Turns out putting the temp sensor in the intake really is no good

[PrairieDrifter]

Side not I believe the factory temp wire is 14 gauge maybe a size bigger. These trucks are old trucks and aren't CRAZY finnicy with stuff like this, it usually comes down to age and deterioration of parts and wires

 

[Swearbody]

Not here to bash on anyone... unless called for....lol. We're here to learn and debate things to figure out what's what.

He didn't state what routing the wire had before he moved it so it's hard to compare, but if it was anything similar to factory it shouldn't have been an extreme difference. He was saying it was pegging but not when, was it always pegged or did the gauge work somewhat properly until engine temps came up and was just inaccurate? The extreme fluctuation is what leads me to believe it was an actual issue with the wire, that resolved itself after being moved around a bunch. If it was just small inn-accuracy then yes the wire for sure could have been the culprit. I believe when the temp wire becomes grounded the gauge pegs to H, I could be mistaking it for the oil pressure sending unit though. That's why I questioned it.

Yeah if its always pegged its grounded out for sure. pretty much any "sender" will have Infinite resistance when grounded. Mine seemed to go half way up then gradually rise to pegged. the odd thing is the two gauges rise together in unison until the factory gauge just keeps on going . I know its not getting any hotter where its at then it would next to the manifold where it was. Id think cooler actually. im gonna look for hotspots with the infrared tomorrow to confirm.

 

[DoubleDingo]

I don't doubt you. Just unprovable by known science. Now... if you really want to make me believe, move the wire all over the engine bay and if you get extremely different readings then I'd be swayed. Or if it could be replicated either by you or someone else

I'd say it's more like the wire was grounding out somewhere that's why it was pegged, and moving the wire around ungrounded it or something like that. Also not knocking on ya, but you can never be sure if you had an air pocket... especially if your switching sensors and plugs around on the system.

Crazy things happen sometimes, and most of the time we don't know the cause, not because we don't know things, but because we can't see the whole picture

Basically what I'm saying is yes, moving the wire may have fixed your issue, but how did it? Science says not because the wire likes to be in a certain position or twisted a certain way. But maybe because of something unseen.

No wire grounding. No instantly pegged gauge. It showed hot at normal operating temperature, and eventually pegged when being driven, then back to hot after stopping. Gauge of wire matched factory specs. Splice was good and solid, soldered and heat shrinked. Changing the routing changed how the gauge read. That's it. I can't explain any better or any different. It's goofy, but it is what it is. I'm now pulling a Ripley... Believe it or not... Because I know what I saw, tested, and fixed.

 

[MikeB]

I use the port near the t-tat housing. That way I read the coolant temperature as it is flowing up from the heads into the radiator. So it's an AVERAGE coolant temperature, not cylinder head coolant temperature taken at one spot on one head. GM does not spec cylinder head coolant temp as far as I know. However, I have found those temperatures get pretty close to the same when the t-stat is open and the water pump is spinning faster than at idle, say at 2000 RPM and above. So don't measure the difference at idle, or with the t-stat closed.

By the way, there is no way the temp at the port near the t-stat is influenced by the radiator temp. I mean we're talking about hot coolant being pumped up thru the t-stat, up the hose, and into the radiator tank.

Oh, yeah, as someone who worked with electronic instrumentation and control systems for 30+ years, I can say electrical wire routing has zero effect on gauge reading. But I'd have to think about that if using a mechanical gauge with a capillary tube.

 

[Swearbody]

I use the port near the t-tat housing. That way I read the coolant temperature as it is flowing up from the heads into the radiator. So it's an AVERAGE coolant temperature, not cylinder head coolant temperature taken at one spot on one head. GM does not spec cylinder head coolant temp as far as I know. However, I have found those temperatures get pretty close to the same when the t-stat is open and the water pump is spinning faster than at idle, say at 2000 RPM and above. So don't measure the difference at idle, or with the t-stat closed.

By the way, there is no way the temp at the port near the t-stat is influenced by the radiator temp. I mean we're talking about hot coolant being pumped up thru the t-stat, up the hose, and into the radiator tank.

Oh, yeah, as someone who worked with electronic instrumentation and control systems for 30+ years, I can say electrical wire routing has zero effect on gauge reading. But I'd have to think about that if using a mechanical gauge with a capillary tube.

I definitely noticed the 2 readings being close once the thermo opened. I rerouted the wire to no avail...gauge still rises to almost pegged(3/4 up or more). The gauge installed in the head is a capillary tube design and is ran straight off the head toward the fender and away from everything. It reads dead nuts according to my infrared.

Ive noticed this plastic capped radiator doesnt register temp the same as the metal capped one did. I mean when I try and read temp at the inlet neck it shows much lower temps than are actually present. I assume the plastic insulates better than the metal end caps because at operating temp the infrared shows the inlet at around 160 but the intake and head temps at 190. The original rads temps readings matched at inlet neck and intake water crossover

 

[fast68chevy]

CTS have different temp to resistance values ranges. make sure it is correct for old school GM guages,,, and you want it to be in the cyl head if at all possible. closer to source of ignition heat and more accurate. when breaking in a motor i like to puill out the thermostat so it wont overheat or at least drill several holes in it. full and intact fan shroud and fan thats pulling good strong air flow throguh radiator.

 

[Swearbody]

CTS have different temp to resistance values ranges. make sure it is correct for old school GM guages,,, and you want it to be in the cyl head if at all possible. closer to source of ignition heat and more accurate. when breaking in a motor i like to puill out the thermostat so it wont overheat or at least drill several holes in it. full and intact fan shroud and fan thats pulling good strong air flow throguh radiator.

