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Old 04-16-2010, 03:38 PM   #1
evonschu
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Arrow Temperature gauge fluctuation

Hi everyone. I have a 94 Bronco 351. I just had the water pump and upper and lower radiator hoses replaced. After all was said and done I'm having a problem where the temperature gauge is fluctuating. Sometimes it goes almost all the way to H which scares me.

So last night I installed a new thermostat and new coolant temp sensor. Problem still there. I'm convinced that somehow theres still air trapped in the system somewhere and thats whats causing the gauge to fluctuate. I've checked the coolant level while its running and its to the cap. Coolant reservoir also is 2/3 full as well.

I have noticed that when I accelerate the majority of the time the gauge lowers almost to below the N in normal. Most of the time it is in the normal range but the fluctuation is not what I've seen before. When it creeps up to around the H level of the gauge it its usually when I'm at idle or not accelerating. When replacing the coolant temp sensor I noticed that I didn't see any coolant come out of where it screws in to what looks like a L shaped flange into the block. Is this normal? Tons of coolant came out when I changed the thermostat so I know thats ok. Is the small tube going into that L shaped flange plugged possibly? Is my water pump not working correctly? What are some tips as far as making sure there is not any air trapped in the system? I've tried squeezing the hoses to attempt to get bubbles out but none do.

Thanks for any input.
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Old 04-16-2010, 04:04 PM   #2
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The only way to remove any possible air in the system is "burp" it by removing the radiator cap COLD of course and run the engine until you see air bubbles in the radiator neck and coolant level bobbing up and down then top it off...make sure the coolant level is just up to the base of the radiator neck.....
You may have to burp it a couple of times......Bronco's get thirsty so top off the over flow tank about 1/4 full..

The "temp guage sending unit" is usually located up front in the lower intake manifold, pipe threaded with a wire lead to the idiot dash guage....check the wire lead for proper voltage and ohms etc...the brass style temp guage unit is the best.


It probably would have been a good idea to "backflush" the cooling system if it wasn't done...

Hopefully who ever installed the thermostat and water pump checked the Haynes Repair Manual to make sure they installed the correct thermostat and water pump for your year Bronco.....?


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Old 04-16-2010, 04:33 PM   #3
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I installed the thermostat & ECT and bought the water pump (WP was installed at a local shop). They were what was what NAPA said is for my vehicle for the WP and what AutoZone said for the thermostat and ECT. The ECT I replaced was brass. My confusion is that where the ECT threads into is not directly in the intake manifold, it threads into what looks like a 2" high post that has a small rubber tube going into it and that 2" post screws into the manifold. That post did not have any coolant in it when I replaced the ECT which I found odd. I might try to replace that small rubber hose to see if that is plugged and not allowing coolant to enter where the post and ECT brass head is. I will also try to burp out the system again. I didn't run it very long with the cap off to see if the coolant went down or not when it warmed up so that might be part of the issue. Should I blast the heater when I do this?

The coolant was backflushed right before I replaced the water pump.
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Old 04-16-2010, 05:44 PM   #4
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What JKossarides is trying to tell you is that you replaced your Engine Coolant Temperature Sensor (ECT), which sends a signal to your PCM to assist in determining proper Air/Fuel Ratio.

I would not have replaced this Sensor unless I received a 21, 51, 61, 116, 117 or 118 trouble code, which would indicate a failure in the PCM coolant temperature circuit. When you receive one of these codes, the next step is to go to the ECT Sensor and test for a resistance value of 58,750-40,500 Ohms cold and 3600-1840 Ohms hot, and replace the Sensor if it is out of range. Then test the harness connector for an approximate 5.0v signal voltage, and look for a short or ground if required.

What he is recommending is that you replace your Coolant Temperature Sender which sends a signal to your dash gauge. These are cheap and frequently go bad giving you an erroneous temperature gauge reading.

Test it by grounding the single wire that goes to the Sender and momentarily key-on your truck. If the gauge goes full tilt, replace your Sender. If all is well and the problem continues, then look elsewhere.

To be clear, the Engine Coolant Temperature Sensor has nothing to do with regulating engine temperature nor signaling the gauge. It only sends information to the PCM.

See Temperature Gauge Sending Unit below.

Photo from Gacknar's SuperMotors web-site.
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Old 04-16-2010, 06:52 PM   #5
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Quote:
Originally Posted by evonschu View Post
Hi everyone. I have a 94 Bronco 351. I just had the water pump and upper and lower radiator hoses replaced. After all was said and done I'm having a problem where the temperature gauge is fluctuating. Sometimes it goes almost all the way to H which scares me.

So last night I installed a new thermostat and new coolant temp sensor. Problem still there. I'm convinced that somehow theres still air trapped in the system somewhere and thats whats causing the gauge to fluctuate. I've checked the coolant level while its running and its to the cap. Coolant reservoir also is 2/3 full as well.

