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It is well known that ethylene glycol when mixed with motor oil will cause catastrophic engine failure. The mix coagulates to a brown-mud that fails to lubricate. But, will a Propylene Glycol / motor oil mix also cause a failure?

The few articles I have been able to find refer to glycol/oil mix. The ethylene seems to be implied. It appears the glycol reacts with the additives in the motor oil. At the micron level, this mixture is very abrasive. I am not finding any info on how Propylene Glycol reacts with engine oil.

I heated a Propylene Glycol/motor oil mixture to boiling. It never coagulated. When it cooled, the mixture separated. I repeated the same with ethylene glycol/oil mix. That mixture failed to coagulate as well. Again, when it cooled, the mixture separated.

Oil pumps generally run in the 20-30psi range. I doubt pressure is needed to cause the coagulation as the oil pan is typically full of mud when an engine fails.

I'm wondering why I can't get the mixtures to coagulate. Does anyone know if Propylene Glycol/motor oil coagulate?

The oil/coolant mixtures sat for a week. Both versions had separated. When agitated, the mixtures would quickly separate. The ethylene version showed signs of mud at the interface layer. The propylene version was very dark in color. Very close the same shade as the oil, but with no sign of mud. When the ethylene version was heated (not to boiling) and agitated, it quickly emulsified into thick mud. When it cooled the emulsion held. When the propylene version was heated and agitated it showed signs of emulsification but when the agitation was removed it would quickly separate. When cooled the separation was more pronounced. It did not coagulate the oil.

Both the EG and PG are a 50/50 mix with water. Update: A week later: The ethylene glycol (EG) is still fully emulsified. It is showing signs of separation but is still fully incorporated. The propylene glycol mixture has coagulated to mud. There is a layer of oil sitting on thick layer of mud with no sign of any unincorporated PG.

Both ethylene glycol and propylene glycol do coagulate when mixed with motor oil.

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    $\begingroup$ Motor oils are carefully designed mixtures that lubricate engines under operating conditions. Adding other ingredients is unlikely to improve their performance under those hot conditions in an operating engine. But the problem is unlikely to be illuminated by simply mixing the extra ingredients: you need to see how the new mix would function in an operating engine with all the friction and heat. Your experiment does not provide much insight into what happens in a real engine. $\endgroup$
    – matt_black
    Commented Mar 7, 2023 at 11:07
  • $\begingroup$ Have you tried intense mixing? Some non miscible liquids form quite stable gel-like mixtures, especially with air, if mixed really well and impurities are present. $\endgroup$
    – Poutnik
    Commented Mar 7, 2023 at 17:42
  • $\begingroup$ @Poutnik I had thoughts the activity in the oil pump may be causing an emulsification. As far as I can tell, once in the mud form, it no longer seperates. In the non-mud form, coolant and oil will seperate. I did vigorously agitate the boiling mixture, and got no change. $\endgroup$
    – Dug
    Commented Mar 9, 2023 at 18:28
  • $\begingroup$ M experience is the violent agitation of a running engine causes the thickening. At that time the engine is also pretty much ruined. $\endgroup$
    – blacksmith37
    Commented Mar 13, 2023 at 19:15

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