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oil light flickers low rpm

24K views 15 replies 4 participants last post by  tommy jingles 
#1 ·
swapped out engine from my old wreck. engine is 98 outback, 150k on it. Did head job at 100k, timing belt and water pump done before install. Engine from wreck ran strong before install. On freeway no signs of any trouble. A few miles off the freeway and the temp starts to climb to 3/4 to top. At stops the oil light flickers , goes off with gas peddle. When i pull the oil dipstick i hear a gurgling sound(seems like suction). Replaced pcv valve,fixed dent in oil pan NO luck. Any ideas aside suicide?
 
#2 ·
Possibly 2 separate problems here but, I think the dented pan 'may' have compromised the oil pick-up tube. The overheating problem 'could' be almost any of the normal culprits. let's hope it isn't the HG again and maybe you just used a non-Subaru t'stat or you have a bad rad cap or something.
 
#3 ·
sounds right on. What do you think about the gurgling sound coming from oil dipstick. Seems to be sucking air (you can hear it)when you pull the dip stick while she's running. When the engine is off it gurgles. Also of some important note, when the oil light is flickering, and you pull the dipstick (relieving the pressure) oil light goes out for a little while.
 
#4 ·
that's weird but might make sense if the tube is cracked. Aspirating air somehow. Some WRX owners are finding cracked pickup tubes and there's a guy making aftermarket ones. You could search NASIOC for that info. But a Soob one should be fine if that's what's going on with yours. Probably won't know till you get your eyes on it again. I've only read about it, but if I ever have the pan off my WRX I might upgrade my pickup tube.
 
#5 · (Edited)
Vacuum in the crankcase?

Double check that the crankcase fresh air intake is working. Not sure about the location on the 98, but based on my 07, there should be a hose off a valve cover that goes to the air intake duct, somewhere after the engine air intake filter but before the throttle.

Also check the placement of the PCV valve. It should be controlling the level of vacuum in the crankcase.

If there's unusual air draw into the crankcase through the oil dipstick tube, there's an unusual vacuum in the crankcase, which I don't think is correct.

Could try the same thing with the oil filler cap -- loosen it and see if the oil pressure light stops flickering.
 
#6 ·
oil light flickers at low rpm

inspected oil pick up tube no damage there. replaced pick up tube gasket. no change better or worse. air intake seems good, having a friend check pcv system today. Also told him to check the coolant for emissions see if the head gaskets are going. if so could that cause crank case pressure? This is all kinda making me feel like I am sitting in hells waiting room with this engine.
 
#8 ·
inspected oil pick up tube no damage there. replaced pick up tube gasket. no change better or worse. air intake seems good, having a friend check pcv system today. Also told him to check the coolant for emissions see if the head gaskets are going. if so could that cause crank case pressure? This is all kinda making me feel like I am sitting in hells waiting room with this engine.
surprising given the symptoms and the dented pan.

At least that's been eliminated. I suppose you've triple checked all the hose routing?

Could the oil pressure regulator be jammed open or missing it's spring? (I had that happen once on a Honda, but it was stuck closed - blew out the filter gasket!)

Can a HG leak to an oil gallery? maybe check the plugs to see if one cylinder looks like it is sucking in oil? weird
 
#7 ·
I have a similar issue. I have low oil pressure at low RPM. Heavy weight only helps marginally at best. After driving it for two years mostly freeway (high RPM) I have come to realize I probably have a leaking oil seperator plate. There is a lot of oil leaking onto the exhast. Replacing it requires removing the engine. Not sure about the sucking noise, maybe that is your wallet :p I would have guessed a broken pickup tube, but you eliminated that.

Also, Subarus are known for overheating after you add coolant. Just add antifreeze, squeeze the upper radiator hose until you see bubbles in the radiator (with the cap off), add more coolant, repeat...
 
#9 ·
ahh haa

my buddy who swapped the engines calls me today and says the pcv system was misconfigured(hoses in wrong spots). Says no more crank case vacuum. Still runs hot, putting OEM thermostat in tomorrow. Says the top radiator hose gets hot, but the bottom one not so much. Let y'all know how she drives tomorrow. Best regards JINGLES
 
#10 ·
my buddy who swapped the engines calls me today and says the pcv system was misconfigured(hoses in wrong spots). Says no more crank case vacuum. Still runs hot, putting OEM thermostat in tomorrow. Says the top radiator hose gets hot, but the bottom one not so much. Let y'all know how she drives tomorrow. Best regards JINGLES
wow.

