Erratic Temperature Sensor Readings

[teletekman]

So lately I have noticed an erratic temperature gauge on my 92 XLT. Once warmed up it will toggle between N and R. This happens about every 5 minutes or so and the swing last about 20 seconds. I have read a bunch of other posts that others have had similar issues with and have replaced the following with mixed results. This is what I have done so far...

Repalced Thermostat with Motorcrat RT-1161 (190F & weep hole at 12
o-clock position. - No Change
Replaced with a Napa Therostat - No Change
Replaced the Temp Sending Unit - No Change
Replaced ECT - No Change

I have also flushed the cooling system twice with no change. Each time I have burped the system using the Lisle Super-Funnel method, both having the Explorer flat and with the nose raised up pretty high. While there was a bit of air being released, after 30 minutes of letting it idle and the thermostat opening and closing this also made no change.

However I did notice something I didn't realize before. With the funnel on and burping the system, the temperature of the ECT according to my scanner in live data mode showed it ranging between 193 & 196F. At this time the gauge was steady between the N & O. I thought it was fixed till I capped the radiator and eventually the temperature started ramping pretty high and then back down. Range is anywhere from 186 to 215F. This is all observed at the gauge so at least my scanner and instrument cluster seem to by in sync.

At this point I am running short of options here. The cabin heat seems to cool off as well with the temp fluctuating. I also ran a combustion test to make sure no exhaust gasses were flowing into the radiator showing signs of a blown head gasket and that passed. It's not losing coolant not is any present out the exhaust.

Can anyone help shed any more light on this? Does this still sound like air is trapped in the system? I let it idle uncapped for thirty minutes so I would assume it would have been plenty of time for it to burp the system.

Sorry for the long post but wanted to make sure I had all my basics covered. I have not tried a new water pump since my flow seemed ok. Nor have I tried the bypass kit mehtod mentioned here but wanted to get a few updated opinions before I went that route.

Thanks,

Sean

 

[surf]

Hav you replaced the radiator cap?

 

[corkey]

if the temp to the heat output goes down, there is still air in the system,, it's tough to get it all out
but it will in time, mine took me a long time on the ohv

 

[teletekman]

Yes, cap has been replaced. Tried both 13lb & 16lb caps with no change.

I was still leaning towards air still being in the system but I would have thought that after a few attempts and it idleing for almost 30 minutes per, it would be ok. I didn't notice any air bubbles after a while.

Is there another trick on these OHV engines or does it litterately take longer to purge the air?

 

[FR-425]

Still sounds like air in the system.

Make sure your heat is on while burping.

Set the truck down level.

Leave the coolant level a couple inches low until it stops bouncing up and down, or you'll just be dump'n coolant on the ground.

Once it settles down completely; Top off, (replace the cap before you shut down)

You won't need to do this again until the next time you drain the rad for whatever reason.

When air gets in the water pump it cavetates (stops pumping) but not completely.

Eventually it passes the air through and the cycle repeats.

Gauge up / gauge down. Weeeeeee!

A very old water pump can cause it's own cavetation, (impeller eroded)

So if no-joy on the air situation - it's water pump time.

 

[teletekman]

The one thing that is puzzeling me and am not sure if this is relevant to air still trapped in the system or maybe my water pump is on its way out, is the fact that while the radiator is not capped, I can keep the coolant at pretty much at a constant 195 degrees. It's not till it's capped does it start fluctuating all over the place.

 

[teletekman]

Thanks FR-425. Looks like I was replying over you.

I'll try a bit more to deplete any more air in the system but the last time I did try and burp any more air out I was using one of those yellow super funnels and letting the coolant settle down which it eventually did as how you had mentioned.

So if I do end up changing the water pump, which is about the only thing I haven't done yet, is there a cheaper aftermarket alternative other than the OEM? I read somewhere Ford redesigned it some time back so would it be safe to say the aftermarket versions have caught on by now?

Napa is usually my parts house of choice...

 

[FR-425]

Bosch;

Also the cheapest; Strange, but Bosch is good stuff.

When I first bought my Ex I replaced the water pump with MotorCraft, but I didn't pay $193.00.

Here's a link to RockAuto to see all the choices.

http://www.rockauto.com/catalog/x,carcode,1119619,parttype,2208

Just personal preference, but I always go OEM on water pumps. (New) not reman.

My original was/is still in working condition and is now my emergency spare.

 

[FR-425]

The one thing that is puzzeling me and am not sure if this is relevant to air still trapped in the system or maybe my water pump is on its way out, is the fact that while the radiator is not capped, I can keep the coolant at pretty much at a constant 195 degrees. It's not till it's capped does it start fluctuating all over the place.

Yep, sounds like the water pump can't handle any pressure.

 

[Texan01]

I burp mine with this trick. Leave the cap off, and with it running fill it to the brim and fill the overflow to the half way mark. Cover the filler neck on the rad with your hand, and squeeze the upper hose a few times, this ill get the air out of the upper hose and into the T-stat, as you do that you should feel cold water running across your palm from the overflow. Then rev it up to say 2,500-3,000 and if there is air it'll suck the upper hose flat, if it does, keep it there and it'll unflatten after a bit. Repeat the cycle till either no more air, or it gets too hot to hold your hand over. If its too hot, put the rad cap on and shut it down till cool.

