supercharged 440 VR1/DBL7349
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#4611442 - 12/22/17 03:52 PM Re: 99 GMC Sierra UOA [Re: tech230]
tech230 Offline


Registered: 11/22/12
Posts: 37
Loc: FL, US

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.

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