The GM Atlas 4.2L LL8 Gerotor Oil Pump Autopsy & Information

mrrsm

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In the wake of multiple Threads lately concerning Questions about the Gerotor Oil Pump and its ancillary Seals and Pick Up Tube Issues, here are the ‘Complete Autopsy Images’ of the Pump along with all of the Dope on What this Pump Does… How it Works… and What its Modes of Failure are:

https://www.flickr.com/photos/126111508@N07/albums/72157713216978312/with/49571444046/

(1) The GM Atlas 4.2L LL8 Gerotor Pump is Mounted to the Inner Timing Cover. The Crankcase-Oil-Pan and Timing Cover MUST be removed prior to it being accessed and serviced. The Gerotor Oil Pump derives its Power from a Crankshaft Mounted Sleeve Gear that Keys Over a Single Steel Pin inserted into the nose of Crankshaft. The Gerotor Oil Pump is held in place by (7) Torx T-30 10.9 Hard Machine Screws that need to have Hi-Temp Thread-Locker applied to their Threads along with a MAX of 106 Inch Pounds of Torque. When removing the Gerotor Oil Pump Cover… the In-Dwelling Gerotor Gears should next be wiped down with Brake-Kleen Spray on a Rag and immediately Marked with a Permanent Magic Marker to locate these Gears in their ORIGINAL POSITIONS on the Two Inner Gear Segments prior to removal and cleaning of the Inner Oil Cavity of the Timing Cover. When replacing these Gears...Remember to install them both with the Beveled Edges toward the INSIDE of the Timing Cover Cavity... AND Soak them Down In Motor Oil FIRST.

(2) The Early Models of the 2002-2004 GMT-360s with the 4.2L LL8 Engine employed a Gerotor Oil Pump having a Shallow Beveled Inlet Manifold at its base and employed a “BLUE” Viton “O” Ring with a short-snout Oil Pick Up Tube. The Later Model Gerotor Oil Pump sports an After-Market (MELLING) Longer Inlet Tube, with a Deeper Inlet Port at the Base of the Oil Pump and uses an Orange Rubber Impregnated Metal Grommet-Gasket as the means to solve a problem with a Design Failure of the “BLUE” “O” Ring Seal on the First Generation Pumps.

(3) The Crankshaft Gear feeds into the middle of a Combination Small-To-Large Gerotor Gear Set contained inside of the Gerotor Gear Pump Housing Rotating Steel Sleeve. These Three Gears work to Propel the Engine Oil from the Wide Spacing in the Open Gearing areas leading to Smaller and Smaller Spaces while attempting to compress the in-compressible Oil Fluid. This action serves to build up Oil Pressure, thus advancing this Oil Stream up, through and out of the Pump and into the Engine Block Oil Galleries. This Animation shows how different the Gerotor Pumping Design is from how The Old Matched Gears, Camshaft Helical Gear Driven Oil Pumps work:

GEROTOROILPUMPDESIGNANIME.gif

(4) The Gerotor Oil Pump Cover is held flush, with NO SEALANTS OR GASKETS in between its smooth facing Flange and the flat, smoothly machined inner surface of the Inner Aluminum Timing Cover. This area of the Timing Cover forms an Oil Collection Chamber that fills with Motor Oil getting vacuumed up from the Oil Pick Up Tube attached at the Base of the Gerotor Pump Manifold. As the Motor Oil is advanced and fed into the Gerotor Gears...it forms a Strong Vacuum that in combination with Capillary Action always present between the Oil Molecules for additional help… Pulls the Motor Oil Up from the Lower Crankcase.

(5) The Gerotor Oil Pump is fitted with an Oil Pressure Relief Chamber and Plunger-Piston-Valve that consists of a Smooth Bore in the Cast Aluminum Body of the Pump that holds a High Grade, Hollow Steel Cup-Plunger fitted with a Strong Coiled Spring. The Nominal Oil Pressure for the Atlas LL8 Engines is 12 PSI at an Idle of 1,200 RPM and 65 PSI at 3,500 RPM. The Coil Spring is held in place by an Aluminum Screw Cap with a 5/16” Hex Wrench Hollow Center. Likewise… the Threads on this Hollow Aluminum Cap MUST also have Hi-Temp Thread Locker applied. Do NOT Exceed 89 Inch Pounds of Torque on the HEX-Cap as this is an Aluminum to Aluminum Fastener.