Takes about 10 minutes to do it this way.

 

[teletekman]

Thanks Texan, I'll try that method and see if it helps any.

Thanks FR as well. I was looking at the picture of that rusted impeller you posted and it made me think a bit. When my dad gave this EX a while back it did have a nasty over heating problem. When I attempted to flush it out a whole crap load of rust water came with it. Ever since then and adding in new coolant it ran great and stayed right about the O & R range. Since this is a recent issue, I am wondering if indeed the impeller is starting to finally give up so I'll try a new pump.

I am leading towards Bosch since Motorcraft is a bit out of the budget right now. I read somewhere that Ford redesigned the pump a while back so is it safe to say that aftermarket manufactures have the updated design as well?

Do you know anything about this redesign?

 

[FR-425]

No I haven't heard of a redesign before you mentioned it.

But I'm sure those cores are no longer available. You'll be gett'n the latest design from any auto parts store. Except Billy Bob's stuf-n-bits.

 

[teletekman]

That's right, I forgot about the re-used cores.

Well I will give the water pump a check this weekend and report back on how it went.

Thanks again for your help, I sure hope this will do it.

 

[teletekman]

One other question, last night went I went to pick up a water pump I was asked whether I wanted one that was "optional severe cooling" or standard.

According to my VIN and from Ford, I have the Super Cooling package. So does this mean I need to go with the severe duty pump or would standard be fine?

Or is there any real difference in the two other than a $40 price difference?

 

[FR-425]

I've heard of the severe duty fan clutch and I run one, but severe duty water pump is a new one on me.

Maybe it's just a higher volume pump that was stock for the optional towing and HD cooling package.

When I went to the ford dealer for mine they asked if I had the heavy duty cooling package (which I do) so may be that was what he was getting at.

It didn't dawn on me at the time, but i guess it make sense that the HD cooling package would have a higher volume pump.

That being said I would spend the extra cash. (better cooling is always; well, better)

It would be awsome if you did and posted up some comparison pictures. [edit] duh you already have the HD pump so no difference)

Let us know what you find out though. Good chance some one put the cheapy on there in the past.

The only visible difference will probably be the impeller design.

 

[FR-425]

Good thing your parts guy was aware of it, huh?

 

[teletekman]

Thanks FR. Ok, when I pulled the old water pump out, the impeller looked fine so I don't think that was an issue. However the bearings did feel a bit tight so I figured it would have been a matter of time before I had to replace it anyways. Replaced the pump, burped the system again the best I could and still issue remains.

I ran a pressure test on it cold this morning and let it sit for over a couple of hours at 15psi. Pressure gauge did not drop at all so I know at least that test passes.

Heat at this point seems to be a bit better. Not near as bad as fluctuation in heat as I did before.

Could it be I still might have a nasty air bubble in it?

I ran one other test and not sure if this is valid or not, but I put my pressure gauge on it while it was warming up. System got to about 14psi before the thermostat opened and then dropped down to about 5psi and then solely back up. Again to 14psi and then dropped to 5psi. The temp gauge seems to creep on its way up between the M and A markings about the time the gauge reads maybe 10psi and then once the gauge goes back down that's when the temp gauge start to fall back to N or below.

Process then continues to repeat itself. Again no coolant loss though the reservoir, no leaks appear anywhere on the engine or ground and no coolant smell from the exhaust.

Does this last test I did mean anything?

 

[FR-425]

No, not really I mean, the pressure will rise/fall with temp variations.

And some temp variation is normal, but that's a huge swing at idle. (this would not concern me if you were seeing that kind of variation while driving in cold weather though)

The only thing left is the gauge itself.

You could buy a cheap manual temp gauge and install it temporarily in place of the temp sensor.

This way you can see if the temp is really changing that much.

STILL SOUNDS LIKE TRAPPED AIR THOUGH

 

[FR-425]

Have you tested for exhaust gasses in the radiator yet?

Auto parts store can rent you a tester.

If there are exhaust gasses in the rad; you've got a blown head gasket/ and possibly a cracked head.

That would be the only other reason for this kind of erratic engine temp.

 

[teletekman]

Ya I have a combustion check tester which I did do just now and fluid still showed a cool blue so I think that test passed.

I did a compression check again for sh*ts and grins and here are the numbers, which I forgot to post earlier...

Cyl_1 - 165
Cyl_2 - 165
Cyl_3 - 180
Cyl_4 - 180
Cyl_5 - 190
Cyl_6 - 180

According to Alldata and Ford specifically, they are within 75% of the highest reading so I think I have ruled out the case of a blown head gasket. I didn't bother with the leak down test again since my compression numbers seemed ok.

I'll try another temp gauge at the sending unit and see how it responds. I'll let you know how this pans out shortly....