oil pressure

[MarcHogenboom]

Finally I got to the piont where I could take the GMC for a testdrive and now it turns out I have way to low oilpressure. Cold it is 4 to 4 bar at idle and 7 at 1500 rpm, but as soor as the engine warms up it drops to about 0 at idle and 1,5 at 1500 rpms. The engine has 2 miles on it! I bought it in the US 2 yaers ago, so no chance os exchanging it umder warenty. Besides the shipping from the Netherlands to the US and back would cost as much as another engine.Its a '97 engine, unused. What can be the cause? is there some kind of presure regulater in the pump that could be at fault?

[eracers999]

You need to check oil psi with a guage (actual pressure type) .

[tom.mcinerney]

The pressure should be 20-30psi{1,5-2bar} hot at idle, and 30-60psi{2-4bar} hot at 1800.

There is a pressure-regulating (or relief) valve integral with the oil pump assembly. These valves have been known to fail (broken spring), causing low oil pressure, and eventual engine death. The crankcase oil pan may be removed and the pump replaced while the engine is in vehicle[at least in 4X4 configuration], but need lower front differential some. The preferred replacement oil pump is 1998 model year.

Bearing clearances worn oversize will leak, rather than develop pressure, of oil. If you replace the oil pump, you might remove a bearing cap to see how 'new/worn' the crank and bearings appear.


There are other pressure regulating valve(s) in a subassembly screwed to the block beneath the oil filter (on the pad the filter threads onto). The OEM manual describes removal and replacement of this relief valve assembly. New replacements can be bought from dealer.

A badly clogged oil filter might cause the symptoms you describe.

[MarcHogenboom]

Today I took out the oilpump. Couldn't find anyting wrong with it. There was no sighn of the relief valve being stuck. Tested it with compressed air and the relief valve opened at 4,5 bar, so that should be good. Next I took of a main bearing cap. It looked a little rough, but the engine has not been ran in yet. There was no sighn of excessive wear as far as I can see, so I dont expect the cause of the low pressure is in the bearings. Now what???

[eracers999]

Marc:

The oil pump you took out, when you look at the 4 screws that hold the top cover over the gears is there a seperator plate in between the cover and the pump body ?? If not then you have the regular pump, if yes then you have the higher volume pump. Recomend genuine GM 1998 assy, comes complete with pickup.

Do you have the oilcooler hooked up?? Resone i ask is that you can really screw one of these engines up by blocking it off it relies on that oil path to feed the engine, blocked off you feed the engine through a very restricted circut.
Kent

[MarcHogenboom]

The pump consists of 3 parts, so it must be the high volume thing? the oilcooler is hooked up, but the engine has a remote oilfilter. The original oilfilter mounting is blocked off, because of the front wheel drive tranny. The oil comes from the cooler hookup to the cooler, than to the remote filter and bach into the block via the cooler return hookup. Could this be too restrictive? If so, than howcome the oilpressure is excelent whenn cold? I must be doing something wrong but what?

[JohnC]

Marc:

I'm not sure I understand exactly what you're saying, but if the oil filter is in line with the cooler then a lot of the oil is not being filtered. The oil doesn't flow through the cooler until the engine warms up. Can you fit a remote filter adapter to the original oil filter pad? This is probably the only way you'll be able to get full flow filtration.

I assume that when you say "blocked off" you mean the oil coming out of the filter adapter is routed back into the filter adapter, right? Otherwise the flow is severly restricted, if it flows at all. This would require an internal bypass valve.


As to why the pressure drops when the engine warms, I'm not sure if this is the answer. I suppose if the oil is being routed to the cooler and the filter is very restrictive it could cause a problem, but I wouldn't expect that since only part of the oil goes to the cooler and (I assume) you are using a full flow filter with an internal bypass valve.

One possibility is that the oil is not getting cooled at all, for whatever reason, and your low pressure is caused by the high oil temps. Do you know how hot the oil is getting?

[eracers999]

Marc:

There is no way that set up will work, you are running completly on the bypass valves. With the oil filter being blocked off you are only feeding oil thru the bypass valve. Oil filter and cooler both have bypass valves in case they are blocked.

John: Correct me if im wrong. My take on the oil circut is that oil runs thru the cooler even when cold, there is no temp sensing valves that open or close, there is only the bypass valves that come in to play when pressure is excessive such as when cold but oil still goes thru the cooler.

Kent

[MarcHogenboom]

If I understood correctly, the oil flows from the pump to the cooler, then to the filter, then into the galleries. So with the original oilfilter blocked ( input connected to output) the oil should go from pump to cooler to filter to galleries. In my mind this should work. Can someone put the diagram from the manual in a reply so that I can so if this is correct?

[Hye]

Here's what the '94 manual says. I'm not smart enough to insert figure 5 here, but I'd be happy to email the diagram to you or anyone who can.


"ENGINE LUBRICATION: A lubrication schematic is shown in figure 5. The gear-type oil pump is driven from the drive gear. The drive gear is driven by the camshaft. Oil is drawn into the pump through a pickup screen and pipe. Pressurized oil is routed to the oil cooler in the radiator. A bypass valve is provided should the oil cooler become restricted. Oil flows from the cooler to a full flow oil filter. An oil filter bypass valve is provided should the oil filter become restricted. Oil flows from the oil filter to the oil galleries, providing pressurized lubrication to various components. The hydraulic valve lifters receive oil from the oil galleries. Oil flows from the hydraulic lifters through hollow pushrods to the rocker arms. Oil from overhead drains back to the crankcase through oil drain holes. The pistons, piston rings, cylinder walls, and connecting rod small end bearings are lubricated by oil splash. The turbocharger is supplied oil from the right side lifter gallery through a braided steel line to the top of the center housing. The oil returns to the crankcase by a drain tube that goes from the turbocharger center housing to the lower right hand side of the engine block."

[MarcHogenboom]

Hye, can you email me the drawing at: hogenboom@dutch.nl .

Thanks