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Engine Running Cooler Than Normal at Low Speeds


biggman100

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My 1994 Dakota is having an odd issue, that noone else around here seems to have an answer to, only speculation, and i have never encountered before, in any vehicle. The issue is due to a loss of heat at city speeds, and the temp gauge drops fairly quickly. I don't lose all my heat, it just drops enough so you can just start to feel it getting a bit colder, and the gauge will drop to around a 1/4 or so, but, when i am at higher RPM's, anywhere above 1800, the heat picks back up, and the gauge climbs close to the middle and stays there, until i am back in an area where i am doing 35 or less. My honest thought is, since i have already replaced the thermostat, twice, which i will explain why in a moment, as well as all new antifreeze, and flushed the system, that it is probably a water pump issue, but, one guy speculated it may be the clutch fan. The reason i replaced the thermostat twice, is because normally, due to high heat issues when towing, i run a 180 degree thermostat, and that seems to keep it a bit cooler when towing, but still gives it decent heat during normal driving. Sunday, i put a 180 degree thermostat in, as well as flushed the system, and refilled with all new fluid. I immediately noticed the gauge wouldn't go above the cold mark in town, and the heat was almost nonexistent, but then, on the highway, at 55-60, i would get decent heat, but the gauge wouldn't go above a 1/4. Today, i replaced the 180 degree thermostat with a 195 degree thermostat, which is oe recommended, and, the heat now is a bit better, but, in town, the gauge hovers just above 1/4, and on the highway, the gauge hovers just under 1/2, but, it still does the, cooler heat at lower speeds thing. I am leaning towards the water pump, since this engine was used, out of a known running truck, that noone can remember how long it sat, but, since i have already thrown money at it replacing thermostats, hoses, radiator (old one was quite twisted from the truck being in an accident before i got it), heater core, since the old one was plugged with rust, and none of that has fixed the issue, i don't want to just drop more money into it and have the water pump not be the issue.

 

This morning, it did do something odd, but, i may already have fixed that issue. I left home, got about 4 miles, and it lost all heat, and the gauge ran way up, and i started smelling antifreeze, but, before i could pull over, the gauge went back down, and the heat came back, so i drove it about 3 miles to a gas station, opened the hood, and the engine was soaked in antifreeze. I think this issue happened though because i had broke the overflow bottle, and plugged off the overflow hose until i could get a new bottle. I also replaced the radiator cap, and it hasn't done the spewing antifreeze thing again, yet. I just think that one is odd due to it only doing it this one time, and the overflow hose has been plugged since august, because i kept forgetting about it. I replaced the overflow bottle, and it hasn't done it again, but only time will tell on that, if it does it again or not.

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Biggman100...A couple of thoughts.  First, I'm suspect of the fan clutch, too.  If it's staying on at low speeds with the engine not laboring, the efficiency of the fan may be great enough to overcome the efforts of the coolant thermostat.  (The water pump is obviously working in this case!  Replace it only if you suspect wear, a seal leak or a bearing issue.)  Test the fan clutch, it should free-wheel with the engine completely cold.  There may be some fan drag when starting up after the engine sets cold, though this should quickly free up.  When we start a very cold engine, the fan clutch will whir the fan for a moment then gradually release.  This is the cold viscous fluid in clutches with fluid drives.  Cold ambient temperatures will exaggerate this. 

 

If the fan rotates under power all the time, the engine may not be able to warm up and reach thermostat temperature.  If the temperature does get a bit better on the road, this is because acceleration and speed requires horsepower, and horsepower equals approximately 45 BTUs per horsepower.  Slight load, low speeds and less BTUs equals a cold cooling system that the thermostat is struggling to warm up.

 

As for the blowing out of coolant, sounds like a couple of possibilities.  One would be an air block in the cooling system from changing the anti-freeze, and the gap could create a pressure pocket within hot coolant.  This erupted as a boil over of coolant under pressure, and the coolant spewed out through the overflow.  With the new cap, repaired bottle and hose, and burping the system of anti-freeze/coolant air gaps, you should have that problem eliminated. 

 

Here's my approach...When changing coolant/anti-freeze, fill the radiator to the top, cap the radiator, and add coolant/anti-freeze mixture to the right level in the bottle.  Initially, make this above the Cold line, as some coolant will certainly syphon into the radiator and engine cooling system.  Turn on the heater control lever to "HOT" and flow coolant through the heater core.  (Some heaters use a shut-off valve, this assures full flow of coolant.)  Run the engine to thermostat temperature, circulate the coolant, then shut the engine off.  Allow a complete cool down with the system sealed and coolant in the recovery bottle.  This will syphon coolant from the bottle until the radiator is completely full to the cap, after which you can watch coolant level at the bottle.  With a sealed cooling system and recovery bottle, coolant level at the bottle is critical, as the radiator must remain full to the cap at all times.  Sometimes, the system will take several heat-cool cycles to reach a stable coolant/anti-freeze level. 

