Oil pressure drops to zero
 

Just put a new 496 stoker in my boat. Everything has been running well. Was out twice with no issues. Went out this evening and the oil pressure readings are screwy. Start and idle reads normal. When i start running it drops down to zero, like it is not reading anything. At around 4,000 RPMs or so, it will start flickering from zero up to 60 and drop to zero again. Will just sit at zero then. Drop back down to idle and gauge reads pressure again.

What should I look at for a possible cause. Everything on the engine should be new. Any feedback?

Wire making a bad connection to the sending unit or the sending unit it self is bad. First thing I would check.

Do not use any sealer on the sending unit when you screw it into the block. It keeps the sender from grounding.

If it was a ground issue...then why would it return as soon as he gets back to idle ? He says he has nothing at 4000 rpm. You guys may be right...but it sounds to me like the pickup came off the oil pump. :eek:

Definitely try a new sender first tho...it's the cheapest. :drink:

screw a mechanical gauge into it ASAP to make sure you really have oil pressure before you cook a motor. provided the mechanical shows everything all good then you can go through the wires, you could have a flaky sending unit or gauge, rubbed through wire someplace, maybe a loose wire in your dash panel, all sorts of possibilites.

just rule out actual oil pressure issues with the mech gauge first and go from there.

my last baja had a flaky oil pressure gauge, 9 out of 10 starts it read fine, the 10th it would show ZERO pressure, after verifying i had oil pressure i ended up just carrying a jumper wire and simply jumped neg battery to the oil pressure sender and the gauge would spike to max oil pressure as soon as i remove the jumper then the gauge would go back to 50-60psi like it usually was. why that gauge liked to stick i have no idea why!

Is your alarm going off, it uses a different sender.

Alarm is not going off, but I am not sure the alarm is hooked up now, as it does not sound when I turn the key to the power on position, which it did with the last engine. Was in it messing around with the wires etc. The way the oil pressure sending unit is set up, it comes out of the engine and is connected to a T type fitting. The oil pressure sending unit is on one side and some other fitting with two wires is on the other side of the T.

When the power is off, the pressure reads between 20-40. Start it up and it goes to 50-60. Review to about 1,000 and it drops to zero. Drop back down to idle and it flickers between 0 and 60. When revving the engine, it will flicker between 0-80. I can't tell if I have more blow by or not. If I stop and restart and stay at idle, it will stay between 50-60.

When I stop the engine and check the oil, the oil is really low, so seems like the oil is getting up into the engine. After a couple minutes all he oil is back in the pan.

helpful advice # 56 ... revving the motor when you suspect no oil pressure is NOT the gold standard for diagnostics...

i have mecanical guage for oil pressure and water temp, to have a second instant diagnostic of my readings in case the dash gauge comes fuzzy...

Just another random thought....you ARE using the correct dipstick right ? And....there is roughly 6-7 quarts of oil in it right ? Are you sure ? :picard1:

(Not doubting your abilities...but I've seen people do worse to their motors sir. ) :drink:

Yea, I have plenty of oil in it. The dip stick is one of the side dip sticks that goes in with little angle. I am well above full. It sucks all the oil up into the engine when it is running. Does anyone have any feedback on the T connector and what might be on the other side of the T. My brother has mech oil pressure gauge, but can't reach him, so will have to pick one up. It does look like a sealant was used on the sender.

:lolhit:, I concur!!!! Before you even think of doing ANYTHING else put a mechanical gauge on it and see wtf is going on, Smitty

"Well above full" is bad. The oil gets frothed up from the crank spinning in it.

The dipstick was just an idea. I saw a guy use a "sweet chrome" one that was too long...and showed full when it was almost empty. You said it shows good oil pressure at idle right ? Is that all the time at idle ?

so ... when you ran it the first two times and the oil drained back fine, what could have changed to make the oil not drain back now... and where do you think it all could be ? the lifter valley somehow ? thats a lot of oil to put somewhere .. about 500 cubic inches worth of space...

All the oil drains back, it just takes time to show up on the dipstick. Mainly because it does not go straight down and in, but at a really soft angle; about 15 degrees off perpendicular to the engine. Just got a mechanical gauge. Going to get working on it and see what happens.

If it aint the sender...then I'm sticking with my first guess that the pickup tube came off the pump. :drink:

Fixx
 

1) im with tin,,your pick up came off or

2) do you have screens epoxied in the drain holes in the front,on top of the cam and rear of the block?

3) or are you using 80w90 for oil :eek:

I put the mechanical gauge on it and I have pressure. Good pressure. I am guessing the sending unit is no longer good. Not sure if it was a new oil pressure sending unit or not. I was pretty sure I had pressure.

