'74 Eldorado 500 - Low Oil Pressure After Warmup

[StremBluey]

Hi All. New-ish Caddy owner. Looking forward to learning from the forum.

I have a 74 Eldorado 500 that starts up and runs fine. Oil pressure is about 30 PSI at startup (from manual gauge piped into the back of the block), and drops to less than 10 PSI after 5-10 minutes of warming up.

The Story:

Made the trip from Kansas City to Tulsa on the day I bought it with no problems (previous owner drove it halfway, I met him and drove it the rest of the way). After a quick stop for food in Tulsa, I began to drive it to where it will be kept. On the way there on the highway all hell broke loose. Started to lose speed, so I gave it more gas, then realized I lost all power. The oil pressure and temperature lights came on and I pulled over as the engine died and the coolant started to boil shortly thereafter. I can't be super precise as this all occurred in a short amount of time on a very busy stretch, so I was more focused on not dying than the car.

The car was in storage (completely forgotten about in a garage) from ~1990-2010. Then was purchased and got running (carb rebuild, new fuel tank/lines, nothing in the engine), but was very rarely driven (less than 500 miles in the past 12 years). It made a 1.5 hour road trip the weekend before I purchased it with no problems.

What I've done so far:

Changed oil - the oil was in very poor shape. Blame the previous owner for having low grade (grocery store) oil in there for years. I put synthetic in with no improvement. Then changed to the highest viscosity oil Cadillac recommended (AmsOil), also with no change.

Compression is fine. No oil in coolant, no coolant in oil. I changed the oil pump and plugs/wires, too: all no changes.

The car had an aluminum aftermarket radiator in it with a pinhole in it (I discovered afterwards), so I put the original radiator in it again, no improvement on any front.

I start the car fairly regularly. Starts easily, runs fine. I never drive it, but let it idle to temp and the pressure just slowly gets lower until the light comes on (I shut the car off once it gets to 10 PSI).

I have not yet checked the RPM but it sounds fine. I can increase the RPM and the pressure goes up, but the speed at which this happens sounds like it's way too fast of an idle.

I also haven't checked the distributor gear to see if all the PTFE covering has flaked off and potentially clogged up the oil pickup screen, but I would expect that to be an issue at startup rather than warm?

I've been told it may be sludge, but I don't know what to expect and that doesn't sound right to me.

So... what's the deal? What do I do now and what's my order of operations? If I had to guess I'd say main bearings, but that sounds like a rebuild. Looking for some help from some experienced Caddy people.

I very much appreciate your help. Hoping I can get this thing running and start enjoying it.

Thanks!

[Wbostoen]

Changed oil to the highest viscosity oil Cadillac recommended (AmsOil), also with no change.

Hi,

What viscosity oil did you put in?
As a reference: I run 10w40 in my '76, and it idles cold around 50psi and when warm I don't think I have seen less than 30psi

[StremBluey]

Hi. I believe it was 20W-50 on this last attempt. That's the most viscous the manual says can be used.

[TJ Hopland]

What usually plugs up the oil pickups on these is plastic pieces off the cam sprocket and valve guide seals.

The oil pump is external on these so its fairly easy to inspect and or replace.  The boss that holds the oil filter is the main body of the oil pump and the engine block is the cover. 

OE pumps were aluminum replacement ones are iron so a quick test with a magnet may give you a clue if its possibly an original or a replacement.  If the pump is pretty worn replacing it may help your pressure issues some but if the pump is that worn that likely means there is wear in other areas in the engine so the real fix will be proper rebuild. 

[StremBluey]

I tried the oil pump just for the heck of it. The one I pulled off looked just fine and the aftermarket one I installed didn't boost it any.

[TJ Hopland]

What did the gaskets look like?  The proper gasket isn't much more than a thick piece of paper.  Some people have reported that there are some gaskets floating around that are much too thick which then leave too much clearance between the pump gears and the cover/block.

Eldo you can't get the pan off without splitting the engine and trans which means one or both has to come out.  If you could get the pan off sometimes you can buy a little time with new bearings but since in this case its got to come out you might as well do it right since its not likely that there is just one problem area, its most likely wear everywhere. 

[StremBluey]

Yeah, I checked the gasket. It was comparable to the OEM. It was from CAD500Parts.com, so I'm assuming they would have the gasket correct (right?). Also, there was zero wear on the original and zero change in pressure.

So does it sound like main bearings? Is there anything else I'm missing on the diagnostics side before making that final?

Assuming it is bearings and I need to rebuild this thing, do you have any resources for this? This was the day I bought it and is my first Cadillac, so my plan of learning the car/engine and building up to something like this is out the window. I'm flying a little blind.

