Low Oil Pressure, hot idle, on 1989 420

[Cigar Havana]

On a 1989 420, using 5w50 synthetic, I noticed that when the engine temperature is 100 or more, usually after heavy stop and go city driving, while at a stop light in 'drive' the rpms are at 500, and the oil gauge indicates 0.5 bar. If I slip it into neutral, rpm goes to about 550-600, and oil gauge reads 1 bar. Is this normal?

When the temp. is around 80, at a stop, and in 'drive', the oil guage shows 1.5 bar. On start-up and speeds above 20, the oil gauge reads 3 bars.

The oil and filter change was about 1,500 miles ago. The filter was changed a second time after 1,200 miles, I thought the synthetic would remove a lot of dino crud since this is the first synthetic oil change. The oil is at the proper level.

I feel the oil guage should never show less than 1 bar so I'm concerned about 0.5 bar at a 500 rpm hot idle. What should I replace or check?

[RKreutzer]

Hi Cigar Havana,
a quote from my manual ('87 420), page 62:
The oil pressure at idle speed may drop to .3 bar (4.4psi) if the engine is at operating temp. This will not jeopardize its operational reliability. Pressure must, however, rise immediately upon acceleration.

Based on that, you are still ok. Was the oil pressure higher with dyno-oil?

Reinhard Kreutzer

PS: my idle is also 500 in D, but 650-700 in N or P. I try to keep the temp under 100 by shifting in 3rd when driving in the city to have the waterpump run faster. AC also works better at higher rpms. At stops I shift into N as recommended in manual to reduce heat from transmission. I can normally keep it at below 95.

[Duke2.6]

[i]

I thought the synthetic would remove a lot of dino crud since this is the first synthetic oil change.
[/B]

Where did you hear this myth?

Duke

[Cigar Havana]

I heard that here, from another posting. It was recommended by another member.

[psfred]

Any old hard sludge or varnish will be softened up and washed loose by synthetic oil, and the hard sludge in particular will end up in the filter.

If you don't believe me, take an older engine run on dino oil with long oil change intervals and pull the valve cover. Note condition, then change to Mobil 1 and run for 1000 miles and pull valve cover again. Will look like a new installation, and guess what happened to all varnish, etc?

This is a known event, not just gossip.

Peter

[Duke2.6]

Known event - my foot! How about referencing a legitimate piece of research, like an SAE paper.

Once sludge or oxidation deposits form on surfaces they are unlikely to be signficantly removed by any kind of motor oil. Synthetic-based SI engine oils have about the same detergent and dispersant additive concentrations as mineral-based SI engine oils, so a synthetic would have no more capability to remove hard deposits than a mineral based oil. A HD diesel engine oil (either mineral or synthetic base) might because they have a greater concentration of additives.

These additives are designed to hold oxidized oil molecules in suspension and keep them from depositing on engine surfaces, but once they do, the additives will remove very little.

Duke

[psfred]

Check with Mobil, this is a known phenomenon. Tests were done about 1970 or so.

Try it yourself, all the varnish will disappear in a dirty engine.

Synthetic base is a "detergent" by structure, not a hydrophobic mineral oil, and will disperse and dissolve considerably more materail than dino oil. Period.

If you don't believe me, go look up the data sheets and recomendations from Mobil or Amsoil.

Peter

[yhliem]

Originally posted by psfred
take an older engine run on dino oil with long oil change intervals and pull the valve cover.

this line right here invalidates the logic because oil changes should be performed every ~3000 miles.

REGARDLESS of what oil you use (dino or synthetic) oil changes should be scheduled frequently and regularly. if the oil changes had been done every 3000-4000 miles the engine should still look like new.

my 1987 2.3-16 has 90,000 miles on it (granted it's not a lot) and has seen all scheduled maintenance at the dealerships and oil changes every 3000 miles and there's NO trace of any sludge. furthermore, i only use conventional oil. no synthetics.

i'm not arguing the "detergent"-like properties of synthetic oils. i'm just stating that the comparison is flawed. synthetics are engineered to last longer than conventional oils and it's well known that conventional oils should be changed frequently.

if you're one of those lazy types when it comes to maintenance and routinely leave the oil in 5000 miles or more then you should use synthetics because they will maintain its ability to lubricate and protect engine components more than conventional oils at high mileage. but if you ARE changing your oil frequently then there's no point in using synthetics. not at 2x the cost.

[blackmercedes]

For those of living in the frozen-belt, the main advantage of synthetics is not extended change intervals, but cold temperature flow ability.

If I lived someplace warmer, I'd probably just use dino-juice and stick to my schedule.