1988 Mariner 150

[imported_ksimpson]

My 1988 Mariner 150 V6 has a good tell tale at tick over but overheats and steams after approx 1 mile @ 4000 RPM. Thermostats, poppet and pump and delivery pipe check out ok. I suspect exhaust gas is forcing into the cooling water and have replaced head gaskets, exhaust divider and manifold gaskets and powerhead and intermediate plate gaskets, also checking all waterways were clean when the heads and plates were off.

One bore has slight scoring but performance is unaffected.

Does anyone please have any previous experience of this and know what else to look for?

 

[rivercookdave]

Re: 1988 Mariner 150

had the same problem on an 89
merc 150v,turned out to be a bad impeller,idle was good,high rpm's didnt put out any pressure. if you dont have a water pressure guage i would add one.

 

[ncfish1]

Re: 1988 Mariner 150

Have had this same problem. Try this so we can eliminate possibilities:
1. Remove both thermostats, install ears to lower unit and start engine and operate at idle speed. You should have a steady (strong) stream of water coming from both t'stat openings. If so, you have eliminated flow restrictions through water passages and proper pump operation. If low flow or no flow is seen at either location, you have a restriction in the water tube or the pump is not properly functioning.
2. If #1 checks ok, I would go back to the poppet. You either have a messed up seal at the water opening at the poppet or the spring has too much tension or the diaphram is not seating or holding.

In my misfortune, I found the poppet spring to be the culprit (had just replaced the entire assy.) and found the spring to be too strong. Reinstalled the old spring and everything went back to normal.