View Full Version : Normal Engine Temp for 86 351PCM

03-04-2005, 10:14 AM
can anyone tell me what the normal engine temp is for 86 351 PCM? mine fluctuates from 158 to 177 is that normal?

03-04-2005, 10:17 AM
Sounds a touch low nothing to worry about I would think. Other's will have more answers.

03-04-2005, 10:33 AM
My '83 (same engine) runs about 165 on average.

03-04-2005, 10:40 AM
does it fluctuate? if i run wide open like when pulling barefooters and bring it back down to idle the temp will rise to about 177. if i am cruising under 35 the temp will hold pretty steady is that normal? or should i look into getting a new thermostat?

Dan K
03-04-2005, 11:03 AM
I would say the fluctuation is normal and really depends on the water temp of the lake your on. This time of year the cooling capacity of the water is far better than say mid August.

03-04-2005, 11:06 AM
My 85' runs right around tthe same temp as yours.Mine will just touch 175 sometimes.Check your belts my temp. started to ride just a hair higher than normal checked everything and found the belts were loose.

03-04-2005, 11:33 AM
well my water temp is about 45 and in the summer it will be about 85. will that make the temp go up or will the thermostat just stay open more? my belts are pretty tight.

03-04-2005, 11:38 AM
I have a 454, which also runs around 165. I've seen the temp climb a bit coming down to idle. I think the t-stat and the lower water flow take a minute to catch up to the excess heat still in the engine. I'd just make sure you don't have anything clogging up your raw water pickup or caught on the screen in the tranny cooler and that you have a new impeller. Only then would I consider putting in a new t-stat if the temp climbs much farther. A weak impeller from my experience will not pull as much water a slow speeds as a new one. Might be a contributing factor.

03-04-2005, 11:47 AM
my impeller is kind of old but looks in good shape i have a new one i have been waiting to put in a little closer to summer maybe thats what the problem is. it is normal to fluctuate though?

03-04-2005, 11:48 AM
Scott did your boat come with that 454 factory?

03-04-2005, 12:27 PM
Far as I know... I bought it 4 years ago so I don't know much about it. The motor box for the 454 is about 8 inches bigger in all dimesions than for the 351 and it looks stock. I think the 454 was a bit more common in days past before they started getting big power out of the 350/351s. I think it was for barefooters and show boats pulling pyramids, etc...

And yes, I think 10 degrees of fluctuation is normal. Do a search on heat sink (I think, or try heat soak) and you'll get a thread on the subject from a while back.

03-04-2005, 01:07 PM
Shouldn't the temperature depend on the thermostat that you have (at least when running steady)? I believe the MC (at least back in 87) had a cool thermostat which causes them to run cooler than what you typically see in a car. I think the standard is like a 149F thermostate(why the odd number I don't know). Next step up is like a 163F.

I don't know why they use a cooler one but I always replace with the same cool thermostat.

03-04-2005, 01:23 PM
My boat runs pretty steady at about 165. The temp will climb a bit after a hard run or sometimes when you first start it up after you have set down a skier (no more than about 180). But as soon as it idles for a few seconds the temp comes right back down. This happens especially in the summer when my water temp is 95 degrees. I replace the impeller yearly so I eliminate the possiblity of trouble.

As for thermostats. I run the 142 in my boat. When I replaced it two years ago I first put in a 165 because I didn't know what the old one was but that high of a thermastat made the boat run at a constant 180. To hot.

From what I know generally your temp runs a bit higher than the thermostat at least in both of my Ford V8s.

03-04-2005, 01:37 PM
thanks for al the info keep it coming

03-04-2005, 01:42 PM
Mine runs a 143,just bought a new one for this season.

03-04-2005, 01:45 PM
is it bad for your motor to run to cool?

03-04-2005, 02:06 PM
The Vortec motors are supposed to run warmer than the older throttle body motors, I was told. But that is in terms of automobiles not boats.

03-04-2005, 03:02 PM
is it bad for your motor to run to cool?
I assume it isn't too bad to run cool if that is what MC / Indmar send out of the factory. Like BrianM, I once ran a higher temp thermostat that I bought at a local auto store but then went cooler when the guys at Rambo Marine said the original is 143 (I mistakenly said 149 above...).

03-04-2005, 03:12 PM
i saw someone mentioned that MC have two thermostats can any one tell me where they are located?

