Fluctuating Oil Pressure

Hi,

My engine has developed an odd oil pressure issue since changing the rear main seal and oil pan gasket.

- When at a stop light after heavy load (freeway at 65-70 for 15min), my oil pressure guage drops very low (almost like turning the car off). it goes back up once I start driving and will happen for a few additional stops before returning to normal.
- This only happens when it is warm outside 80F+
- Shifting into neutral at the stop keeps the oil pressure at normal levels.
- If I stay on the side roads the pressure is fine.
- I have only verified this with the guage on the dash, I don't have another gauge yet.
- Car is running on 10w-40 conventional oil.
- Oil is filed to the full line on the dipstick.

I am not sure where to look for this issue, but I am guessing the oil pump. Does anyone have any other ideas? Should I use 10w-30? I am planning on going back in there to fix the rear main seal next weekend so I would appreciate any suggestions that I should look out for.

Thanks.

Oil pressure will go up and down with the engine RPM. So how low is it? Normal oil pressure at cruse is 35 to 55 PSI. If you don't have that and it's a high mileage engine you might try some 20 W 50 in the hotter weather. If you do plan to install a new oil pump you could also slip in some new bearings if yours have a lot of miles on them.

I do not have an RPM guage. The guage I have is the stock dash one and the needle goes from about 1/4 from the low mark (safe zone) to the very left of the L if I let it. The engine was rebullt about 12 years ago and has a little over 90k miles.

Do you think the oil weight will help this problem? I know it will cause more pressure overall, but I am worried about why it is in the safe zone on the freeway, but danger zone once I stop after getting off.

Those gauges are sometimes not so accurate, are any other gauges acting funny? If so might be time for a new voltage regular for the gauges or if its only the oil the gauge sender unit could be going bad (at engine block) also check that wiring connection at the sender they can be loose and cause intermittent operation you can tighten that up with a pliers. Or you could think about getting aftermarket oil gauge or a small triple gauge set there are some that are low priced. The heavier oil will help some, after that other things would require opening up the engine to check.

Thanks, I plan to keep the original gauge in the dash and if I can't figure this out I'll buy a cheap gauge and run it temporarily. I am going to tighten up the sender and it's wires to see if it improves things. It is odd that I can reproduce it under certain conditions and that it is not random.

I will also be going into the engine soon for the rear main seal, anything I should look out for? I will check the oil pump screen for any clog, but I was just in there a few weeks ago and it looked clear.

Might be worth pulling a couple main and rod caps to take a look at condition of the bearings

Update:

I replaced the oil sender, after checking the wires, and the problem still exists. The rest of the gauges are acting normal so I don't think it is the voltage regulator. This started happening after I replaced the rear main seal so I think the bearing is somehow misaligned (even though I didn't take it off). I am going in there this weekend to fix the rear main seal that continues to leaks and I will check this bearing.

When I was in the engine this past weekend, I thought the oil pan may have been sitting to close to the oil pickup (was dented). I took a rubber mallet to the dents and made as much space as possible. Unfortunately, this was not the reason for the dropping oil pressure.

Now that the engine is back together, the mechanical oil gauge is the next step. If this gauge also shows the problem, where do I look? I am guessing either the oil pump or main bearings. Also, would the oil pressure light switch cause an issue? There are no lights on my dash for oil pressure, but this is hooked up to a wire.

As for more details, it looks like the gauge is shorting out when keeping it in drive or reverse at stops and this only happens after the engine is warmed up. The needle on the gauge cluster will drop to the end if I let it.

I hooked up a mechanical gauge and the same thing happens. Stopped in drive idles at less that 10psi. I also tried 20w-50 oil with no noticeable improvement. I am down to the main bearings and oil pump. I am thinking it is the main bearings as the rear one was scratched up quite a bit when I checked it this past weekend. Can these be changed with the engine still in the car? Does anyone think that this may be caused by the oil pump?

Yes you can put new main and rod bearings in the car but also put in a new oil pump with them. However if there's any deep groves in the old bearings how about the crankshaft condition are there also scratches? If so than at that point your probably better off pulling it out and doing a full bottom end rebuild.

I looked at the crankshaft briefly and did not see any scratches. Do I have to remove the crankshaft to access the top part of the main bearings? They don't just slide out like the rear main seal right? I will also have to read up on how to measure them so I know which to buy.

You think a rookie could do this or should I take it somewhere?

I looked at the crankshaft briefly and did not see any scratches. Do I have to remove the crankshaft to access the top part of the main bearings?

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No if careful (don't nick the crank you can use a small drift pin punch or a flat blade screwdriver to drive the bearing round enough so the bearing tang is released (only one direction will work). Install new bearing in the opposite direction until the tang is seated. Did you see copper color across the bearing shell? That would be a sign that bearings are worn badly and the clearance maybe excessive.

