rebuilt 390 losing oil pressure-thoughts?

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   Sorry for the long story, but i wanted to be thorough.  I'm on my second rebuild on my 390 just completed this may.  New edlebrock performance plus cam and lifters (they came as a set), new piston and rings, new bearings, oil pump checked and serviced.  So here's my story...1. engine broken in on the stand by my reputable engine builder.  2 hours run time, good oil pressure (50+). used correct oil for break in.
2. put engine in car, drove 240 miles in one day, all good, oil pressure (sun mechanical gauge) at 2300 rpm freeway speed (5 speed) was always 49-52 psi.
3. changed oil and filter. used mobil 1 15/50.
4. drove 2/3 way to St. Louis meet (about 400 miles), noted the pressure slowly dipped at 2300 rpm freeway speed from 50 down to 28 or so in about 4 minutes.  Freaked out - stopped on side of road, engine off for about 10 minutes, called the engine builder.  The response is while not good need to get to a city (I was somewhere in the middle of nowhere).
5. When I got back on the road, oil was right back to 50 psi at 2300 rpm.  WTF?  Figured maybe a bit of stuff from the engine re build got in the oil sensor line (1/8" diameter).  Watched that gauge like a hawk all the rest of the way (another 150 miles).
6. Returned from St. Louis (another 600 miles), good pressure, 50 psi at 2300 rpm, no issues.  figured strange, but maybe a one off and meant to put on new sensor line, but didn't.
7. Drove around town each of the next 4 weekends, no issues.
8. Yesterday went to drive to Detroit show.  Got 50 miles, pressure dips pretty fast from 50 to 20, pulled over.  let it sit for 10 mins.  Decided to turn around and limp home.  Whole way was at 16 psi at 2300, engine sounded fine.  Figured it was a clogged line again or maybe the gauge was bad.
9. Got home, no issues, engine sounds good.  Then one by one I did the following, replaced the line to gauge, no change.  Put air pressure to the gauge, it worked fine and matched my pressure gauge on the compressor.  Put gauge back in, no change, still 16 psi.  Changed oil filter, no change.  Took out the rebuilt bypass with the heavy spring and the 15/16 nut, it looked good, put back in, no change, still 16 psi.
10. Today I plan on just buying a cheap oil gauge and seeing if by some magic the old gauge is just faulty, but I'm not holding my breath.
So the question is, how did I get this drop in pressure over 800 miles ago, then it "fixed itself" for no apparent reason and now it has returned?  Has anyone seen this when a cam bearing went bad?  Other thoughts?