03-04-2005, 05:33 PM
Follow BrianM's post. That is how the system should respond. Engine temp should remain steady weather cruising or running hard (lake temp is really not as much of a factor). This indicates the system is working good. Yes, it will heat-soak after you shut the engine off after a run. Turn the key on and temp guage will register the hotter temp from the transfer of heat to the system (engine not running), but once the engine is re-started it should drop smoothly within several minutes. If the temp is fluctuating while running or cruising, then the system is not woking properly. Do the easy stuff first (belt condition/tension, impeller condition/replace, thermostat). 9 times out of 10, one of these will be the culprit.
In addition, boat thermostats are different than cars. Do not substitute.

03-04-2005, 05:59 PM
jayderwin1, your 351 should only have one thermostat and it is located in the little triangular housing centered at the top front of your engine (there are two hoses attached to the cover). There are three bolts holding the top down. If / when you replace the thermostat you should replace the gasket.... you can use the paper ones or form-a-gasket seems to work well.


03-08-2005, 08:47 AM
i saw someone mentioned that MC have two thermostats can any one tell me where they are located?

This is only for the LT-1

03-08-2005, 08:51 AM
Running too cool is bad for the motor. Unburned fuel can pass thru and cause serious damage to pistons and potentially even bend up valves, etc....
Your temps don't seem to bad. Thermostats can be temper mental little buggers. They are cheap, never hurts to replace one, especially if its older. Check your impeller too!

03-08-2005, 09:11 AM
i just bought two 143 t-stats from skidim and a couple of gaskets. the 143 will not be to cold will it?

03-08-2005, 09:31 AM
i just bought two 143 t-stats from skidim and a couple of gaskets. the 143 will not be to cold will it?

I dont think so, thats a little cooler than I want mine to run but I think it's still within the safe zone. You might get a hair better mileage with a little warmer stat but it could be such a small increase that it wouldn't really be noticeable.

03-08-2005, 09:35 AM
Jrandol what t-stat do you have in your boat?

03-08-2005, 09:39 AM
99% sure it is a 160 degree, if not it is a 165%

03-08-2005, 09:44 AM
I think the 143 is fine. The 160 made my boat run to hot.

east tx skier
03-08-2005, 11:24 AM
Hey, Brian, got the manual. Thanks.

Sorry for the threadjack.

03-08-2005, 12:54 PM
my boat came factory with a Indmar 454 and it usually ran around 120. It had a leaky water pump and just replace them and the thing runs so cool! It runs pulling barefooters and four tier pyramids running at just about 100. It was the best $140 I've ever spent. It's hasn't seen warm water yet so i'm sure that will change but thats what it's running now! The boat is still broken so i'm not running it and i'm hopeing with it running better after the cables and plugs are replaced and it will warm up! I might even have more trouble it that doesn't fix it!

03-08-2005, 01:00 PM
100 seems awfully low working temp for an engine???

03-08-2005, 03:20 PM
Running too cool is bad for the motor. Unburned fuel can pass thru and cause serious damage to pistons and potentially even bend up valves, etc....

I'd say that's a bit of and exageration there. No need to scare people.

Folks, running an engine too cold for a long, long time can (doesn't mean that it will) cause premature piston and bore wear. This can occur because the piston doesn't heat up enough, and therefore doesn't expand up to it's engineered size for 'normal' operation. This can cause the piston to rock in the bore and result in above average wear. Keep in mind that this would have to be a pretty COLD engine, run for years and years under that condition. I personally think that 120* should be the lower acceptable limit for an engine operating temp. The advantages of lower temps are more power, better resistance to detonation, and a greater margine of safety in the event of a cooling system issue. The dissadvantages are incresed fuel consumption, crankcase condensation, and emissions. These variables are MINOR however, in both directions, and the resulting unburned fuel from cooler temps will NEVER cause piston damage, oil contamination, bent valves, etc. There just isn't that much fuel (left after burnging) to cause that.

On the other side of the spectrum, these SAME engines are run in later model cars and trucks w/thermostats designed to keep the engine operating at 210*-220*. This higher temp is not detrimental to the engine, and is engineered into cars to provide better emissions, and lower fuel consumption. Obviously the down side is lower margin of safety, increased propensity for detonation, and slightly less power.

So anywhere between 120* and 220* is fine, IMO, with short episodes tolerable on either end under some circumstances. Basically the closer you go toward either extreme, the closer you should monitor the temp to make sure it's not roaming into ranges you'd rather not see.

Inlight of this, you can see that the original posters temps are perfectly fine, running in the middle of what most knowledgable mechanics would consider to be a fine, safe range.