You think a rookie could do this or should I take it somewhere?

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Yes if you take it apart than its not all that hard a job to do and if you work carefully you should have no trouble. I don't think it would very easy to find a shop that would do the job like that so might be expensive. Leave the front an rear main to hold up the crank loosely. Than after you get those uppers in replace a few caps and than take down the remaining mains. Rods are very easy to do cause you can move the rod up some to pop an upper bearing out only thing be careful not to push rod up when a cylinder is at TDC or you could maybe get a top ring out of the bore. On bearings if you look at the back side you will see the size an oval with FoMoCo in it that means its never been apart, if it's been rebuilt before it could have Std. (this is the stock size) or an undersize from .010, .020, .030, and maybe even .040 Good luck

The one main bearing I pulled is scored up, but the rod bearing looks OK. Do you recommend I keep the rod bearings if they all look this way?



Both are FoMoCo bearings, but the engine was rebuilt (firing order different). Either the re-builder kept the same bearings or replaced it with old stock. Anyway, the std main bearings that I need are part number 4290M correct?

67-Conv-200":ljhi2nyq said:

The one main bearing I pulled is scored up, but the rod bearing looks OK.

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bubba22349":ljhi2nyq said:

Did you see copper color across the bearing shell? That would be a sign that bearings are worn badly and the clearance maybe excessive.

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Those bearings are shot. Replace them all, if the crank is still smooth and round you could be successful.
Replace the mains first as they are the hardest and if you mess up you won't lose any work you did on the rods.
Be sure to use plasi gage to check cleareness. If successful be sure to run the oil pump to prime the bearings before start up.

You might want to replace the oil pump just to be sure.

With so much wear on the bearings is the engine a candidate for a complete rebuild?

X2 on those rod bearing's they are long gone

Is 4290M the part number for the std main bearings (eBay #370470058359) and 2380CP for the std rod bearings (eBay #251040277484). I have to order and wait impatiently. Ordering from Napa would take longer than eBay.

I'll have to check the crank better to make sure it is smooth after I remove all the bearings. The previous owner told me the engine was rebuilt around 2000 (90k miles ago), but apparently they either kept the bearings or used old stock. The car runs nice and strong (before this oil pressure problem), so I don't think it is time for a rebuild yet.

Any thing I should be aware of before I put them in? Do I dunk the bearings in oil? I will obviously research more and read the Ford manual before I put the new bearings in.

Those would be the right part numbers on the rods. They would also be the right numbers on the mains if your engine is a 1965 to 1975 block so check the block casting number if you are unsure 1967 car engines will have a C6 or C7 casting number but even if its been swapped out for a newer block up to a D5 they could work. To do a simple check on the crank run your finger nail across the journals if it catches on anything or feels rough you should have it micro polished if it's minor and still mic's in size spec or have it reground or swap it out for a crank kit IE includes a reground crank and new bearings. No big deal in putting them in but be sure the bearing journal is clean wipe it off so there is no git oil etc. before install of a bearing than set bearing being careful to start the tang into its cut out side it in. When installed the bearing half sits flush with the blocks main or the rod cap. Use some assembly lube, White grease or at lease some clean oil on the top of each bearing before you bolt the caps back on and torque to the proper specs, you should also prelube the oil pump and for best results prelube the engine before starting using a electric drill or speed handle so you have oil pressure and turning the engine by hand a 1/4 turn at a time until you have at least 2 revolutions. Good luck

Thank you. Engine is a C6 and looks to be the original one (date code is 7B and car was built in March 67).

I have ordered both bearing sets, hopefully they are here by next weekend and I won't have any trouble or need to refinish / replace the crankshaft.

I received the new bearings and on the new ones the tops look like the bottoms. Meaning there is a hole on the bottom bearing while there was no hole on the bottom bearings on the old set. Will this matter?

Also the since the new bearings look the same, how can I tell the bottom from the top, or does it matter? Looking at the part numbers, half have HO while the other have BW above STD. I am guessing the HO is the top while BW is the bottom. Is this correct?

I received the new bearings and on the new ones the tops look like the bottoms. Meaning there is a hole on the bottom bearing while there was no hole on the bottom bearings on the old set. Will this matter?

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That's OK it won't matter!

Also the since the new bearings look the same, how can I tell the bottom from the top, or does it matter?

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It won't matter!

Looking at the part numbers, half have HO while the other have BW above STD. I am guessing the HO is the top while BW is the bottom. Is this correct?

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Again it won't matter what's is important is that the oil galley passage in the block and the hole in the main bearing line up so it gets oil to the bearings and that the clearance is within specs. Be sure to prelube that the new oil pump and the engine after assembly before starting it. Use a good quality oil filter that has a anti drain back valve too like Mortorcraft or NAPA / Wix to keep those new bearings from getting worn again on a dry start. Good luck