It's a 98 F-150, 4.2L motor. I have owned my own shop for over 20 years
and this one has me stumpped!
Customer replaced the thermostat because engine temp. read cool and no
heat from the heater. When he came to me, I recommended he replace the
thermostat again. He did, but problem still exists. I can not get the
engine temperature to rise above 153 degrees. And that is after I covered
the radiator with paper to restrict the air flow. I went as far as
pinching off the upper and lower rad. hoses. After 10+ minutes and
holding the rpm at 2000, the engine temp. came up to 180. As soon as I
removed the restriction from the hoses, cooled immeadiatly back to 150. I
have ensured the system is full. I as using a infa-red temp sensor, the
temp. guage on the dash and a scanner to check engine temperature reading
of the computer, as well as checking temperatures with my hands. I can
not get this truck to heat up!! Any ideas??
Did you check the radiator temperature? Assuming the radiator is at
the correct temperature, the problem would have to be in the heater
circuit. Things to check:
- Air in the system?
- Water valve operating properly?
- Blend doors operating properly?
So if the hottest part of the radiator is close to the engine operating temp
and the rest of the radiator temp drops from that I guess it's safe to say
that the radiator doesn't really have a correct temp?
Do you have a correct operating temp? If you swallow a temperature probe,
the temperature will shoot up to about 37 or 38 C (99 or 100 F) for a day or
so. before it drops to whatever the sewer water temp is. But if you hold the
temperature probe in your hand, the probe will be a degree or two warmer
than room temperature. And if you put it in your sock, it will be 3 or 4
degrees warmer than room tempature. Your body has a correct operator
temperature. But it depends on your activities and where on (or in) your
body you measure it.
Likewise, the radiator does have a correct operating tempature. It should be
warm near the water inlet (close to the engine temp) and near air temp at
the radiator outlet. If the inlet is not close to the engine temp and the
outlet is not close to air temp (I am not sure how close, I would imagine
within about 10 degrees), something is wrong.
What you describe is the normal operating temp of the radiator, hottest at
the inlet and coldest near the radiator outlet.
You missed my point, Ed asked the OP if the radiator was at the correct
operating temp. Since the temp varies greatly depending on where you check
it, that's not really a good way to check the engine operating temp.
And you missed my point. The operating temp of the radiator varies depending
on where you measure it. And, there should be a variation. While it is not a
good way to check the engine operating temp (actually, it is not a bad way,
although the temp might be off a few degrees), it is not really a bad way.
It is also a way to check the functioning of the radiator.
Another way: Once the engine has been running for a while, if the inlet hose
to the radiator is hot and the outlet warm, the radiator is working and the
On the other hand, I have never seen this mentioned as a way to check the
functioning of the radiator.
Last resort. Is the pump correct? If it pumps backwards to the intended
flow, this may happen. Can you look in the radiator or feel water flowing
in the hoses? How about an additional radiator mounted beneath for extreme
Vehicle came to my shop again today. Deductive reasoning tells me that the
thermostat is not opening. We discussed the fact that the water maybe by
passing the thermostat because it is incorrect. Customer is going to try
an OE thermostat and see if it makes a difference. My biggest mistake
maybe the fact that the customer is changing his own parts, but I want to
give the guy the "benifit of the doubt".
Radiator temp. is about 135 degress and I checked the temp. of the therm.
housing--145!! It has to be the thermostat.
I have a '73 Chevy pickup that has been that way for the last 10 years
- it WILL NOT heat up. The water temp runs 135-140 all the time. The
thermostat has been changed multiple times and it has a new radiator.
I finally gave up trying to fix it.
Brink wrote: 98 F-150 4.2L
Heater will not get warm.
Engine temperature will not rise above 153 degrees.
Replaced thermostat twice - no improvement.
With upper & lower radiator hoses pinched, temp
will slowly rise to 180 degrees.
I can not get this truck to heat up!! Any ideas??
The hose pinch test showed that the problem
is too much coolant flow through the radiator.
Possible ways to limit coolant flow:
1.) Install the correct TEMPERATURE thermostat,
oriented in the correct DIRECTION.
2.) Install the correct DIAMETER thermostat.
3.) Rework the thermostat housing or add
an internal spacer ring to RESTRAIN the
thermostat from FLOATING.
4.) Install a RESTRICTOR in the water pump bypass
hose, if so equipped.
Tell the "customer" to stop driving the truck YESTERDAY. Have him TOW IT to
shop. He's on borrowed time and actually owes the car gods a major sacrifice if
has gone on for more than a month or so.
That engine has an air lock that can't be cleared by normal means. The
isn't telling you(he doesn't know) the whole story. He very likely gets white
from the exhaust on startup that lasts a little longer than the normal "steam"
one would expect from a cold engine. It probably runs a little rough until it
(the cylinders clear).
4.2s of that vintage are notorious for leaking intake manifold gaskets. When
engine is shut down hot coolant dribbles into the intake manifold. As the engine
off, it sucks air into the cooling system causing airlock. One morning he is
start the motor. It will spin about 3/4 turn and hydrolock, taking a connecting
with it. Or, the engine will suddenly self destruct driving down the highway
intake gasket totally gives way and fills a cylinder with coolant.
Pull the intake immediately and replace the gaskets with the updated set FROM
Change the oil, run the vehicle to operating temp for an hour, then change the
again. Repeat. (Coolant runs past the rings and can play hell on the crank and
I can't stress enough that tomorrow morning could be the time that the
hydrolocks and ruins the motor. It happens, to most drivers who don't know the
To verify this, check out http://www.ford-trucks.com/ and do a search for "4.2
engine" . Add terms like "hydrolock", " intake gasket", "losing coolant", etc.
get lots of relevant hits. Add "class action" and you will get some interesting
Good used 4.2's are around $2000 right now, if you can find one. I have 2
that are waiting for replacements here in Northern Ohio. Remans are +$3000 with
decent warranty. The rebuilders have trouble finding good cores.
Hope this helps, Tom (been there and done that) Adkins
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