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Water temperature fluctuating again


josh817

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So last time I had this problem I thought it was the thermostat getting stuck open or foam bubbles forming around the sensor.

 

Essentially what happens is my temperature drops way low, like in the cold section of the gauge, where its at when you first start the car.

 

Today I got hard on the brakes and came to a complete stop. When I looked down the temperature had dropped below what the gauge reads (like when you first start the car). As soon as I bring up the idle, it rose up slightly. When I pulled away and was cruising at 30mph, it read fine again.

 

Any ideas what this could be?
 

  • faulty sensor or bad wire coming from sensor
  • bad thermostat
  • foam in the cooling system (when the sensor gets dry, it will read cold from my experience on other cars)
  • faulty voltage regulator on gauge cluster (fuel gauge reads fine)

 

This is all I can guess. I highly doubt its the thermostat. The speed the needle dropped and rose back was far too fast for the temperature to actually do that unless the sensor did go dry and then when water made contact again it picked it back up.

 

 

I must add that I run a 185ºF thermostat. This usually runs right on that middle line or just below it. When this problem started, it was running cooler, half way between the greenish/blue "cold" section and the middle line.

 

I illustrated it below. The red line is where it likes to read right now. I usually have it right at that black middle line. The blue line is where it sometimes drop to when I come to a complete stop. Never happens while cruising. The gauge should peg out on hot if grounded, so maybe I have a bad connection on the sensor? The old sensor I had, had this problem. You would wiggle the wire at the base and see the gauge go all over.

KxuoejS.jpg

 

I appreciate any input. With these old cars its imperative that you trust the information it gives you. With a faulty reading I begin to doubt what temperature I'm REALLY running at.

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Ground the wire going to the senser, it should show pegged hot when you do this.

Maybe the wire that plugs onto the senser is lose, when one lets off the pedal the engine shifts slightly., and when you give it the gas it also shifts slightly, so make sure the senser wire plug is clean, also the senser post the wire plugs onto, when the engine shifts the wires move a little, that can cause issues sometimes, my issue was the matchbox wires, when ever I let off the gas, the connection was broken to the distributor and the engine would quit hitting, and when I down shifted it would re-connect, I think it actually re-connected when I pushed in the clutch to down shift. once I got the connections sorted out I never had the problem again.

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You have 3 vehicles on your Intro. We can assume [?] thatyou are not complaining about your BMW aren't we? SPECIFIC ON YOUR COMPLAINT" We are Not mind readers! If you want help think before you type! We don't have limits on charachters like your cell phone! Think, don't just punch your keys!

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OK, I am not saying this is what is happening, but I am telling you this to help you understand what can happen so you will have more ideas/info of what can happen.

I have a 1968 Datsun/Nissan U20 roadster engine in my 67 roadster, it has an aluminum thermostat housing coming off the side of the head just like the L series engines, I figured out that the thermostat housing was not making a good ground to the head, I confirmed this by touching the senser body with a wire that was grounded to the battery, not the wire post, the senser body that is turned into the thermostat housing, when I grounded it out the gauge worked again.

I tried turning the senser into the hole another 1/4 turn, but that did not fix it, maybe if it is the issue, you can remove the senser and clean it up and re-install it, but first check it out to make sure the housing ground is the issue by grounding it out directly with a wire.

I knew you were talking about a 521 because of the photo of the 521 dash/instrument cluster.

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Waynos theory sounds either dead on, or very close. You are looking for a connection issue. It could be to ground, to the spade connector, to the back of the gauge itself, the wire between the gauge and the sensor. If not this, the gauge or sensor could possibly be going. If you have a volt meter, clamp the leads to the back of the gauge while driving. If the needle falls, and the voltage does not change, you have a bad gauge. If not, run a jumper wire to the sensor so both the volt meter and gauge are connected. And ground the other lead. Same test while driving. If the gauge drops, and the voltage remains, you have a problem in the wire between the gauge and sensor. If the gauge drops, and the voltage drops, it's is the housing ground or sensor. Sensors are dirt cheap, and new bolts for the housing are too. Replace both.

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In my experience, a 521 is under engined, and over radiated.  This is what I mean by that.

In the 1970's and into the 1980's I used to show horses.  I had a lightweight two horse trailer, that I towed behind a 521, initially with  the stock L-16/4 speed,  later with a L-18/5 speed combo.

 

These were full sized riding horses.  15 to 16 hands tall, not pony’s. Over 1000 pounds each.  Yes. the trailer had brakes.

 

The temperature gauge would stay in the normal range almost all the time.   In extreme cases, climbing Cabbage hill, east of Pendelton, OR, in 95 degree heat, in second gear, about 5,000 RPM, almost full throttle, I would turn on the heater to keep from the red zone on the temp gauge.  That would bring the needle back closer to normal.

 

The temp sender is in an aluminium thermostat housing, bolted to an aluminium head.  Even if there are air bubbles, or the coolant is low, but not gone, the temp gauge will read pretty close to the temperature, if everything is electrically working.

 

A 40 ohm resistor connected between the temp sender wire, and ground, should make the temp gauge read in the middle.  Conversely, I would think at normal operating temp, the resistance between the terminal on the temp sender, and ground should be 40 ohms.

