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Ranger: Odd Thump Occasionally at Idle or After Shutdown

ajskyman

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We have a 1991 Ford Ranger with the 4.0L v6. It never warmed up and I finally pulled the thermostat to find it stuck open. After replacement, the engine warms up great and the heater puts out heat for the first time in years. The dash gauge for coolant temp gets into the normal and then fluctuates all over the place....but since it was always cold, I feel the sensor is probably just bad and never noticed because it had always been on C. The radiator is full and the reservoir fills and drains properly. The coolant is quite dirty even after first flush but that will be taken care of completely once I have a bit more time. Where the concern comes in is that occasionally there will be a thump that can be felt/heard in the vehicle. This occurs when the engine is at idle, and when driving, the problem stops. Also, after the engine is shut off, this condition will continue for around a minute or so after shutdown. This never happened before the thermostat was replaced, so is it just the shifting of mounts that have been cold for years? Or perhaps slight boiling of the coolant? I am not sure that the coolant is boiling because the reservoir does not overflow...but perhaps the cap is slightly under pressure allowing for the boiling point to be a bit lower and the condition never showed due to constantly low engine temp.

Any thoughts are welcomed and I appreciate all member's time!
 
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ajskyman

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ajskyman

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New Cap--Not resolved

New radiator cap did not resolve the issue. Gauge still fluctuates and the sound still occurs when it is very cold out. This summer I will do a complete flush of the system and if it persists the water pump will be replaced. My next move will be a new temperature sensor to see if that stops gauge fluctuation. If it doesn't then the coolant really is varying wildly in temperature. However I feel as if the sensor is what is faulty because the heat coming out of the heater is consistently quite hot and does not increase or decrease as the gauge does.

I will post back over the months with any findings.
 
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Anime

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Gauge fluctuation is usually caused by the thermostat. Cheap, thin thermostats with weak springs allow the temperature to vary, and so the gauge goes up with warmer temps sitting at stoplights and moving slow, and goes down when moving fast with air going through the radiator for cooling.

Usually the only gauge that gives a solid reading in the middle is the factory Motorcraft gauge, which is also the only 190 degree gauge there is, but it's $20+ a pop and they don't always last over a year or two. A Stant Superstat is a good choice for the ~$5 they cost, they are a 195 degree thermostat but still well within operating temp. They still fluctuate a little, but not nearly as much as the cheap thermostats.

Temp fluctuations can also be caused by the efficiency of the cooling system. A poorly maintained system with old coolant and buildup in the cooling passages, old hoses that expand with pressure, and a worn out radiator cap will cool ok when conditions are right, then not do so good when things aren't perfect.

It's best to just do a complete drain and flush of the system, throw in a brand new high quality thermostat and seal, use all new hoses, making sure the clamps seal well, and add new coolant following the factory procedure. If the cooling system still needs help after that, sometimes the new radiator cap and water pump does the trick. After that, it's new radiator/heater core time.
 
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ajskyman

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Thank you!

Gauge fluctuation is usually caused by the thermostat. Cheap, thin thermostats with weak springs allow the temperature to vary, and so the gauge goes up with warmer temps sitting at stoplights and moving slow, and goes down when moving fast with air going through the radiator for cooling.

Usually the only gauge that gives a solid reading in the middle is the factory Motorcraft gauge, which is also the only 190 degree gauge there is, but it's $20+ a pop and they don't always last over a year or two. A Stant Superstat is a good choice for the ~$5 they cost, they are a 195 degree thermostat but still well within operating temp. They still fluctuate a little, but not nearly as much as the cheap thermostats.

Temp fluctuations can also be caused by the efficiency of the cooling system. A poorly maintained system with old coolant and buildup in the cooling passages, old hoses that expand with pressure, and a worn out radiator cap will cool ok when conditions are right, then not do so good when things aren't perfect.

It's best to just do a complete drain and flush of the system, throw in a brand new high quality thermostat and seal, use all new hoses, making sure the clamps seal well, and add new coolant following the factory procedure. If the cooling system still needs help after that, sometimes the new radiator cap and water pump does the trick. After that, it's new radiator/heater core time.

I did recently (less than a month ago) replace the thermostat with a Stant , but the cooling system was neglected for years. Next up is the flush for sure. I will post back here with any results. Thank you very much for the helpful information!:salute:
 
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ajskyman

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Update

Just wanted to come back after some work had been done to the truck and post what is going in case this information is of use to anyone else viewing.

