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Oil pressure light at higher speeds - Eagle |
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AMCVT
AMC Apprentice Joined: Nov/18/2008 Location: Milwaukee, WI Status: Offline Points: 69 |
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Posted: Jun/03/2018 at 4:34pm |
So my Eagle has started something interesting.... The oil pressure light is coming on while on the highway, and it only happens after I've been on the highway for a while. Maybe after 10 miles or so. Once I slow down significantly or get off the highway the oil pressure light goes off immediately. Just driving around town it doesn't come on either, only when I'm on the highway. It's now done it three times in the last week.
I know the easy answer is "don't drive on the highway", but any thoughts? It's an 81 Eagle with the 258 and the auto transmission. The oil level is fine, engine temp is fine, and there are no sounds out of the ordinary.
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1976 Pacer DL
1981 Eagle Sundancer |
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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How many miles are on the engine? 258s are pretty "bullet-proof" but that doesn't mean something hasn't gone goofy or a previous owner didn't help it wear out somewhere.
You answered my first question - oil level is ok. Has the engine been worked on recently? Do you have access to a mechanical oil pressure gauge so you can check the oil pressure with a real gauge - to be sure the oil pressure switch isn't at fault?
If the engine has high miles I wonder if the pump isn't worn and not keeping up with the oil leakage past bearings at speed. It won't necessarily be noisy - my own Eagle's 258 would drop oil pressure below ten pounds at stop lights and it never made a tick or other sound. (but at 160,000 miles, I wasn't surprised at all) If you have any more information, such as what oil you are running, miles on the engine, how's the filter (some go bad and cause troubles) and can you access a real pressure gauge to screw in in place of the oil pressure switch and check the TRUE oil pressure........... (preferably with a long hose you can run into the car and watch the pressure as you drive it) |
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6881 |
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As with the V8 higher RPM ongoing theories/modifications ...
Your situation seems very similar to looking at the oil pressure bypass and more specifically the bypass spring. It could be weak over time and cycling. The other possibility is bearing wear , but unlikely if the light goes off at temperature and just lowering speed. You have not mentioned mileage, oil burn, recent oil grade change, filter change. A bad filter or lowering grade oil can cause this. |
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pacerman
Supporter of TheAMCForum Joined: Jul/03/2007 Location: United States Status: Offline Points: 9057 |
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Install an oil pressure gauge and don't guess. Don't depend solely on the oil pressure warning light. Joe
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Happiness is making something out of nothing.
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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Yup - that was among my first suggestions - don't make the engine health rely on an electric device that in itself could be faulty. Use a good mechanical gauge with hose and check for sure.
If after a highway drive things are ok with a REAL gauge, then we look elsewhere. This is what I mean by a real pressure gauge, with a long hose - so you can tell what the pressure REALLY is.......... Note the blue hose, and the gauge here. |
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19676 |
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For a quick check, just replace the oil light sending unit. It might be faulty, and they are pretty cheap. But you really need to put a mechanical gauge with a tube on it. You can put a T in where the light switch is and run the switch and a gauge. Gauge is pretty cheap too. Mounting it temporarily under hood won't help much, can't read it at highway speeds. Yours is an interesting problem. Usually the light comes on at idle because the pump isn't building enough pressure, as in Bill's case. I've rarely seen one come on at highway speeds and not also at least flash at idle. Could be an oil pump issue, the relief spring sticking, or even a partially clogged oil filter. When was the last time oil was changed? Might want to just change the filter even if it was recently changed... could be faulty if not clogged. Or just a faulty light switch.
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Frank Swygert
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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Let's investigate the theory of the pressure relief valve some have.............
