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Oil pressure variation


peter88's Avatar
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08-10-04, 05:25 PM   #1  
peter88
Oil pressure variation

A 1978 F150 pickup, 302 engine. When oil is cold, there is no pressure light present. After warm up, at an idle, the light flickers constantly. I've flushed the engine oil but still no luck. Is there any other potential cause other than an oil pump? I would have thought the pump would work easier when the oil in hot and thin rather than thick and cold. Your thoughts would be greatly appreciated before I pull it apart for the wrong reason.

 
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08-10-04, 06:21 PM   #2  
Johnc280
Think of it this way....the oil pump is trying to push oil through a bunch of tiny passages (bearings, pushrods, filter etc.). It will build up a lot more pressure trying to push the cold thick oil through than it will hot thin oil. There are two reasons you are losing oil pressure when the engine warms up.

1. The oil pump is wearing out and not functioning as well as it used to. Still well enough to provide pressure with cold oil but not well enough to keep up once the oil heats up and gets thin.

2. The bearings in the engine (mains and crank mostly) are getting worn and allowing too much oil to "flow through" and not allowing pressure to build up.

You have a couple of options depening how much you like the truck and how much money you want to put in it:

1. Replace the engine (obviously the most expensive but will certainly give you the best results)

2. Replace the oil pump and bearings. (cheaper than a whole engine but you could still have problems with other components)

3. Use a thicker oil. If you just want to get the most you can out of the engine you have then try using a much thicker oil such as 20w50. You can even use additives such as STP.

Hope that helps,

Johnny

 
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08-11-04, 05:32 AM   #3  
you don't say...

...whether or not you have verified the actual low pressure condition. first before you tear down anything, buy, beg, borrow a mechanical oil pressure guage and check your oil pressure with it. it's possible that you have a sending unit going bad or some other electrical anomaly. always double check with a mechanical guage.

is the engine noisy at all when hot and idling? noise (lifter or bearing) also indicates a real low pressure situation.

 
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08-11-04, 05:02 PM   #4  
peter88
Thanks for the help. I think I'll try thicker oil.
Posted By: Johnc280 Think of it this way....the oil pump is trying to push oil through a bunch of tiny passages (bearings, pushrods, filter etc.). It will build up a lot more pressure trying to push the cold thick oil through than it will hot thin oil. There are two reasons you are losing oil pressure when the engine warms up.

1. The oil pump is wearing out and not functioning as well as it used to. Still well enough to provide pressure with cold oil but not well enough to keep up once the oil heats up and gets thin.

2. The bearings in the engine (mains and crank mostly) are getting worn and allowing too much oil to "flow through" and not allowing pressure to build up.

You have a couple of options depening how much you like the truck and how much money you want to put in it:

1. Replace the engine (obviously the most expensive but will certainly give you the best results)

2. Replace the oil pump and bearings. (cheaper than a whole engine but you could still have problems with other components)

3. Use a thicker oil. If you just want to get the most you can out of the engine you have then try using a much thicker oil such as 20w50. You can even use additives such as STP.

Hope that helps,

Johnny

 
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