Is this the best forum around for 500 engine rebuilds (I'd like to do it myself)? Any other resource that may help me along?

Thanks for your help.

[James Landi]

These engines have an oil pressure of about 10 pounds at idle when hot... and it's usually the case that pressure will get up to around 30 pounds above idle. My theory is that GM used "idiot lights" so that customers would not stress out when their Cadillacs were in traffic with the a/c on.   Regarding the interior coolant temperature, your service manual will provide some "limits"--- HOWEVER--- replacement radiators are a constant source of frustration and old radiators need to be THOROUGHLY CLEANED as the internal core builds up a crusty accumulation. Issues always occur during long trips, during the summer with a/c on. SO a thorough cleaning of the water jackets and radiator will provide that additional margin against boil over.  Some of us have been known to run theheater on full during heavy summer traffic to avoid  a stalled over heated engine   You can use single viscosity high detergent oil during the summer, and that will provide about 5 extra pounds at your oil pressure gauge, but that's not recommended for the engine (only for your head!)   Hope this helps,  James

[StremBluey]

Just one more bump to see if there was any more input.

I follow on the overheat thing. But it's currently going below 10 PSI oil while sitting idle in the driveway after it warms up - zero engine load outside of just running. I'm not sure how low it will go as I shut it down once it crosses the 10 PSI threshold. No temp light has come on since the initial event.

Thanks!

[eldofever58]

Just one more bump to see if there was any more input.

I follow on the overheat thing. But it's currently going below 10 PSI oil while sitting idle in the driveway after it warms up - zero engine load outside of just running. I'm not sure how low it will go as I shut it down once it crosses the 10 PSI threshold. No temp light has come on since the initial event.

Thanks!

Are you running the gauge in addition to the warning light? Could the gauge be off?

The old 'rule of thumb' is 1 PSI per hundred RPM. 10 PSI would certainly get my attention, but doing some research on this recently, I learned quite a few GM V8's are set to trigger the low-pressure light at 4 PSI.

[James Landi]

Try running straight weight 30  high detergent oil.  In the 1980's Cadillac was ""suggesting" this "summer weight oil" single viscosity for the 4100's.  You'll likely pick up another 3-5 #'s of oil pressure at idle, and achieve some "peace of mind."  If you use the car during the cold seasons, then switch back to multigrade.   I've owned several inboard boat GM gas engines, and back in the 1950's through the 80's, the recommendation was NOT to use multigrade oil. Of course it's a different kind of application, however, I can personally confirm that as a "newbie" to boat motors back in the 1970's, I changed oil and used multigrade, and went into a panic when I watched the gauge drop to 5 pounds at idle...  hope this helps,  James

[TJ Hopland]

The not recommending multigrade was common for engines that were likely to run hot or often be under heavy load. This was often air cooled engines like a classic VW or lawn and garden equipment.  Marine engines also tend to run under heavy load because unlike a car the load doesn't decrease much once they get moving.  Similar thing with heavy equipment.

The reason there was concern there is especially early multis were really prone to breaking down when exposed to high operating temps.  To understand what was breaking down you need a basic understanding of oil and how a multi viscosity works.

This is really really basic but gets the concept across.  Think of the oil as balls or ball bearings.  The weight is the size of the balls.  Regular oil its an average size and synthetic its the exact size.  Smaller numbers are smaller balls.  This is why the engine build and somewhat condition determines the grade.  A new tight high tech engine may only have 5 weight gaps in it.  If you put 10w in the balls will be too big to fit where the lube is needed.  If you have an old or worn out engine it may have 50w gaps so if you put 5's in they don't even slightly slow down passing through.

Where it gets tricky is cold oil doesn't like to flow and cold engines the gaps shrink so a 30 is going to take a long time to get pumped places and when it gets there may not fit where it needs to go.  Once the engine is hot pumping isn't a problem but that gap may have heat expanded to a 50 so that 30 flows through pretty fast.  You now have wear issues at startup because the oil can't get where it needs to be and you have issues at temp because its too thin.  If only there was an oil that was smaller when cold and bigger when hot?

Again its way more complex than I'm making it out to be but I'm just trying to get a concept across. The multi viscosity oil is basically made up of the smaller number but they add I think a polymer to it which is basically a 6 pack holder.  Say we have a 5w-30.  Its 5w balls but in a 6 pack they act like a 30. The magic part is the 6 pack holder operates on temperature.  When its cold the 6 pack holders expand and let the balls go free so you have the grade 5 balls acting like 5w balls.  As the temps come up the 6 pack holders start to attract and hold the balls together and when they fill up it acts as a 30 size ball.