Pressure occurs when you have volume working against a restriction. A pump puts out volume, not pressure. It moves fluid. If it is moving fluid through an open pipe without hindrance, no pressure is built but what it takes to shove it on its way. Now pinch the end of the pipe and the pump trying to move the same volume of fluid will be hitting that wall and pressure will build as the pump keeps shoving fluid into the pipe that now has a restriction. If the pump were strong enough and nothing broke, the pressure would be limited only by the physical strength of the pipe. Pressure relief is simply a valve or gate working with a calibrated spring. Let's say the spring was set for 70 pounds and the valve was exactly one square inch in area on the end - when the oil had 70 psi pressure there would be 70 pounds of FORCE pushing on that valve and thus the spring and things would be equal. Get to more than 70 psi and the oil pushing against the 1 square inch valve would be exerting over 70 pounds of force against the spring and the valve would move, opening up a hole. If the hole was enough to allow the pressure to drop, things would balance out. The pump at idle doesn't move enough fluid to move that valve as it can't build 70 psi - there's not enough volume against the restriction - it moves through the engine and builds less than 70 psi. I'm sure many have experienced a car with power steering and there's a bit of a problem with the idle situation and the engine speed isn't sufficient to spin the pump to put out enough VOLUME so it doesn't build PRESSURE and you find you can't turn the wheel or it's sluggish or jerky. That's a pump not putting out volume so it doesn't build the pressure needed to aid in steering. Back to the oil pump..... So at idle, you want that relief valve closed so the fluid isn't let out as if it was it would be like a leaking hose and oil would never get to the end - it would dump out the relief valve and not get to the last bearings, etc. If the relief valve is stuck open, at lower speeds, lower RPM, you lose enough volume through the stuck relief valve you have LOW pressure at idle. But rev it up and the pump can put out enough VOLUME that it can actually build pressure - you now have more volume, fill the pipes and build pressure. If the valve was stuck very open, you'd have no pressure at idle because the pump can't put out enough volume to fill the system and build pressure - volume against resistance, there's no resistance. If it was stuck a little open, you'd have lower pressure all around but a fast spinning pump may produce enough volume that you are still ok at higher speeds. My point is that a stuck open relief valve will impact you at idle speeds, but one the highway, you spin the pump fast enough to still have pressure. Volume builds pressure - try it - rig the valve so that it's partially open all the time, you'll see no or really low pressure at idle but spin the thing up and there's enough oil being pumped that some can leak past the valve and still have pressure - maybe LOWER pressure, but above the 8 to 10 psi needed for the light. At idle, however, you can't move the volume needed because it's bleeding out past the partially open valve. It's exactly like an engine with WORN BEARINGS - you see the light come on at stop lights, but rev it up and the pump can overcome the leaking bearings. The relief valve if stuck open is the same as bearings that are worn - it would let oil bleed out and not let pressure build because the pump at idle can't keep up, but rev it and it has enough volume to build pressure and shut the light out. I'd do what farna suggests, and it's a cheap test- CHANGE the filter. Buy a good new filter and swap that on. We've actually seen filters come apart inside. If the filter were plugged, the pump may push enough oil to keep the system filled at lower speeds but maybe enough oil can't get through the filter to satisfy an engine at higher rpm because there's more oil leaking past bearings, lifters, etc. I can see how a filter could in theory do this, but a stuck relief valve should do the opposite - lose pressure at idle. Again, also use a mechanical gauge to know for sure......... |
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vinny
Supporter of TheAMCForum Joined: Jan/05/2012 Location: Calgary Status: Offline Points: 2837 |
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It could be that the oil is not getting back to the pan fast enough or the valve seals are clogging the intake screen.
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akimmet
AMC Nut Joined: Aug/02/2012 Location: Republic OH Status: Offline Points: 428 |
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There may also be valve seal debris in the drain-back holes of the cylinder-head. |
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6881 |
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The bypass spring or pressure thread if reviewed in detail discussed the situation when the spring was weak, not broken or stuck open.
It acted as described above but it's pressure limit at higher RPM was too low. A weak spring that only puts out 40 PSI would be fine under lower RPM conditions. At higher RPM 40 PSI is not sufficient. If this bypass valve operates more like a relief valve (when it has a weak spring) it may open further under high flow conditions and actually reduce pressure with more flow. Many steam and hydraulic relief valves are purposefully designed like this and only re-seat at a given lower pressure. When setting these valves you set both opening and closing pressures. Agree, a pressure gauge is the best option for diagnosis.
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