The Ranger was in the garage for a new rear main engine seal and some other various things. They flushed the coolant system and refilled. The gauge still fluctuates yet the sound has not returned. I feel as it approaches winter here in Colorado, the sound will return, as it only seemed to happen in the cold.

I am suspecting a faulty thermostat, as new ones are sometimes flawed out of the box.

Heater temperature is still very warm and steady, so as long as the heat is good, I will probably wait until the next coolant replacement to investigate further.
 
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ajskyman

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Another Update

Well here in Colorado, the average temperature has been about 10 all week and the thump has returned, only towards the end of the week and it isn't so bad. The gauge still fluctuates as well. The truck still provides good heat and runs well. Apparently the garage replaced the water pump, so that isn't the issue. I am going to replace the thermostat and replace the coolant (which looks nasty again) over the holidays and I'll report back again. Looks like the neglect of the cooling system has really been the issue. The garage did flush the coolant, yet it looks brown again, making me think that the radiator/block is coated with that nasty old coolant that was in it when I first started the project.
 
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OneofMany

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Brown? rust from inside the block. cant fix that just ignore it. you may also have a lot of junk in the heater core tho.
 
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ajskyman

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Another Update

Replaced the Stant with the OE Motorcraft thermostat and seal today, burped the system, and unfortunately the fluctuation still exists. Next up is a replacement of the hoses because they are cheap. Possibly also new temperature sensors because they too are cheap. After that, probably looking at a new radiator. I do not suspect the heater core as there is hot and steady heat in the cab.
I will check in again once I am able to conduct more work.
 
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ajskyman

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Yet another update

Just wanted to post back again in case anyone else has this issue. I have replaced the temperature sensor now and the system is still fluctuating. At this point, I think when summer rolls around I will throw in a new radiator and hoses. Until then, it works and the heat from the heater core is consistent, so I can ride winter out.
 
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ajskyman

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10/6/14 Update

Hoses and radiator were replaced this summer and the system was burped. The gauge still fluctuates and the thump will come back very occasionally. I have done some research and found this forum thread: http://www.ranger-forums.com/4-0l-ohv-sohc-v6-tech-33/temperature-gauge-fluxuating-130343/

One of the users had this to say:

Today I was successful. I got thinking about the coolant flow and what the added bypass kit that Ford had once offered did. The coolant bypass when the thermostat is closed is limited to the heater core circuit. If the heater core becomes restricted it limits the flow. The heater core feed comes off right by the temp sending unit and the thermostat and the return goes right to the pump inlet. If there is a pressure delta it will create and air pocket by the thermostat and temp sending unit. This caused the dancing gauge because the thermostat can not react in steam.

I replaced the heater core today even though I had good heat. It took care of the problem. No more dancing gauge, no more collapsed hose or coolant blowing out of the cap due to air pockets.

The hardest part of replacing the heater core was draining the coolant. Remove the vanity panel under the gove box. 4 screws to remove on the 1990 and there is the heater core. 20 minutes job and the part was half the price of the pump.

Hope this helps and possibly works for you as well.


I am thinking of trying this as every other piece of the cooling system has been replaced (pump was replaced at the shop visit previously mentioned). Also, since the cooing system was so neglected, I wouldn't be surprised if there was restriction in the heater core.
 
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jd4242

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How about the fan?? Has it been replaced? It could be that the fan thermostat is locked on causing the fan to run full speed.this would keep the water in the radiator very cool, once the motor reaches temp and tstat opens you get a quick burst of extra cool water, it then would close very fast..this would show a fluctuation on the temperature gauge and also can cause bubbling or sounds from too cool of water hitting a hot motor or mixing with much warmer water..in winter it would be even worse as the ambient cold air and the fan running on high can make the water VERY cold in the radiator. .in winter the fan should barley be spinning
 
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ajskyman

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How about the fan?? Has it been replaced? It could be that the fan thermostat is locked on causing the fan to run full speed.this would keep the water in the radiator very cool, once the motor reaches temp and tstat opens you get a quick burst of extra cool water, it then would close very fast..this would show a fluctuation on the temperature gauge and also can cause bubbling or sounds from too cool of water hitting a hot motor or mixing with much warmer water..in winter it would be even worse as the ambient cold air and the fan running on high can make the water VERY cold in the radiator. .in winter the fan should barley be spinning

