Erratic/cycling idle

[AeroRamer]

Erratic/Cyclic RPM idle and Overheating problem

About a week ago I started noticing the Ex idling erratically, with the RPM’s going back and forth or cycling between normal idle of about 600 RPM to about 1200 RPM every 3 to 6 seconds, sometimes a few more seconds longer in between.

This appears to happen only when the truck is in gear and I am stopped. I have not noticed this while the Ex is in neutral or parking, or when coasting. It appears to happen only in first gear. My Ex is auto.

One interesting thing I noticed this morning when I was parking the Ex is that the jump and cycling of RPM’s is happening right after the temperature indicator needle starts moving noticeably fast to the hot range. At that time, the RPM’s jump and the temp needle immediately moves back to what it was before. It is like the engine is sensing a higher temp and increase the RPM’s trying to cool the engine. For comparison, it behaves similar to turning the A/C on and the compressor cycles from on and off and the engine RPM adjusts to it.

I have yet to check for any codes as I just noticed this last week but I wanted to check with you all and see what you think. Any comments on this issue?

 

[RomeovilleIL]

To clarify, you are stating the engine begins to heat up quickly after you come to a stop followed by the RPM rising?

To find out the source of the surging, you may best start by checking why you are overheating. The engine will race when it overheats. When the engine is cool, check your radiator and overflow. Are they both full? If not, fill and see if 1. issue goes away and 2. it leaks out somewhere.

If the coolant system is not low, try turning heat on max and see if it gets hot and stays consistent when the engine is running. There is a good chance it could be simply a back thermostat. << bad thermostat

 

[AeroRamer]

Yes. The engine begins to heat up quickly after you come to a stop followed by the RPM rising. Two days ago it was not that bad. The temp gauge increased slightly. I took a video of it and uploaded to YT as unlisted. To see it go here: http://www.youtube.com/watch?v=N_omxYaJLvE

Today, is a different story. I had to go on a 90 miles drive on mostly Interstate and now the temp gauge is almost straight up just shy of the red hot zone. Again, a few seconds later the needle moves back to the middle range and cycles. To prevent the engine from overheating I used the old trick I learned a long time ago of turning the A/C in hot. That kept the engine temperature in the middle range until I got home.

I stopped at the local auto part and got a new thermostat. I will going to replace it and see where that takes me.

 

[swetrid]

Sounds like typical thermostat concern.

 

[AeroRamer]

I just replaced the thermostat. No luck. The problem persists.

Not sure what else to check now. Even though I have no 'Check Engine' light on I will try to see if there are some codes. Maybe the light does not work. It does light on when I start the car though.

BTW, as a side note, it took me about 1.5 hours to replace the thermostat. I took some notes and pictures along the way. I'll try to remember to post them later.

 

[swetrid]

Make sure you bleed the air out, also what engine are you working on 4.0L SOHC?

 

[AeroRamer]

Oops! Yes, 4.0L SOHC. It is a 2000 Limited, 2WD, Auto.

Bleed the air out?! I feel ignorant. I have not heard about that before for a coolant system. How is that done? By opening the drain plug on the radiator? Would not any air get out as the system circulates?

On a similar note. When the thermostat is removed, is there supposed to be a lot of coolant coming out from the housing area? Instructions call for draining the coolant but I did not have to do that. There was no overflow coolant.

 

[RomeovilleIL]

Usually when you remove the thermostat you will have the contents of the system above it drain out unless you drained. So that plus the empty overflow indicates you have a leak somewhere which needs to be found and fixed. Fill the system, pressurize, and look for leaks.

As for getting air out of the system, I would start first and foremost by getting the system full. Fill the rad and overflow and run with heater on to see if the system purges itself through the overflow tank. Level should drop as liquid is pulled in. Refill and see if your problem corrects.

If not, then you likely need to burp the system. Top off radiator, fill overflow, put on radiator cap half way to first click and run a little while from cold start at 1500-2000 rpm. The air should burp out after a few 5 minute sessions. This is messy so have a large pan under the truck.

 

[AeroRamer]

Well, I got me a new Actron CP9580 OBD II and ran an auto scan hoping to find something that will help me T/S the overheating problem. The scan came out empty. No codes found. I ran the Real-Time option and checked the Coolant Temp Sensor data. From the chart display you can see the temperature increasing, and decreasing as RPM's increase.