Wonder if you could need a good throttlebody cleaning now?

at least there's progress. good luck.
 
#11 ·
Says no more crank case vacuum.
The way it should be. Interesting to see how the incorrect build up of vacuum in the crankcase prevented the pump from being able to maintain sufficient flow and pressure. Shows how problems in one area can often be due to something quite different, and when both explaining symptoms and diagnosing, every detail, however remote and unlikely it might appear, can be important.

Look forward to hearing the result after the thermostat is replaced with a Subaru unit.
 
#12 ·
all good

drove her around today and all is good, temp stays perfect doesn't move. Gonna take her up to lake tahoe after new year. Really have to say it's been nice having ya'll in my corner. Thought I had seen it all after putting nearly half a million miles on my 71' VW microbus. Ya know like having to pull your engine in a Dennys parking lot in east who knows where, just to fix the fan housing. When I let go of the bus I swore I was done with doing my own wrenching, but I guess if you want anything done right you have to do it yourself. I also used to think I was going to retire at 30, ha guess I will be dusting off my box. best regards Jingles
 
#13 ·
Great. Enjoy the ride.

Incidentally, do you know what hoses were mis-configured and how they were in the wrong spot? Just wondering if there are some that are relatively easy to install incorrectly. Including, in the thread, what was incorrectly installed might help someone else in future having similar, low pump pressure, vacuum in the crankcase, symptoms.
 
#14 ·
^^^that

I'm guessing now that the gurgling was actually airflow into the dipstick tube and exiting into the oil in the pan. But, I don't quite get how the vacuum could lower the oil pressure. I suppose it could be a false reading without full atmospheric pressure 'on top' of the oil 'column'...? That is, the pressure switch can only compare to atmo press on it's 'dead' side so, actual oil flow may never have been poor - just a 'fooled' press switch?

weird
 
#15 ·
the pressure switch can only compare to atmo press on it's 'dead' side so, actual oil flow may never have been poor - just a 'fooled' press switch?
That might well be the case.

Looking at the attached diagram of a typical Subaru pcv system, it's apparent that the crankcase should be at or close to atmospheric pressure because of the air inlet tubes going to the valve covers. The pcv valve, being connected to the intake manifold vacuum, draws fumes from the crankcase, but this is compensated by air coming in the inlet tubes.

But if the inlet tubes were, in an unlikely event, connected to the manifold vacuum, and the pcv valve hose connected to the engine air intake duct, the vacuum in the crankcase would hold the pcv valve closed. With it closed, and the oil fill tube and dipstick tube more or less sealed by their o-rings, there's no way for sufficient air to get in to compensate, and the crankcase vacuum builds up.

The vacuum in the crankcase operates on the overall lubrication system (it being more or less closed in this case), and therefore the pump works within this system. The pump might well be circulating sufficient oil, but as you note, the oil pressure switch measures pressure relative to atmosphere (which is normally where the crankcase is assumed to be), and as the oil lubrication system overall is at a relative vacuum, the switch senses a low pressure relative to atmosphere.

There might be some other combination of mis-connected hoses that would give a similar result. Consequently the interest in finding out how the hoses had been connected in this case.

Incidentally, and as you indirectly alluded to, had the oil pressure switch measured pressure between the crankcase and the outlet of the pump, it might not have shown a low level at idle, despite the vacuum in the crankcase.
 

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#16 ·
misconfigured pcv system

My buddy who did the install, owns a general auto repair business. He does my work at cost, in return I refer him quite a bit of business. I never really expect a whole lot.He told me that the pcv system hoses were not routed through the air filter. When I picked up the car, there were some customers around and I couldn't get an exact explanation. He was a bit embarassed. When or if I get the info I will post. I did notice before I dropped the car off, that there was an outlet on the airbox with nothing attached. I'm not really sure why I never checked the hose config. Thanks best regards Jingles
 
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