Erratic coolant temp

[bmeyer29]

My coolant temperature is very erratic sometimes. Generally it runs around 80C after the engine is warmed up, but occasionally it will bounce up to 90C (in about 5 seconds) then drop back to around 80C (again in about 5 seconds). I haven't worried about this and assumed that was just the normal opening and closing of the thermostat (180F). It did the same two days ago at highway speeds (over a 25-30 minute drive), but today on the same route, it it went up to 90, then down a little, then up to 95 and down a little, and up and down... It got as high as 110C. I had been cruising on the highway for 20 minutes or so, and was going around 70mph in 5th gear at around 2000 rpm. I pulled off the road and let it cool for a while, and then headed home. Going home it was back to staying at about 80C, and then every few minutes it would bounce quickly up to 90C then back down to 80C ( over a total of about 10 seconds).

Is this a sign of a bad thermostat?

Details:
351W rebuilt locally, dynoed on the stand at about 450HP, about 350miles on the engine since the rebuild, 750 Holley carb, large aluminum radiator, ducted fan (and fan on!), 180F thermostat with small check valve at 12 o'clock, 50/50 mix, coolant within 1/2" of radiator cap when cool, 16lb radiator cap, metal pipe for most of the lower radiator hose so unlikely to have collapsed from heat/suction.

My gages are Autometer and in Celsius, so I have to translate them to more familiar numbers:
80C=176F
90C=194F
100C=212F
110C=230F

Many thanks for any suggestions. I am always impressed with the willingness of forum members to help out others!

[csx wnab]

it is quite possible you have an air pocket in your coolant. the high reading is the air pocket, and the lower reading is the water temp. burp your system at its highest point. once you get the air out you should have steady readings. good luck

[marcalan]

I had a similiar problem in my 67 mustang. It was an air pocket, in addition to getting the air out, you may also want to drill a bypass hole in the thermostat

[olddog]

If the engine gurgles when you shut it off hot, it deginately is an air pocket.

I had to back my cobra over a steap bank (hill) to get it to burp out, as jacking the front end up a foot wouldn't get it to burp.

[vanoochka]

Here's another vote for an air pocket. You described exactly the problem I had. I finally got tired of dealing with it and installed a fill tank.

Besides making the problem vanish never to return, it was a good excuse to put another shiny goody in the engine compartment.

[bmeyer29]

Many thanks for the responses!

I don't hear any gurgling when I shut off the engine. I was hoping the check valve in the thermostat would eventually bleed out any trapped air, but perhaps not.

@olddog How steep was the hill that finally worked?

[Juggernaut]

Bob, one more vote for an air pocket.

I drilled a small hole in my thermostat & installed it with the hole at the 12 o'clock position. Left the cap off, filled up the system, started the engine, when the thermostat opened & the coolant level dropped, I topped off the system, filled the overflow tank up about 1/3 & shut everything down. Started it again & let it run a few more minutes, saw the coolant in the overflow bubble a little & knew any remaining air was coming out.

No problems at all!

Randy

[olddog]

Originally Posted by bmeyer29

@olddog How steep was the hill that finally worked?

Hill is beside my dads garage. I would say it dropps about 8 feet in 24 feet. It was likely much more steep than I needed, but it worked great.

[bmeyer29]

Thanks, you guys were right. I had convinced myself that the check valve already in my thermostat eliminated an air pocket as a possibility.

The radiator was full, but the overflow tank was empty, so I made sure my overflow tank was about 1/3 full. Then I burped the engine by unscrewing the water temperature sensor attached to the intake manifold near the thermostat housing. (Clois had suggested this technique some time ago, but again, I thought my check valve had eliminated an air pocket as a possibility). I got the coolant up to temperature, parked the car on an incline for 20 minutes with the engine off to let it cool. Then I started it back up and drove it for about 20 minutes. Once the oil temp was up, the coolant temp was rock solid at 82-84C. I'll drive it again in the next day or so to confim the results, but so far so good.

Thanks again to all!

[WarrenG]

Check all of your hose clamps also. I had the same problem last year. Burbing the system solved the problem, but at the same time I found one clamp just loose enough to seep without leaving anything on the garage floor. The only thing I could figure was that it let out enough coolant while driving to allow air into the system but wasn't a big enough leak to leave coolant on the floor when the engine wasn't running.

[bmeyer29]

WarrenG, Thanks for your suggestion. I'll take a look at the hoses, as well.

[Jac Mac]

Old Dogs 'Hill' is about 1 in 3 gradient so based on the 90" WB the front tyres would have to be raised about 30" off the ground. That should be plenty to free any air caught in the water jackets etc.

[donraye]

