96 V-8 GC Oil/pres gauge reads "0"

[dannah]

Help! My 96 v8 GC oil pressure gauge drops to goose egg when I come to a stop. Sending unit's been replaced. If I slip trans into neutral while slowing down sometimes prevents the o/p from reaching zero. O/P never drops for the first 10 minutes or so (while warming up). There's no knocking, leaks or anything indicating problems. The pressure has been tested and it was good. Other than burning 1 qt of oil every 1500 miles, she runs like a champ! I put 3-4k miles on her per month w/o any probs other than the "dings" from the check gauge indicator. Does ANYONE have a problem like this and if so, what was the fix? Thanks, and happy 4 wheelin'!

[murpht3]

dannah:

I doubt that the oil pressure is actually going to 0. If it was, that would mean that no oil is traveling around the engine and it would probably seize. If everything else is good, it might be your oil pump. But like I said, this seems unlikely. Does the engine jerk or anything when you notice the oil pressure drop? If not, I would say that the gauge or the wiring from the sending unit is faulty. If you were actually dropping to 0, your engine would likely crap out in short order!

[dannah]

No, no jerking. She purrs like a kitten. The gauge drops only when the engine is warm. If wiring was the case wouldn't the gauge drop more randomly (or gauge itself for that matter)? The sending unit had recently replaced too. Seems the "dinging" happens more frequently now too, only happens when stopped.

[JDPascal]

you say the pressure was tested and was good...

Did they test it cold AND at operating temperature.

Those are the first tests I would do. Get a Mechanical master gauge that has a range of 100 psi or less.. One that is used for fuel pressure will also work.

The reason the oil pressure drops to 0 when warm is that the oil is now thinner than when cold. It flows through the worn clearances much easier and the pressure is less.

two areas where there could be a problem is wear in the pump or wear in the bearings.

0 oil pressure will not cause the engine to seize quickly but it will cause extra wear on the moving parts.

Get the oil pressure checked cold and hot and drive the car while the gauge is hooked up to see what is actually happening then go from there.

JD

[dannah]

JD, Thanks. This makes a boatload of sense! I am POSITIVE the pressure wasn't checked while the Jeep was being driven... only while it sat idle in the shop.

[murpht3]

dannah, I have seen stranger things happen. The sending unit on my grand cherokee went and the gauge only fluctuated when I came to a stop or accelerated....times when the oil pressure is changing. This change must've been enough to make the gauge read either full scale or **zero** scale.

I would agree with JD that the oil is thinner when it is hot. However, in all my time working on cars I have never seen a car run to 0 psi without the engine seizing. 0 psi means the oil pump is not running, not that the oil is thin. Your oil pump should maintain a nominal pressure in the lubrication system, and this should be maintained through all driving conditions. Just because the oil is thinner doesn't mean the pressure will go to zero. For example, you could pressurize maple syrup (thick) in a container to 30 psi. If you then replace the syrup with water (thin) you could still pressurize that container to 30 psi. Again, just because the liquid is thinner doesn't mean it can't be pressurized.

If your oil pump is shutting off intermittently, this is a BAD problem. I would doubt this is the problem since you notice no change in engine performance. All of my bets would go to a problem with the gauge, sending unit, or associated wiring. But, this is just my opinion.

[mcscholz]

i had the same thing happen and spazed. the connector to your oil pressure sensor which is behind the distributor (V8) needs cleaning. clean it with a wire brush, put dielectric grease on it and put it back together, you'll be fine. I added a mechanical oil pressure gauge just to be safe, yet i have not had a problem with stock gauge since i cleaned terminal. advice, use synthetic oil so if you do lose oil pressure you won't trash the engine. with conventional oil, if you lose oil pressure for 5 seconds; your engine will be severely damaged.

[mcscholz]

when idle, if your oil pressure is zero, turn the car off. one of two things is certain. 1st you oil pressure is zero and the engine will seize. 2nd your gauge is broke. the difference between hot oil and cold oil should be no more than 25-30 pounds. if it is, you are using the very wrong weight oil or have major wear and or oil pump issues. be wary of what people say. fords (are junk) run as low as 9 psi, jeeps stay above 20, always. my jeep grand cherokee, v8, towed 8000lbs 1500 miles, with mobil synthetic and the oil pressure never went below 40 psi. it is 65 when cold.

[mcscholz]

this reply is obviously from an engine builder that wants your business. an engine can not run with less oil pressure than it takes to lift the weight of the crankshaft. a v8 (four pins) crank weighs about 40-45 pounds. the total area of 1/2 the main bearings circumference will determine the oil pressure at which failure is imminent. go to school and stop giving people bad advice!

Originally Posted by JDPascal

you say the pressure was tested and was good...

Did they test it cold AND at operating temperature.

