Low Oil Pressure....351ci

F

Fortunateson

Well-known member
I’m way more interested in my flatties, Y-block, and my 390 but I have a question about my ‘87 351. The gauge is showing low oil pressure... In searching for a solution the easy/quick ideas are change to a little thicker oil or change the sending unit. The sending unit is around $100 CDN. I don’t have time right now to be checking the oil pump so besides these two suggestions are there others? Kind of due for an oil change anyway but what is “thicker”? Currently I’m running 10w30. How do you check oil pressure?

I appreciate all good natured responses!
 
Remove the sending unit and install a mechanical oil pressure guage to test the accuracy of the original sending unit and dash guage.

Sal
 
351m or 351c or 351w. 351m has a history for low oil press, all mine did, bad crankshafts, with a lower end rattle.
 
The 351M/C oil pressure sender is located at the very end of the oil galleries above the last cam shaft bearing and gives low readings when the engine has some age and miles on it. If the oip pressure is really too low the lifters will start to rattle when first started.
 
M engines gone by 83 replaced by W, best to try mechanical gauge. At some point around late 80's and early 90's even though you had a gauge, low or high readings the sending unit was basically the same as a red warning light sender. If you had any pressure the gauge would default to a normal reading. If your sender looks like a half of a D cell battery with a cone at the wire end it's regular gauge sender. If it's the size of 5 nickels stacked you have the no real info sender. W engine sender is right above oil filter.
 
Yes it is a Windsor engine. And yes the sending unit is right above the oil filter, more or less and it looks like 1/2 of a D cell. So the 20-50 oil may help? I’ll see if I can borrow a pressure gauge to check what I currently have...

Thanks for all the replies!
 
Before getting too excited, hook up a mechanical gauge and see what you really have for oil pressure. Ford electric gauges are basically for looks, far from accurate.

If your engine is a pre 1994 351, then it has flat tappet lifters. I'd recommend a higher zinc content oil. Castrol markets 20W-50 high zinc in 5 qt. jugs, bought some on Amazon the other day, $24.28 a jug. (I'm not a spokesman for either of them)
 
That sounds like way too much for an oil pressure sending unit. Go to Rockauto and look it up. You should find it in the engine electrical section under switches.
 
My 63 Galaxie with 390 had the light come on. New sending unit didn't help. Put gauge on it, was low. Now it gets fun. Installed new oil pump, light still on. Pulled distributor, oil drive shaft was worn and slipping in oil pump drive. At least, I changed the motor mounts while I was under there.
 
Well I did purchase a sending unit from RockAuto and saved a bundle! However, it really didn’t change a thing so the cost really doesn't matter as it was inexpensive.

Now, I don’t really want to replace the pump at this stage, I’m presuming it may be the oil pump, so would 20w40 be a good choice to thicken up the oil to see if that helps? Would Marvel Mystery Oil be useful? I use the truck 2-3 times a month and don’t want to get too deep into it right now...
 
Back in the day, the first solution was a straight 30 wt oil, for low pressure or consumption. I guess it would depend on where you live and when you drive. 30 wt is pretty stiff at 0 degrees in a rig that hasn’t run in a month. Either way, watch the zinc content for a flat tappet engine. Those oils are out there. Unless it’s getting fully warmed up, I’m not sure a multi viscosity will help as much.

I’m not trying to start an oil discussion here.
 
The pump is the most lubricated part in the engine. Rarely the pump unless trash went through it. Check the bearings. If thicker oil raises pressure it's probably bearings. My Dad bought a used truck and the pressure was fine. PO didn't tell us the crankcase had more STP in it than oil. 2 oil changes later and no pressure. I pulled engine and replaced the shot bearings. It had great pressure after that and was on the road for many more years.
 
Th oil pump drive shaft is 5/16" on the 351W, it likely hasn't "rounded" out like the smaller 1/4" shaft of an FE or 302. It makes no sense to start tearing this thing down or even changing the oil viscosity until your actually check the pressure with a mechanical gauge. I mentioned earlier, Ford electric gauges are more of an ornament than accurate. Also, this thing sees very little use, it won't grenade if it drops to 20 psi at a hot idle
 
Well this has taken a bit of time... a fellow club member is going to lend me a oil pressure tester and then I’ll report back “with the rest of the story...”.

If the main and con rod bearings are a bit buggered can they be swapped out with the engine still in the truck? To me it seems doable.
 
Finally I tested the oil pressure with a mechanical gauge. At cold idle it was 35psi then dropped down to 30 then 25 and finally after warming up and choke kicked off it settled down at 5psi which I know is not good at all! A club member advised trying 15/40 or straight 30 oil and a new filter. I’m going to do that and then re-check.

We discussed the possibly new bearings route which I may have to try. Don’t really want to put a lot of money into a truck I only used a few times a month if that.
 
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What’s it show up at 1000 rpm or so? If the pressure comes up and you don’t hammer or lug it, it might go a long time in light usage.
 
I have a 351W in my 94 F250. I started it a couple yrs. ago and oil pressure gauge hardly moved off of L. I quickly shut it off and checked many posts on Google about the problem. I finely decided the easiest thing to do was install a new pressure sender. As soon as I did the oil gauge went up to just below H. The sender was the problem, the truck only had 100K miles.
 
Well this is what has been done...

A. Replaced sender and no change.


B. Test with mechanical gauge and got 35psi on cold startup which after went to 30, 25, and eventually 5psi.

On advice of an old time mechanic I changed oil to 15w40 along with new filter. Cold startup was 50psi but eventually dropped down to 7-8psi after about 20 minutes.


On the HAMB a fellow posted that after using Rislone zddp additive his pressure went up by 10psi which he really did t want. I’m no mechanic and I don’t believe in “rebuild in a can” remedies though I may try the Rislone. The fellow stated that it was quite thin and not the STP molasses kind of stuff.
 
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