(6) In order to Remove the Gerotor Oil Pump HEX-CAP holding in the Plunger Spring… the (7) Torx T-30 Gerotor Oil Pump-to-Front Timing Cover Machine Screws MUST be unwound and the Outer Pump-Cover-Assembly MUST be removed FIRST. The reason for this is because there is a LEDGE Cast into the Lower Horizontal Flange of the Aluminum Front Timing Cover-Gerotor-Oil-Pump Assembly designed in as a “Last Ditch” effort to keep the HEX-CAP from completely coming off of its Screw Mount over the Coiled Spring should it become loose. It IS possible to insert the 5/16” Hex Wrench Socket into the Hex Hollow of the Aluminum Screw Cap... but as the the Cap is being Unwound Counter-Clockwise… that small Cast Aluminum “Ledge” WILL Break Off th
at Piece of the Lower Aluminum Flange.

(7) The Gerotor Oil Pump is capable of Pumping 11 + Gallons of Motor Oil Per Minute during its normal operation. However, if the Oil Pressure were to exceed 65 PSI… The “Lost Foam” Cast Aluminum Engine Block Oil Galleries could be overcome by Excessively High Oil Pressure ...and Crack them Open like Egg Shells (Edited as per Instructions after Peer Review by @Sparky and @Mooseman NOT to include Personal or Speculative Information... Thank You, Gentleman.)

(8) The Oil Pressure Relief Valve is designed to use its Plunger or Piston to squeeze down against the Strong Spring just enough to allow the Over-Pressurized Motor Oil to By-Pass the Main Oil Pick Up Chamber and return the Oil to the Incoming side of the Oil Pump. This happens automatically and even though the Plunger-Piston is in the constant presence of the lubrication offered by the Engine Oil… if Sand, Grit or Metal Particles get Vacuumed into the incoming Oil Stream...Those Gritty Particles CAN get JAMMED in between the Steel Plunger-Piston and the Aluminum Bore and cause the Plunger to cant sideways and get JAMMED AND STUCK INSIDE of the Bore.

(9) If this Stuck Open Oil Pressure Relief Valve event lasts for even a small amount of time, a Catastrophic Drop In Oil Pressure can put the Engine Cylinder Walls, Pistons, Rings, Crankshaft and Connecting Rod Bearings at risk of Seizing Up. IT IS VERY IMPORTANT TO REALIZE THAT THE OIL GETTING VACUUMED UP FROM THE LOWER CRANKCASE AND FLOWING IMMEDIATELY INTO THE GEROTOR OIL PUMP IS UNFILTERED.

(10) It will NOT be Possible to Remove the High Grade Steel Plunger-Piston through the Upper Threaded Bore in the Base of the Gerotor Oil Pump as that component is TRAPPED below the Thread Lines for the HEX-CAP. However, It WILL be possible to use a Pencil Magnet to Check the Piston for Freedom of Movement gliding in and out in that Chamber. But If that Piston is found to be JAMMED solidly inside the Body of the Gerotor Oil Pump… REPLACING THE GEROTOR OIL PUMP & TIMING COVER CASE AS A COMBO-UNIT: GM Part # 126 28565 should be considered. An Alternative choice is to keep the Timing Cover Case and ONLY replace the Gerotor Oil Pump using the Sealed Power Kit Part # 22453582 which includes the Inner Gerotor Dual Gear Set, The Outer Pump Cover, the (7) 10.9 Hard T-30 Torx Fasteners and the all important Oil Pump to Engine Block Black Rubber Grommet-Seal. Before Installing the Dual Gerotor Gear Set into the Front Timing Cover Inner Cavity... Soak them both down in Motor Oil.