Those are the first tests I would do. Get a Mechanical master gauge that has a range of 100 psi or less.. One that is used for fuel pressure will also work.

The reason the oil pressure drops to 0 when warm is that the oil is now thinner than when cold. It flows through the worn clearances much easier and the pressure is less.

two areas where there could be a problem is wear in the pump or wear in the bearings.

0 oil pressure will not cause the engine to seize quickly but it will cause extra wear on the moving parts.

Get the oil pressure checked cold and hot and drive the car while the gauge is hooked up to see what is actually happening then go from there.

JD

[JDPascal]

Originally Posted by mcscholz

i had the same thing happen and spazed. the connector to your oil pressure sensor which is behind the distributor (V8) needs cleaning. clean it with a wire brush, put dielectric grease on it and put it back together, you'll be fine. I added a mechanical oil pressure gauge just to be safe, yet i have not had a problem with stock gauge since i cleaned terminal. advice, use synthetic oil so if you do lose oil pressure you won't trash the engine. with conventional oil, if you lose oil pressure for 5 seconds; your engine will be severely damaged.

Are you saying that similar oil pressure for 5 seconds will not produce the same damage when using synthetic oil as you get with conventional oils???

What about 45 seconds?? or longer????

I've inspected engines that ran for over an hour at highway speeds with 0 oil pressure on the gauge. A pushrod had popped out of the rocker arm in a new Cadilac that came into the dealership I worked at at the time. That allowed the lifter to be thrown out of it's bore, opening the oil gallery to all flow from the pump. We inspected the bearings and NO damage as there was sufficient oil at the bearings for the load

That does not mean I recommed running with no oil pressure. By all means, shut down the engine and have it tested and repaired.

Engines do not seize instantly when they loose oil pressure. Yes damage can happen eventually. That varies with the circumstances. The type of oil does not have that much effect - synthetic or conventional.

Several car cruzes I've been to have had pools on how long it will take an engine to fail when all the oil and antifreeze have been drained out and the throttle is pinned wide open. Every time, the engine lasts at least 30 to 45 minutes before total failure. And YES there is damage happening continually while the engine is running under these conditions.

JD

[JDPascal]

Originally Posted by mcscholz

i had the same thing happen and spazed. the connector to your oil pressure sensor which is behind the distributor (V8) needs cleaning. clean it with a wire brush, put dielectric grease on it and put it back together, you'll be fine. I added a mechanical oil pressure gauge just to be safe, yet i have not had a problem with stock gauge since i cleaned terminal. advice, use synthetic oil so if you do lose oil pressure you won't trash the engine. with conventional oil, if you lose oil pressure for 5 seconds; your engine will be severely damaged.

It could well be the sensor or the wire to it or even something with the pcm, but the most important thing is to varify the actual oil pressure under hot and cold operating conditions and compare that to the activity on the gauge in the instrument cluster.

Once that is done, there is enough information to decide on the next step.

JD

[JDPascal]

Originally Posted by mcscholz

when idle, if your oil pressure is zero, turn the car off. one of two things is certain. 1st you oil pressure is zero and the engine will seize. 2nd your gauge is broke. the difference between hot oil and cold oil should be no more than 25-30 pounds. if it is, you are using the very wrong weight oil or have major wear and or oil pump issues. be wary of what people say. fords (are junk) run as low as 9 psi, jeeps stay above 20, always. my jeep grand cherokee, v8, towed 8000lbs 1500 miles, with mobil synthetic and the oil pressure never went below 40 psi. it is 65 when cold.

I agree with most every thing said here.

On the topic of engine seizure, see my posts above.

Fords are.......... well Fords. When times were tougher, I once ran a F150 for over 10000 miles with "0" hot oil pressure at idle and a max of 20psi at highway speeds. It was still in use long after I could afford to replace it.

My 96 jgc with 5.2 has similar numbers with mobil 1 and 120k miles too. Hope I don't have to use it till it gets like that old Ford.

Rule of thumb is a minimum of 10 psi for every 1000 rpm of engine speed.

JD

[JDPascal]

Originally Posted by mcscholz[SIZE=5

]this reply is obviously from an engine builder that wants your business. an engine can not run with less oil pressure than it takes to lift the weight of the crankshaft. a v8 (four pins) crank weighs about 40-45 pounds. the total area of 1/2 the main bearings circumference will determine the oil pressure at which failure is imminent.[/size] go to school and stop giving people bad advice!

First of all I am not in the business of engine rebuilding and I am not out for anyone’s business. Like most of the others who answer questions on here, my responses are based on a desire to help those struggling to maintain their vehicles in the face of limited finances and sometimes questionable shop practices.

The answers are not always as comprehensive as they could be and some times they are based on incorrect assumptions about related abilities and knowledge. my appologies for when that has or will happen again.