I did in fact eventually have to drill two holes in the tstat to break the engine in. The temp sender was matched to the ohm range for my truck. I suspect its the gauge at this point. It now rises to about 3/4 for 190 where before it was around 2/3 for 190. But the aftermarket sender/gauge in the head reads 190 dead nuts so ill just run that one permanently.

 

[Rich VandeWater]

I just put a new 350 in my '86 Suburban. The dash gauge from the cyl head read 220f....so I shut down the engine. Checked the coolant, all was ok so restarted and it did the same. Called Pace Performance who built the engine and they didn't know except to say it had a 195f tstat in it. Unfortunately even though I had been looking at that gauge for 15 years I cant remember what it used to read except it was always consistent. So I did the same as you, installed a Bosch water temp gauge and it reads 190-195 when hot under load. Then I put the dash gauge in the intake and it still read 220! So I have both gauges working and the engine is fully broken in and they never vary. 190-195 and 215-220. I believe the dash gauge was only ever designed to show if you were over heating not exact temp and as others have said being so old it is great that it works at all. My youtube channel orvillespooner1 has several of my engine install videos.

 

[bucket]

I have a couple thoughts to add to this as well. On the fuel injected trucks, the gauge sender was in the head, and the sender (CTS) for the ecm was in the intake. The CTS harness ended up bundled up with the rest of the engine harness, laying on top of the engine and exposed to heat. It works fine that way.

Also, was the temp sender possibly bottoming out on the intake?

 

[Shorty81]

My sensor is in my intake on my bbc. It runs on the cool side. I've checked the heads with my infrared heat gun and the temp doesn't vary but a few degrees.

 

[59840Surfer]

Keep in mind, your cylinder head temp can be quite a bit hotter than the intake temp. Cylinder heads is where all the combustion takes place and the sender is in between 2 cylinders generally. The intake is right next to the thermostat so as the thermostat pops it can be sitting next to cooler antifreeze.

Uhmmmmmmmmmmmmmmmmmmmmm mmmm..... I don't think so...............

For the record, I have dual temp sending units and gauges.

I find that the sender in the intake crossover before the thermostat ('86 K-350/383 stroker, Edelbrock Lo-Riser-QJet) reads about 250°F until the thermostat opens (200°F) and then drops until the 'stat closes again and that crossover coolant temp goes back up.

OTOH - the head position sender unit where the factory puts it, reads what the thermostat setting is (again 200F) once it's warmed up and running or standing (electric fan).

So - unless I've got is exactly backwards - and I'm pretty sure I'm OK here with this memory-thing what I'm seeing is the opposite.

.... just sayin' .............

 

[HotRodPC]

Uhmmmmmmmmmmmmmmmmmmmmm mmmm..... I don't think so...............

For the record, I have dual temp sending units and gauges.

I find that the sender in the intake crossover before the thermostat ('86 K-350/383 stroker, Edelbrock Lo-Riser-QJet) reads about 250°F until the thermostat opens (200°F) and then drops until the 'stat closes again and that crossover coolant temp goes back up.

OTOH - the head position sender unit where the factory puts it, reads what the thermostat setting is (again 200F) once it's warmed up and running or standing (electric fan).

So - unless I've got is exactly backwards - and I'm pretty sure I'm OK here with this memory-thing what I'm seeing is the opposite.

.... just sayin' .............

That is a bit strange. Normally the one in the head reads hotter because it's right there at the combustion chambers where the engine is hottest.

 

[Swearbody]

Uhmmmmmmmmmmmmmmmmmmmmm mmmm..... I don't think so...............

For the record, I have dual temp sending units and gauges.

I find that the sender in the intake crossover before the thermostat ('86 K-350/383 stroker, Edelbrock Lo-Riser-QJet) reads about 250°F until the thermostat opens (200°F) and then drops until the 'stat closes again and that crossover coolant temp goes back up.

OTOH - the head position sender unit where the factory puts it, reads what the thermostat setting is (again 200F) once it's warmed up and running or standing (electric fan).

So - unless I've got is exactly backwards - and I'm pretty sure I'm OK here with this memory-thing what I'm seeing is the opposite.

.... just sayin' .............

Uhmmmmmmmmmmmmmmmmmmmmm mmmm..... I don't think so...............

For the record, I have dual temp sending units and gauges ALSO

I find that the sender in the intake crossover before the thermostat (84 c1500 355,summit heads 2.02in/1.60ex,comp 1.6rr,comp springs,summit chromoly .83 pushrods,comp guides,xe262h cam,air gap intake,1906 carb, recurved timing...blah blah blah its completely irrelevant)reads no more than 200°F until the thermostat opens (195°F) and then maintains that temp until I turn the truck off.

OTOH - the head position sender unit always registers higher than the intake and has on every sbc ive ever owned. In fact its pretty common knowledge that there is an tangible difference in the two. This was never really in question. This was a thread in regards to the possible resistance increase across the wire due to heat.

So - unless I've got is exactly backwards( sorry, Unless i am completely wrong along with tens of millions of other vehicle owners with dual thermostats on a sbc) - and I'm pretty sure I'm OK here with this memory-thing what I'm seeing is the opposite.

So...in closing
SOMETHING IS WRONG WITH YOUR THERMOSTAT OR GAUGE...BRUH

.... just sayin' .............

 

[Swearbody]

That is a bit strange. Normally the one in the head reads hotter because it's right there at the combustion chambers where the engine is hottest.

you are not crazy. that is strange