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Oil pressure loss SB Chevy

28K views 30 replies 14 participants last post by  tresi 
#1 ·
Hello, I am new to this board. I've been lurking for a while now, but now I ran into a problem and decided to ask this board for help. I tried the search function and searched the Knowledge Base, but did not find any clues.

I just finished building a 383 cid small block Chevy. The engine has a '72 350 cid block (.030" stress bored), an Eagle cast crank, 6" Eagle I-beam rods, KB pistons etc. Everything is practically new except for the block.

I started the engine for the first time on Saturday, and it seemed to run fine. I have a new Melling M55HV oil pump (with the heavier spring), and the oil pressure was 60 psi at idle when cold and 40-45 psi at idle when warm, rising to about 60-65 when driving. I drove it around for a while and everything seemed fine. The oil in the engine is a mixture of two mineral oils, some 20W-50 and some 10W-40. I thought I'd break the engine in and then change to synthetic.

Today I took the car out, having driven only a couple of miles on the weekend b/c the hood was still off the car. I went to a test drive with a friend, and while warming the car up the oil pressure was a constant 60-65 psi. When the engine had warmed up, I took it to around 4500 rpm on 2nd gear, and when I changed to 3rd I noticed the oil pressure gauge falling rapidly. I let off the gas pedal, and the oil pressure stopped falling at 20 psi and stayed there. I idled back to the garage (1/2 mile), the gauge at 20 psi all the time and shut the engine off.

We opened the hood and checked for visible leaks. None were to be seen. I checked the fittings on the oil pressure gauge, an Auto Meter mechanical unit, and they were fine. I started the engine again, and now the pressure rose slowly to 40 psi at idle and stayed there. I tried 2500 rpm and the gauge stayed at 40 psi and then fell slightly. The engine idled and ran fine, with no extra mechanical sounds, but the pressure was down.

I have an extra gauge, so I decided to try that. I put the other gauge on, with a different piece of copper tube, and started the engine. Same results: slowly to 40 psi at idle and a slight drop when at 2500 rpm. Again the engine seemed fine, with no extra mechanical sounds.

I took the car in, drained the oil: it seemed fine. No metal shavings were visible, even after pouring the oil through a paint filter. The oil had a strange, bronze-ish color but otherwise it seemed fine.

I'm at a loss. What could cause the oil pressure to drop all of a sudden like that?

I'm sorry for the long post, but thank you if you made it this far. Thank you even more for any ideas.
 
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#5 ·
Re: Oil

The oil filter was a regular Fram PH13. I did not cut it open, only removed it and poured the oil out. The oil in the filter was the same color as in the pan: bronze-ish with no visible metal shavings.

The oil galley plugs at the front were replaced by a previous owner of the engine. They looked fine and intact so I left them as they were.

Any other ideas about the cause for this type of sudden pressure loss? Like I said, the pressure was fine at first, on Saturday (the initial first start) and today. The pressure dropped from ~60 psi to 20 psi while accelerating on 2nd gear to 4500 rpm with the engine warmed up. After that the pressure rises to 40 psi at idle but doesn't go up with revs.
 
#6 · (Edited)
You and everybody else with an Eagle rotating assembly are having the same problem. I have just hashed out two of my customers' low oil pressure problem and have come up with an answer. The rod side clearances obtained with the Eagle components is too much. Factory clearances are from .010"-.020". With an Eagle assembly, you will get clearances from .024"-.035". That is where the pressure loss occurs. Melling says they are getting Beaucoup calls from frustrated Eagle customer with the same problems. BTW, stock specs. for SBC rod big-end width is .945" Eagle rods measure .940". That stacks up to .010" MORE side clearance than stock.

tom
 
#8 ·
I just got off the phone with 4 different con-rod suppliers. ALL of them gave .940" as the width of the big end on the SBC. With stock at .945" to .948", the differences can be substantial. At this point, I guess I will have to advise using a high volume pump to anyone using these types of assemblies, which is something I heretofore stayed away from.