03-08-2005, 03:46 PM
Would you say that some thing is wrong when an engine is running at 100 ?
Gauge,sender,water flow,tharmostat?
My boat run at 160 all the time!

03-09-2005, 12:01 PM

It sounds like your engine's operating temp is completely fine.

As for one that is operating at 100* (Assuming it IS 100 and ghe gauge/sender is fine), I would say there is nothing wrong w/the ENGINE, and it'll run fine like that, but as mentioned above, EVENTUALLY, it could cause increased rate of piston/cylinder bore wear.

I would say that the problem is indeed waterflow: there is too much of it, and a different thermostat would be in order to help regulate the water flow an increase the engines temp.

03-09-2005, 12:27 PM
Just to throw another fact/statement into this discussion is that my brother's '79 Barefoot Natu., with a 454, was overheating. It was the thermostat. He removed the stat, closed the lid, and has been running without a thermostat ever since. :steering: That was in 1983, and over 2000 hours ago.

03-09-2005, 01:53 PM
To add a question to that. Some people remove them to keep them cooler. But i'm not sure that there is one and that might be my problem. Is it possible that with out one, it would run too cold?

03-09-2005, 04:15 PM
Without a thermostat it will definitely run cooler. Whether it is 'too cold' I guess is a matter of opinion. You can certainly bet that the ngineers that designed these engines wanted them to to run warmer than 100F otherwise they would have never developed (nor designed into the engine) the thermostat in the first place.

I would guess it is missing altogether if it is running at 100F but there is also a chance that it is just stuck open. When you have problems with sticking thermostats you definitely want to replace it because it is only a matter of time until it is stuck closed and you get an overheat. At about $5 apiece I put this on my list of yearly consumables.


03-09-2005, 09:36 PM
Where should I look for getting one? And how do I know what kind mine is?

03-09-2005, 09:46 PM
SkiDim.Com is probably a good source. here is a link to the thermostats they carry for Indmar:


They have a similar page for PCM. Someone on this thread indicated that you shouldn't use an automotive thermostat but I must admit that I have used automotive thermostats without a problem. i will say though that I didn't use it too long because it was the higher temperature version (like 160F). Once I realized that my boat came with the 143 I ordered one from Rambo marine. Rambo has very knowledgeable staff which can tell you what you need but they don't have an online presence like SKIDIM.


03-09-2005, 09:51 PM
Older boats that are running FORD 351's came with a 142deg Marine thermostat. The temp guage would show a running temp of about 160-175 depending on water temp and how accurate the guage was. Those same FORD 351's that came installed with a heater ran a 160+ marine themostat. The guage would read somewhere around 180-190 deg. Helped the heater core get hotter. And then there is the FORD 351 with NO Thermostat it will run about 100deg and be less fuel effiecent. Maybe OK to run in Florida in the summer but not up north. In other words Georgia and NORTH. There is a difference between a Marine and a AUTO thermostat RUN A MARINE ONE. Marine thermostat can close shut but there are 3 small holes in them to allow water to always flow through.


03-10-2005, 07:50 AM
As soon as I read the last few responces, I started to look for a stat on skidim.com The problem I have is that they don't seem to have any that would work on a 454. They have some that say GM 350 or LT1 but not not anything about a big block 454. I want to fix the problem but I just need to get pointed in the right direction. (Jim) Do you have anything like this that I could order straight through you? Thanks Guys!

03-10-2005, 08:14 AM
E mail Jim directly with infos at baws@verizon.net.
He will help you out!

03-10-2005, 08:55 AM
Thanks... I need to talk to him about some other things too! We have been playing phone tag for about a month about new skins! I hope he can find one because i was worried when I didn't find one on skidim.com

03-10-2005, 08:59 AM
I've found that I can't find a lot of stuff on skiDIM.com for my 454 (or at least can't decipher their descriptions to see if it will fit). Many of the PCM 350 parts work in the 454 but aren't described as such. So now I don't bother with the website and just call. They fix me up everytime and ship immediately.

03-10-2005, 03:50 PM
The temperature gage on my boat never read about 100 when I bought it. If I ran it for awhile, then shut off for a few minutes and then started up again, the temperature gage would read fairly high for a few seconds before dropping back off to the bottom. I replaced the gage with two other gages to no avail. Decided to check the thermostat to find that the previous idiot, I mean owner, had removed the thermostat and the gasket. Somehow, he got the thermostat housing to seal with no gasket, just metal on metal.

I bought both the 160 and 142 degree thermostats just in case. Been running the 160, yielding a operating temperature up around 190. Might switch to the 142 three this year.