 

By the way, if you disconnect the temp sender wire, and hook a test light between it and ground, the light will flash.  This is caused by the gauge voltage regulator functioning.

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Not from my experience. I've driven around normally only to find the rad core exposed and needing more than a quart of water to fill. The pump pushes a lot of water into the block and up into the head keeping the air pushed out into the rad. Best not to ever run it this low.  

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My apologies for not getting back. We had a tornado or something go through campus here in Arlington. Luckily the Engineering Dungeon is pretty solid, so was the ol' Datsun outside.

 

The problem is with the 521. It has an L20B with a Fiero radiator, plastic tanks. Have an overflow tank but that may have dried out.

 

I will try again tomorrow, all the things you guys have recomended. I suspect a connection issue, myself.

 

So...

  1. check fluid level
  2. ground sensor body; check if it reads correct
  3. ground sensor wire to gauge; check if it pegs
  4. hook test light to gauge wire and see if it blinks
  5. wiggle sensor wire near the base of the sensor and see if gauge freaks out
  6. for shits and giggles do another leak down

 

As always thanks for the speedy input.

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  • 5 weeks later...

I checked the levels, it took more water. Was about 3/4 full in the radiator. Temperature doesn't fluctuate now.

 

I had wiggled the wire to the sending unit and it did nothing at the gauge.

 

My next concern is the load from the alternator due to the headlights being on. If the engine shutters or has a low rumble idle while the lights are on, the belt will squeal when RPM's increase. It's enough to raise the temperature substantially. Probably a glazed belt. I can tighten it but to get it tight enough to not squeal I wind up getting noise from the alternator like I'm killing its bearings.

 

But that is a different problem.

 

I have to do another leak down test and maybe see where the water is leaking. Obviously its a slow leak, as I haven't needed to address fluid levels for a year. The heater core has always blown fog. Maybe it has some sort of seepage.

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"The heater core has always blown fog. Maybe it has some sort of seepage."

 

This tells me your heater core is leaking.  A very small amount of coolant leaking can make a lot of fog.

 

The instrument cluster has its own voltage regulator.  It regulated the gauge voltage down to about 8 volts.  If your engine cranks, and starts, there is enough voltage for the gauges to read correctly.

 

Go and get a new fan belt.  It should not have to be that tight, to run the alternator without squealing, unless you are running a huge alternator. 

 

Pressure test the cooling system.  But if your engine has poor compression, that will make the engine run hot, because hot gasses are slipping by the piston, and instead of the heat being used to push the piston down, it just goes into the cooling system.  A similar thing happens with retarded timing. The piston is too far down in the cylinder bore, and that exposes a lot more cylinder wall to the heat of the burning gasses.

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Timing is good, probably just the belt. The cold doesn't help right now either. Use to squeal when the engine shuttered while pulling away from a stop with the lights on. Now just sitting at idle with the lights on (if that's not enough then turning on the heater or a turn signal will do it) the belt will start to chirp.

 

Your opinion, would this problem be alleviated by putting a newer alternator on there (KA24 or Saturn 90A conversion)? In my mind, energy is energy. If it takes 12 volts @ whatever amps to power the lights then it will load the same on any alternator IF the efficiencies are the same. I don't see it being a fix... I need more power anyway though. Currently have a '78 (or something later) 35-40A internally regulated alternator. It's from a 720 or a hard body, I can't remember.

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Replace the belt. Yours may be damaged or glazed. Tighten the belt until you can only deflect it about a finger width using only thumb pressure.

 

A higher output alternator will need about the same amount of power to run the headlights but.... after starting the engine the battery is going to be slightly discharged from sitting over night and cranking the engine and the alternator will start to recharge it. Because it can supply more power than the alternator you have now it will need a lot more drive power from the engine and the belt may be over loaded. I have a 90 amp on one of my 710s and it always starts with a screech even though a new tight belt on it. This is why the newer high amperage alternators are all driven with a wide serpentine belt and not the old V type. The serpentine belts have better traction to drive them.

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I will put on my L16 pulley. I tried to avoid it since it is a pulley and not a damper, which is what I have now.

 

Datzenmike... that's really unfortunate. I want to get over this squeal. Mine too squeals when I first start.

 

I will start the motor, the IGN light will be on indicating no charging. Blip the throttle to tickle the alternator and it will start to charge, and usually wait about 10 seconds at idle. If I don't wait, it will squeal when I pull away. Of course with the engine cold you almost always get a shudder when you pull away. I wonder what our options are if the V-belts can't handle much load... I was planning on adding an electric fan, already have a radio, and electric fuel pump. Doesn't sound like it will turn out well if I had everything turned on at once.

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The L16 doesn't come with a crank shaft damper. The internal parts are very well balanced and matched at the assembly plant. Use the crank pulley with confidence.

 

Perhaps a good quality brand name Vee belt is needed. Like over $10.

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The L16 doesn't come with a crank shaft damper. The internal parts are very well balanced and matched at the assembly plant. Use the crank pulley with confidence.

 

Perhaps a good quality brand name Vee belt is needed. Like over $10.

I am running an L20B, that's why I hadn't used the L16 pulley initially.

 

The highest RPM my engine has seen is probably 6000 RPM, but with the low 521 gearing and the 5 speed (the early one where 5th gear is very close to 4th), I cruise on the highway routinely at 3000-4500 RPM.

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