Heat is turning cold... then hot... then cold... etc.

[andrewbourdeaux]

Hey there. I'm new here and this is my first post.
I have a 1994 Ford E350 7.3L IDI, non turbo ambulance. After starting up, my heat begins warming up and as I am driving down the road, it will suddenly start blowing cold after maybe 20 minutes. It will blow cold for maybe 10 minutes, then immediately becomes hot again for 5-10 minutes, then cold and it's a back and forth cycle that goes on indefinitely. I have had this problem since I bought the ambulance three years ago. Coolant is at the cold fill line when the engine is cold and there are no leaks.

Another thing to note that could be related is that I can roar down the highway at high RPMs (not sure exactly what because there is no tach) or hammer it up a 6-8% grade here in Colorado and my engine temperature gauge will not exceed the letter 'O'. Often times it doesn't even exceed 'N'.

As for the heat going cold issue, when it switches back and forth, there is a different sound coming from the air ducts. You can hear when it switches. I've also tried running heat on A/C mode as well as Vent mode.

Thank you for any advice you have!
Andrew

 

[Clb]

First factory "idiot" gauges are not accurate.
Sounds like a stickie t statt..
Hit the tech section, and read the stickies.
Tons of helpful info

 

[greenskeeper]

Hey there. I'm new here and this is my first post.
I have a 1994 Ford E350 7.3L IDI, non turbo ambulance. After starting up, my heat begins warming up and as I am driving down the road, it will suddenly start blowing cold after maybe 20 minutes. It will blow cold for maybe 10 minutes, then immediately becomes hot again for 5-10 minutes, then cold and it's a back and forth cycle that goes on indefinitely. I have had this problem since I bought the ambulance three years ago. Coolant is at the cold fill line when the engine is cold and there are no leaks.

Another thing to note that could be related is that I can roar down the highway at high RPMs (not sure exactly what because there is no tach) or hammer it up a 6-8% grade here in Colorado and my engine temperature gauge will not exceed the letter 'O'. Often times it doesn't even exceed 'N'.

As for the heat going cold issue, when it switches back and forth, there is a different sound coming from the air ducts. You can hear when it switches. I've also tried running heat on A/C mode as well as Vent mode.

Thank you for any advice you have!
Andrew

HVAC control issue.

Blend door is probably not holding and bypassing the heater core at times making the heat go to cold air and then back again.

 

[Clb]

Oooh
that vacc line under the pass side corner of cowl was bad on mine,(had to go into the loom to get good material to fix to....

 

[andrewbourdeaux]

Oooh
that vacc line under the pass side corner of cowl was bad on mine,(had to go into the loom to get good material to fix to....

What exactly is this line you are referring to?

 

[Clb]

Vaccume line, runs the hvac ,open hood, look in the left corner at the cowl small white line (later rigs) .
No ac, follow the heater box vac. Line thru the firewall ...
If the vac. Is intermittant the door will open/close dependant upon vac. Signal.

 

[snicklas]

Do the brakes work OK, or do you have a really stiff pedal? Or, does the heat, or which vents changes when you press the brake pedal?

On the IDI's the HVAC doors and the power brakes are run by vacuum from a betr driven vacuum pump. If your pump is getting weak, strange things can happen with the brakes and/or the HVAC. If the brakes are fine all the time, then it's a vacuum leak of the white line, mentioned above, or the vacuum pot on one of the doors. If the brakes and HVAC are acting up, it could be the pump. If your trick has hydroboost, then the brakes are run by power steering pressure, and not vacuum.... I had a 92 F-250 that had a "lazy" vacuum pump. When the engine was cold, it would take a bit to build vacuum, and then you would only get 1 or 2 brake applications and then it lost all assist. It would come back in a few seconds. But when the engine and pump were warm, it worked fine......

BTW - I moved this and the glow plug threads to the proper area... you have an IDI... they were in the PSD section....

 

[Isaac Ristow]

My 86 plow truck is doing this now. Mines the hi output heat without ac no vacuum controls. My temp gauge will sit on the cold line if it idles or drives gently/normal if I pound on it she will warm up to normal operating temp. Sometimes it will run normal temp idling. seems that the thermostat is sticking open sometimes. I have a new motorcraft one to go in I can stop back and share results if I remember

 

[andrewbourdeaux]

@snicklas My brakes are very stiff only when it's super cold out right after starting. Maybe 20 °F and under? After she warms up they work fine. My BRAKE warning light on the dash recently started turning on then immediately back off ... but more recently it comes on then off then on then off even when she's been running down the highway for 6 hours straight. Everything should be warm by then, eh?
Thank you for moving post by the way, I couldn't find the IDI section.

@Isaac Ristow Yes let me/us know what happens with the new thermostat. I am very interested because I've had many symptoms that all point to my thermostat being bad.

 

[Clb]

Change t statt and coolant (read up on sca's first) unless ya know how old it is...

 

[andrewbourdeaux]

@Clb Do you have a good resource to read? What do I need to know? I read something about using test strips..?

 

[The_Josh_Bear]

Do the brakes work OK, or do you have a really stiff pedal? Or, does the heat, or which vents changes when you press the brake pedal?

On the IDI's the HVAC doors and the power brakes are run by vacuum from a betr driven vacuum pump. If your pump is getting weak, strange things can happen with the brakes and/or the HVAC. If the brakes are fine all the time, then it's a vacuum leak of the white line, mentioned above, or the vacuum pot on one of the doors. If the brakes and HVAC are acting up, it could be the pump. If your trick has hydroboost, then the brakes are run by power steering pressure, and not vacuum.... I had a 92 F-250 that had a "lazy" vacuum pump. When the engine was cold, it would take a bit to build vacuum, and then you would only get 1 or 2 brake applications and then it lost all assist. It would come back in a few seconds. But when the engine and pump were warm, it worked fine......

BTW - I moved this and the glow plug threads to the proper area... you have an IDI... they were in the PSD section....

@snicklas My brakes are very stiff only when it's super cold out right after starting. Maybe 20 °F and under? After she warms up they work fine. My BRAKE warning light on the dash recently started turning on then immediately back off ... but more recently it comes on then off then on then off even when she's been running down the highway for 6 hours straight. Everything should be warm by then, eh?
Thank you for moving post by the way, I couldn't find the IDI section.

@Isaac Ristow Yes let me/us know what happens with the new thermostat. I am very interested because I've had many symptoms that all point to my thermostat being bad.

@snicklas you're good man...called it!

Do the ambulances use hydroboost brake units?

I'd grab a vacuum gauge and test your vacuum pump, if you don't have one you should buy one that does fuel pressure and vacuum like for an old carburated gasser. Will come in very handy for this vehicle! They are around $15 IIRC.

 

[andrewbourdeaux]

Something I should add is that I have had this issue with the heat going cold way before ever seeing that brake light on the dash. The brake light has only been within the past few months. I'm going on three years with this ambulance. I figured the stiff pedal in the cold had to do with slow moving cold fluid or something, but now that I think about it... that's doesn't make a ton of sense. Haha!

I believe it does have some form of brake hydroboost or powerbooster. Are there a difference in these two terms?

 

[The_Josh_Bear]

Hydroboost is Ford's hydraulic-assisted brake booster, runs off the power steering pump.

A normal F or E series rig will have a vacuum booster instead.

 

[andrewbourdeaux]

@The_Josh_Bear Okay, so given this is an E350 with the ambulance package from Ford, do you think that I would or wouldn't have this hydroboost? Anything I can get up under and look at to confirm?