ChevCorvette89

I am new at the Forum, although I have owned a 1989 C4 for 12 years. The car recently started showing very low oil pressure 3-4 psi at idle when hot, goes up to 20 or higher when driving.

My mechanic tested the oil pressure manually at 18 psi at idle when hot. Could the problem be the car's gauge? Any comments will be appreciated.

RF1

It is likely the sending unit.

vader86

Sending unit, which is right beside the switch and people do get them confused, and its right behind the distributor.

It can be the connector to the unit too, so you may have to splice a new one in.

ChevCorvette89

Thanks Guys

Sincerely appreciate the comments.

ChevCorvette89

Hi Guys - the sending unit has already been replaced last week. Further facts; this issue with low oil pressure at hot idle started very quickly, not gradual. Also at cold temperature; and when running over 2,000 rpm the pressure is fine.
Any comments will be appreciated.

WVZR-1

Was the oil viscosity and brand changed recently maybe?

ChevCorvette89

The oil was not changed before the pressure started dropping. It has been changed since, to 20W50.

ChevCorvette89

Summary:
- For 12 years this car has always read oil pressure at 540 kpa / 78 psi. Then suddenly a few months ago it started dropping to 4 psi at warm idle. Installed a new sending unit and filled up with synthetic 20W50. Still the same.
- At cold temperature; and when running over 2,000 rpm the pressure is between 20 - 44 psi. It does seem to fluctuate a lot.
- My mechanic tested the oil pressure manually at 18 psi at idle when hot. Could the gauge be faulty?

I need to sell the car, as we are down-sizing, but do not want to sell in this condition.
Any ideas?

ChevCorvette89

Forgot to mention that the car has done only 110,000 km.

Whaleman

Did you put 20w-50 in to try to raise the pressure? Dan

ChevCorvette89

Yes Dan. Will replace the oil pump; hopefully that will solve it.

RF1

You may find that the oil pump pick up cracked, or fell out.
Oil pumps themselves rarely fail.

ChevCorvette89

Thanks - I will get the mechanic to look at it tomorrow. Appreciate it.

Whaleman

Just a thought. Oil pumps are designed for the weight of oil being pumped. It was designed for 5w-30. It is not impossible that it is cavitating with the thicker oil. Was it low pressure with thinner oil? Dan

ChevCorvette89

Yes Dan. No difference in oil pressure between the 2 oils.

Whaleman

If the sender was reading correctly I would expect some difference in pressure on cold startup with the two oils unless the oil filter bypass was broken or weak and allowing all the oil to skip the filter. Dan

ChevCorvette89

Hi Guys. Short summary: the sending unit and oil pump were replaced. Still the same issue with pressure dropping to 3 psi at hot idle and then back to +32 psi at acceleration. Could it be the bearings going? Appreciate your responses.

Tom400CFI

Doesn't seem likely that low oil pressure developed suddenly, due to bearings....but you've also got no bearing noise? Possible, doesn't seem likely. Talk to us about this:

It seems clear to ME, that you've actually got ~20 PSI, hot/idle....but you've got a gauge in the car, showing 3-5 pounds. Seems like a gauge, to me. I'd probably get a "3rd opinion"...another mechanical gauge, to confirm which of your two readings are more accurate.

ChevCorvette89

Thanks for the comments. I will test the oil pressure with another mechanical gauge.

Gale Banks 80'

Keep in mind there are two sending units at the Distributor. You might of replaced the wrong one. I thought way back in your post that the Oil pressure was 18 psi at Idle despite what the gauge sez.