The Gerotor Oil Pump Autopsy Images can be seen as linked below via my “Flickr-Bucket”:

https://www.flickr.com/photos/126111508@N07/albums/72157713216978312/with/49571444046/
 
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aaserv

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Dec 1, 2019
408
N of Baton Rouge, La.
Thats excellent info. Im guessing the only thing keeping thousands of these motors running is the difficulty of accessing the pump compared to a V-8 LS style pump. Otherwise virtually every mechanic without this knowledge would be boosting that spring pressure ala LS pumps with disastrous results.
Ive told people for years that the only way to destroy these motors was to run them low on oil, until now I never knew why exactly. And the 12 to 65 variation in oil pressure finally explains why the oil pressure sending unit feeds its info thru the computer and then to the dash gauge. The constant swings in oil pressure would have every customer who owned 1 bringing it back to the dealer before they even got it home!
This info would have been good to know 10+ years ago but if you kept the oil changed with a top quality oil filter you would likely never need to know it. Thank you for posting this. I certainly learned something new about the 4.6L today.
 

gmcman

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Dec 12, 2011
4,656
Nice write up @MRRSM .

I wanted to mention since I'm running an aftermarket oil temp/pressure gauge, I will see a little over 100 psi at the oil filter housing when reading the inlet side of the oil filter if the temps are cold (around 40 deg or less) and the RPM's are around 4K.

I surely don't drive it like this on a regular basis, but noticed once leaving work.
 

Reprise

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Wanted to chime in as well - great article. :thumbsup:

There's one item I'd like to nit / pick on, however - the oil galleries bursting with pressure in excess of 65psi. Do you know where you saw that? I'd be interested in reading more on it.
 

mrrsm

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The Thin Cast Aluminum "Lost Foam" technique is 'marvelously economic' in its use of this Metal to create the hidden inner structures of Atlas LL8 4,5 & 6 Cylinder Engine Blocks during the Burn Out of the internal foam planks dissolved away during Molten Aluminum Pours. This leaves very modest Oil Channels for the non-bored oil galleries to contain the Motor Oil moving through the Engine Block. The Head GM Engineer Ron Kociba over The Atlas Engine Development Team must have had a Very Good Reason for setting the Upper Limit for Maximum Oil Pressure at 65 PSI and I suspect that these issues must have been part of their factoring.

My ad-hoc impression is not based upon any documented "Pressure Test To Failure Analysis of the All Aluminum LL8 Engine Block" ...but rather an understanding based upon the Common Knowledge we all have of the Strengths ...and Limitations... of Thin Aluminum Structures vs. the Sheer Power of Hydraulic Pressure very easily exceeding that limit. I would NOT want to risk having a Sudden Blockage of the Motor Oil in a system capable of moving a Fluid Steam at over 11 Gallons Per Minute as the Gerotor Oil Pump gets driven by 285 Horse Power at 6,000 RPM taken directly off of the nose of the Crankshaft... and NOT have that Relief Valve there working properly to deal with any over-pressure events happening.

In @gmcman 's situation... it is important to consider that it is just as possible for the Plunger-Piston to get Stuck in the CLOSED Position as it is for the Valve to get stuck in the By-Pass Mode if any Sand or Grit has frozen that Plunger in the Aluminum Bore of the Pump. The excessively High Oil Pressure he is reading leads me to think this is why he is occasionally seeing 100 PSI ... or perhaps because the Gerotor Oil Pump is trying to move Very Thick, Sluggish Motor Oil through an Ice Cold Engine being revved up to 4,000 RPM and the Relief Valve is too small of an opening to allow it to get through.

One last note concerning the OTHER Oil Pressure Relief Valve being present adjacent to the Welded Steel Oil Filter Canister. It is there for the express purpose of "Preventing the Oil Canister from BURSTING due to Excessive Hydraulic Oil Pressure in the Line".
 
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Mooseman

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Dec 4, 2011
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Ottawa, ON
If you think of the pressure produced by brake systems and power steering systems on their thin steel tubes and rubber lines, the solid cast aluminum oil passages, unless they are paper thin (which is not likely) would likely take thousands of PSI to break, which is far beyond the capability of the oil pump. It would also take a solid plug to cause such a possible high oil pressure. With bearing clearances, there is always leakage allowing pressure relief. There are other things would blow out first like the oil filter, pressure switch, CPAS and such.

