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91 Crown Vic no heat

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Dec 14, 2007, 11:18 PM

Post #1 of 10 (1317 views)
91 Crown Vic no heat Sign In

I've had car for several years no problem. Starting last winter, no heat. Heater core not plugged on water or air sides. Blower works, air distribution doors all work.Have physically opened damper to full heat mode, 3 t'stats. Heater hoses changed summer before losing heat. Checked, and are firm and will flow water. There is no heater valve in hoses. Fan clutch good. Problem I believe is engine doesn't reach high enough temp, although gauge shows low side of normal, maybe a little higher. Can block radiator with cardboard, no change in gauge setting or heat output. Heater hose ports to pump and intake both open. Heater hoses warm, not hot, to the touch. What are your thoughts?

thanks in advance



(This post was edited by phil63146 on Dec 14, 2007, 11:23 PM)

Tom Greenleaf
Ultimate Carjunky / Moderator
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Dec 15, 2007, 2:02 AM

Post #2 of 10 (1308 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

That is a 5.0 - right? Heater hoses are a total PITA on those - if this just happened with changing those and you read steady temp there must be something wrong with them. There's a metal tube going under upper intake manifold to rear where a heater hose clamp/hose is near impossible to deal with, with the manifold in place. If this is the 4.6 please disregard that comment,

Long retired now


Dec 16, 2007, 1:13 AM

Post #3 of 10 (1301 views)
Re: 91 Crown Vic no heat [In reply to] Sign In


5.0 yes. PITA, yes. Had all 3 hoses off for weak spots or any obvious defects. Hooked garden hose to hose that comes off of intake, water flows thru hose, metal tube, hose, heater core, and return hose in generous volume. I can't to raise the temperature in the engine to save my life. Thanks for your ideas.

(This post was edited by phil63146 on Dec 16, 2007, 1:14 AM)

Tom Greenleaf
Ultimate Carjunky / Moderator
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Dec 16, 2007, 2:57 AM

Post #4 of 10 (1299 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

I've owned about 400 of those engines - just three right now! System known full, start engine from cold and hold the upper hose near thermostat - the sign that says "caution-fan" means it as you know. That hose should stay stone cold all the way to the radiator till thermostat starts to open - period. If not the thermostat is broken and I don't care how many you have put in it's supposed to stay shut.

These thermostats are easy to let fall out of their spot when just tightening up the housing and a good one will be indexed as to what it TOP with a teeny, tiny ball check to let air out. As always the spring and wax pellet go towards the engine.

This about must be a thermostat problem. Test it in a pan of water on a stove (may create domestic problems especially around holiday cooking times) and watch it open. T-stat should be 192F -- 195 is fine and more common. Clean and dry the housing, use minimal sealer to hold and lock the t-stat in the housing with the gasket. Bake to 350F for two minutes (JUST KIDDING !!) but let it get good and stuck in place. Wire wheel brush the bolts and put a smear of lube on them as you need miniature fingers to get them started when putting this back on car. Another trick is use lashing/mechanics wire to hold bail of thermostat tight inside cover and bend wire ends over unhosed outlet of cover till this is snug against engine and you can pull the wire out. Test that with wire as it must be soft wire or this becomes the problem if it doesn't just come out easily.

You MUST know that the cover is holding the thermostat in place as the slightest slip will allow coolant flow and the engine won't warm up in moderate to low temps. The 1/2 inch timing wrench is good at getting to the harder bolt. There are two styles there and one doesn't allow sockets at all to attach that bolt.

From there you can fight with the by-pass hose and finish. Aim all hose clamps so you can tighten them with 5/16-8mm 1/4 socket on extention when all together as room stinks around there.


Note II: The metal hose under intake plenum has a 10mm coolant hose out of sight in it to keep upper clam shell warm and it is impossible to replace that without removing the manifold upper part. Hit back and we'll find a new metal tube if yours if NFG for some reason. I doubt they are available new but might be - haven't checked lately.

Note III: If you have Done all the heater hoses you will note a metal cork/plug in the inlet hose with a pencil size hole to slow coolant flow down to save the heater core. That could easily be plugged. If you have ATC the thing with two wires just delays fan till warm coolant is there and can be jumper wired - common problem and it only controls the fan not restrict coolant. So far anyway with those.

If this is not the thermostat from what you said I'd be surprised but we can get thru this. Upper hose test will tell,

Long retired now


Dec 19, 2007, 12:06 AM

Post #5 of 10 (1289 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

Hi Tom,

I've worked on a ton of these too, and have never, ever had this kind of problem!

I went out and bought a Stant thermostat (baked at 350 per your recommendation), and that sucker consistently opened at 195.