Second is that while it would seem that the crankshaft is supported by one half of the circumference, in reality only about 10 to 15 degrees of the circumference supports the most of the forces that the crankshaft carries. This is due to the force vectors acting at 90 degrees to the surface. Furthermore, the location of the area supporting the crankshaft and the forces it carries do not align with the center line of the engine or even the cylinders as it moves with the location of the piston in the bore and the angle of the connecting rod to the centerline of the crankshaft.

Third, the weight of the crankshaft has very little to do with determining the oil pressure observed on the oil pressure gauge as there are many other places for the oil to go if a crankshaft gallery is blocked. The actual pressure in the oil trapped in the main and rod bearings of a running engine can run into several thousand PSI depending on centrifugal and combustion forces. The crankshaft and rod bearings are actually supported by a wedge of oil as the surface of the crankshaft travels in relation to the bearing much like a boat planes on the surface of the water when the speed is great enough. This is called hydrodynamic lubrication and at low engine speeds it can be present with very low supply pressures.

The factors that determine the oil pressure in the engine, as observed on the oil pressure gauge, are the clearances in the engine bearings, the volume of oil supplied by the pump and last of all, the pressure regulator valve at the outlet of the pump.

The engine oil pump is designed and selected to supply more than the bearings will allow to pass when the engine is new. The pressure regulator valve in the outlet line from the pump allows the continuous flow of this excess oil back to the oil pan. If the regulator valve sticks closed, the pressure spike may result in an expanded or burst oil filter at about 200 to 250 psi.

As the engine and oil pump wear, the bearing clearances get wider and will allow more oil to pass through. As long as the pump is still capable of producing more oil than the clearances will allow to flow, the pressure will remain with in the range limited by the pressure regulating valve.

When wear increases clearances and decreases flow sufficiently, the regulating valve will no longer open to return oil to the pan. This happens first at idle speeds. At that time the oil pressure will be less than the specified range for that engine. The gauge might even read 0 at certain RPM’s. That does not mean that there is not any oil in the bearings. There may not be sufficient for the hydrodynamic wedge of oil to separate the moving surfaces. That is when what is called boundary lubrication occurs. Boundary lubrication allows some metal to metal contact and wear occurs at that point. Boundary lubrication also happens at engine startup - when the majority of engine wear occurs.

This is just a small introduction to some of the lubrication happenings in an engine. Sorry it turned into an epistle.

JD

[mcscholz]

you have some valid points. if you have hydraulic lifters, with zero oil pressure, and any oil available, your not going to get far. with solid lifters, with synthetic oil you can go quite a ways. my experience; had a honda 4 trax that leaked oil profusely due to cracked side cover. forgot to fill it with oil one day and rode it for 4 hours in 90 degree heat with no oil in it. I never knew the difference - this is why i like synthetic. other experience. rebuilt a honda motor (250xr) that had dropped a valve due to improper assembly. castrol syntech had been used in the motor. the problem i had trying to rebuild it was that i could not get the oil of any of the parts. brake cleaner and gasoline would not clean the parts enough to stop water (sweat) from beading. as far as how long an engine will go without oil. I have a friend who's engine we just rebuilt. 4.3 vortec. oil line to his oil cooler seperatated, his blazer's oil light came on and before he could turn it off, it seized. 2 mains spun. this was with castrol conventional. these are just my experiences. i am not an expert on oils but my experience leads me to believe that synthetics are worth every penny. ford emdo (engine manufacture and design) did a study on new york cabs with 250,000 miles on them. the ones with mobil syn showed no measurable wear after the 250k miles. with conventional, they showed 30 to 50% (of service life) wear.

Originally Posted by JDPascal

Are you saying that similar oil pressure for 5 seconds will not produce the same damage when using synthetic oil as you get with conventional oils???

What about 45 seconds?? or longer????

I've inspected engines that ran for over an hour at highway speeds with 0 oil pressure on the gauge. A pushrod had popped out of the rocker arm in a new Cadilac that came into the dealership I worked at at the time. That allowed the lifter to be thrown out of it's bore, opening the oil gallery to all flow from the pump. We inspected the bearings and NO damage as there was sufficient oil at the bearings for the load

That does not mean I recommed running with no oil pressure. By all means, shut down the engine and have it tested and repaired.

Engines do not seize instantly when they loose oil pressure. Yes damage can happen eventually. That varies with the circumstances. The type of oil does not have that much effect - synthetic or conventional.

Several car cruzes I've been to have had pools on how long it will take an engine to fail when all the oil and antifreeze have been drained out and the throttle is pinned wide open. Every time, the engine lasts at least 30 to 45 minutes before total failure. And YES there is damage happening continually while the engine is running under these conditions.

JD