I have noticed that when I accelerate the majority of the time the gauge lowers almost to below the N in normal. Most of the time it is in the normal range but the fluctuation is not what I've seen before. When it creeps up to around the H level of the gauge it its usually when I'm at idle or not accelerating. When replacing the coolant temp sensor I noticed that I didn't see any coolant come out of where it screws in to what looks like a L shaped flange into the block. Is this normal? Tons of coolant came out when I changed the thermostat so I know thats ok. Is the small tube going into that L shaped flange plugged possibly? Is my water pump not working correctly? What are some tips as far as making sure there is not any air trapped in the system? I've tried squeezing the hoses to attempt to get bubbles out but none do.

Thanks for any input.
So... while driving, the temp drops but, when sitting at idle, it climbs, right?

I am sorry if I come across like an ass.. I don't mean to... Honestly.

OK.. what is the major difference between moving and sitting still?
Air flow.

You probably see your fan moving (you do have a mechanical fan, right?) when the engine is running but, it probably is not moving fast enough to give sufficant air flow to the radiator. That would indicate a bad fan clutch.
As you are moving down the road however, the wind is hitting your radiator and therefore cooling it down.

It happens often enough.
The fan had to come out in order to put the new water pump in and if the mechanic dropped it or someting, the clutch could have gotten damaged.
A new fan should be cheap enough to replace and it is something that you can probably to in an hour or so with basic hand tools.
The plus side is that the bolts should be rust free (thanks to the water pump replacement)
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Old 04-16-2010, 07:06 PM   #6
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The temperature sometimes goes up though even when I'm moving fast but not accelerating. Although most of the time it is when I reach a stop. I'll try replacing that sender unit first and see if that helps. Then I'll look into the fan clutch. Thanks for the help and no I didn't think you came across as an ass at all.
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Old 04-16-2010, 07:55 PM   #7
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an easy way to see if it is the fan clutch is to "lock" the fan to the clutch. Use a heavy tie wrap or piece of wire to lock the two together so that the fan spins at engine speed no matter what. If your over heating problem is solved then it is obviously the clutch, if not then you have other issues like; air in system, bad water pump, bad temp sender, etc. Do this and then post back after driving for a bit.
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Old 04-16-2010, 08:13 PM   #8
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I would try the fan clutch test first. I had one do the same thing a few years ago.
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Old 04-17-2010, 03:11 AM   #9
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So the clutch is working cuz the fan spins faster as I accelerate. Also the temp gauge was going up and down even when sitting at idle so I doubt its actually a problem with the fan on the system. So I replaced the sender unit for $5 and now the temperature is not fluctuating, but is remaining high at around the L or above in Normal. It doesn't seem to change much whether I accelerate or not now though. Its higher than I'm used too but within what the owners manual considers the normal band. When the bronco is cold it shows cold on the gauge and then warms up but just goes too high once it does warm up. I was thinking of maybe doing a coolant flush myself even though I had one done at jiffy lube right before I had all the hoses and water pump replaced. I can see lots of gunk floating around with the coolant in the reservoir. The coolant is still way green though. Would this even make a difference to the problem I'm having? I thought maybe the radiator might be clogged up with gunk. But that wouldn't make sense cuz they sucked all that crap out at jiffy lube which I'm sure wasn't as good as an actual flush with the hose but I was in a hurry. I'm really just fed up with this issue. Any more suggestions?

If I do do a flush I was gonna use the hose and go off what the haynes manual says. Maybe squirting a bunch of water up the lower and upper hoses into the engine. How much coolant would I need and what mix of water would I need to do this for just the radiator assuming the rest of the coolant in the block is at the correct mix ratio? Whats the exact capacity of the radiator? The manual says to look in the refill capacities but I see nothing in there about coolant fill amounts. To make matters worse I just broke the lcd on my blackberry when I was leaning over to check one of the sensors yesterday. *Tired and Frustrated*
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Old 04-17-2010, 10:24 AM   #10
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Also put a bottle of Redline Waterweter in with the collant it helps ...
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Old 04-17-2010, 03:44 PM   #11
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The ECT and Temp Guage Sending Unit are 2 different items but both are affiliated with the lower intake manifold....

The ECT= engine cooling temp sensor threads into an octagonal shaped TREE which inturn threads directly into the lower intake manifold and if you look at it closely you'll see the rubber hose is for the lower throttle body cooling tube, a design for colder regions to heat the TB. However be aware that these small "metal" cooling tubes are pressed into an aluminum TB block and rust and dissolve over time creating leaks there..... so that small tube on the "octagonal tree" for the lower cooling tube will do the same but you can drill-tap threads for a short 3/8 brass tube there and be done with it, that's what I did on mine and it makes disassembly a lot easier not having to "turn" the distributor out of the way......

I'm not sure what anyone here has done about those TB cooling tubes, can they actually be eliminated like blocking them off etc. short of replacing the TB itself....like I did..

