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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 05-11-2010   #1 (permalink)
hka9394 is offline Rookie

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Default New oil pump, low oil pressure 1966 289 4V

Hello, we have been cleaning and restoring our '66. The old oil pan was dented badly so decided to change it. While the pan was off, we decided to swap the oil pump as well since we have only owned the car a year and don't know the history. We swapped hoses, transmission lines, cleaned up power steering and all components, cleaned the distributor, changed transmission fluid, radiator fluid, and went from 15W 50 oil to 10W 30. Finally found TDC last night and started the car. When sitting at idle, the oil pressure was a little less than halfway across the guage. After the engine warmed up and we took it for a spin, noticed that the oil pressure was almost down to the first hash mark of Low...not quite in the warning area, but almost. Before we made all the above changes, the oil pressure was about halfway between low and high when running. Could the grade of oil really make such a difference in pressure, or could our new oil pump be bad, or what else could be going on? BTW the car still runs very hot...between M and P on the temp guage when warmed up. Any ideas of where to start looking (with the least amount of taking apart) would be appreciated.
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Unread 05-11-2010   #2 (permalink)
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The 10W-30 might give slightly less oil pressure than a 15W-50, or then maybe not much. Neither the stock oil, nor temperature gauges are all that accurate; they are more for relative indication only; i.e. is it the same reading as yesterday. So unless you measure pressure and temp with something else you don't know if you have any problems or not.

Stock oil pressure should be 55 psi at 2000 RPM when hot although Ford allowed weasel room for warranty purposes and said it could be as low as 35 psi. 55 psi can be nearly anywhere on a stock oil pressure gauge so without a true gauge hooked up you don't know what you have. Warm oil pressure will always be lower at idle than at 2000 RPM. That will be difficult to see in normal driving using a stock gauge since the gauge takes a minute or so for the needle to move down and by that time you usually are no longer idling the engine.

Same with the temperature. The needle was supposed to be about in the center of the gauge but it may be anywhere on the gauge depending upon several factors. The best way to measure temp is with an IR gun looking at the back side of the thermostat housing. The coolant was supposed to run in the 190-200F range when new. The very top end of the gauge was usually around 250F where pressurized antifreeze boils. 200F is not all that hot, it was designed to run that way. M or P on your gauge could mean anything from 150F up so is truly useless without other data.

New senders for either the stock oil pressure (usually read low) or temperature (usually read high) gauges are notoriously inaccurate so a new one of either will only give you more confusion instead of answering your questions.

Measure both pressure and temp with something accurate and then see if you still have a concern.

BTW: Oil pumps are usually replaced at overhaul time since they are cheap. Rarely is the pump the cause of low oil pressure; usually its worn bearings.
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Unread 05-11-2010   #3 (permalink)
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Thanks so much, ivy66gt. Will check both of those, and can borrow an IR gun so will do that.
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