Low oil psi and some start-up issues

[cdmcfarland]

Would adjusting the idle make any difference if the oil pressure is at 0 when it's hot though? It idles fine when the engine isn't at 200, but as soon as it hits 180 you can feel the sputters start to kick in.

 

[mtnwhlr]

It might help wouldn't hurt to try.

 

[tarry99]

Change the oil go to straight 30 weight........motor sounds like it is worn out or has excessive bearing clearance. IE: Gets hot, oil gets thin no pressure.
Could have a vacuum leak at the carb or incorrect Idle mixture........all else fails rebuild the carb.

 

[cdmcfarland]

I've been using Rotella 10w-30, should I go heavier?

 

[tarry99]

Well it seems you are chasing a few Items, but from what you described that when the motor warms up the pressure falls, usually it is only one thing and that is the motor has excessive bearing clearance, either at the rods,crank or cam and once the oil warms up it losses its viscosity / weight and then thins out. Not sure what the average ambient temperature is there but for summers I would try a heavier oil.........perhaps a 20-40 or a straight 30 weight, and then in the winter go back to a little thinner oil. That will help your low Idle pressure.

 

[cdmcfarland]

Alright, I'm just trying to make the thing keep turning until this summer and I'm planning on purchasing a rebuilt AMC 258 i6 / 4.2l to put in it. So I'll give it a shot! Thanks.

 

[tarry99]

10-4 sounds good.

 

[cdmcfarland]

Would the excessive bearing clearance explain occasional drips of oil from underneath when it's been ran for a while? Only happens, like with the pressure drop, after it's been driven for a while.

 

[tarry99]

No, I think as the motor warms up and thing expands you just have some gaskets that are leaking.
Some engines as they become worn out , the piston rings leak during the compression cycle just a little and pressurize the lower crankcase. When that happens seals and gaskets that never leaked before will leak because they have pressure behind them. Once you rebuild it you'll notice a big difference.

 

[Casuall454]

My thought the use of 10w-30 oil is to light. While the motor is cool = oil having more viscosity your pressure stays higher, yet the motor gets hot the oil thins and your pressure drops.

As long your driving in warmer weather have you thought about 15w-40 I know that is diesel weight oil, or maybe STP / motor honey to increase viscosity of your oil ?

> Side not at one point on my '77 with AMC 258 i6 / 4.2l liter I used 20w-50 for couple of months till I saved enough to buy my rebuild kit and pay for the machining work. <

You spoke of "drops of oil" from the bottom of the motor as you noted 119,000 miles main bearing wear can cause drop in oil pressure, although due to increased gaping of the bearing / main journals the pressure usually stays low regardless of engine RPM.

 

[cdmcfarland]

Not sure why I typed 10-30, but I've been running 15-40. Brain fart. Should I swap it out for 20-50? Or just try to add some of the stp to make it a bit thicker?

 

[tarry99]

Listen, you're just trying to make this thing last a few months till you change it out Right?Put some straight 30 in it or the 20-50...........in either case this will help you low oil pressure at Idle............in either case it is just a band-aid!
Question: Does you motor run hot water temp wise? over 190?

 

[cdmcfarland]

It runs about 220-230 normally, doesn't get any higher though. Once it hits the 210+ mark is when the oil psi drops and it starts stalling in idle.

Edit: I just did some research on this and the temperature should be at around 180-190 for a 3.8L, yeah? So that could mean that my engine may not be about to explode, I think.

 

[cdmcfarland]

Gonna' take it into the shop tomorrow to check for cracked head or head gasket. After taking it out today, temperature was pretty damn high. 2 mile trip and when it parked was sitting at around 220. All else fails, heavier oil it is.

 

[Casuall454]

Do to higher temps and such a short drive, stop driving it.

Save the motor possibly the head.

If the temp was fine and your just dealing with oil pressure dropping after the motor gets warm, sure a temporary fix is acceptable. However that isn't what you have.

 

[tarry99]

CD its real helpful when you tell all about the motor.........you never mentioned High Temp until a just asked.......sounds like you cooking the motor. 180-190 is about normal for that vintage.
Get the high temp. problem fixed first, might be radiator, thermostat,cracked head or gasket Etc.
Once fixed you may not need another motor for a while.

 

[IOPort51]

I believe I have noticed that high engine temp can cause low oil pressure with the change in viscosity or perhaps the change is bearing clearance.

someone is welcome to tell me that it is all in my mind, I am not prepared to back this up with any hard evidence.

 

[tarry99]

IO,

Not sure I understand your question, are you saying you do not believe that high temp will lower oil pressure?

 

[IOPort51]

not a question.

I am saying that high engine temp can cause low oil pressure.

but I have nothing to back the statement up.

IO,

Not sure I understand your question, are you saying you do not believe that high temp will lower oil pressure?

 

[tarry99]

IO,
Well I don't think we have enough time here to teach a class in Thermodynamics but here are a few facts to think about.
When oil heats up it's viscosity will decrease. Engine oil does two main things, lubricates and transfers heat away from the lubricated part. Having to much oil pressure is just as bad as not having enough....reason is that you need the oil to move through at a speed that allows the heat to be taken away and still do its lubricating job.
Oil is designed via various base stocks to reduce friction at a normal specific operating temperature range's. Normally oil likes to operate about 50+ degrees above water temperature. Take it out of its designed range and it breaks down and can no longer perform notwithstanding that the block and associated parts become heat sinked which allows greater clearances that the oil can no longer protect. Add various additives and the oil can work in almost any temperature environment.
Not to bore you but the design of oil is quite complicated in the since that in the multi grades its designed to first while the motor is cold run like water to lubricate all the parts before damage occurs. This is also part of the bypass oil filter system so that oil gets in the motor quickly.Then the oil thickens up as temperature increases to again provide that envelope of protection. Take it out of its happy place and there is Limited protection and damage occurs.
Hope that helps a little.
I race drag cars and I am forever chasing lubricating problems, we actually try to dry the motor up internally for two reasons, one is parasitic drag created by the weight of the oil and the other is combustion contamination caused by the oil during the firing cycle. We run Dry sump systems along with crankcase evacuators that suck negative pressure in the crankcase. Because of the lack of oil in the crankcase we use a lot of high tech coatings on piston skirts and wrist pins to make thinks live longer.