Edited by 390amx1 - Aug/19/2019 at 7:21am

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390amx1 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jul/22/2011 Location: Medina, OH Status: Offline Points: 525	Post Options Post Reply Quote 390amx1 Report Post Thanks(0) Quote Reply Topic: rebuilt 390 losing oil pressure-thoughts? Posted: Aug/19/2019 at 7:11am
	Sorry for the long story, but i wanted to be thorough. I'm on my second rebuild on my 390 just completed this may. New edlebrock performance plus cam and lifters (they came as a set), new piston and rings, new bearings, oil pump checked and serviced. So here's my story... 1. engine broken in on the stand by my reputable engine builder. 2 hours run time, good oil pressure (50+). used correct oil for break in. 2. put engine in car, drove 240 miles in one day, all good, oil pressure (sun mechanical gauge) at 2300 rpm freeway speed (5 speed) was always 49-52 psi. 3. changed oil and filter. used mobil 1 15/50. 4. drove 2/3 way to St. Louis meet (about 400 miles), noted the pressure slowly dipped at 2300 rpm freeway speed from 50 down to 28 or so in about 4 minutes. Freaked out - stopped on side of road, engine off for about 10 minutes, called the engine builder. The response is while not good need to get to a city (I was somewhere in the middle of nowhere). 5. When I got back on the road, oil was right back to 50 psi at 2300 rpm. WTF? Figured maybe a bit of stuff from the engine re build got in the oil sensor line (1/8" diameter). Watched that gauge like a hawk all the rest of the way (another 150 miles). 6. Returned from St. Louis (another 600 miles), good pressure, 50 psi at 2300 rpm, no issues. figured strange, but maybe a one off and meant to put on new sensor line, but didn't. 7. Drove around town each of the next 4 weekends, no issues. 8. Yesterday went to drive to Detroit show. Got 50 miles, pressure dips pretty fast from 50 to 20, pulled over. let it sit for 10 mins. Decided to turn around and limp home. Whole way was at 16 psi at 2300, engine sounded fine. Figured it was a clogged line again or maybe the gauge was bad. 9. Got home, no issues, engine sounds good. Then one by one I did the following, replaced the line to gauge, no change. Put air pressure to the gauge, it worked fine and matched my pressure gauge on the compressor. Put gauge back in, no change, still 16 psi. Changed oil filter, no change. Took out the rebuilt bypass with the heavy spring and the 15/16 nut, it looked good, put back in, no change, still 16 psi. 10. Today I plan on just buying a cheap oil gauge and seeing if by some magic the old gauge is just faulty, but I'm not holding my breath. So the question is, how did I get this drop in pressure over 800 miles ago, then it "fixed itself" for no apparent reason and now it has returned? Has anyone seen this when a cam bearing went bad? Other thoughts? Edited by 390amx1 - Aug/19/2019 at 7:21am
	69 BBB 390 Tremec 5-speed AMX #13584 since 1978 (166,000 miles) 08 Zo6 Corvette 22 Ram 1500 24 CT5-V Blackwing #317 Yamaha Supercharged Jet Ski's

Midnight Rambler Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jan/17/2011 Location: SoCT Status: Offline Points: 1670	Post Options Post Reply Quote Midnight Rambler Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 8:07am
	Oil pressure sensors on the 4.0L sixes go all the time...I found that out while losing my mind watching the gauge jump between 5-15psi and thinking I was going to toast it before I got home. It was just the sensor/sending unit, I swapped it out and no issues. I'd start there, it's a quick and easy process.
	'66 American 440 Convertible 290/M-40/AMC 20 3.15/PS/PB '04 Jeep Wrangler X Rocky Mountain Edition 4.0 5sp

390amx1 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jul/22/2011 Location: Medina, OH Status: Offline Points: 525	Post Options Post Reply Quote 390amx1 Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 9:15am
	Thanks for the thought but it is a mechanical aftermarket Sun gauge, nothing electrical about it. I just replaced that mechanical gauge with another new gauge - same thing 16 psi.... its pretty clear it is low pressure. The question remains how did it go low pressure over 800 miles ago, recover in a few minutes, then be fine and return and now stay...
	69 BBB 390 Tremec 5-speed AMX #13584 since 1978 (166,000 miles) 08 Zo6 Corvette 22 Ram 1500 24 CT5-V Blackwing #317 Yamaha Supercharged Jet Ski's

bbgjc Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/24/2009 Location: No.VA Status: Offline Points: 1660	Post Options Post Reply Quote bbgjc Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 10:15am
	Are you running synthetic? You should have at least 500 miles on a new rebuilt engine using conventional oil, especially with flat tappet engines. I did see you overall mileage, I don't know when you switch to Mobil. I personally would not run synthetic oil in that engine. Not to question your abilities, but if you have any air in the gauge pressure line, you'll get an incorrect reading.

azfletch Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Mar/11/2008 Location: SoCal Status: Offline Points: 829	Post Options Post Reply Quote azfletch Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 11:03am
	Is there any chance the fuel pump is leaking fuel into the oil and deluding it?
	Mark Fletcher 72 AMX 401 with 6K original miles. 70 Mark Donohue Javelin, 390 4 speed with 47K miles. 70 AMX, 390 AT, AC Randall car. 67 Rogue Barbados Blue 290 225HP 4 speed convertible 71K miles.

