Oil pressure sending unit problems...
 

Hey guys, wanted to give a heads up about a problem I just had with my Mercruiser big block. Just built my motor over last winter. Installed a brand new Sierra oil pressure sending unit (bell style). Motor never had great oil pressure from day one. 45 psi cold, 25 psi hot. I wasn't happy, thought a new motor should be better. Used it all season, until last week, pressure started diving off under throttle...took a few years off of my life. I hooked up a mechanical gauge, and it had 60 psi cold, and 45 psi hot!!! Installed a new Merc seding unit...BAM...60 psi. It's getting hard to work with these junk new parts, it can make you crazy!!! Hope this helps somebody.

Does not surprise me, Sierra stuff is mostly junk.

Learned that the hard way!!! lol.

Don't use their exhaust manifold gaskets either. Coooked a set out in less than a season.

sending unit must also be compatable with gauge as well..not all will work together..

Using thread sealant on an electric sender can cause this problem too.

Yah, I'm just glad the motor is healthy!

how do you guys check a mechanical gauge while your up on plane? I've been wanting to try this as well.

Buy a proper length of copper tubing and wire tie the gage so you can see it at WOT.

Or you can just put a mechanical gauge in the dash. One of the fitting companies has a nice cloth braided hose, -4 is the smallest it comes in I think. Or you could have your local Parker store make you up one.

I have stock sending units in on the side of my block. Can't figure out how to replace them with ones that match the auto meter marine gauges I have. I know that the stock aren't compatible cas the gauges peg when the key turns to accessory. Then when the boat fires the motors read 65-70 when cold and at 3000 rpms it drops to 25. After the motors warm up they shoot back up to 65-70 at idle and drop when rpms are up. It's like they are wired backwards. Any chance the may be wired wrong at the gauge. At this point I think I'm going to look for stock gauges rather then try to swap these sending units.

Yep, the gauge sender is working in reverse. There is not really any way to wire tehm backwards to do that. When I got my Pantera, it had a VDO sender going to Gaffrig/Livorsi gauge and it did the same thing. I would call Autometer and see what sender you need.

My AutoMeter Proc Comp Marine gauges work fine with my factory Merc sender, but mayber like Griff mentioned at some point somebody changed your senders out to match a previous set of gauges.

http://www.summitracing.com/parts/ATM-2242/

Per Rex Marine's website, the ACU 900 will work with AutoMeter gauges:
http://www.rexmar.com/page60.html
http://www.rexmar.com/page57.html

I have the new automated gauges just can't figure out how to swap them out. The originals I can't seem to get a wrench on and to be honest there just isn't a lot if room.

Where are the senders at currently, down by the oil pan/oil filter adapter? That is where they usually are, however on my GenV 330 the oil pressure sender was plumbed at the top of the block, back by the distributor. You could always leave your existing ones where they are and put your new senders up top.

You probably need to remove the power steering pump/alternator to access them from forward.

yup
 

once you remove the alternator you have tons of room on a 311