1999 4.0 OHV gauge goes to hot and steadys out about midway

All,

I have a 1999 Explorer 4.0 OHV. Mileage is around 135,000. Had the heads
replaced around 120,000. Both heads had a fine crack. This was done about
14 months ago.
Fast forward to 3-20-08. While driving the Explorer after it had been
sitting for a few hours and engine was cold, the temp gauge would rise to H
and go back to the mid point where it always would rest at. When I reached
a safe spot to pull over, I checked under the hood and my radiator cap had
released some coolant. After it cooled off, I checked my level in the
radiator and the coolant tank. Both seemed fine. Started back up drove for
until it warmed up, same scenario, except this time, it came out the coolant
tank. Now, it has not overflowed out of the radiator cap or coolant tank
anymore.
What I have done is replaced the radiator cap. Replaced coolant lost during
the times it overflowed.
Will replace the thermostat today thinking that it may not be opening up
like it should.
Any ideas on what else would cause it to heat up rapidly and steady off on
the gauge. I thought the gauge may not be reading correctly, but if that
was the case, would that cause the engine to send coolant out the tank or
the radiator cap?
I was thinking the heads that were replaced may be having issues that are
the root cause. If the heads were cracked, again, or gasket issue, would
that let combustion gas into the coolant and cause it to heat up? But, if
that happens, why would the temperature gauge go back to reading normal?

Thanks for any suggestions.

Lance

"Lance" > wrote in message
.. .
> All,
>
> I have a 1999 Explorer 4.0 OHV. Mileage is around 135,000. Had the heads
> replaced around 120,000. Both heads had a fine crack. This was done
about
> 14 months ago.
> Fast forward to 3-20-08. While driving the Explorer after it had been
> sitting for a few hours and engine was cold, the temp gauge would rise to
H
> and go back to the mid point where it always would rest at. When I
reached
> a safe spot to pull over, I checked under the hood and my radiator cap had
> released some coolant. After it cooled off, I checked my level in the
> radiator and the coolant tank. Both seemed fine. Started back up drove
for
> until it warmed up, same scenario, except this time, it came out the
coolant
> tank. Now, it has not overflowed out of the radiator cap or coolant tank
> anymore.
> What I have done is replaced the radiator cap. Replaced coolant lost
during
> the times it overflowed.
> Will replace the thermostat today thinking that it may not be opening up
> like it should.
> Any ideas on what else would cause it to heat up rapidly and steady off on
> the gauge. I thought the gauge may not be reading correctly, but if that
> was the case, would that cause the engine to send coolant out the tank or
> the radiator cap?
> I was thinking the heads that were replaced may be having issues that are
> the root cause. If the heads were cracked, again, or gasket issue, would
> that let combustion gas into the coolant and cause it to heat up? But, if
> that happens, why would the temperature gauge go back to reading normal?
>
> Thanks for any suggestions.
>
> Lance
>
>

Since you had the heads replaced I would suspect the radiator cap. But
since you replaced that I would suspect the heads or head gaskets again.

My '91 will do that with the temperature gauge but I think it's just my
thermostat getting stuck when the engine is cold. To me it sounds like they
may be two different problems. Flushing the cooling system might help. I
had success using some radiator stop-leak on my Explorer but I only used
about 1/4 can and it seemed to take care of the leaking heads. Too much
stop-leak and it might clog your heater core etc. But it might tell you if
you do indeed have leaking heads. From what I've read if you do a
compression test head leaks usually indicate low compression on one cylinder
or two adjoining cylinders. Supposedly there are tests strips you can get
at auto parts stores that will test for exhaust gases in your coolant but
I've never used them.

Thanks for the reply Ulysses. After replacing my thermostat, I only noticed
one thing when driving it for about 100 miles. My check engine light
clicked on for a second and was off again. The temp gauge did not move from
its normal position(which is midway) after it was warm. Once cooled, I
checked my coolant and it was down a little. I am still leaning towards a
failed head gasket or heads. I will let the group know what I find out.

"Ulysses" /> wrote in message
...
>
> "Lance" > wrote in message
> .. .
>> All,
>>
>> I have a 1999 Explorer 4.0 OHV. Mileage is around 135,000. Had the
>> heads
>> replaced around 120,000. Both heads had a fine crack. This was done
> about
>> 14 months ago.
>> Fast forward to 3-20-08. While driving the Explorer after it had been
>> sitting for a few hours and engine was cold, the temp gauge would rise to
> H
>> and go back to the mid point where it always would rest at. When I
> reached
>> a safe spot to pull over, I checked under the hood and my radiator cap
>> had
>> released some coolant. After it cooled off, I checked my level in the
>> radiator and the coolant tank. Both seemed fine. Started back up drove
> for
>> until it warmed up, same scenario, except this time, it came out the
> coolant
>> tank. Now, it has not overflowed out of the radiator cap or coolant tank
>> anymore.
>> What I have done is replaced the radiator cap. Replaced coolant lost
> during
>> the times it overflowed.
>> Will replace the thermostat today thinking that it may not be opening up
>> like it should.
>> Any ideas on what else would cause it to heat up rapidly and steady off
>> on
>> the gauge. I thought the gauge may not be reading correctly, but if that
>> was the case, would that cause the engine to send coolant out the tank or
>> the radiator cap?
>> I was thinking the heads that were replaced may be having issues that are
>> the root cause. If the heads were cracked, again, or gasket issue, would
>> that let combustion gas into the coolant and cause it to heat up? But, if
>> that happens, why would the temperature gauge go back to reading normal?
>>
>> Thanks for any suggestions.
>>
>> Lance
>>
>>
>
> Since you had the heads replaced I would suspect the radiator cap. But
> since you replaced that I would suspect the heads or head gaskets again.
>
> My '91 will do that with the temperature gauge but I think it's just my
> thermostat getting stuck when the engine is cold. To me it sounds like
> they
> may be two different problems. Flushing the cooling system might help. I
> had success using some radiator stop-leak on my Explorer but I only used
> about 1/4 can and it seemed to take care of the leaking heads. Too much
> stop-leak and it might clog your heater core etc. But it might tell you
> if
> you do indeed have leaking heads. From what I've read if you do a
> compression test head leaks usually indicate low compression on one
> cylinder
> or two adjoining cylinders. Supposedly there are tests strips you can get
> at auto parts stores that will test for exhaust gases in your coolant but
> I've never used them.
>
>