The possible problem especially in the early days was that 6 pack holder was fairly brittle especially when it came to prolonged heat.  Other contaminates also weakened them.  Fairly quickly they would break down.  Apparently they still held the balls fine but they would break apart so even at higher temps you were left with the 5w balls which were too small to protect most engines. 

With typical car engines it was felt that it was worth the trade off for cold starts and there wasn't usually enough heat to prematurely break down the 6 pack holders during a normal oil change period.  For the high stress applications there was concern that it would break down too early and on those high stress applications and if it did break down you would get catastrophic damage from the thin oil due to the high loads.

Oil tech has improved a ton.  Anyone who changed oil back in the 80's probably remembers times you were changing hot oil and it came out faster than water. This was because either due to high temps or contaminants all those 6 pack holders were broken so it was a 5 weight oil. You have to have pretty beat oil these days to have that kind of break down and usually by that time you will have so much sludge things kinda thicken up again.                                

[James Landi]

TJ,


Thanks for taking the time to explain these differences.  The original "poster" is concerned about low oil pressure at idle in his big block.  I'm not certain that he truly has an engine problem, for, as you well know, these old engines at idle during summer driving don't develop much pressure.  As I mention, he could pick up a few pounds of pressure with a single weight, but from reading your excellent explanation, you don't recommend this approach as the cold start with single weight is, potentially, damaging.  I appreciate your wisdom.    James

[TJ Hopland]

I'm no oil expert that 6 pack thing is just how a smart person described it to me so that I could wrap my small brain around it.  It was mostly the 'don't use a multi viscosity' statement I was trying to address and why that was a legitimate thing at one time but like many things oil tech has come a long way.

     

[V63]

You could try LUCAS oil stabilizer that is a viscosity improver. Their entire product line is great!

[StremBluey]

Good stuff here.

I'm pretty sure the gauge is fine. It was brand new and has performed exactly the same every time. I almost always shut the car off before the low oil light comes on, but early on in diagnostics the gauge would go somewhere below 10 (the needle starts to bounce below 10), and the low pressure light would begin to flicker on. Again, I shut it down when it hits 10, but this is indicative that the gauge and light are working the same.

That was an excellent explanation on multi-viscosity oils. I have a little background in rotating equipment lubrication and that was the best explanation I've heard so far. Yes, it's polymers that change with temperature that allow multi-viscosity to work, and nowadays, work well. Also the fact that I've tried three different oils, the most recent being a 20w-50 AmsOil leads me to believe this is not the case. It also does this in summer or winter - doesn't matter. And there is never a load on the engine. It never does more than idle out of the garage and into the driveway.

Thanks for the fruitful discussion.

[TJ Hopland]

I'm thinking you are probably OK.  Another way to look at is is what if its not?  What would your next move be? Rebuild the engine? If that is in your longer term plan/budget then whats the worst that can happen if you keep running it the way it is?  Engine has a catastrophic failure?  Leaves you stranded and you have to pay for a tow?  Compared to what a rebuild is gonna cost a tow is cheap.  Even if its a really catastrophic failure that destroys the block a rebuildable block should be easy to find, likely cost more to ship it than it will cost you.           

[StremBluey]

I'm thinking you are probably OK.  Another way to look at is is what if its not?  What would your next move be? Rebuild the engine? If that is in your longer term plan/budget then whats the worst that can happen if you keep running it the way it is?  Engine has a catastrophic failure?  Leaves you stranded and you have to pay for a tow?  Compared to what a rebuild is gonna cost a tow is cheap.  Even if its a really catastrophic failure that destroys the block a rebuildable block should be easy to find, likely cost more to ship it than it will cost you.           

Well... That's a perspective I hadn't considered. Fair point. I guess my concern would be the unknowns. Like, okay, it kills the block and I have to find a new one. But what if it wipes out the block, crank, and heads? Then that's a considerable upcharge.

If I had more confidence in what would actually fail I may be more inclined. But I'm brand new to this engine and there's a whole lot less info available than a SBC, for example.

But I like your style

["Cadillac Kid" Greg Surfas 15364]

Please note, a "good" block, crank, and rods at the going rate will ONLY set you back about $1,000 plus the shipping. Not saying you need to do a rebuild but going to catastrophic failure seem (to me) a bit cavalier.
Greg Surfas

[TJ Hopland]

Its pretty rare to have one of these engines that isn't rebuildable.  Most of the time cranks just need a polish and the only thing that requires major boring of cylinders is if one or more sat with water in it.   

I think more often than not when one of these engines 'fails' in a Cadillac it just starts running poorly and making not so good noises.  Now if its been swapped into a mud truck or race car, ya those do sometimes end up with pieces missing.