1996 GMC Sierra



March, 8, 2011 AT 6:37 PM

NEED some advice and help. 1996 GMC Sierra Z71, 4 wheel, extend cab, 5.7L, 302000 miles. Yes that is the mileage.

I'm extremely mechanically inclined and enjoy doing my on vehicle maintenance (as long as I have the tools and knowledge to do so#. However I do know when it's time to let the experts do it.

I just recently replaced the 2nd radiator on this vehicle which has been on here for ONLY 3 years; the original had been on here up until then. This second one began leaking anti-freeze into the oil, and I #along with a 20 year mechanic friend) was pretty sure it was the internal seam on the oil cooling side of the radiator (that has been since verified by putting a new radiator in along with a complete FLUSHING of the oil system prior to a NEW Radiator installation) because it also was leaking externally around the seam (very small amount) as well.

Prior to this problem my oil pressure has been very unstable At start-up it races up to about 60 lbs and then slowly declines as the vehicle warms up, after the truck warms up it idles at about 15 to 20 lbs of pressure. However after it has been driven for a while on the hwy the oil pressure would be between 22 - 30 lbs; but at idle or stop (not moving) it will be about 8-10 lbs.

I recently was told that this oil pressure problem was probably attributed to the oil lines coming from the block near the oil filter were not completely secured (tight#, the same ones that go to the radiator. We could noticably see this leakage when the truck was on the service rack #also the small puddle in my drive#. The mechanic suggested replacing the lines or at a minimum tighten them ASAP. I haven't had the funds to do this yet #UNEMPLOYED#. Back to the NEW radiator.

Presently the oil is clear now after 100 miles of use with the NEW radiator #radiator number 3#, but now it has a leak at the seam on the transmission side. I purchased both these radiators from Radiator Depot. Has anyone else had any BAD experiences with inferior radiators from this company? They swapped me a NEW one when I asked to exchange radiator #2 after 3 years. I'm receipt hoarder, so I was able to do this with NO problem. They were extremely nice about it. BUT now I have another BAD #I'm pretty sure) radiator.

Sorry for this novel.

Can anyone verify for me that this is NOT attributed to bad or inconsistant HEAD pressure? This truck gets an oil change every 3,000. It gets a oil flush every year or at 15,000.

With 300,000 miles you can tell I baby it. I use Quaker State High mileage with Rislone. Half can of Seafoam every other oil change and half can of Seafoam in the gas every 3rd tank full. I presently don't have the funds to get a head pressure check, but I'm pretty confident with NO oil leaking in other places (heads, valve cover (only normal age) or intake) or signs of anti-freeze in the oil.

I appreciate any help and yor patience. This is my only vehicle until I'm back working, but even then I still want to keep it around to restore.

Thanks to all who read this and reply.



5 Answers



March, 8, 2011 AT 7:24 PM

Are you sure that's engine oil going through the cooler in the radiator tank? Every vehicle I've ever seen runs transmission fluid through there. When GM used oil coolers, they were a disc with coolant running through it that sat under the oil filter.

Haven't heard anything about low-quality radiators. If there was a big problem the manufacturer and the suppliers would be aware of it. Is the shop you're dealing with a parts store? If so, you might consider visiting a radiator specialty shop. They've seen and heard of anything you can dream up and will be able to give you a better answer.

To add a different dimension to your story, my '88 Grand Caravan with 379,000 miles hasn't had the oil drain plug removed in over eight years. When the hydraulic lash adjusters start rattling and asking to leave the engine, I start thinking about adding two quarts of oil. That's at about every 3,000 miles. No fluid drips, no smoking, and lots of power. We all understand that's not neglect; that's abuse, but it shows what these old engines are capable of. I don't treat any of my other cars that way. Oh, I DO replace the oil filter every other year, if I think of it. I also use that van to drag around a tandem axle enclosed trailer that's bigger than the van, and the transmission fluid and filter has been changed only once in its 22 year life, only because the fluid fell out while replacing a $3.50 side cover that rusted out. This van is my daily driver. I have a newer van but I would never trust it as much.



March, 8, 2011 AT 8:02 PM


I'm positive. Obviously this transmission / oil cooling combination radiator type is a first for you? Not trying to sound sarcastic.
This 1996 GMC 1500 Sierra Z71, 5.7L Vortec truck was manufactured and equipped for towing. It was manufactured using a radiator that on one side (passenger side)is for transmission cooling, the other side (driver side) was for oil cooling during towing.
Is there any other USEFUL diagnostic information you can give me about head pressure issues? I appreciate your help but I need help (experiences or etc.) To know if this is A HEAD PRESSURE issue? I believe the intake manifold is a NON-issue. It was replaced around 100,000 miles (typical of this model year), and it leaked on the backside and the top of the engine (near the firewall). I won't say it is completely ruled out, but NO anti-freeze is leaking into the oil or externally any other place.
Thanks for your effort.



March, 8, 2011 AT 8:19 PM

What do you mean by "head pressure". The cooling system pressure is limited by the pressure relief valve in the radiator cap. Even if you had combustion gases getting into the cooling system from a leaking head gasket, which isn't common with your engine, the pressure still can't go any higher than the rating of the cap. Oil pressure is limited by the pressure relief valve in the oil pump. 60 psi is kind of high but not outside the realm of possibility. It will be lower when the oil warms up and thins out so those observations are normal. If the pressure comes up when you increase engine speed, that would rule out a restriction in the cooler or lines going to it. I would suspect bearing wear as the cause of low pressure long before I'd suspect a cooler problem. We also always discuss a worn oil pump but that isn't terribly common either.



March, 8, 2011 AT 9:36 PM

I was refering to cylinder (my apology, head pressure might NOT be the right terminology) pressure. This engine has overheated before due to a failed water pump and air conditioning compressor clutch going out causing the serpentine belt to break.
You came up with an excellent point maybe with regards to combustion gas. With this many miles on this VORTEC engine we could have gasket issues, bearins or oil pump. I thought GM was high (proud of the fact) on these VORTECS about how they could be EXTREMELY low on oil or run EXTREMELY HOT for miles. I'm not really noticing any loss of power when excellerating, so my compression should be fine? I'll look into these other items and get back with you.
Thanks for your help. I may be back on here tomorrow.



March, 9, 2011 AT 10:11 PM

There really isn't any common history of these engines developing head gasket issues but there is a quick test that will indicate if combustion gases are leaking into the cooling system. The tool, (I call it a "leak sniffer"), is a glass cylinder with two chambers partially filled with dark blue liquid. Air is drawn from the radiator through that fluid while the engine is running and warmed up. If combustion gases are present the liquid will turn bright yellow.

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