390amx1 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jul/22/2011 Location: Medina, OH Status: Offline Points: 525	Post Options Post Reply Quote 390amx1 Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 11:22am
	Mobil was put in after 2 hours of break in and 250 freeway miles plus about 50 miles around town. The builder said that was enough miles. I bled the line of air then connected gauge, same 16 psi. I'm not concerned with Mobil 1 in the engine overall; that's what has been used in it for the last 30 years (15/50 Mobil has the highest ZDDP of all Mobil 1- its 1100 PPM).Didn't think of the fuel in the oil. Will check that (I'm going to do a Blackstone before I pull the engine). It was a new pump 2 months ago but I've heard they can have problems...
	69 BBB 390 Tremec 5-speed AMX #13584 since 1978 (166,000 miles) 08 Zo6 Corvette 22 Ram 1500 24 CT5-V Blackwing #317 Yamaha Supercharged Jet Ski's

JGRANTAMX Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jun/02/2017 Location: Maine Status: Offline Points: 1703	Post Options Post Reply Quote JGRANTAMX Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 11:31am
	If you are not running a WIX or NAPA gold filter you might want to try one. If the engine runs and sounds good I would upgrade the oil pump to an adjustable one and increase the PSI to where it should be then keep an eye on it. J Edited by JGRANTAMX - Aug/19/2019 at 12:25pm

Greyhounds_AMX Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/14/2009 Location: Kansas City Status: Offline Points: 1268	Post Options Post Reply Quote Greyhounds_AMX Report Post Thanks(0) Quote Reply Posted: Aug/19/2019 at 6:11pm
	The recirc valve could cause low oil pressure if it sticks. And with thermal cycling it can pop loose again and work like normal. I'd try a different one if possible. It doesn't sound like the problems you get if the pickup button is missing, but it would be one thing to check.
	1968 AMX 390 w/T5

390amx1 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jul/22/2011 Location: Medina, OH Status: Offline Points: 525	Post Options Post Reply Quote 390amx1 Report Post Thanks(0) Quote Reply Posted: Aug/20/2019 at 7:54am
	Checked the oil for gas, very sure its not gas in the oil. We replaced the whole oil filter housing assembly with a new one (new spring and all). Heh, used a drill on a dizzy and couldn't get it to prime. Will try again tonight. Its got to be the 75 PSI pressure relief valve. I believe if it goes off or is clogged it recirculates oil to the suction of the pump and therefore kills the pressure?
	69 BBB 390 Tremec 5-speed AMX #13584 since 1978 (166,000 miles) 08 Zo6 Corvette 22 Ram 1500 24 CT5-V Blackwing #317 Yamaha Supercharged Jet Ski's

Jmerican Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Mar/29/2016 Location: Seattle, Wa Status: Offline Points: 585	Post Options Post Reply Quote Jmerican Report Post Thanks(0) Quote Reply Posted: Aug/20/2019 at 10:29am
	It can take a while to prime, and just mentioning ... but don’t put any weight on the drill downward; but if you ran the drill for awhile, that could be telling you something. Have you gently put a wrench on the timing cover bolts? Hopefully not a gasket issue. There will be many suggestions on this here, but be patient, as you are doing. I don’t see how it could be your “75 psi spring”. It could be a sticking valve, but you are on to that thought. When you say you put a new cover on, with spring.. do you mean a reproduction new piece? * What filter are you running? Did you replace it with the new cover adapter? * Have you checked the clearance of the gears to cover/adapter housing, (end clearance, with gasket) ** this is a goose chase item, prob not it, but needing a look in terms of running the engine long term. If it was way off, it could be an issue in prime. I might be leaning toward the timing cover gasket failing. Some here might lean toward a bad pickup tube. Some have seen it. Both are some tear down, and goose chases are often ridiculous, eh? Might just need to motor the pump more... You could try to lightly pressurize the crankcase via the Pcv grommet, block the oil fill, run the drill - good luck if alone. See if prime happens. It sure doesn’t fix it though. Nor the pressure drops you saw. Tricky. Good luck