 

Another possibility is that too much anti-freeze is in the solution or became isolated without mixing.  If you're not using premix 50/50, your own mixture of water and anti-freeze should not be too concentrated or poured into the cooling system as pure anti-freeze.  (Mix coolant/anti-freeze and water before pouring it into the radiator or bottle, just to be safe.)  I'm not sure what your winter ambient temps do at Upstate New York, you may need -45 degree F protection.  If the concentration is too great (read as protection even lower than -60 degrees F or so on a simple hydrometer test), the anti-freeze solution will not expand within itself and can either "boil over" or actually cause a casting crack just like a system full of water only.

 

Here is a PEAK Anti-Freeze protection chart.  This is typical for modern anti-freeze types with 50/50 being -34 degrees F protection.  I never run protection below -62 degrees F or 60% concentration.  If I were at Fairbanks, Alaska or northern Canada, maybe lower temperature protection would be a consideration.  Note the need for a quality, good sealing pressure cap.  Consider the cap part of the boil over protection:

 

What is the proper mixture of antifreeze to water for adequate freeze protection in my area?
While temperatures and climates vary, most vehicles can use our conventional antifreeze protection chart:
Antifreeze%20protection%20chart.PNG

 

Let us know what you find...The 195-degree F thermostat is a very good idea, if you run a 180, there's risk that the engine may not come off its enrichment warm-up cycle.  If so, you'll run rich and use more fuel, there's even risk of carbon buildup.  The PCM is depending on a warmer engine temperature signal to advance past the cold-start and warm-up phase.  As an example, on the Mopar EFI conversion for the inline Jeep 4.2L sixes, Mopar insists on a 195-degree F thermostat for just these reasons.  Your Dodge Dakota EFI/MPI system has similar programming.

 

Moses

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Moses, we keep antifreeze mixed to right around 50/50, and when tested it is good for at least -32 degrees. Luckily, unless the wind is really bad, it doesn't normally get to more than 5 degrees, and that is usually only at night. We rarely see below 0 Temps here luckily.

Now, on to the fan clutch. When we put this engine in I'm pretty sure I put it in with the fan that came with the engine, so, it may have issues. I did notice it does turn kind of hard, but it didn't seem excessive, so I just left it on. I have the original fan that was on the old engine, that didn't have cooling issues, so, tomorrow, I'll swap it just to see what happens. I'll keep you posted.

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I swapped the fan this morning, and it seemed to have gotten a bit better heat wise, although the gauge stays in the 1/4 range, and only goes near half after extended highway driving. One odd thing i did notice though, the bottom half of the radiator seems quite a bit cooler than the top, so i am wondering if it isn't an issue like the 1995 had with the heater core. The radiator was out of the truck for almost 3 months, and mud wasps are a huge issue around here. They will make nests in anything and everything, and i never covered the radiator inlets. On the 1995, it had an issue where the gauge would go to half, the hoses would get warm, but, it wouldn't get any heat inside, so i unhooked the hoses from the engine, filled the heater core with some over the counter radiator flush, let it sit about 45 minutes, and flushed it out with clean water and then blew through it with an air hose, and what came out looked like mud, and it has had good heat inside ever since, so i am wondering if they didn't somehow plug part of the radiator. The part about that that does puzzle me though, is that when i filled it with the new antifreeze, it took about the amount that the book says it should, which is right around 1 3/4 of a gallon, according to my manual.

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The latest info i have gotten on this one, one part really astounds me, the other does make sense. I asked my brother, who worked at a dodge used vehicle dealer for 10 years, what he thought. His first thought was that maybe the impeller is going bad in the water pump. He said it is common on the older dodges for that to happen over time. OK, that one, i can possibly agree with. His other thought though, is that the timing chain may be going bad. I don't see any way that a bad timing set could cause the heating issue i am having, and his explanation, as usual, made no sense at all, so i figured i would put it on here and see what everyone else thinks.

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Biggman100...A worn water pump impeller and poor coolant circulation are always a possibility.  That's worth checking out, though a bad impeller should also cause summer overheating.  At least confirm adequate circulation. 

 

Often overlooked is the thermostat alignment mark.  In addition to installing the thermostat with the correct side up, many thermostats have an indexing mark for setting the clock position of the 'stat.  Is this an O-ring seal thermostat, a gasket or RTV sealant alone?  A concern is whether the thermostat stays seated and sealed in its ledge and does not "float" around with the thermostat housing installed.  If the thermostat is loose and allows coolant to circulate around its edges, at low engine rpm the coolant could be bypassing the thermostat instead of being held back by the closed thermostat when the engine is cold.