Stock crank, stock rods are a lot less problematic, and don't need the high-volume pump.

tom
 
#9 ·
Another possibility is that you could be pumping the pan dry. If you are running a stock pan with HV pump, it can happen and will look just like you described. I know it does not help, but I had this issue when going to a solid cam in a SBC a long time ago and restrictors did not help. I ended up putting the cam in a new engine and selling the old long block. The new engine is still running with no oil pressure problem and the old engine ran two seasons in a circle track car. I think that my problem was the HVHP pump with solid cam and stock pan. I came up with this since the new engine used standard pump and had no issues. At the time it really sucked because I could not figure it out.

Chris
 
#11 ·
i have seen more than once the oil pump HP bypass hang open with some very small piece of trash and cause low press. Pull it out and inspect but don't bump or drop the pump removing or it may close up and you will be tearing down the engine tring to find the problem.
 
#13 ·
Thanks everyone for your input. Here's some answers to the questions raised:

Rod side clearance is a good suggestion, but would that cause these symptoms, i.e. pressure was good at first and then rapidly dropped?

The oil pump is a high volume, high pressure Melling unit. It is new, along with just about everything in the engine. But the pickup is stock, as is the oil pan. I have a Milodon baffle at the rear to prevent oil from climbing up when accelerating.

The pressure relief valve at the pump was one of my suspects. That could sort of explain why the pressure was good at first couple of startups but then suddenly dropped while driving the car.

I will take out the engine on Thursday to drop the pan and check the pump. Any new ideas are still welcome until that.
 
#14 ·
I`m not saying this is the problem, but don`t count it out. Years ago we did a rebuild for a friend, was a warmed over 350, no big deal. He came back to us after a week and said there were times when the oil pressure gauge read 0 psi and would stay there, but the engine still ran good and he never heard it rattle, we did the usual checks and etc and came up empty handed, he drove the car more and said sometimes it had great oil pressure, other times the gauge would go to 0 and stay there, then go back to where it should be, he swapped gauges and the same result, that weekend we pulled the pan and replaced the oil pump, he came back 2 days later with the same problem, so as a last resort we pulled out the oil gauge bung on the back of the block and discovered it had trash it in, bits of RVT sealant was found and as well as other particles, we cleaned it up and reinstalled it, the oil pressure stayed at the normal reading from there after.
 
#15 ·
filter

I am not saying this is your problem, but i went through almost the exact same thing about 2 years ago. I had a motor i did for my self. I had been driving it for awhile and all of a sudden it lost oil pressure. The same deal as yours. The gage would go to about 20 psi and that was it. I switched gages, changed oil and filter but i put a fram back on. I then switched to a different brand of filter (k&N) and the pressure came right back up. I have found that the fram filters get clogged with the break-in lube real easy also....

Just something to think about.

keith
 
#16 ·
Thanks for your input.

I did remove and check the gauge bung yesterday, but it wasn't clogged. After that I tried the engine again with the different gauge, but with same results.

I have used Fram filters for 10+ years in my Chevy engines with no problems. This is the first time, however, that I have a high pressure oil pump. I will look into this as well.
 
#17 ·
filters

I have used the frams for longer then 10 years with no problems also. But they have changed ..... On another forum 3/4 guys all had the same deal happen to then as me and it was all around the same time. I had the same deal with 2 of my customer motors with in 6 months of my problem. I had them all switch to napa or k&n filters and had no more problems.

Once again this may not be your problem but a new filter is alot cheeper then tearing the motor apart if you don't have to... If it's not the problem your only out a few $$$$$$

Keith
 
#20 ·
Finmike:
I had the exact same symptoms, but I am sure that the causes are different. Regardless, IMO, the end result (unfortunately) is the same. I believe that if you pull the pan, and also check your bearings, you will discover that they are fried. My pressure would drop just as you described. Despite the fact that it never hit zero, it would rise slowly on restart just as you describe. I ended up having my bearings replaced. Luckily, the crank only needed polishing. I base my opinion on the following comments:

Finmike wrote: “When the engine had warmed up, I took it to around 4500 rpm on 2nd gear, and when I changed to 3rd I noticed the oil pressure gauge falling rapidly. I let off the gas pedal, and the oil pressure stopped falling at 20 psi and stayed there. I idled back to the garage (1/2 mile), the gauge at 20 psi all the time and shut the engine off.”… “I started the engine again, and now the pressure rose slowly to 40 psi at idle and stayed there. I tried 2500 rpm and the gauge stayed at 40 psi and then fell slightly. The engine idled and ran fine, with no extra mechanical sounds, but the pressure was down.”… “The oil had a strange, bronze-ish color but otherwise it seemed fine.”

