i was thinking of buying a 95 mazda v6 but the guy said the oil pressure was reading low after driving for a while any information on that would help? he also has had it parked for about a year and has not crunk it. any info on that?

 

Sitting is bad on a engine but not the worse thing. I just bought a truck that had been sitting for 2 years and it cranked right up, even with the 2 year old gas in it. The oil pressure could be several things, first the simple. It could be as simple as a bad sending unit, but not likely. More severe options, and the most likely ones, would be a worn out oil pump, worn rod and or main bearings or worn camshaft bearings. If you buy this truck I would expect to do a engine overhaul.

 

As an engine warms up the oil clearances increase, and the oil gets thinner, this could be the source of the decrease in oil pressure. If this is the case If I bought the vehicle I would drive it until the oil consumption became unbearable. I wouldn't drive it like it was a brand new engine of course. It's worn, take it easy.

 

I bought a 90 Cherokee with a similar problem. I went ahead and found a old engine and started slowing rebuilding it. The failing engine lasted a good 10k miles until I had the rebuilt engine ready. The engine still was not totally dead when I pulled it, but the bearings had started rattling pretty bad.

 

Do a diagnosis before spending the time and materials to do a rebuild. Check the pressure with a gauge after changing the oil and filter. The Vulcan is hard to break, if it is a 3.0, and the 4.0 is a pretty good engine too. As noted it could be the sending unit. It could also be the gauge, the electrical voltage regulator, or the condition of the alternator and battery. There was a problem with the 2.3 pan gasket material flaking off and plugging the oil pump pickup, but I haven't heard of any v6 problems like that.
tom

 

tomw could be onto something there. I also bought a 94 Grand-AM that was doing the same thing and it eventually lost all pressure. I was getting ready to replace the engine when I decieded to pull the oil pan to take a look, it was full of old sludge that had plugged up the pick-up tube. I cleaned the engine out with kerosene and a screw driver, replaced the oil pump and pickup for good measure and put it back together. I then proceeded to add another 60K miles to the odometer before I sold it to a high school kid, that drove it through college before he traded it off. What I had to do was not very expensive, a little time comsuming digging all the gunk out. It could be a hit or miss sitsuation.

 

g_k, the engine clearances actually close as the engine heats up. The oil does get thinner, to pretty close to the viscosity of water, when it gets hot.
What you do depends on your time/$ ratio. If you have a lot of the former and little of the latter, you can try the investigating the necessity of the de-gunking process. I would check wiring, sending unit, regulator and gauge functionality first.
tom

 

How do you know for sure the oil pressure is in fact low? Does it have a real gauge or is it the factory dummy? My understanding is that the dummy gauge only needs 8 psi to register in normal range. Many moons ago I installed a real under dash gauge and my 94 w/4.0L carries about 20-25psi at idle, running speed about 40-45psi.

 

Yep Check it with a real gauge. Unless some one has modified it it came from the factory with an idiot gauge. as soon as the pump makes 5-8 psi the gauge goes to half scale. I have seen these gauges fluctuate because of electrical issues, or the sender itself being bad.

 

wrong, better check with your thermodynamics teacher, I think that grade you got might be due for an adjustment. Holes expand when they get hotter, rod big ends are holes, crankshaft bores are holes.

Enough about holes.

 

g_k after I posted that --- I thought about it when going to sleep. BOTH expand. The hole itself may get bigger on the connecting rod, but the crankshaft will ALSO expand. The cylinder walls may get bigger, but the piston expands too.
The engineers that design the engine specify the bearing clearances and viscosity based upon the 'end state' desired clearance, the lubricant specified, the load placed on the product, the expected rpm, the BMEP, and how long they have to warranty their build. They have to allow for heavy pumping losses when the engine is cold, the oil very thick, and very hard to pump through the lubrication system. They have to allow clearances large enough to get the cooling effect of the oil, yet small enough to maintain a thin film of lubricant in shear. Then they have to allow for the expansion of the crankshaft and camshaft journals, the big end of the rods, the bearing inserts and the block and bottom end. The clearances change. But, not all get looser. To state that as a fact is just not right. Some get tighter. It all depends, as they say. The engineer tries to constrain the temperature range by using a coolant and a thermostat to regulate the operating temperature of the engine, but in reality, has to guesstimate what the real world will bring up. I have yet to see an engine with any miles on it that has maintained the original clearance specifications for the connecting rod and main bearings. They have all, each one, been out of tolerance, per design, but the engine did not knock or rattle. My equipment did not include a micrometer to measure journals, but the insert wear could be observe by inspection. They ain't spozta do that, as they are 'assumed a full pressure lubrication by means of the attached oil pump'. Kinda think engineers specify gravity, and 'make it so' pops into their head... whatever.
If you are old enough, you might remember the Chevy Vega. Aluminum block and cast iron head. Or the Honda Civic with the CVCC engine. Both had severe cylinder head leakage problems because of the different expansion rates of Al and Fe. Some number of pretty smart guys designed head gaskets that worked after the problems became apparent.
I don't think thermodynamics is the area of study that covers expansion and contraction of metal alloys. I think it has to do with energy, as in fuel and air in, combustion, how much heat is generated, how much is thrown off as useless waste, entropy, final states, etc. Metallurgy and properties of materials would be more appropriate topics to study when designing engines.
I do not claim, and never have, to be an engineer. Though, with a lot of products, I sure can see a lot if deficient designs and a lot of places to easily improve things. I have read some of the bound proceedings of the SAE that my dad brought home from work, such as the design of the Cadillac OHV V8 engine, the metal properties of flat tappet camshafts, and a whole lot more back when Packard was a going concern.
I never took thermodynamics. Did you?

tom

 

I took Thermo and passed it, barely.
Consider this, a long thin rod is heated, it expands, it expands more in the length than in the circumference. There's a reason for that.
Now consider a cube of the same material as the rod, it expands equally in all directions.

The thin rod expands more in the length because there are more sides of the equal sized cube in that direction.

Now take the long thin rod and bend it into a circle, heat it and it expands. The hole gets bigger.

The exact prediction of how the metal is going to expand is an intergration of the thermal expansion over the shape being heated.

That's a long story. "You're right, I'm wrong" would have sufficed.

 

fwiw i didn't take thermodynamics but i stayed at a holiday inn last night and found if a rod has been in the hole over and over numerous times the hole is definitely bigger than it should be in most cases.

 

That's what she said.

Holiday Express.