My apologies for not bringing this up earlier, so let me take a moment and summarize what has been replaced.
Before the truck was getting worked over at the shop, I replaced the thermostat. It had been stuck open for years, meaning that the vehicle never warmed up. First heat up after replacement and the behavior started.
The shop replaced the water pump and fan/clutch assembly among many other things that it needed, not related to the cooling system.
After we got the truck back, the behavior wasn't any different.
Since this initial thermostat replacement, and shop work, I have replaced:
  • Thermostat (motorcraft part)
  • upper and lower hoses
  • radiator
  • radiator cap (motorcraft part)
  • temperature sensor for gauge (motorcraft part)

The fan seems to operate normally. When the truck is running, the fan is inaudible even when accelerating hard, making me think that the clutch is okay. Since the behavior was the same before and after the new fan and fan clutch, I am doubtful that the fan is at fault. An interesting bit of information however was that the system seemed to stabilize early this summer when I replaced the radiator. Upon inspection, the radiator was only about 3/4 full after all of the air had left the engine. When I topped it off, the issue resumed. Less coolant in the radiator could mean that it cooled down less, however being that low could cause a whole host of unpredictable factors. Also, I did the full burping procedure recommended on this vehicle after topping it off, so I am fairly confident that all air has been evacuated. So the only piece of the cooling system left to change is the heater core. My last post and the first reply when I first made this thread both point to the heater core having the potential to cause issues. Due to the age of the truck (nearing 24 years old) and the fact that the cooling system was so neglected, I would not be surprised if the heater core is original and restricted inside. I appreciate the thoughts, and I have a new heater core on the way (only $25 brand new). I should have it in by Friday and I will post back when I get the chance. If this does not fix it, I will be completely stumped.
 
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jd4242

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Aww didnt see the fan had been replaced. Heater core may have been the easiest thing to replace. ..but the fact your getting good heat makes me think the flow is ""alright""
 
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ajskyman

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Aww didnt see the fan had been replaced. Heater core may have been the easiest thing to replace. ..but the fact your getting good heat makes me think the flow is ""alright""

I'm with you, but since someone else said they had good heat and a replacement fixed the problem, I thought $25 was a reasonable price to give it a try.
 
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jd4242

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I'm with you, but since someone else said they had good heat and a replacement fixed the problem, I thought $25 was a reasonable price to give it a try.

O absolutely. .

Just a fyi im getting the same exact thing your explaining but I run a efan...my tstat for fan is in my heater hose not lower radiator hose where it should be..I just haven't had time to get right fitting. .my other had same issue. .once I put it in right spot it went away BUT I did replace heater core same day..Heater core is easy so hope it fixes this ;) thanks for keeping us updated for so long tho, its crazy things like this that seem to never get posted how it or if it was ever solved

Right now my fan tstat is reading the engine water not radiator water..once fan gets water down to temp the engine tstat closes and controls the engine water around 185, higher than my efan stat kick off..so my fan runs for extra time cooling my radiator water way beyond what it should..tstat starts to open again and I get a burst of cold water doing what I explained earlier. .bubbles and causes a fluctuating temperature gauge..just some info for the rare cases
 
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ajskyman

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Promising Results

After replacing the heater core, burping, and testing, the issue seems to have been fixed. Still find it odd how the heater core was the issue, but with as bad as the cooling system was, I'm not exactly surprised.
The results are good, fluctuation is much better. Still a slight bit of fluctuation, however it only happens slightly under the N in NORMAL and goes away once the truck has been running for a while. The temperature seems to be happy on the left side of the N instead of the O, but I can't complain. If any more issues arise with thumping or fluctuation as the Colorado winter temperatures set in, I will be sure to report back.

Thanks for all of the help everyone, and I hope this thread can be of use to someone else battling this frustrating issue.
 
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natenkiki2004

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For what it's worth, mine fluctuates when it's still warming up as well. Mine has an aftermarket radiator, I believe the larger one. Especially with getting on the freeway before the thermostat opens, it seems like it opens and closes 2-3 times before it's truly warmed up. It goes up to the A or L then opens and drops down to N then up to R/M then back down to N/O. I think as long as it goes away shortly after the initial opening, it's not something to worry about, especially in colder climates. It might help to put a higher temp thermostat in there, I think the factory was 180 degrees and you can get 195 degree ones, something like that. Build up a little more heat before it's flushed into the radiator.

Anyway, very interesting that the heater core would cause that issue but glad you got it figured out.
 
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