 

As for the timing chain, the only relationship between a loose/worn chain and an engine running cold would be lack of manifold vacuum and essentially loss of compression if severe enough.  This would also cause the engine to be sluggish and not produce full horsepower, and here we're back to BTUs: less horsepower output, less engine heat output.  The timing chain would have to be pretty loose to impact horsepower and cooling.

 

My quick check for a loose timing chain (engine still assembled with timing cover in place) can be found at: http://forums.4wdmechanix.com/topic/174-quick-test-for-engine-timing-chain-wear/?hl=%2Btiming+%2Bchain.

 

Moses

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Moses, the thermostat doesn't have any kind of locating mark, and is just a regular gasket type. As for it overheating, when I first got it together, and was driving it, it was late july, early august, the hottest part of the summer here, and it never once overheated on me. Being in sort of a rush, and needing it finished kind of quick, as much as I hate to admit it, I also cut a couple corners, such as, leaving the old thermostat in the engine, and not mixing a proper amount of water and antifreeze, being that I had a bottle that was about 3/4 full, so I just used that, and filled it the rest of the way with water, and the only thing I noticed there was that the temp gauge would stay on the low side. It never seemed to run abnormally warm, or even hot. I also ran it for almost 2 months with no overflow bottle, and the overflow hose plugged off, and still no issues with it running hot. As for the timing/vacuum correlation, I don't see it being that, even though I haven't checked it, due to the fact that it runs and idles very well, gets very good gas mileage, and, being that I live mainly in a hilly, mountainous area, and I never have issues with it losing power going up some of the steep hills, even towing small cars, or full of wood. One thing I did notice though, when I finally changed the thermostat, the old one was quite rusty, which is why I flushed the system before I put the new thermostat and antifreeze in, but, when I did flush it, not a lot of rusty or dirty fluid came out either. Given the symptoms, versus stuff I have found with it, this is why I am at a loss as to what it could be.

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I think what I am going to do, even though it is a bit of a job, is change the water pump anyway, due to it not being an overly costly part, as well as not truly knowing the condition of the pump, being that it was a used engine, even though it may or may not fix the problem, it would still be peace of mind, due to having some long trips coming up with it. I won't be doing it for a few days, but, i will keep everyone posted as to what happens.

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  • 2 weeks later...

I finally got back to this, after needing to use the truck non stop the last couple weeks, and found the problem through trial and error. Ultimately, the issue turned out to be a plugged radiator. I completely drained the cooling system, and also made sure to slowly blow out all the antifreeze with an air nozzle, to make sure the system was completely dry, i then filled the heater core with straight radiator flush, and let it sit for about 20 minutes, reattached the heater hoses, filled the system up with water, and ran it, with the water/flush mix for about 30 minutes. The first 15 minutes, no heat at all, but, the last 15 minutes, i ran it at a steady 2000 rpms, and the heat not only started to work, but worked very well, and, being that it was only 34 degrees out, still made it very warm inside the truck. After i let it run with the water/flush mix, without waiting for it to cool down, which yes, i know you aren't supposed to do, but, i drained the radiator, and for the first couple of minutes, the mixture that came out was ice cold, then as more came out, the warmer it got. I then blew out the entire system, to make sure it was completely drained. The reson i did this was to gauge exactly how much antifreeze/water mix it took. It is supposed to take, according to the factory service manual, just over 3 gallons, and it didnt. It ultimately took just under 2 gallons, so i started investigating further, but i couldnt see anything wrong. I then drove it a round trip of 95 miles, in which it had heat the whole time, but the gauge still fluctuated. After this trip, i started checking, mainly by feel, to see if the hoses, radiator, and other things were warm. During this check, i noticed, the top half of the radiator was almost too warm to touch, but, as i went towards the bottom of the radiator, it cooled down, to where the bottom of the radiator felt like it had sat in ice, and was about the same temp as the outside air, after being checked with a temp gun. This led me to believe the radiator was plugged. Another thing that led me to conclude the issue is the radiator, is due to the fact that the truck was hit in the front right, significantly enough to bend the frame, and also the core support. Those issues were taken care of over the past summer, but, in putting the truck back together, i reused the radiator that was in the truck when it got in the accident, being that it was new not long before the accident. I never suspected that it getting bent a little would cause this issue. The only other thing i did notice, in putting the hoses back on this last time, i reversed them. I dont know if that is why my heat got better, but, i am going to put them back the way they belong, and see if the heat still works the way it should.

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  • 2 weeks later...

Moses, i finally got this issue solved. I changed the radiator, which caused the needle to now stay in a more steady position once it warms up. The needle now stays pretty much in the middle, and doesnt drop off even at slow speeds. I also put the heater hoses back the way they belong, and the truck still has good heat. This has lead me to believe that after flushing the system again, and then reversing the hoses, it backflushed the heater core, and removed whatever obstacle there was. Now it is on to the next project, whatever that may be.

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