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Engine Runs Cold No Cabin Heat

Hello All,

I have a 2003 Ford E-150 Service Van with 174000 miles. Started out thinking it was a blocked heater core due to lack of heat. But after having the heater core flushed at least 3 times with no help it was determined that the engine itself is not getting up to temp. Then the T-Stat was replaced 4 times with 3 different manufactures. (Delco, Stant, Motorcraft) all 195F. Nothing has helped. Except for this temperature issue the truck runs just fine.

Statistics after running for 20+ minutes.
Ambient Temp 30ºF
Internal Heat Air Temp maybe 80ºF
Temp Gauge (Middle/Normal)
Climbs from full bottom cold to middle in about 4 to 5 miles – or 5 to 6 minutes.
Top hose Warm 135ºF – Bottom Hose Cold 50º-70ºF
Top Hose easily squeezable (not pressurized)
Has been run with cap off reservoir to bleed off any air.
Heater in hose temp. about 135ºF
Heater out out temp. about 100ºF

Have replaced the water pump since we did not know if it was the issue. It was not, made no difference. The blend door and actuator have been checked as well as the heater core for blockage and all is in good order.

It seems that the coolant temp under run conditions is about 40º-50ºF lower than would be expected. All the while the temp gauge appears in normal range. Not that I am trusting the temp gauge but…

Blocked off radiator approximately 70% with some Coroplast sheets.
Drove 200 miles with morning temperature at about 35ºF
Temp Gauge was a frog hair higher than normal.
Inside air from vents reached about 75º - 85ºF Higher if fan was on low.
Stopped at about 60 miles and checked top hose. Was pressurized,
had a temperature of about 185º - 190ºF which is higher, maybe closer to normal.

Next day, removed Coroplast sheets ambient temp 40ºF Inside air from vents
reached about 75ºF Drove about 20 miles and checked top hose was not pressurized,
had a temperature of about 145º - 160ºF which is lower than previous day.

My question is: What could be causing this low engine coolant temperatures??

Except for my driving and temp checks using a infrared sensor and a multi-meter with a “K” temp probe all the work was completed by my local shop. They are good quality shop and have experienced tech’s. They are stumped as am I.

Anyone here have any suggestions on what to investigate??
Thanks in advance.


If you remove the thermostat housing and check the recess(es) in the housing and intake you might find that there is a gap that requires a seal ring.


I have not viewed personally but I believe that the T-Stat was and is sealed.

But, I will verify with the shop.



I don’t know what sort of crew you are dealing with but I am not going to replace a thermostat 4 times for anyone. You plug in a scan tool and read the operating temperature, this will show that the temperature is normal.

Heater hoses insulate the temperature, I can wrap my hand around a heater hose with an engine operating at 210F on a 100 degree day and not burn my hand. The heater hose temperature can prove that the inlet temperature is greater than the outlet temperature but this is not the actual temperature of the coolant.

I would like to recheck the blend door operation and servo motor but I think at this point I would be replacing the heater core.


Sorry, I did not enter every detail of what has been done over the last 4 months, aka winter. If I had it would have been a novel and may end up that way soon. The shop has had the scanner hooked up a number of times and the readings were abnormally cold for standard running conditions.

As for the Stats, the original was removed and the first new installed(Stant). After running for a couple of days it showed no improvement so it was assumed it was possibly a bad Stat so it was removed and replaced with another Stant. Again after a number of days/weeks (with ambient temps in the 50’s -60’s all was good. Then as it got cold again noticed the same thing. It was then decided that maybe there was a bad batch of Stats so again it was replaced this time with a Delco. The next day after making 50 mile trip one way the engine overheated and cooked out coolant through the reservoir cap. Engine running very rough decided to call AAA and roll-backed the 50 miles back. Next day all was found to be normal (BUT) to play it safe replaced the Stat again because nobody was sure what had happened but assumed a Stat problem. This time with a Motorcraft.

As for the temps and conditions that I am reporting I am stopping on the road, jumping out, pop the hood in freezing temps or in a rain downpour to check thing as best I can. Can’t say I can trust the reading every time but it is the best I can get. However I do trust my a$$ every time and it reports COLD!!!

