96 5.0FI Weird Temp Readings?

NickH · **Joined:** Sat Aug 02, 2014 7:55 pm **Posts:** 67

So I finally launched our Sundowner this past weekend and all in all was a great time on the lake. Part of the winter service was me replacing the thermostat and temp sender that hasn't worked since we bought the boat last summer. The temp seemed to be fine most of the weekend (okay, so I really didn't pay too much attention to it...) then I noticed that it was running hot at idle/headway speed nearing 190-200 degrees and then dropped to 120 at plane. The Water temp was a balmy 51degrees here in New England. I'm going to bring my IR temp gun up with me this weekend to confirm the temps on the t-stat housing and risers. Nothing "seemed" hot, the engine area or rear seating area all seemed fine, and the boat ran superbly all weekend, like it was brand new.

Sorry for rambling...

I guess my question is, is that much of a temperature swing normal? Or should I be suspect of the gauge which is part of one of the world-famous automotive style clusters? What would normal operating temps and temp differentials be?

GottWhat · **Posted:** Tue May 26, 2015 9:11 pm

How old is your impeller? Higher temps at idle could mean you are not getting sufficient water circulation.

LouC · **Posted:** Tue May 26, 2015 9:15 pm

If you have a 160* stat you should see temps of 160-175*. If it's varying that much then either its a problem with that gauge cluster (one of their not so hot ideas), or a stat that is sticking open (120*) or closed (200*). And as noted a weak impeller and/or wear plate will cause hot running at idle and low speeds. Use an IR temp gun and that will tell you what's what.

NickH · **Joined:** Sat Aug 02, 2014 7:55 pm **Posts:** 67

Thanks guys. The symptoms definitely point me towards the impeller, what really has me baffled is the range. It seems like temp senders are pretty generic, I ordered the one for my engine. It would seem that the incorrect sender (if there is such a thing) would make the ranges on the guage way off.

GottWhat · **Posted:** Tue May 26, 2015 10:46 pm

Please follow up after you get it all sorted.

Surface Interval · **Posted:** Wed May 27, 2015 3:54 pm

As previously stated, I would start with the water pump. This pump is mounted on the right front lower corner of the engine. You could have a worn or bad impeller, or maybe even a worn pump housing. You say the temp drops to about 120 at cruise. My experience with that engine was that the thermostat has some grit, sand, or rust that is holding it open a bit. They put a 145 degree thermostat in my '95 190 with the 5.0 EFI Ford. I'm guessing yours came with the same. If possible, I would recommend installing a new one. Running at 120 should not hurt anything, but it would run better at normal temp. After making these changes go out and try it again.

NiagaraChillin · **Posted:** Thu May 28, 2015 6:37 am

Impeller would be my first guess. When was it last changed? Its a very easy change out, about 15-20 minutes with basic hand tools.

Jdpber · **Posted:** Thu May 28, 2015 8:31 am

i think it is your flux capacitor.

NickH · **Joined:** Sat Aug 02, 2014 7:55 pm **Posts:** 67

Jdpber wrote:

i think it is your flux capacitor.

Unfortunately I don't think that ol' girl's hitting 88MPH anytime soon...

NickH · **Joined:** Sat Aug 02, 2014 7:55 pm **Posts:** 67

I was able to run the boat through its paces for about an hour tonight. Once it warmed up i idled around, brought it up on plane for a period, putted along at headway speed again, repeat, you get the picture...

Sure enough the guages fluctuated between 120 And 210 being hotter at idle after planing.

BUT the IR gun told a very different story. I found another thread where LouC posted a procedure with different locations on the engine/cooling system to test and what the temps should be. Well, the temps on the motor itself were right where Lou said they should be. Risers were between 80-100, the manifold right at the temp sender never crested 120 and thr T-Stat housing reacted to the conditions appropriately.

At this point, unless I'm missing something (besides pulling the Impeller, I'll need to bring my tools up for that) I'm going to chalk this one up to a wacky guage and/or sending unit (which I JUST replaced a couple weeks ago).

- Nick

GottWhat · **Posted:** Fri May 29, 2015 8:53 pm

When was the last time the impeller was changed? Manufacturers recommend they are replaced every 2 years. Most people extend it past that. If you don't know when the last time they were changed, it is worth swapping it out for the peace of mind. If your old one is still good, you can keep it as an emergency spare.

NickH · **Joined:** Sat Aug 02, 2014 7:55 pm **Posts:** 67

Supposedly last spring, I will probably do it anyway next weekend.

LouC · **Posted:** Fri May 29, 2015 10:10 pm

If you're getting a reading of 145-155 right under the 'stat housing on the intake manifold, and the gauge says 210, then the gauge or sending unit are bad or have bad/corroded terminals/grounds. On the temp sending unit, from my OMC manual, it should have the following resistance readings at these temps:
100*F-448 ohms, 160*-128 ohms, 220*-46 ohms. You can see that the cooler the temp, the higher the resistance. Actual readings may vary from these by about 10-5%. The gauge needs 12 volts and a good ground to function properly.
I see from your early posts it has the one piece cluster, I'm thinking that's really your problem. These were know to be troublesome.

NickH · **Joined:** Sat Aug 02, 2014 7:55 pm **Posts:** 67

Thanks Lou,

I had suspected the guage all along, but I wanted to make sure.

LouC · **Posted:** Sat May 30, 2015 1:19 pm

You might even try checking the ohm reading of the sending unit when the dash gauge is reading 220; if it's close to what it should be when the engine is really at 160 then you know it's really the gauge...

www.iFourWinns.com

96 5.0FI Weird Temp Readings?

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