Fixx
 

may not be the correct unit to gauge,wrong ohme range,,,gafrig and livoursi has their own senders for their gauges.

I'm glad you have good pressure. :drink:

Seems funny tho that you said you had pressure numerous times at idle...then at 4000 rpm's you got zero. :rolleyes:

Run i till it won't run, the you know you have a problem

in spite of what some want to tell you these senders go bad all the time. I have changed out dozens over the years and all acted like yours. oil pressure will not jump around that rapidly or radically in a motor. if you are losing pressure it will go up and down but not "jump" or only work at certain rpm. the sender however will develop dead spots and as pressure changes by even a half pound could put it a dead spot. any time pressure is jumpy you should change out the sender before you worry about anything else.

I believe we said to change the sender first...numerous times. Thanks for the input tho. :drink:

theres a trick to making all these piezo type sensors for oil press and fuel pressure etc live.... assuming you don't want to run mechanical gages , you use a short piece of high pressure rubber line with the appropriate fittings to remote locate the sensor from the motor and you externally ground it. what this does is isolate it from all the vibration and heat that kills these things and gives it a nice solid reliable ground.. these consumer grade electric sensors are just scrap on their best day anyway and personally, i would never run anything other than a mech oil pess gage anyway but thats just me. on all of these applications you can just remote mount the sensors out of harms way and it will make all the dif in them living and dieing all the time...

That was my first guess also. I've replaced gages with the same part exact part number and the new gage would not act right with the old sender. New gage and sender is in order.

I like the electric gauge in the dash, but also like to mount a mechanical gauge in the engine room for double checking.

Something like this (ignore the STBD one, I dropped it and the needle was off. It was replaced shortly after the pic):

http://i116.photobucket.com/albums/o...y/IMG_0194.jpg

I like that idea sir. :drink:

sure. i understand completely... but you aren't standing in the engine bay when you are going 75 mph at 5300 rpm... and wouldn't it sort of be a better idea if the gages you were actually looking at and relying on to save your ass actually worked ? think about it... with this set up you are saying " gee the gages in my dash are such sht that i have to have a back up set somewhere else " huh ? that's like the guy that told me the reason he loved the MSD stuff was because it was cheap so he could carry spares and it was easy to replace every time it failed...

Here we go :food-smiley-007:

I understood that several recommended changing the sender. my point was for all those who kept trying to overcomplicate things and find dramatic issues before going to the source of the problem (the sender) first. there are a lot of knowledgable folks on this board but there are some who either dont like to listen to them or are bound and determined to prove them wrong. the biggest example is the guy looking at putting the alpha drives on the 38 foot 6 ton boat and basically ignoring everyone who told him it would be lucky to get on plane. no offense to anyone meant.

It is/was the sender. Checked voltage, etc. Changed out sender, but was not able to find the correct one. Have one in now that at least relays, so i know if I lose pressure, etc. I have stewart warner gauge, so trying to find a sender for it. If not able to find the correct one, then will get new gauge and sender. So problem solved. Thanks!

Maybe so. Who said it was to be looked at, at 75 mph? For the most part, electrical gauges are usually pretty reliable and it is a lot easier as far as routing to run a wire up to the helm than a -4 hose. I never had any trouble with the electrical gauges, but for my own reason I like to have something to double check the electrical from time to time.

....Or another reason is if I happen to be in the engine room priming an engine with the distributor out or rolling it over with a remote starter and I can't see the gauge at the helm, be it electrical or mechanical. All I was doing was offering an option. Personally I wouldn't mind having an entire extra set of gauges in the engine room for when I am back there working on things but that's not really practical.

i understand all that and its a great idea and well executed ... for that purpose. but my point is simply that
you have about 80 grand worth of motors , 40 feet away from you that you can't hear. your only information at speed is what you see on the dash. for the extra 100 bucks, why wouldn't you run a piece of rubber -4 ( so no chaffing) 200 psi hose, that 40 ft to a gage that you can believe and trust ?

its the same with the water temp. i replaced some of the gages on mine when i got it and the values were all over the place ( gaffrig) so i took the sensors out and put them in boiling water and one was wrong by 10 degrees cold and the other 15 degrees hot. for me, unacceptable. thats with the senders that came with the gages... the ones that were original were closer... inside of 5 degrees ... close enough but the point is that , for the money that gets spent on these motors, i am really surprised how nonchalant people are about the accuracy of their dash or , in a lot of cases, the instruments they aren't carrying at all ( fuel press and oil temp). sure... i can seem to be obsessive but in reality it just seems crazy to me do do things badly when it is very easy and reasonable to do things well. i would rather have gages that actually work rather than ones where the bezels happen to match my interior.