Mercruiser 5.7L Strange Gauges

[Scorpion8]

Hello all! First post from SouthEast Alaska. I recently bought a 2003 model-year Trophy (by Bayliner) 2359 hardtop for fishing in my area. Does a superb job (in fact, brought back 3 Coho's on her first run today!) but am getting odd action from my gauges. Engines starts up and runs 40-50psi oil pressure, by gauge. But as the engine heats and we're running back and forth from the salmon grounds, the oil pressure drops to zero and even below zero at 3,200 rpm or above. I stopped twice to confirm that I was not running the engine w/o oil, there's plenty in the Mercruiser engine sump, and it's warm. Engine makes no strange noises and runs like a top.

Is my sending unit faulty or flaky? Or since I've also noticed that the gas tank gauge is "off" by quite a bit, is Trophy just using el-cheapo gauges on these boats?

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges

Taking the boat back out today. Would appreciate any thoughts.

 

[searay3]

Re: Mercruiser 5.7L Strange Gauges

Look behind the guages and look for corrosion on the contacts, especially the ground. There is no 'body' ground like on an auto, a wire completes the circuit. Also check the cannon plug for tightness/corrosion. If you find all is well do the wiggle test. Make your way thru the helm wiring/engine and see if you wiggle connections if you get a reaction on the guages. It is possible you have a bad guage, but if more than one is acting up, look for connection problems. You can also buy/borrow a mechanical guage to determine actual oil pressure. Although if it is showing zero but running well, I doubt the oil pressure itself is an issue...

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges

Thanks, I'll do that. I also had a "new one" today, where the tachometer stuck while we were trolling for salmon and stayed at 1200rpm until we reached the boat harbor and turned the engine off. Then it went back to zero and when I restarted the engine, it seemed to work fine.

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges

Checked all the connections today, and found no loose connections or rubbed-grounds. Everything was tight. No wiggle-excitement anywhere. Started the engine, and the oil pressure reads a firm 40psi at idle. Didn't take her out today, but have an excursion planned for tomorrow. I really hate running around w/o a true indication of engine oil pressure. Any more thoughts?

 

[littlebookworm]

Re: Mercruiser 5.7L Strange Gauges

Marine fuel guages are notoriously inaccurate. They're basically the same time of float system you have in your toilet at home and you know how well that works! So, let's take one thing at a time. Since you say there's no corrosion or loose wires, the next logical suspect is the sending unit. If you don't have a Mercruiser manual, get one. You can download one for free from a number of sites. Find the sending unit on the engine. Disconnect the wire from it. Turn on the key and ground the wire. The guage should go off scale. If it does, I suspect the sending unit is bad and recommend you change it. If it doesn'e, you either have a bad guage or a broken worn to the guage. A bad sending unit is more likely. There've been threads on how to check the gas guage; it's similar to checking the oil pressure guage. Do a search and you'll find them. Hope this helps. Hy

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges

Hope this helps. Hy

Thanks, will give that a try. On another aside, the Mercruiser 5.7 manula says "minimum oil pressure at idle" is 4 psi. Shouldn't my oil pressure go UP as I advance the throttle instead of DOWN?

 

[04fxdwgi]

Re: Mercruiser 5.7L Strange Gauges

Thanks, will give that a try. On another aside, the Mercruiser 5.7 manula says "minimum oil pressure at idle" is 4 psi. Shouldn't my oil pressure go UP as I advance the throttle instead of DOWN?

You may have a flakey sender or maybe it's not grounding to the engine block very well. Check to see if someone may have put teflon tape on the sender threads, which is a no-no. If yes, then remove sender, clean threads of any teflon and just use a dab of non-silicone sealer on it, like permatex and put sender back in. Also try installing a "T" at the sender and stick a cheapo (you can get em for 10 bucks) mechanical gauge in the "T" with the electric one and compare readings.

 

[LilRedNeckGirl]

Re: Mercruiser 5.7L Strange Gauges

speaking on oil pressure, lack there of when the engine gets hot.... did you change oil & filters when you bought the boat. An old trick is putting STP or Lucas oil treatment in, which increases pressure, till it gets hot. Kinda hides issues that normal oil would show such as taps in the valve, smoke, or low oil pressure.
Gas guages. lol, on old boats, they are to " give you a reasonable idea on how much fuel you have". We tote 100 gal in a tank thats bout 24 inches wide, 16 high, and probably 7 feet long. so settin still, we have a half tank, on plane, its 3/4 full, and on a good start up onto plane, we briefly have a full tank.... Ahh the science of old fashion fuel sending units, and toilet tank valves.

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges

You may have a flakey sender or maybe it's not grounding to the engine block very well.

Thanks, I spent some time on the boat hanging upside down in the engine compartment yesterday (we planned to go out today) but couldn't find the sending unit. The Merc 5.7 is kind of shoe-horned in there and a lot of clearances are 'tight".

.... did you change oil & filters when you bought the boat.

No I did not, and that bothered me. But we bought the boat in the water, sans trailer (which is on order), so I had no opportunity to check it over as carefully as I would a new "used" car. The engine runs great, doesn't smoke, makes no noises otherwise and the mechanic who did the "once over" found no issues to advise me of.

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges FIXED!!!

Re: Mercruiser 5.7L Strange Gauges FIXED!!!

Okay, at least one issue gone. I found the oil pressure sending unit. Man, that thing is hidden away. Anyway, the electrical connector is not captured with a nut, but has a slip-on connector like a spark-plug onto the end of the stud protruding from the sending unit. It was "loose". I pulled it off, cleaned it some, and then squeezed it down a bit so it fit snugly when I pushed it back on. Now the oil pressure goes to 40 psi at idle, and 60psi as I throttle her up leaving port. Stayed at a nice 40psi or higher at all times.

Whew!

 

[cpubud]

Re: Mercruiser 5.7L Strange Gauges FIXED!!!

Re: Mercruiser 5.7L Strange Gauges FIXED!!!

thats where mine runs 40 to 60 psi depending on rpm

 

[Scorpion8]

Re: Mercruiser 5.7L Strange Gauges FIXED!!!

Re: Mercruiser 5.7L Strange Gauges FIXED!!!

thats where mine runs 40 to 60 psi depending on rpm

Thanks, that's a good confirmation.