70hp thermostat compatibilities?

[johnyrude200]

I have a 1990 70hp, and am confused as to which thermostat and spring I should be using. The engine is running cold, barely reaching 90° in warm water at idle, and should be around 130° after a minute or two. At WOT, it may rise to 100°.

There are two different springs included in the water pump repair kit, the cone shaped one (yellow/green, shorter) and the larger cylindrical red spring.

My confusion is that the thermostat orifice on the cylinder head seems to have 3 different versions to accommodate 3 different thermostats:

1. original big plastic white one with thermoferm, not serviceable (replace full assembly)
2. 2nd generation big plastic black one with thermoferm, but a little shorter (replace full assembly)
3. newer style (through end of production) small plastic one with thermoferm that can be disassembled, including a small spring inside

So when I did the water pump repair kit, the directions explain to switch out the thermostat cover and use the cone shaped spring. However, the picture shows #3 style thermostat (newest style). This will not fit in the original thermostat orifice in the cylinder head. I’ve observed the cylinder heads have the same part numbers with a ‘ – 1’ or ‘ – 2’ or ‘ – 3’ to indicate the different style orifice hole.

The #1 and #2 cylinder heads utilize a cup and grommet to seal up the thermostat to the regulate water flow and use a smaller orifice. The #3 cylinder head does not use the cup and grommet and has a larger diameter orifice to accommodate the thermostat.

What am I supposed to do to get the temperature up on this motor? It runs OK, but is clearly ‘chugging’ at low RPM due to a cold running condition. Should I swap the cylinder head out with a ‘ – 3’ style one and be done with this or can the first two versions be updated to get a warmer motor? The cone shaped spring and replacement t-stat cover are not compatible with the older style thermostats (big white or black plastic ones which are one assembly).

Can’t really get this thing calibrated correctly until it’s at correct operating temperature.

[dan-in-tn]

Two different style cylinder heads. I’ll call you. Be happy to explain to anyone who is working on one!

Dan in TN

[fleetwin]

Well, this is a most confusing topic, even with the directions right in front of you for sure. The first thing we need to know is if this engine still has its original powerhead… The reason I ask is because if the powerhead was replaced, it may have the later style head that uses the thermostat set up with all the cups/diaphragms, etc. Someone may have mistakenly installed older thermostat components into a newer head, if that is even possible.
The original set up used a vernatherm encapsulated in a white plastic housing that could be unthreaded so the vernatherm can be replaced. This was combined with a hard rubber thermostat seat that was retained with a SS retainer that was press fit into the head. The problem with this set up was in salt water. Salt could collect between the head and rubber thermostat seat, that would deform/compress the seat. The thermostat had long flutes that helped align it into the seat. When the seat got deformed/distorted, it would jam on the long flutes which kept the thermostat from "blowing" off the seat at high RPMs. The "fix" for this issue was a redesigned thermostat assy, black in color, and a redesigned thermostat seat. The black thermostat did not have the long "flutes" that could get jammed in the thermostat seat. The replacement seat was a very hard rubber/plastic that was driven into the head without the SS retainer. The problem with this set up was that the seats were often distorted when driven into the head, they had a tendency to pop out also. The newer style black thermostat was more likely not to seat properly due to the lack of alignment flutes. This situation resulted in cold low speed running, the opposite condition to what happened with the original set up.
I usually ended up just using the original seat set up with either of the thermostat set ups. The problem was trying to remove the SS retainer, near impossible without a big mess….Kind of like trying to removed gearcase seals. So, I usually just fished the rubber seat out of its pocket without removing the SS retainer. I then got inside and cleaned the salt build up out of the head before replacing the original style rubber seat into/behind the SS retainer.
The next issue was changes to the water pump design. Many of these changes were brought about in an effort to deal with powerhead failures on these engines. This is why the water pump kit is so complicated/confusing. These kits include so many different pieces to deal with all the various designs/updates. I only installed a few of these kits, and was confused, and felt the directions were not complete enough to deal with all the various combinations of pieces parts that were used during the years. Some of the kits used a three blade impeller, which I was totally against. One of the other changes that was made during the years was the location and design of the overboard indicator hose. Early engines used a smaller ID elbow mounted on the side of the exhaust manifold. This setup had a restrictor plastic nipple that snapped into the lower pan, just like most of the other OMCs of this era. Later models used an larger diameter overboard hose with a larger ID elbow that was mounted on top of the exhaust manifold. The hose went through the pan with no plastic restrictor, it just slid through a rubber grommet through the pan. This change was made in an effort to remove air pockets from the manifold and block which may have been causing hot spots. The problem here was mismatching systems and water pumps/thermostat set ups. The original style overboard indicator created more of a restriction and higher water pressure at low speeds. So, using the older indicator components with a newer style pumps/thermostats might create a high water pressure situation that blows the thermostat off its seat at low speeds, cold running. Using the newer style overboard indicator pieces on older thermostat/pump set ups might create a low water pressure situation at high speeds, or hot running at higher RPMs.
So, one of the hardest things to do is to figure out if your engine is running cold at low speeds due to a high water pressure situation, or a mismatched/improperly installed thermostat/overboard indicator.
To be honest, you really need to be able to identify the various versions of these components just by looking at them…This is near impossible.
I wish I could provide pictures, worth so much more than the volumes I have written here…
Hopefully Dan Anderson will chime in…..D

