Low oil pressure at idle

[rwmastel]

Hello List:

Oil pressure when driving is fine, it reads max pressure of 3 bar (45psi).

At idle in neutral or park (700rpm), pressure is just slightly below 1 bar (15psi).

At idle in gear with foot on brake (600-650rpm), pressure is at .5 bar (7.5psi).

These two measurements are with the engine warm, with fresh oil, and with proper seals in place around oil filter.  Actually, these measurements were the same with the old dirty oil and without the upper filter seal.

I believe this to be too low of pressure.  Where should I start investigating?

Rodd
1966 230SL Euro
1994 E420

[n/a]

it doesn't sound horrible to me, based on (relatively) modern cars I've driven with oil pressure gauges...but only if the car is very much warmed up, and the ambient temp is high.  If you are not hearing knocking or clunking, just drive the thing....in fact, maybe put in a heavier oil, and just drive it, until things get worse (and they most likely will)

-Lewis

'66 230 sl 113042-10-016238
67 saab sonett II #43 of 258
99 saab 9-5
00 saab 9-3 viggen conv

[Bill Rader]

Hi Rodd,
My oil pressure is about the same.
What weight oil are you using?
You could use a heavier weight oil to bring up the idle pressure but I don't think thats necessary.
I just got back from South Carolina and drove 80mph through the mountains with no problems.If my oil pressure was incorrect I think I should have had some problems then.
Bill Rader
68 250SL

[n/a]

I have the same. the pressure goes up to normal whenever I press the accelerator, either in gear or at idle.
I believe I have seen that on quite a few cars, but I will sure check with fellow owners at the next gathering...

Fabrice

Silver 1971 280 SL
New York City

[rwmastel]

I am using Mobil-1 10w30 "Drive Clean" oil.  This is not synthetic, it is just your basic normal oil, about $1.30 per quart.

Rodd
1966 230SL Euro
1994 E420

[Naj ✝︎]

Hi Rodd,
In my view, an SAE 30 is too light for an oldish motor.
I asked Cees: I would be interested to know what oil he chooses for his rebuilt engine. No doubt he'll tell us when he's decided.
I use Castrol Classic 20W50 in my  230sl while in the 280sl, I'm trying Mobil Sport 15W50 synthetic (recommended by an oil expert, but then he works for Mobil!). Oil pressure stays at the top of the gage unless driven hard at warm ambient temps when they will be just above 2/3 on the gage at idle.


naj
'Kloines Scheisserle'
65 230SL
68 280SL

[n/a]

I have heard a few different stories about why manufactureres took gauges out of cars, and installed warning lights. You would get two neighbors comparing gauges, and one of them complaining to the dealer that "Mr. Jones" had higher oil pressure than he did, so he wanted it fixed or he wanted a new engine. Did you know that a lot of gauges in today's cars aren't really representative of what they purport to be measuring? They are still warning lights, htey jsut needles instead of light bulbs. My two cents is your oil pressure is totally adequate. It's not the same as a new car, but it's not a new car.

Rudy
Los Angeles
1971 280 SL

[Benz Dr.]

Nope. Sorry, but that's low. These cars usually hold full pressure until they warm up and good ones never drop at all. You should have full peg by about 1,500.00RPM or when you throttle up a bit.
 I once had a car come in and it was doing something similar. However at road speed ( 50MPH) the pressure would actually start to drop down to about 15 or 20 PSI. When I took the engine apart there wasn't one piece of battit left on the bearings - just the copper shells. I had to bore from standard to max oversize pistons all in one shot. My supplier said '' that's not possible , they won't run with that much wear!'' Well, it did. But not well. Strangley it didn't knock too much or smoke a lot.
 This is a sign of worn bearings and worn oil pump. You could install a new pump by pulling the sub pan and that may help out. You must keep in mind that the pressure reading you see is right off the main oil galley and that the spinning crank multiplies this somewhat.
 While you have the sub pan off you could always pull a main bearing cap and see how it looks. If there's wear or scratches in the bearing you can assume they're all like that.
 Dan c

Daniel G Caron

[n/a]

I sit corrected. If Dan says it's low....it's low.

