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Finmike 06-08-2004 11:36 AM

Oil pressure loss SB Chevy
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.

EBlack36 06-08-2004 11:47 AM

Make sure that you have clearance under the pick up tube, if it is too close you will not have good oil pressure. Believe it or not I saw that on American Hot Rodder, so you might want to take that with a grain of salt. 06-08-2004 12:13 PM

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. Check also for a front oil galley soft plug blown out (next to cam).

k-star 06-08-2004 12:21 PM

What kind of filter is on the engine????

Did you change that when you drained the oil???

Did you cut the filter open???


Finmike 06-08-2004 01:25 PM

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.

machine shop tom 06-08-2004 01:33 PM

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.


killerformula 06-08-2004 02:22 PM

quality control anybody? What is it with these aftermarket companies selling these out of spec high-dollar parts?! Not cool.


machine shop tom 06-08-2004 03:05 PM

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.


TurboS10 06-08-2004 03:35 PM

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.


edge 06-08-2004 06:50 PM

I have some ideas and suggestions, but I would first like to know what oil pan and pickup you are using. Your post describing the symptoms could have been mine a few months ago. Ed.

roys63 06-08-2004 07:08 PM

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.

85406 06-08-2004 07:21 PM

I wasn't going to go there but the relief can cause lots of similar problems. Was the pump new. I have got an extra 30000 miles out of junk engines by doing the relief valve/ spring.

Finmike 06-08-2004 10:56 PM

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.

DoubleVision 06-09-2004 12:47 AM

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.

k-star 06-09-2004 06:34 AM

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.


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