So I put it in the car and drove it for 15 minutes and guess what? The gauge went up maybe the width of the needle. Blew cold air, upper radiator hose COLD. Pressure in the system (upper hose nice and stiff, still had pressure 15 min later) radiator full, and overflow 1/3.

The heater core was replaced 3 years ago. the 1/4 hose to the EGR cooler replaced maube 2 years tops.

Something wierd I remembered and forgot to mention earlier.. On at least 2 of the 3 (I believe it was all 3) of the prior thermostats on the first warm-up the gauge would go to the half-way spot, then settle to the bottom end of "normal" never to go that high again.

A few weeks ago I had the garden hose hooked up so water ran through the 3 hoses and metal pipe. Even reversed the flow, and it was very good. One thing I didn't realize was the cooling system would fill up via the EGR cooler hose. The fan clutch is good, too.

What now? I'm stumped.

thanks in advance


I just checked the old stat. Opened at 182.

(This post was edited by phil63146 on Dec 19, 2007, 1:23 AM)

Tom Greenleaf
Ultimate Carjunky / Moderator
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Dec 19, 2007, 4:28 AM

Post #6 of 10 (1280 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

Hi Phil,

This is causing major hair loss!

Review: 5.0 Ford runs cold, no heat, thermostat not believed stuck open, cold upper rad hose.

The burning question is...... WHERE IS THE HEAT GOING?? 50% of the energy of burning fuel is making heat!

I've just spent an hour looking at this thread and still wonder. Is the lower hose warm by any chance? Is it extemely cold where you are right now? It is here but things still warm up to norm temps even at zero F out.

Again, I'm a bit lost too! Where is the heat going? Feel around hoses and see where it's going. It cant' all be going out the exhaust. Virtually every possible problem would make an overheat, like air, pinched hoses out of sight, blockage in water jackets, water pump failures etc, etc.

If cold enough a heater core would suffice to cool the whole engine but the heat would be going somewhere. Is one of those(heater) hoses (5/8" from water pump is heater inlet hose) hot and the other notibly cold even though no warm air output?

Do hit back with this. I want to know what the heck is causing this!

Long retired now

Double J
Veteran / Moderator
Double J profile image

Dec 19, 2007, 3:26 PM

Post #7 of 10 (1278 views)
Re: 91 Crown Vic no heat [In reply to] Sign In tom,i've been reading this one over and over...racking whats left of my brain.....remember one from the old days..had a pontiac...with a pontiac engine in it...thats far back......but had one where it wasn't getting heat....
inspected everything or so we thought...finally took off the water/coolant pump....found the plate on the back of the pump that covers the impeller was rusted thru...pump/impeller were working but wasn't slinging the coolant because of the rusted thru hole.
i'm probably in left field but just a thought...Crazy


Dec 19, 2007, 10:43 PM

Post #8 of 10 (1271 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

thanks for the replies and encouragement. Thank goodness I have a spare vehicle (with a great heater:).

I replaced the water pump in 6-06 after #2 son let the engine overheat so badly the engine stopped. I checked the compression afterwards and all 8 holes were between 180 and 200. Still doesn't use any oil (but leaks it)

When I recently had the heater hoses off, I stuck a stiff wire into the water pump to make sure it wasn't plugged.

As far as the temperature difference across the heater hoses, I didn't even check last night as the engine didn't warm up. When checked with the previous t'stat, the hoses were lukewarm.

The temperature during last night's test flight was around 35 ambient. I forgot to mention (because I forgot) that over 1/2 of the radiator blocked off with cardboard for some time now.

After the holidays I will be pulling the engine for a re-seal and overhauling the transmission. I guarantee I'll have less time in that than in the heat problem.

I'll check the lower hose Thursday night and post results

thanks again. I appreciate your efforts.

ps to Tom: a Pontiac with a Pontiac engine, those were the days!. Changing the cam gear in 389s sure put a lot of bread on the table
(edit by Tom G) That was "jimn" with the Pontiac. Ya, there was a while when GM put anything in anything..

(This post was edited by Tom Greenleaf on Dec 21, 2007, 5:09 AM)


Dec 21, 2007, 12:25 AM

Post #9 of 10 (1260 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

Son took off with car at 9AM and won't be back until after my bed time. will try for Friday night


Dec 21, 2007, 9:00 PM

Post #10 of 10 (1254 views)
Re: 91 Crown Vic no heat [In reply to] Sign In

#2 son just returned with car, and at 7:50pm in St. Louis the ambient is 55 degrees!

Both radiator hoses hot and heater working real well. I guess we'll have to wait till it gets colder.(it doesn't take long here). I'll post then.

Until then...thanks,



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