idiot light or gauge, which to believe?

[Rattlenbang]

I'm in the middle of taking my E350 motorhome on her inaugural trip into the BC interior. We did a fair bit of climbing today with the longest, steepest section several kilometers long, outside temps about 28 C. During this climb the temp gauge, which usually sits at about 1/4 to 1/3 deflection hit just over halfway. You could feel the heat coming from the doghouse of course, but we didn't have any problems. It was only quite awhile later that I noticed the overheat light had come on, and stayed on until I turned the van off and restarted it. Pretty sure it came on during that foot-to-the-floor climb. I guess they don't clear once triggered unless you shut off because temps were back to normal and it was still on.

But of course nobody likes conflicting indicators like this, and wondering if I shouldn't trust the temp gauge anymore. I dont have my infrared gun with me to test accuracy. AFAIK all the senders and gauges are original. I'm pushing a 26' motorhome with an '86 6.9 so I'm expecting a lot of pedal to the metal driving in the mountains.
In terms of temps anyone know what the upper safe limit is?

 

[Philip1]

The factory temp gauge is wildly unreliable and inaccurate. The best solution is to buy an aftermarket temp (and oil pressure gauge, its equally unreliable) and install it. I've heard that around 230-240 or so is absolute max you want to see your temps to get.

 

[franklin2]

I 2nd that, buy a aftermarket gauge set for the temp, oil pressure and volt gauge. I have seen a lot of money and time wasted on what the factory gauges were saying.

If it turns out the overheat switch was telling the truth, get ready to do the clutch fan song and dance. Very common problem with these trucks.

 

[saburai]

@Rattlenbang NA or blown?

Definitely worth it to have a good engine monitoring. Transmission temperature if auto too.

 

[Fredrickson]

For my simple brain.. and to help answer the likely follow up questions.

I found this thread from a several years back
https://www.oilburners.net/threads/oil-pressure-sensor-location.36373/

 

[Cubey]

My RV with a weak fan clutch got up around 3/4 of the way on it's factory temp gauge when driving in very high wind (like, 45mph wind, with higher gusts) on a flat back highway in SE SD. I could barely do 45-50 safely due to the wind, partly for safety sake since it was a cross wind. It never overheated, just ran warmer than normal. When I saw it climbing, I'd let off the throttle and pull off the road, if I could. (Did that 2 or 3 times on that drive). "Normal" with that weak fan clutch was about 195-210, so it was probably getting up to 230ish at least. I knew the "normal" temp after installing an Equus mechanical water temp gauge while the old fan clutch was still on it. A new fan clutch & thermostat dropped it to about 180-195 on flat highways with minimal to no wind. Got up to 200-210 in a heavy head wind where 50-55mph was the top speed the poor ol' thing could manage.

 

[Cubey]

@Rattlenbang NA or blown?

Definitely worth it to have a good engine monitoring. Transmission temperature if auto too.

I've read some opinions that monitoring the fluid temp in the trans cooler line before the fluid gets cooled is the better place to monitor than the pan, since that's the actual highest fluid temp in transmission. The pan is gets cooled down fluid dumped back in, showing a median temperature.

Monitoring the trans pan temperature is probably like monitoring the radiator temperature instead of the coolant temp in the engine.

 

[Rattlenbang]

6.9 N/A.
I'm hoping I dont need the fan clutch. Testing it, its pretty stiff, not much freewheeling, no signs of leaks, although its damned old. I already gave it a new T stat.
Hard to say if the cooling system is underperforming; I'd expect a steep climb, buried pedal, and hot day would bring up the temps. The big question is how much is normal?

I think you guys are right, time to pop in a set of gauges so at least I can know what's really happening.

 

[saburai]

I've read some opinions that monitoring the fluid temp in the trans cooler line before the fluid gets cooled is the better place to monitor than the pan, since that's the actual highest fluid temp in transmission. The pan is gets cooled down fluid dumped back in, showing a median temperature.

Monitoring the trans pan temperature is probably like monitoring the radiator temperature instead of the coolant temp in the engine.

It's good to know that before you install your sender on the transmission in the recommend port on the driver side of the tranny! Otherwise, you might find yourself freaking out about the high temperatures until you figure out that the temperatures are substantially higher when monitored at that port. Ask me how I know

 

[saburai]

6.9 N/A.
I'm hoping I dont need the fan clutch. Testing it, its pretty stiff, not much freewheeling, no signs of leaks, although its damned old. I already gave it a new T stat.
Hard to say if the cooling system is underperforming; I'd expect a steep climb, buried pedal, and hot day would bring up the temps. The big question is how much is normal?

I think you guys are right, time to pop in a set of gauges so at least I can know what's really happening.

Motorcraft t-stat I hope...

 

[Cubey]

It's good to know that before you install your sender on the transmission in the recommend port on the driver side of the tranny! Otherwise, you might find yourself freaking out about the high temperatures until you figure out that the temperatures are substantially higher when monitored at that port. Ask me how I know

From what I've read, 150-175 is "normal" for idle/flat road use, while 200-250 is heavy loads/long hill climb. Those ranges are give or take of course, depending on all factors like the vehicle, the load, flat roads/hills/wind, ambient temperature. I read that 270*F is the red line you don't want to cross, like 240*F for IDIs.

 

[firehonomichl]

If you were pushing the motor that hard you really should get an exhaust temp gauge. You can do a lot of damage if your EGTs get to hot!

 

[Cubey]

If you were pushing the motor that hard you really should get an exhaust temp gauge. You can do a lot of damage if your EGTs get to hot!

Agreed.

The aftermarket ATS 085 kit installed way back when the RV was new came with Isspro pyrometer and manifold pressure (boost) gauges, since they are shown on the parts diagram for the kit. I moved the pyrometer up to the A pillar since the steering wheel blocked it down low like it was installed (as in pic below).

The boost gauge can't be moved without more tubing, but it's a bit less important on this kit. It's not a super powerful kit for a 9x7x27' brick and it usually tops out at about 8-10 psi, well below the max pressure (~12psi) for a head bolted 6.9. Isspro must gold plate their plastic tubing since they want $35 for 10' of it, with a few brass fittings I don't need. It's like a coffee stir stick straw for crying out loud. How can it cost much that much? They don't sell the tubing by itself either, only with the fittings.

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