I will try topping off the radiator fully and try your recommendations. I am certain there is a leak somewhere as I can smell evaporated coolant. But the coolant level is not noticeably dropping to make me believe that the cooling system is starving for coolant. Perhaps the water pump is going bad? The belt is loose? One weird thing thought...I can hear a strange noise that appears to come from under the System Check display. Funny thing is that it sounds like mice making noises. I tell my kids they are hamsters I keep there.

 

[RomeovilleIL]

You are likely hearing air pockets going through the heater core. When the rad is full and engine is hot, try squeezing the upper and lower rad hose to see if they have pressure. Also, take a few minutes to watch the hoses as the temp rises. If you have a big enough air pocket, the hose may partially collapse for a moment.

It may very be that your pump is shot, but I would concentrate on finding the leak first before you start tearing things apart and buying parts.

 

[AeroRamer]

I thought of adding an update. The problem went away by topping the radiator with water/coolant. The hamsters are gone and everything is back to 'normal'.

One thing I do not understand is if the radiator was low on water/coolant, why it did not pull water from the reservoirs? Or, is the reservoir just for overflow?

 

[yavapaires]

No, the reservoir is intended to be a holding tank for coolant. The coolant will pump INTO the tank when hot, and PULL OUT of the tank when cool, thus allowing the radiator to remain full at all times. Look closely at the reservoir, you will see a marking cast into the tank. It should say full cold. Keep the coolant level at this mark, and you should not have to remove the radiator cap.

 

[Lube]

No, the reservoir is intended to be a holding tank for coolant. The coolant will pump INTO the tank when hot, and PULL OUT of the tank when cool, thus allowing the radiator to remain full at all times.

True assuming the radiator cap is working correctly and is the right one. Rent a radiator pressure test from parts store. It has an adapter to test the cap, and it will allow you to test the rest of the cooling system as well. Looking for a leak on a hot engine is a good way to get burned. The tester allows leak checking on a cold engine.

 

[yavapaires]

True assuming the radiator cap is working correctly and is the right one. Rent a radiator pressure test from parts store. It has an adapter to test the cap, and it will allow you to test the rest of the cooling system as well. Looking for a leak on a hot engine is a good way to get burned. The tester allows leak checking on a cold engine.

That's why the more the merrier on this forum. One person does not have all the answers or, in my case,remember all the possibilities for a problem.

 

[AeroRamer]

No, the reservoir is intended to be a holding tank for coolant. The coolant will pump INTO the tank when hot, and PULL OUT of the tank when cool, thus allowing the radiator to remain full at all times. Look closely at the reservoir, you will see a marking cast into the tank. It should say full cold. Keep the coolant level at this mark, and you should not have to remove the radiator cap.

That is exactly what my understanding is of the radiator system. That is why I found it odd. When I noticed the radiator needed fluid I noticed that the reservoir was to the FULL line. Expecting to see an overflow I proceeded to top off both the radiator and reservoir to the absolute top. After a few months, the gremlins came back telling me the radiator was running low on coolant. When I checked the reservoirs it was still completely full of fluid to the top. The radiator needed about 1.5 gallons of coolant.

True assuming the radiator cap is working correctly and is the right one. Rent a radiator pressure test from parts store. It has an adapter to test the cap, and it will allow you to test the rest of the cooling system as well. Looking for a leak on a hot engine is a good way to get burned. The tester allows leak checking on a cold engine.

I know there is a leak somewhere but can’t find it. After I replaced the thermostat obviously I did not installed it right and I can see coolant dried up around the housing. The leak is probably very small. I can smell the coolant emanating from the engine but cannot locate it. Maybe (I hope) the problem was with the old thermostat or housing and with my bad install the problem remains. I need to fix that first before T/S for another leak or cap problem. I did not know of a tool to check for that. Thanks.

 

[yavapaires]

Another thought. Check the overflow tubing for a kink or any damage. Radiator would probably NOT pull coolant from tank after shutting down until coolant cools. If you had air in the block, you will have to keep watching the reservoir level until all the air is expelled.

 

[bobflood]

Another thought. Check the overflow tubing for a kink or any damage. Radiator would probably NOT pull coolant from tank after shutting down until coolant cools. If you had air in the block, you will have to keep watching the reservoir level until all the air is expelled.

Still another thought: Make sure there is not a small crack at the nipple on the radiator neck the overflow hose fits to. If there is, expansion would push coolant out that crack to evaporate away, and never pull coolant in from oveerflow tank. Over time, radiator coolant will go down, but overflow tank always looks fine. Don't ask me how I know this.