Hi all, Not sure if my problem is related to an "air pocket" or not?
A few days ago I was out on the backroads for a drive and one of the blades on my 16" puller fan broke and took out all the other blades with it. The 351 quickly overheated and by the time I limped into a service station the temp was just about pegging the guage at around250 degrees. There was water/steam being lost and by the time I got it cooled down some of the coolent had either boiled off or run into the overfloww tank.
Since then I of course replaced the fan with a new monster pulling over 3150 FPM and it works fine, BUT...
Here's the problem: Ever since that incident the cooling system has been doing some erratic and odd things and I do not know why or what to do to fix the situation. You need to know that I am NOT a mechanical whiz and this is a new type of problem for me and I have no experience in this area to date.
The engine is a 351 Windsor roller making 425 HP. It used to run at between 170 and 180 degrees (after fully warmed up) under all conditions from feeway speeds to cought in trafic at rush hour. Now suddenly it's running at 195-205 degrees in all conditions even on the freeway at 70 MPH and around 2500 rpm. Oddly it sometime runs at about 170-180 for the first 20 or 30 minutes (or longer if very cool ambient temps are present like cool mornings/evenings) but then it still eventually get up to the 200 degree range and just stays there. Sometimes if will actually cool down when you are stuck at a long light or at idle (when it should run hotter not cooler).
Any idea what is going on with this change of temps after the overheating incident? It sometimes seems to give erratic reading as well until it eventually get into the 200 degree area where it stays most of the time now.
Could it be that the overheating incident caused the thermostat to become faulty and no longer open where it used to? Could it be that the sending unit fo my water temp gauge is damaged form the incident and is no longer giving me a correct reading(telling me it is at 195-205 degrees when it is actually at 170-180 as it used to be)?
Or could it be this "air pocket" issue that you have talked about here in this thread?
Any suggestions on what I might do to find out what is going on with the system would be appreciated.
I was thinking to start by replacing the thermostat to see if that corrected the problem and if that did not work then to go on to replace the sending unit, etc. I have never heard about this issue with the air pocket so that's a new one for me to learn about. How would I check for or correct that issue and how does it occur?
Please remember you are talking to a newbie and a marginal mechanic at best, so make your suggestions or answers fit for an unsophistocated weekend warrior type mechanic with no experience in this particular area.
Don

[bmeyer29]

Update:
Since my last post the erratic nature of my water temp came back. Someone suggested it could be caused by a short in the wires, and noticed there was teflon tape around the threads of the water sensor. It needs a ground through the block, so we took the teflon tape off, cleaned the sensor up and screwed it back in the block. We burped it again and started driving. Still erratic for a few minutes, and then the gauge or the sensor gave up and dropped to zero (well, really 60C). This is an Autometer Cobra style electric model. We will reorder a gauge and sensor and restart the whole process. I'll keep you posted.

[donraye]

Originally Posted by bmeyer29

Update:
Since my last post the erratic nature of my water temp came back. Someone suggested it could be caused by a short in the wires, and noticed there was teflon tape around the threads of the water sensor. It needs a ground through the block, so we took the teflon tape off, cleaned the sensor up and screwed it back in the block. We burped it again and started driving. Still erratic for a few minutes, and then the gauge or the sensor gave up and dropped to zero (well, really 60C). This is an Autometer Cobra style electric model. We will reorder a gauge and sensor and restart the whole process. I'll keep you posted.

Have you changed your radiator cap? I know it sounds a bit too simple but I had the same problems you are experiencing and went through a lot of hoops trying to fix it including new temp sender unit and new thermostat, draining and refilling systems, etc., with no real improvemnets until I decided to try a new radiator cap(even though the old one seemed/looked OK). I put on a new 16lb. cap and It instantly cured all the symtems and the preciously erratic readings were rock solid again as before the whole event started.
It's a cheap thing to try and it may be the answer to all your ills with the system. The cure for mine after the new cap was instant and dramatic! Runs a rock steady 180 now in all conditions as it always had before.
Don

[bmeyer29]

Just wanted to give folks an update about my original post. A couple of weeks ago, I made several changes to my configuration, and the erratic water temp issue appears to be solved. My car has a heater and originally both heater hoses connected directly to the water pump. My water temperature sensor was plugged into the intake manifold directly behind the thermostat housing, and the small port on the thermostat housing was stubbed off.

Now, my temp sensor is on the driver side of the intake manifold, one of the heater hoses is connected where the water temp sensor was, and I have a short bypass hose between the water pump (where one of the heater hoses was attached before) and the port on the thermostat housing. PLUS, I added a Kirkham small block surge tank to eliminate the chance of an air bubble in the intake manifold and the hassle of burping and reburping. It looks great and I don't have to keep looking at the temp gauge every few seconds!

Please see the thread about small block surge tanks started by David Kirkham or my gallery for pictures.

Many thanks to all that have given suggestions about how to solve this problem!

[Rick Parker]

There are 2 main types of radiator caps and it's possible to have the incorrect one installed. If you have a coolant recovery or over flow tank mounted below the upper level of the cooling sytem the cap will require a seal on the inner part of the cap as well as the outer part of the radiator cap to maintain a seal on the top of the neck so when the system cools it will be able to maintain a vacume and allow coolant from the overflow tank to be pulled back into the radiator.
If you have the tank, and the cap being used only has the seal "inside" the neck, it will maintain pressure in the system until the rated pressure is exceeded and it will allow coolant to be purged but not to be drawn back in. If you do not have an overflow tank the cap used will not require the outer seal on the cap. This is what was used on all cars in the 60's, because very few had a recovery tank.

[bmeyer29]

Summer Update

The outdoor temps have changed dramatically from my post in March about "problem solved". Yesterday I was at the Hallett track for a drivers school, and the air temp was around 100F, with a heat index of around 107F. HOT!!!

The engine temp drifted from ~80C (with a 180F thermostat) up to about 90C (sorry, my gages are in C not F) after several laps, but nothing erratic. Yeah!!! The oil temp normally stays within a few degrees of the water temp in moderate weather and when just driving around town. On the track, it got up to ~110C.

Pictures of my mods from March are located here:
Small Block Surge Tank???

Quick note: I made several changes back in March, so I don't know if just adding the surge tank was all that was necessary to tame my erratic temp. It may be that moving my water temp sensor, rerouting the heater hoses, and adding a bypass hose had a big part in the solution.

Thanks again to all that helped with suggestions.