Rudy
Los Angeles
1971 280 SL

[rwmastel]

Dan, you really make my day.   :evil:   I have ordered a lower oil pan seal because I want to take it off and examine things out of curiosity.  When I get it and get time to remove & inspect everything, I will ask here about how to access and remove that bearing cap.  What material are thoes made of?

Rodd
1966 230SL Euro
1994 E420

[Benz Dr.]

You know , I'm not sure. The block is cast iron and the bearing caps are either cast iron or forged steel - not sure.
 The bearing shell sits inside of the block on one side and the cap on the other side. A small recess or divit holds the bearing in place so it won't start to spin ( they can too) You can pull at least one cap but I wouldn't remove a rod bearing cap - it might not go back together properly. I mean , it should - just might not and then you have more things to deal with than you'd ever want.

 Main bearing cap removal should be safe and they can only go back on one way. DO NOT remove the bearing , just look at it. It should be a dull gray with no copper showing through. It may have lines running across the face where dirt scratched the surface - you can live with that for a while probably. If the bearing is worn only on the ends where the two halves meet this could indicate an out of round condition possibly from improper assembly. Too many things to consider but for now just look for over all wear as that's where the oil pressure is probably going. If the compression is low and it's smoking , that's a sure sign of general wear.
Dan c

Daniel G Caron

[Benz Dr.]

Just a bit more on this;
 your engine isn't terminal but it is worn. They can run a long time in this condition as long as you don't push them too much.
 Most of these old engines break in the same way. The cylinders get worn and the tayper is so bad that a compression ring ( usually the top one) will crack at high speed. The vibration from expanding in and out will do it every time.
 The ring will groove the cylinder wall , sometimes deeply , and will slowly migrate up the side of the piston until it comes out and starts rattling around in the cumbustion chamber. This sounds like a death rattle and it is. Car may actually quit this sound after a while and this almost always happens after prolonged high speed driving. The car will now smoke and you'll have one cylinder that has a fouled plug all the time. I've seen this several times.
  Only once did I see an engine spit a rod bearing out. In this case I did the head and the bottom end couldn't take it at 4500 RPM. I've seen this and old head gaskets blow out after the car sat for a long time. This old stuff just can't take it. If you drive easily it will , if you drive hard it won't. Solid engines will take whatever you can throw at them.
Dan c

Daniel G Caron

[Ben]

My engine displayed the EXACT symptoms and damage that Dan points out and I believe incorrect ignition timing was much of the cause.

My motor now hits 45psi while spinning on the starter and it stays that way.

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Benz Dr.]

Hi Ben,
 this may look similar yet is quite different. In this case the rings crack from fatigue,age and cylinder wear. The ring breaks off in a nice clean line and it will wear a spot on the clinder wall. Wear will be severe but very localised.
 In the cae of running to lean or ignition advanced too far you will find that the top of the piston may be pitted or even holed. If there's a fair amount of cylinder wear the whole side of the piston will be completely melted away and the rings will be melted too. In most cases the spark plug electrodes will be melted as well. The extremely hot gases will find the weakest place to exit which can also be a head gasket or valve. Rapid falure is almost always a burned piston and rings.
 This is severe in all cases but not really very common. I most cases the spark plug will tend to wear away quickly and the engine will miss and run poorly. Removal of the spark plug will show over heating and too lean or ignition problems. The spark plug may be speckeled with black spots which are actually small pieces of aluminum that have melted from the piston and end up on the spark plug. This is a sign of this gone bad and may be already too late to fix.
 Reading of the spark plugs is very important on these cars. Always remove them in order and keep them that way. I keep the old ones and compare them each time I change plugs to see if anything major has changed. If you don't know how to read plugs have a mechanic look at them. Many charts have been developed for this purpose.
Dan c

Daniel G Caron