Mirage 3A92 Engine repair in process. (Problem: low compression)

[Mechanic]

Inuvik,
. . . . Possible but I don't think so. Remember that at 200F 50 weight oil pours like water. When I tow I change the oil every 3K miles. When I change the oil at 7.5K miles the oil is just a dark brown color. The engine doesn't make any abnormal noises. The compression just falls off. These engines do not have sleeves. The aluminum surface is treated to harden the cylinder walls and to provide a wear surface. This surface is only a few thousands of an inch thick and once you wear through it bye bye engine!
GOD Bless and Thanks,
rich!

[7milesout]

Rich - There's a thread on here asking you about your transition to Texas. It is below:

https://mirageforum.com/forum/showth...TX-relocation!

We are awaiting your response kind sir...

[inuvik]

Inuvik,
. . . . Possible but I don't think so. Remember that at 200F 50 weight oil pours like water. When I tow I change the oil every 3K miles. When I change the oil at 7.5K miles the oil is just a dark brown color. The engine doesn't make any abnormal noises. The compression just falls off. These engines do not have sleeves. The aluminum surface is treated to harden the cylinder walls and to provide a wear surface. This surface is only a few thousands of an inch thick and once you wear through it bye bye engine!
GOD Bless and Thanks,
rich!

You are correct but the overwhelming majority of owners haven't had any issues with compression or engine longevity. The one variable that separates you from the other forum members is the use of 5w-50 so I'm just pointing out that with all other factors being identical (driving, changing the oil) it definitely could be a contributing factor. Or it might have nothing to do with it, dunno.

[Mechanic]

Inuvik,
. . . . There are several other factors. I sleep in my car for 6 to 8 hours and I leave it running. I don't think that anyone else does that. I drive at 75 to 85 mph for hour after hour. Not unusual to run for 4 hours without a break. Lots of desert and desert mountain driving with outside temperatures of 105-115 and coolant temperatures of 220-225. A lot of high rpm work. I pull to the redline through the first three gears when I am getting on a highway. So there are other factors besides the 5-50 oil. I have been running Castrol Syntech 5-50 oil for over 30 years and this is the first car that I have had where I lost an engine. Now the other engines were either a rotary or they had cast iron blocks. This is my first all aluminum engine.
GOD Bless and Thanks,
rich!
BTW: Mark, how much snow do you guys have? It was 55 here today. 25 degrees cooler than 2 days ago! Love, rich!

[DirtGearTv]

I've thought for awhile that some of Mechanic's engine problems were due to running 5w-50. Is it possible that the engine is oil starved on the bottom end & cylinder walls during extended high rpm operation using 5w-50? Since at operating temp the oil has the viscosity of a 50w oil and the Mirage having only a 3.2 quart oil capacity is it possible that during high speed operation the oil return galleys may be designed for flow from lower viscosity oils (smaller diameter) which would cause the majority of oil to remain in the valve train starving the bottom end? I don't know, just conjecture on my part.

The oil viscosity is primarily based on bearing tolerance. Since bearing tolerance is so small we see manufacturers recommending different oils for climate. A 20 to a 30 weight change in a hot climate or boost situations like mine isn’t a terrible idea. But 20 to 50 weight where cold temps can be a factor is something i’d steer clear from myself

To clarify further if the oil is too thick it will not have sufficient flow between bearing tolerance. If it too thin it cannot support the load applied to the bearing and metal to metal contact happens. Either is a good way to catch an uber to the local repair shop.

[klroger]

I have been running Castrol Syntech 5-50 oil for over 30 years and this is the first car that I have had where I lost an engine. Now the other engines were either a rotary or they had cast iron blocks. This is my first all aluminum engine.

Maybe the technology over the last 30 years has changed??? Time to re-think??? & did they really have Synthetic 5w-50 oil 30 years ago???

[Mechanic]

Dirt,
. . . . This where a lot of people get confused about how multi-viscosity oils work. Remember the first number is what applies when the oil is cold. So when it is cold it pours like a 5 weight oil at that low temperature. It then pours like a 50 weight oil when the oil is at operating temperature. Otherwise the viscosity doesn't change as much as a 5-20 oil would that is subjected to the same temperature change.
GOD Bless and Thanks,
rich!

[Mechanic]

The Castrol Syntech of today is not the same as the Syntech of 30 years ago. It is better and it was very good 30 years ago. I have been using Castrol for almost 50 years. I used Castrol GTX 10-40 during the winter and 20-50 during the summer in my RX-3 until Syntech came out. I then used 5-50 all year long in my RX-3. So that would be over 35 years. When I got my 92 323 I used GTX in it for the first 25K miles and then I used the Syntech 5-50 until I sold the car with 325K miles in 2001. It still wasn't using oil between changes. I bought a Protégé. Again I used GTX in it for the first 25K miles and then went to the Syntech. When I traded it in on my first Mirage it had 147K miles on it and it didn't use any oil between changes.
GOD Bless and Thanks,
rich!

[DirtGearTv]

Dirt,
. . . . This where a lot of people get confused about how multi-viscosity oils work. Remember the first number is what applies when the oil is cold. So when it is cold it pours like a 5 weight oil at that low temperature. It then pours like a 50 weight oil when the oil is at operating temperature. Otherwise the viscosity doesn't change as much as a 5-20 oil would that is subjected to the same temperature change.
GOD Bless and Thanks,
rich!

May I just ask why are you using thicker oil when they call for 5-20? I know for a fact after just having mic’d these bearing specs a 50 weight oil is not going to lubricate the 3a92 bearings very well. They are tight tolerance made for 20weight

[Mechanic]

Dirt,
. . . . Because all of my education, training, and experience tell me that a 50 weight oil at 200F provides better bearing protection than a 20 weight oil at 200F. There is a lot of difference in viscosity between a 20 weight and a 50 weight oil at 200F. There is almost no difference in viscosity between a 5 weight and a 0 weight oil at 20F. What is the nominal clearance for the mains and rods in this engine? I couldn't find that number on the web. Is it 1 to 2 thousands?
GOD Bless and Thanks,
rich!