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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old March 16th, 2020, 09:54 PM
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Default 305d V6

Hi all

I have a 1962 step side long box. Had a great weekend getting some work started on it. Until I noticed running a bit rough. Found cylinders 1 and 6 have low compression 30 psi. I pulled the valve covers to see if all parts are moving as they should. I noticed a lack of oil. Seems like it is starving for oil on the top end but Oil pressure gauge is running high??? 60psi. Never run into this before, Looking for some experience hear before I tear this down. Possible main journal blocked? Thanks in advance for any insight.
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  #2  
Old March 16th, 2020, 11:30 PM
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Default Re: 305d V6

The low compression has nothing to do with oil pressure. You may have blown head gasket or bad valves/springs and all that stuff. Don't run it too much until you figure it out. Check your oil for milky look.
The fact you have very low in 1-6, which are in the firing order sequence is a hint (165432). Their location is far apart. So possibly is not head gasket, bad piston, broken rings, I.E. cylinder related. It could have something to do with timing. Chain/gear slip, broken. To further my timing related suspicion, the two PSI's are WAY low but equal. Just guessing. What are the other 4 cylinders?
It could also just be a coincidence. Could be that they are at the ends of camshaft/crankshaft. Check the valve springs closer. Also could be valve seat, bent valve............
I don't know, just trying to start a brainstorm.

Last edited by AZKen; March 16th, 2020 at 11:51 PM.
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Old March 17th, 2020, 10:59 PM
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Default Re: 305d V6

I would pull off the rocker arm shafts. They could be full of sludge, which will starve your valve train of oil. You may want to look at the hole that supply the rocker arm pedestals (the end pedestals for cylinder 1 and 6) to see if it is plug too.

The oil supply come up from the cam bearings through the heads above cylinder 1 and 6 (In fact it is the hollow head dowel pin that feed the oil to the heads).

Assemble the well oiled rocker arm shafts back onto the heads, check/set the valve clearance. After you do that, continue rotating the engine and take note on how far the valves are opening. They should be opening about the same amount.
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Old March 18th, 2020, 01:54 AM
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Default Re: 305d V6

I understand there may be an oiling problem. How does that make 1 & 6 have 30PSI? The issue is the actual PSI reading by the poster. The bit about "lack of oil" is the posters observation vs an actual gauge reading. Need to address the 30 lb reason before the oiling. Lack of oil will not stop the valves from operating mechanically. Some mechanical or gasket failure somewhere will. Find that. More inspection and testing is needed before tearing too much apart.

Last edited by AZKen; March 18th, 2020 at 02:06 AM.
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Old March 18th, 2020, 04:13 AM
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Default Re: 305d V6

I was going to add if the engine hasn't been run in awhile it could have a stuck valve.
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Old March 18th, 2020, 02:49 PM
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Default Re: 305d V6

try a little oil in cylinders, see if compression comes up, could be washed cylinder walls. or stuck rings
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Old March 18th, 2020, 05:56 PM
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Default Re: 305d V6

Thanks for the suggestions.. I too believe more testing and diagnostics is required. My plan is to bore scope the 1 and 6 cylinders then do a leak down test and see if I can locate the pressure loss source. I have worked on small block V8's but nothing with solid lifters.
I do not want to run the engine any further, can I pull the distributor and prime the engine by rotating the oil pump? Or is this pump driven by something else.?
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Old March 18th, 2020, 10:13 PM
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Default Re: 305d V6

Quote:
Originally Posted by Cfriis View Post
I do not want to run the engine any further, can I pull the distributor and prime the engine by rotating the oil pump? Or is this pump driven by something else.?
You are correct to assume the distributor drive the oil pump.
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Old March 23rd, 2020, 05:43 PM
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Default Re: 305d V6

Just wanted to update everyone on work completed on the 62 this past weekend. Bore scoped all six cylinders nothing to report there. All looked good no scaring or dimpling in the pistons or cylinder walls. Performed leak down test on 1 and 6. found on each intake valve leaking. backed off on each valve tappet both cylinder at 125psi. Cleaned rocker shafts as suggested, all seems good. Here is the bad. found on the #4 cylinder one of the push rods was damaged/bent. Looks like some one tried to straiten and re-installed. I checked the lifter and no signs of wear. I also got a good look at the cam lobe. Nice and clear as well.. Checked the back lash in the timing chain it too seemed like it has little to no wear.(replaced? maybe) Does anyone have a good supplier for new push rods? The only ones I have located so far are from EGGE and state for 305E.

Thanks again for all of the support.
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Old March 23rd, 2020, 11:54 PM
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Default Re: 305d V6

https://www.rockauto.com/en/catalog/...,push+rod,5648


Pushrods from RockAuto
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