If the engine temps are cold the heater core is going to be cold. If the heater core is cold the cabin temp is going to be cold. I bought this van 2003 with 1750 miles in on it and previous winters it has (as it should) taken a long time to heat up in the cabin. Which makes sense due to the lack of insulation and the volume of air inside that needs to be warmed. I get that!! However, this year has been a different issue. Having a 70F air temp off the blower does not make a happy driver.

Yes, at this point replacing the heater core may be the next step. But, since it has be flushed, has good flow, and the engine temps are cold then what would be the benefit??

Now that it is April it is almost a non issue but still a problem.

Thanks for reading and providing any input that may help.


The detailed “statistics” indicate that coolant is bypassing the thermostat.

When you check under hood for things and “trust the reading”? What is the reading? What do u mean by that?


I can accept that as a good possibility. Would you care to elaborate on how or where that could be happening??

I do know that there are two(2) hoses that come off the water pump. One(1) goes to the heater core and the second goes to the back of the block somewhere as maybe a bypass in case the heater core would block up there would still be flow.

This hose was clamped off for a day and I noticed that it took a lot longer for the upper hose to start warming up. Maybe an additional 10 minutes. The clamp was removed since it did not help pinpoint anything.

Don’t know of any other hoses.




I am using my infrared (no touch) temp probe looking at the hoses, top of radiator, heat hoses,
engine block, etc.

Top hose Warm 135ºF – Bottom Hose Cold 50º-70ºF


If the top radiator hose is 135* and the lower hose 50*-70* it would seem quite conclusive that coolant is flowing through or around the thermostat. After 4 thermostats I would assume that the problem isn’t a defective thermostat but an improper installation of the thermostat.

Does your current thermostat have a rubber seal ring on it like this

If not, that is your problem.


That has been asked a number of times. Although with out the blessing of x-ray vision I can be 100% certain but I am very confident that the Stat is installed properly.

Out of the box the Stat does not have a seal for this model. There is a counter bore area in the flange pipe that fits the Stat and secures in place. At one point it was thought that there may have been an obstruction in the orifice and a new flange pipe was order to evaluate. It was not installed and was returned since no obstruction was found and all looked good.



Regardless what your thermostat had “out of the box,” indications are quite strong that your problem is exactly what occurs when the thermostat is installed with the “gasket” only


I understand. Again to confirm, I will ask the guys in shop on Monday.

However, please be patient here… :confused:

IF the gasket and gaskest are indeed installed correctly and the Stat is sealed as it should be then what other thoughts would you have on the topic??

What else could be going on??



my Taurus had a heater bypass assy. any chance your ford van has a similar hose assy?


Good question. Honestly I don’t know if what is there is that convoluted. The shop guys had removed the air cleaner box, etc. to get a good look and to follow hoses and the report was just the two hose off the water pump going to the heater core and to the back side of the block. Then of course the upper and lower hoses to the radiator.

I did hear from another source that on some engines behind the water pump there is a plate that if either eroded thru can cause a short loop of sorts in the coolant flow. But it was also said in the same breath that the 4.2L may not have that.

Thanks for the input.


Well my bet is the thermostat has no seal ring.

If you start the engine cold, remove the radiator cap, increase the idle to 1500 rpms and find that coolant is flowing from the top hose I win.


Sorry, no radiator cap. This has a reservoir.
So looking at the flow is part of the challenge because it can’t be done. Wish I could!!!

It has been discuss about making some kind of a sight glass with some plumbing parts to insert into the upper hose to do exactly what you are suggesting. At this point it is only a discussion.

Have not yet gone to Lowes and sort though what is in the plumbing department for the necessary fittings to see if it is even possible.



This isn’t rocket science. If the upper hose is 135* what mysterious route could it have taken to get there?

Try this.

Get the engine as hot as possible.

While the engine is idling, loosen the upper radiator hose clamp and slip a small flat bladed screwdriver in between the upper radiator hose and hose neck on the radiator.

Allow the engine to idle until any air that might be trapped in the system is purged.

Remove the screwdriver and tighten the hose clamp.


But then he could have a left handed hose clamp on a right handed hose.

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