BOBCRMAN@aol.com wrote: “Better pull the pan.. That bronzish color is usually fine aluminum particles from a bearing failure. The sudden oil pressure drop is definatly not good.”

I would check out your pick-up to pan clearance. I once had an engine with too much clearance because I used a one piece Fel-Pro gasket. It is much thicker and does not crush. I’ll bet you have too much clearance and by the time you hit third gear, the G-force is sloshing the oil (what’s left in the pan) to the back, exposing the pick-up; which then sucks air into the system, causing the pressure drop, and subsequent damage. I hope I am wrong concerning the probable damage to your bearings and possibly the crank. Good luck solving this problem, Ed.

www.edgesz28.com
 
#21 ·
UPDATE

Last night we pulled the engine and took off the pan. The oil pump pickup was not loose, and we measured the pickup height. The bottom of the pickup was 1/4" from the pan, meaning that the screen on the pickup is maybe 1/2" from the pan. Is this OK, too much or too little?

We removed the oil pump and checked the pressure relief valve. It seemed to be seated, and moved in it's bore when pressed with a screwdriver.

We removed rod caps, starting from the front. There was some sign of wear on the #1 and #2 bearing, but nothing dramatic. Smaller signs of wear were on the rest of the bearings. Two friends who are professional mechanics were helping me, and they both agreed that the wear wasn't big enough to lose oil pressure, at least in any significant way.

We took off the cyl heads, rods, pistons and cam and removed the crank next. The main bearings were fine, but there was some wear on the rear main bearing thrust face, the one that points to the rear of the engine. That would explain the bronze-ish color in the oil but again, this would not explain the sudden pressure loss. Or would it?

I'm still puzzled. If the oil pressure loss was due to the HVHP oil pump pumping the pan dry or the pickup being too far from the pan, the pressure should have come up after shutting the engine off and letting it sit for a while. There was not enough damage to the bearings to explain why the pressure did not come up again.

If the pickup was too low or the rod clearances too big, the oil pressure should have been low from the first start, no?

The oil pump pressure relief valve was not stuck open, at least as far as we could tell.

I have new bearings on the way, but I would like to find out why I lost oil pressure before trying again. Any input and/or new ideas would be appreciated.
 
#22 ·
Check your crank thrust flanges. I have seen some poorly finished ones on aftermkt cranks. There should be no wear on the thrust with this low hour engine operation. Was the thrust set properly? You may try using a file and modifying the rear bearing to get more oil to the thrust surfaces.
 
#23 ·
I am curious to see what you are calling where. It is perfectly normal for the white(not sure what it is) substance to wipe of the bearings and it could be mistaken for wear. Apparently some people like to scuff off the white stuff so that the bearing surface is exposed. If there are actually wear marks like scratches, I would be concerned on an engine with that few miles.

Chris
 
#24 ·
I'd like to also stress what K-star mentioned. I also had issues with Fram filters lately (within the last year) it sounds too simple but your symptoms sound painfully familiar. Try a Wix or a Napa or a Mobil 1. I literally tore an entire motor down short of pulling the pistons only to have the same problem show up again after restarting. The problem went away when I put a Wix on.
 
#25 ·
TurboS10: By "wear" I meant there was more signs than just the white substance gone. This was evident on #1 and #2 rod bearings. But even these two bearings were not too bad. Scratches etc. were not to be seen, and the crank was fine.

BOBCRMAN: Thanks for the tip. I will check the thrust flanges. The thrust was set properly, but I didn't measure the endplay. There was endplay, i.e the crank was not obviously too tight by looking at it. My bad...I meant to measure it but you know how things go...too much in a hurry to get the engine ready.

Engineczar: I think I will try a Wix too...just for safety :thumbup:
 
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