Jumping Temp Gauge Reading - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

Styles · 09-29-2002, 17:53

Hi All

To get started, I have a 90 EA MPFI. I noticed on the weekend that the temp gauge on the weekend was doing funny things. First of all the gauge would be fine when below the normal running temp. Once it was into the normal operational reading it started to do funny things. Even when driving, not just at traffic lights the gauge would jump up to 3/4 of the way up in about 2-3 seconds, sit there for a while and then drop back down to the normal range, But when it dropped it would go down in about 2-3 seconds. This did not happen continuously but did happen every once in a while.

My question is, do you think that this is a problem with the temp sensor or is it a problem with the thermostat. My reasons for and against is that the warming/cooling is too quick to be the thermostat. But the sensor is close to the radiator inlet so when the thermostat opens it could be a rush of cold water.

I checked the ohms on the sensor and it seemed to vary all the time (I think that it was between 2-6 ohms) when it was hot but when cold it seemed to be steady (1ohm)

Thanks in advance.

EvilEA · 09-29-2002, 19:27

hey guys,
yeah mine does the same thing. I have changed the thermostat and still it does the moving around thing..

Any ideas on what to check next??

Thanks

09-29-2002, 19:51

This question is asked so many times Jack should add a "jumping temp guage" link on the front page of Eafalcon.com ;-)

Your thermostat could be on it's way out but more likely you need to modify your thermostat to make it EA compatible. Take the thermostat out of the thermostat housing and check to see if it's still working properly, if not buy a new one.

The problem with standard thermostats is that they don't allow the water on the other side of the thermostat to heat up any before it opens up. Which means on one side of the thermostat you'll have 20C water and on the other side 92C water. When the thermostat opens the water pump pushes all the 92C into the radiator and the 20C water back into the motor, which explains why the temp guage falls very quickly. I don't think this is very healthy for motors, because sudden temperature changes like that cause metal to go brittle. Thats why you should modify your thermostat.

With the thermostat get out a drill with a very small drill bit and drill a hole through the side of the metal plate. Put the thermostat back in and hopefully all your problems should be fixed. It will now let a tiny amount of water through, just enough to preheat the water on the other side, and your guage shouldn't jump all over the place anymore.

Aussiblue · 09-29-2002, 21:00

Hmm I have yet to see a Thermostat yet that didn't have provision for water to move from one side to the other. Either a hole with a jiggle pin or a special ecentric shape to the part (seat or wax pellet plug part) that opens so even fully when closed there is a small gap on one side. If you look carefully at the T stat as it comes out of the bubble pacm or box you can usually see the air gap; if not pour some water on one side and see where it trickes through.

It sound to me like you have bled the system correctly when topping it up and you have an air lock in the system.

Styles · 09-29-2002, 21:08

I see your point Aussiblue about the air lock.

Sorry if this sounds stupid, but how do you bleed the cooling system?

Also do you guy's know that ohms the temp sensor needs to be when hot and when cold?

Thanks

09-29-2002, 21:49

AussiBlue, yes the thermostats have that provision, but they don't work.

I've bought a expensive thermostat(think it was like $30) from autobahn and it was meant to have a special V-shaped mouth so that a small amount of water can be let through even when closed. Did it work? Nope. The one I pulled out of my motor from the previous owner also had a hole drilled so no doubt they found out the same thing.

I haven't tried one of the jiggle pin before, but you'd probably find the same thing on them as well as they aren't made to let much water through at all.

Other members will agree with me that the only way to cure a jumpy temp guage is to drill a small hole in the side. In fact it was reading the posts in this forum which was the reason to doing that to my nice brand new expensive thermostat to fix my jumpy temp guage problem.

Aussiblue · 09-29-2002, 23:11

There is a procedure see:
http://discussions.fordforums.com/sh...hlight=coolant

Ok; it covered before somewhere but here goes:

1. Park car on slope with nose upwards (aids air bleeding),
2. Put heater on highest setting.
3. Remove thermostat and thermostat housing top.
4. Fill to top of thermostat housing via thermostat housing.
5. Replace thermostat (make sure its a dual acting one with
jiggle pin).
6. Replace thermostat housing.
7. Fill rest of system using a funnel to full from the small
bleeder hose that is on top of the thermostat housing and runs
to the coolant overflow bottle; hold hose in air above highest
point of engine or radiator while doing this).
8. Replace bleeder hose and top up over flow bottle.
9. Screw cap on overflow tank loosely one turn.
10. Start engine (heater still on max setting) run for 10
minutes at 2000 rpm or go for a 10 km drive.
11. Let cool top up and tighten cap.
12. Have a beer and toast the Ford engineers who designed such
an air lock prone system.

If you're changing coolant brands flush with distilled water
before doing this (i.e. fill with distilled water and go for a
20 km drive, let cool then change coolant).

If you don't follow the correct procedure (which originally was
on a sticker on that coolant bottle) those air bleed hoses may
eventually purge the system but don't count on it; and in the
meantime risk overheating or doing the pump in. The plastic
supply tank has a min and max mark on the side but fill it to
about and inch below the cap; just so the tips of your fingers
get damp when you stick them in.

A hole in the T stat might help accelerate the system self bleeding but....I've never needed to do that yet.

Also make sure you have the original type dual acting thermostat
fitted. See http://www.vtr.org/maintain/stag-cooling.html

Aussiblue · 09-30-2002, 00:02

Quote:

The problem with standard thermostats is that they don't allow the water on the other side of the thermostat to heat up any before it opens up. Which means on one side of the thermostat you'll have 20C water and on the other side 92C water. When the thermostat opens the water pump pushes all the 92C into the radiator and the 20C water back into the motor, which explains why the temp guage falls very quickly. I don't think this is very healthy for motors, because sudden temperature changes like that cause metal to go brittle. Thats why you should modify your thermostat.

Donut I have some problems with the logic of this reasoning as follows:

a) the thermostat opens slowly as the water temp rises so there shouldn't be a sudden rush of cold water into the motor; or at least not to the extent that would be materially changed by having a small hole in the thermostat. To have holes big enough to have an impact would effectively counter the positive role of the thermostat (i.e. keeping water recirculating through the block until the engine and coolant is warm).

b) Hot water will also naturally stays on top of cold water so at best you will have some hot water in the top of the radiator and a whole mass of cold water at the bottom ready to flow into the engine through the bottom radiator hose when the thermostat opens.

b) the jiggle pin or standard hole is there to prevent air locks so water can tickle through the hole when you fill the system and when coolant is scavenged from the overflow tank ant cool down. I suspect if an extra hole does anything positive it provides another escape route for air that might otherwise be locked below the thermostat. The jiggle pin as fitted to genuine Ford Thermostats acts like a valve closing the air bleed hole when the pump pressurises the system.

I might be wrong (yet again) what do others think?

TopGhia · 09-30-2002, 00:42

Here is another slant on it
www.troubleshooters.com/tlitthypothesis.htm

Aussiblue · 09-30-2002, 00:59

Hi Topghia; certainly a possible explanation given the history of Ford I6 heads.

I must conced that Donut has some hole drilling supporters and fans as well see:
http://www.zzperformance.com/zzp/pro...thermostat.htm
and http://www.turbokits.com/honda_turbo_kit_faqs.html
and
http://www.mgfcar.de/thermostat/
and
http://www.f-max.com/HONDA%20FAQ.htm

Some people are even making $$s selling thermostats with holes drilled in them though in some cases the reasons seem to be added cooling and less restricted flow with thermostat open as well as closed. And I ahve to admit I have done it myself as a temporary measure for a car with a partially blocked radiator.