Please do not over-speculate.
 

cornchip

Member
Jan 6, 2013
637
Yes...it's all about 'area' when it comes to pressure. A small hole can handle much more pressure because it has less surface. Personally I'd be careful about starting another urban myth about blocks failing because of this. You know how people are on the internet. :Rumour:
 
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Sparky

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Dec 4, 2011
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As they say at my workplace (where I work in quality and market issues over a wide span of failure modes), "Stick to the facts." If you have a theory, sure test it, but do not state that the theory is anything beyond that without testing.

Yes they had a reason to limit to 65 psi, but we don't know the reason. To assume it is to prevent block bursting based on the assumption the channel walls are very thin without any hard data to back any of that up is a bit much, especially considering we have no known cases of that happening anywhere.
 
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Reprise

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Thanks (to all) for the clarifications. Given the new info, I'd suspect that the upper boundary on oil pressure was spec'd based on potential to introduce aeration / foaming, based on what I've read about it other places (so, nothing scientific on my part, either)

That's also the main reason why I'm leaning toward just a high volume pump for my 6.0L (in-truck) build, rather than a HV/HP, since I've got really good oil pressure on the engine (high 30s @ idle; near 50 under load), despite it's mileage.

TBH, I'm debating whether I should update anything but the O-ring for the pickup tube. We'll see what I find out once the front cover comes off.
 

mrrsm

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There is one thing to be cautious about when installing High Volume Oil Pumps though:

At higher RPM, they can pump so much of the Motor Oil up and out of the Crankcase into the two Engine Heads that the volume of the Motor Oil below can drop significantly and suddenly before that oil can drain back down and refill the Reservoir. If this happens when you are on a steep grade and the truck is under stress with less than a Full Boat of Oil in the Pan... the Pick Up Tube can scavenge air unexpectedly. So you have to be very fastidious about Checking the Dipstick much more regularly.
 
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mrrsm

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Thanks Very Much to @Reprise for raising the Question regarding the potential for hydraulic damage occurring from excessive pressure and for @Sparky and @Mooseman for "Keeping The Hands on the Tiller" for the sake of accuracy in putting Articles forward for Publication on GMT Nation. Thanks also to @Mooseman and @cornchip for advancing the logic of 'smaller area' within and along the length of Oil Galleries as being more structurally sound than I would have proposed. In hindsight... the Physics of that idea makes much better sense.

Even though I have very strong feelings about this matter based on experience with these conditions... the only way to keep the Article factual and scrupulously free of speculation or opinion is to make any Edits in the collective discovered by the Mods prior to its acceptance as being "Gospel". Please let me know what you find and either delete those portions necessary or I can deal with them first hand. I very much appreciate this.
 
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Reprise

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There is one thing to be cautious about when installing High Volume Oil Pumps though:

At higher RPM, they can pump so much of the Motor Oil up and out of the Crankcase into to the two Engine Heads that the volume of the Motor Oil below can drop significantly and suddenly before that oil can drain back down. If this happens when you are on a high grade and the truck is under stress with less than a Full Boat of Oil in the Pan... the Pick Up Tube can scavenge air unexpectedly. So you have to be very fastidious about Checking the Dipstick much more regularly.

Well, I could very well fall into this category, since this particular truck is a dedicated tow vehicle, and I do climb grades.

I do have a habit of checking oil / coolant before each day's travel, when on a multi-day trip.

I suppose a larger sump would help avoid this situation as well, and I do have clearance for it. Will look into that (it would keep the oil even cooler, and extend oil life / stress it less).
Appreciate the heads-up.

Thanks Very Much to @Reprise for raising the Question regarding the potential for hydraulic damage occurring from excessive pressure

I feel bad about asking about it, now, since a couple of folks took you to task. Usually, I've heard something about most automotive topics -- but that particular one was definitely 'new to me'.
Thanks for the clarification. I did find the balance of the post useful / interesting.
 
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