Chevy 6.5 Diesel has no oil pressure again!!! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 12-30-2006, 02:00 AM
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Chevy 6.5 Diesel has no oil pressure again!!!

ok, so here it goes. back in march of this year my dads 1994 chevy K2500 with a 6.5 turbo diesel cracked a piston. so we yanked the engine and had it completley rebuilt with a rebuild kit from diesel direct, and had all the machine work done by Polk Diesel in Madison. they also magnafluxed and pressure tested the block. we checked all the clearances with plastigauge, and buttoned the enigne back up. after we got it back into the truck, we fired it up and it ran really good, but then it lost oil pressure and shut it off. At first we though that we had a bad sending unit, so we replaced that, but that didnt solve the problem. next we replaced the oil pump and pickup screen but the problem continued. we also replaced the oil cooler lines and made sure that the oil cooler was free flowing. after all that, it just sorta sat int he garage during summer and fall. then this christmas break my dad decided to pull the engine back out again, and see if we couldent find the problem. we looked at the bearing to make sure that they where ok, and checked the tolerances again, and they where within specs. while cleaning out the oil pan my dad found a welch plug that goes into on of the oil passages. so he ordered the part and decided to put in the new one, and then, put one more in on top of it so it would never come out. so after all that hassle, we put the engine back in, hooked it all back up, and fired her up. At first the oil pressure was at 40 psi, but then it slowly dropped over the next 4 or 5 minutes to reading no oil pressure. before we replaced the plug the lifters had bled down and began knocking, but this time they knocked just a little bit until it stopped and sounded really healthy, even when reading zero oil pressure. Also if you disconnect the oil feed line to the turdocharger before we did not have any oil flowing, but this time, we have a nice high volume stream. we also verified that all the passages in the engine block where free flowing, and that no assembly lube had gotten stuck in a passage or somthing. also, we replaced the oil, and oil filter after the inital startup. the dealer suggested that teh bolt that holds on the oil pump may be too long and could restrict oil from passing through, but after ordering the part and comparing it to the bolt that was in there holding the pump in, they are both the same. It dosent make any cense that we would have oiil off the turbo feed line, if we didnt have any oil pressure, and it runs too well this time to not have oil pressure. has any one got some suggestions????

thanks,
Jordan

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Last edited by dorksrock; 12-30-2006 at 02:04 AM. Reason: wanted to add something i frogot to mention
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Old 12-30-2006, 09:04 AM
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Old 12-30-2006, 03:09 PM
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I thread all the plugs I can and install allen plugs. On any engine.

Turbo. Pressure is the resistence to flow.

Depends where the oil pressure is read and where the turbo line gets fed. You can have volume without pressure. Look at 1/2 volume out of a garden hose. If you read the pressure in the hose it is low to zero because the end is wide open with no restriction.

Combining your internal leak and the turbo hose, you read about zero but volume is still pumping.
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Old 12-30-2006, 08:39 PM
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the turbo is the last thing the oil goes to before it hits the pan. before we replaced the welch plug, there was no oil coming at all to the turbo line if we disconnected it. now with it replaced, we get oil at the turbo line when we disconected it. there should be no internal leaks other than the normal loss on lifters, mains, rods, etc now that the welch plug has been replaced. generaly by the time the oil reaches the turbo, it is around 10 psi on a normal engine. the 6.5 is a high volume, low pressure system, and they normaly run 50psi cold. when we started it up we had 40psi and it slowly dropped off over the next few minutes. i wonder if it would build any oil pressure if i plugged the oil line to the turbo, just to see if this where a problem may be. I am all out of ideas of what it may be, and so is my dad. we have rebulit many engines before, and my dad knows the 6.2/6.5 engines very well. I just dont know what to think, and my dad is sick and tired of working on it and wants to just sell it as is, but I really want to try to fix it, but it seams that nothing is making cense any more. re replaced the oil pump once after the rebuild, because we suspected we had a defective pump, but this didnt cure the problem.
any suggestions will be greatly appreciated!
Jordan
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Old 12-31-2006, 07:30 AM
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Old 12-31-2006, 08:17 AM
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Ok a rather strange idea but this does sound like the oil pump relief spring may be an issue here...? I know you put another pump in but it not to say that it did not go bad or has the wrong one in it...? It does sound like its by-passing somewhere somehow...? When the oil gets hotter it drops in pressure from what you describe as lower than normal to begin with and finaly to no reading on the gauge... One other question that I wonder is where are you measuring the pressure from...? I`ve a couple of 6.2`s but its been a while since I messed with them...
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Old 12-31-2006, 10:51 AM
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the oil pressure is mesured at the rear of the block and is located after the oil passes through the filter and cooler. We suspected that the releif valve may be opening prematurly, but when we had the engine apart this last time, we checked it out and it seemed to be working properly. I just have no clue now, and am getting really frustrated with it.

-Jordan
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Old 12-31-2006, 10:54 AM
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oh yeah also, the oil pump didnt go bad, but when we had no oil pressur after the rebuild, we suspected an oil pump defect and dropped the pan, and put a new pump into it, only to find out that the other oil pump was good, and that this didnt solve our problem.
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Old 12-31-2006, 11:29 AM
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Then you might try removing the cooler adapter if its like a reg Chevy SBC or BBC and just use the filter only...? Most had the 2 long bolts holding the adapter in place and once removed the reg adapter used 2 shorter special bolts to hold it and had the filter screw on nipple also...

Something to eliminate...?
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Old 02-01-2007, 10:29 PM
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....did you find the problem yet ? I had a 350 go zero oil pressure while driving back in '82. It rattled already ,stopped it, checked for oil , restarted a couple of times, no pressure. Started again and pressure normal, but engine damage had occured already. Started again and no pressure......like wierd. Tore it apart , motor looked ok ( oil pump etc ) except for one rod brg, but found a faulty valve in the oil cooler adapter between the block and oil filter. Valve would flip and just let oil pass through as fast as the pump could push thus no pressure. When it reset itself properly then pressure was back to normal. Seen this happen twice in my day. So check the filter base for a bad valve in there is my best guess.
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Old 02-02-2007, 12:39 AM
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this may sound silly but.......bad pressure gauge?
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Old 02-14-2009, 07:10 PM
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I know this thread is old and dead but maybe someone will get use from this info some day. The 6.5 diesel is known for the oil pressure sensor going bad. This sensor is both the sending unit for the oil pressure gauge and the safety cut off for the fuel supply pump. However, its pretty useless for that, the injector pump will pump fuel through the supply pump. Truck will still run although it may or may not run the best.
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Old 09-02-2012, 08:51 PM
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There should be back pressure on input oil line on turbo to hold bk free flow. have turbo tested for leaky seal if truck has cat still on it you may not see oil out tail pipe at first . but theres three wires running oil pressure sending unit. the two outside wires run your fuel pump sender is a natural open switch oil pressure closes it fuel pump starts that is the shut down switch when theres no oil pres. remove sender turn motor over to see if oil sender port is clogged . happy head banging
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