You'll also notice there is a "longer metal tube" on the left side of the Tree which has a heater hose attached to it....stupid design IMO but it is what it is.......lol lol

Good Luck ~
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Old 04-17-2010, 07:07 PM   #12
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Flushed the radiator and hoses thoroughly and put some fresh coolant in. Will see if it makes a difference over the next 100 miles or so... I've already replaced both the ECT, thermostat, and sender unit JK. Thanks for the info though.
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Old 04-17-2010, 08:06 PM   #13
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have you taken the upper hose off and looked inside at the condition of the core? Hows your fan shroud?
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Old 04-17-2010, 11:22 PM   #14
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my truck is kinda weird. when i replaced the t-stat it would do what the OP's does. then i replaced the sender unit, same problem. let it idle with the rad cap off for about an hour. now what it currently does, is in the mornings or just the first time i've driven it during the day, the needle on the dash will go just barely into the H zone for about 10-15 seconds but then back off. for the rest of the day it will just fluctuate around the middle (straight up). i dunno why it does that but i don't think it's a major problem

ok hijack over
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Old 04-18-2010, 03:49 AM   #15
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Schucks sold me a crap sender unit. Replaced it AGAIN with a NAPA one and now everything is working as it should and any fluctuations stay within the NORMAL band of the gauge.
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Old 04-18-2010, 05:37 AM   #16
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Schucks sold me a crap sender unit. Replaced it AGAIN with a NAPA one and now everything is working as it should and any fluctuations stay within the NORMAL band of the gauge.
If the engine is not actually overheating, then it is either the sender or the gauge.
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Old 04-18-2010, 09:16 PM   #17
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Glad you got it sorted out Brother ~
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Old 04-20-2010, 03:23 AM   #18
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Hi there, I think im having a sort of similar issue and Im glad there's a current thread here. My idiot gauge has run -very- cool since an incident a long while back where I overheated , it would never go past "the N" on Normal. a few times since then it has heated up past there and gone all the way to L and flunctuated pretty wildly , and its usually been a sign (even though my pressure gauge showed normal) that I needed more oil, check the oil and sure enough oil solved the problem and the temperature would stay at "N" again.

I just a tune up about 3 or 4 weeks ago, and no difference until this week, suddenly now Im fluctuating again. today i filled my radiator overflow to the recommended level (seemed low) ,checked oil : oil is on-the-dot perfect levels and not burnt or anything.

Yet I still head up to the L and then it goes back to the middle then back to L , does NOT correspond to whether im moving or idle. totally random. sometimes it will stay on L sometimes it will hover in the middle of Normal.

My question is, is it the ECT sensor you guys are discussing? thermostat potentially? I will double check radiator fluid again but im pretty sure thats not it. SeattleFSB suggests if the engine is not actually overheating then its probably the sensor or the gauge...what is "overheating" defined as? all the way to H? or is hitting the end of the L on normal overheating? If its the gauge what do I have to replace? do I actually have to crack open the dash at that point? (gawd hope not)

Id rather stay on my kick of fixing this stuff myself when possible instead of taking it to the shop, you guys have been pretty great about advice.
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Old 04-20-2010, 05:08 AM   #19
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Hi there, I think im having a sort of similar issue and Im glad there's a current thread here. My idiot gauge has run -very- cool since an incident a long while back where I overheated , it would never go past "the N" on Normal. a few times since then it has heated up past there and gone all the way to L and flunctuated pretty wildly , and its usually been a sign (even though my pressure gauge showed normal) that I needed more oil, check the oil and sure enough oil solved the problem and the temperature would stay at "N" again.

I just a tune up about 3 or 4 weeks ago, and no difference until this week, suddenly now Im fluctuating again. today i filled my radiator overflow to the recommended level (seemed low) ,checked oil : oil is on-the-dot perfect levels and not burnt or anything.

Yet I still head up to the L and then it goes back to the middle then back to L , does NOT correspond to whether im moving or idle. totally random. sometimes it will stay on L sometimes it will hover in the middle of Normal.

My question is, is it the ECT sensor you guys are discussing? thermostat potentially? I will double check radiator fluid again but im pretty sure thats not it. SeattleFSB suggests if the engine is not actually overheating then its probably the sensor or the gauge...what is "overheating" defined as? all the way to H? or is hitting the end of the L on normal overheating? If its the gauge what do I have to replace? do I actually have to crack open the dash at that point? (gawd hope not)

Id rather stay on my kick of fixing this stuff myself when possible instead of taking it to the shop, you guys have been pretty great about advice.
Theres the ECT which supplies info to the computer and then theres the sender unit that looks really similar. It sends info to the gauge. I had to replace that twice (Cuz Schucks sold me a bad one), but also replaced the ECT and thermostat in the process. Mine still hangs around the L in normal sometimes and fluctuates down as I accelerate and no doubt cycle cooler fluid through the system by the thermostat and water pump. The manual describes anywhere in that bracketed area as normal although I don't know how accurate that could be. Honestly I think the whole gauge setup is s**t. I might just add in an actual coolant temperature gauge to know what the real value is at some point IDK.
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Old 04-20-2010, 03:35 PM   #20
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Honestly I think the whole gauge setup is s**t. I might just add in an actual coolant temperature gauge to know what the real value is at some point IDK.


Just saw this thread... I would strongly suggest you use an aftermarket gauge so you know what the temp actually is. And FYI, the temp on my gauge fluctuates from 195ish to 205ish as the tstat opens and closes...
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