[fleetwin]

quote Dan in TN:

Two different style cylinder heads. I’ll call you. Be happy to explain to anyone who is working on one!

Dan in TN

OK, sure wish I knew you were posting this simple answer while I was busy blabbering….Will call you later also…D

[johnyrude200]

Well Don, Dan called me and explained how to modify the vernatherm to stop it from blowing off those white plastic thermostat housings and discussed your points.

But you brought up something totally different that now I will have to look for!

I updated this motor to relocate the overboard water indicator to the top of the exhaust manifold, using a big fuel hose to let the water flow out directly. It comes out like a fire hose! The cylinder head is original to the motor and utilizes the plastic housing thermostat that comes apart with the vernatherm.

When I ran the motor on the test prop in the river, I could not get it to rise above 90° at low RPM (<1500), and it would barely climb to 95-100° holding steady in above this, say 4000-5000.

I’m hoping the fix Dan explained will remedy this, as we all know it’s futile to calibrate these when they are cold. I’ll run it hard later this afternoon with a prop on the boat and see how it behaves across all RPM’s, temperature-wise.

[fleetwin]

OK, well adding the big diameter overboard hose would lower the water pressure…This would not create the condition you are experiencing…And, the engine runs cool at high speed, so I would not worry about it.
You have the original style white thermotat assembly…I thought I remember that you could unthread these to get at the vernatherm. What style thermostat seat is in place? Like I say, I prefer the original style with the SS retainer over the "updated" hard rubber plastic seat that was pressed in without the retainer.
So, am guessing that the vernatherm is not seating properly inside the white thermostat assembly, or the thermostat seat is mismatched/messed up.. That fancy water pump kit included a replacement blow off spring for the original style thermostat as well, the best I remember. I don’t recall if the replacement spring was stiffer or weaker than the original though.
Will call you later, don’t want to confuse this situation further….
Adding the wide diameter overboard indicator would have lowered the water pressure slightly, so this would not cause the situation you are experiencing…

[jerry-ahrens]

Yet another advantage of using OEM parts! The Sierra water pump kit for these engines do not include the service bulletin, or the Tstat housing springs, etc. I still see some of those 3 cyl. motors out there, that have not had the tell tale hose relocated to the top off the power head. I’ve fixed a few of those myself over the last few years.

[fleetwin]

Joel mentioned that he is using non Q plugs in this engine. Advised this will cause all sorts of errant horn warnings and ignition problems….
I know this is hard to believe, but I have experienced it more than once on later model OMCs….Have been working with him over the phone…

[george-emmanuel]

Loving this discussion! I have a 70 on my Whaler and have had idle issues due to running cold. My problem has been different….buildup of stuff holding the t-stat open. The problem is I only use this rig about once per year. It is my salt water rig. The white corrosion that forms in the block breaks loose and stick in the t-stat. Casey removed the housing, cranked the motor and blew out a bunch of stuff and solved the problem, that is until I start to use it again in a year…but now I know what to do. BTW, wow, will that motor blow water! Looked like a geyser when he did it.

[johnyrude200]

If anyone has a copy of service bulletin 2221 that might be of interest if willing to share.

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