My car is a 1989 Volvo 740 GL 2.3 SOHC Red Block non turbo with 210,000 K on the odometer.

The Mann oil filter gets changed every 5000 Kms

Mobil 1 10/30 Synthetic oil gets changed every 20,000 Kms.

Two weeks ago a condition with my warning light started as if someone had suddenly turned on a switch. The oil light was suddenly staying on at idle and would only go out if the revs were increased to 1200 Rpm where it stayed out until the engine revs dropped back to idle. There was no prior warning of this condition.

However this condition would go away sometimes and the oil light acted as normal remaining out at idle. Then next time the car was used the condition would re-appear with the light being on while the car idled and going out again when revs increased.

I changed the oil - no change in behavior.

I changed the filter- no change in behavior.

I thought that perhaps the oil pressure sensor could be failing so I replaced it. No change in behavior.

Since the oil pump delivers the oil via a steel tube that is sealed to the pump and the block gallery at each end by O rings, I wondered if the problem could be the O rings which would mean removing the oil pan and replacing the O rings. I also thought that perhaps the oil pressure relief valve in the pump might not be re-seating itself properly allowing a pressure drop.

If I was to replace the O rings, I also ordered a new oil pump to ensure that the pressure would be the best it could be.

After all the work to install the pump – No change in behavior.

Today coming back from work the oil light was on while idling at intersection lights going out as soon as the revs rose above 750 RPM. Drove home at speeds of around 70 to 80 KPH for about 5 K after which the light behaved normally when stopping for lights etc. and did not come on while idling. Everything worked as normal.

So what I am wondering is: does anyone one have any ideas on what would be causing this intermittent condition?
I’m open to ideas as frankly it’s got me baffled.

Cheers,
Stephen

You could have sludge build up in the oil galleries restricting flow. How did the bottom end look when you had the pan off? Sloppy clearances will also kill pressure. Also, drop the oil when you change the filter. 20,000 km is way to long for service intervals in any engine IMHO.

Waaaaay too long an oil change interval. Changing the filter at 5000 km is a start, but the oil itself is well and truly done by that point.

What did the oil pan look like when you dropped it? Without a gauge, the only warning is the light, which operates just like a lightswitch. You could be at 6 psi normally, but one day it gets a bit hotter, drops to 5 psi, and your light comes on. However, my thought process mimics yours: oil pump, transfer tube and orings, fresh oil & filter, it SHOULD be good. Clogged oil galleries should increase pressure at the sender, not reduce it.

Too bad you didn't roll new bearings into the crank while you had the pan off, or at least remove a couple caps to see what they looked like.

Unless the gallery is so clogged that it sees no oil...then no pressure. My guess is that the bearing clearances may be large enough to dump the pressure.

UPDATE ON THE ISSUE

Hi all, the problem persisted until about three weeks ago when it suddenly disappeared. The warning light is behaving just as it did when I first bought the car. The light now goes out before the engine actually fires up when starting. If the revs dip down to 300 RPM as it does on some occasions the light does not come on.

The only thing I can think of is that it somehow got an air bubble in or near the pressure sender that did not allow a proper pressure reading as the air compressed.

So needless to say I'm a happy puppy.
Thanks all.
Stephen