Cabin Heat slow to warm up

[DennisG01]

Just curious if I've got a problem, or if everything seems normal. It seems like the cabin heat is taking longer to heat up as of recently, but I've really just started to pay attention to it as it hasn't been too cold until lately.

The water temp gauge seems to climb at the normal rate, eventually reaching between 180 and 195, depending on if I'm sitting still, going slow or at higher speeds (and higher RPM's). Although, maybe it's climbing a little slower than normal, not positive.

Other than a blockage in the cabin's heat exchanger, any ideas? It does eventually blow nice and warm, but will cool if the RPM's slow down.

I flushed the cooling system twice over the summer, which is why I'm wondering about other possibilities. I realize a blockage could still be the problem, but am curious about other things.

[JohnC]

Try bleeding the cooling system. If it has an air bubble the heater can blow cold even when the engine is warm.

If you find air, the next thing to do is figure how how it got there....

[DennisG01]

Good idea, and easy enough to do. Thanks, John. I will do that... but... would an air bubble work it's way out ? I ask because the "slow to heat" is pretty stable, meaning the same thing every morning. Takes a while to get warm, then only stays warm with increased rev's. It doesn't turn "cold", but it does get noticeably cooler as I see the water temp gauge dip down to the 185* mark, or a bit below (when in stop and go traffic, or idling, for example).

[leolkfrm]

parking it with the nose up hill might help burp it!...heard it has worked on some

[DennisG01]

I loosened the bleed screw on the t-stat housing with the engine running and a solid stream of AF squirted out. I did it a few more times and kept getting a solid stream.

I also visually verified that the "recirc" door is operating. Although, I figured it was as I can hear a noticeable change in the air/blowing sound when it press "recirc".

??? I think I might clamp off the rear heat lines in the engine bay. That should at least eliminate a variable, right? But, the rear is another problem. I've never had good (well... decent, anyways) heat coming out until traveling at highway speeds. I've flushed that system as well.

[DennisG01]

I clamped the rear heat supply/return lines. No change in heat performance of the front heat. It still only gets somewhat warm, but not until I'm traveling at consistently higher RPM's (highway speeds). Once I slow down, there is a noticeable drop in warmth. It does not get "cold", but I would refer to it as lukewarm, at best. I'll take my IR gun with me and jot down some actual numbers.

I would really be surprised if there was a blockage in the front heater core, since when I flushed it this summer, water seemed to flow through it very easily. But, could it be anything else?

[DmaxMaverick]

Thermostat. If it isn't closing (or is missing), the pressure differential may not be enough to force much coolant through the core.

Verify the return line isn't blocked. If coolant can't get out of the core, it can't get in, either.

Is the hose routing original? Was the pump replaced at some point before/during the problem?


Try removing the heater hoses and route them to a container. Idle the engine for a few seconds at a cold start and actually see where flow is going. You should see significantly more flow from the supply than the return. If equal, there won't be much, if any, flow through the core.

[DennisG01]

OK - I can do that.

Concerning the T-stat issue... does that still hold true since I have 2 stats? If it matters, the temps I see on the dash gauge are where I have always seen them (ie, they have not changed since this problem started). My ScanguageII gives a water temp readout that will vary between about 183* to 190*, once the engine is warmed up.

I replaced the water pump when I put in the Schoolcraft engine, about 3 years ago.

Although they have been off an on a few times, the heater hoses are still plumbed the same way they have been since I got the truck at 60K miles (about 125K, now).

[DmaxMaverick]

If it worked last winter, and you haven't (intentionally/knowingly) changed anything, then something has changed you don't know about. External changes are fairly obvious. Internal changes not so much, but happen over time.

Dual stats are no different than single in operation, other than having one more. At any rate, they should be closed until the coolant temp nears their rated opening temp. They weaken over time, and tend to either begin opening too soon, fail to open smoothly, or don't open at all.

Without something jumping out and taking the blame, effective diagnosis involves breaking down the system by function, and testing each function. Usually starting at the functions with easiest access and the least effort. Restriction is easy to test and see without special tools. Only time and effort is required. Start there. If it checks out, I suggest replacing the thermostats (even though your temps seem to be normal). Use Robert Shaw OEM spec stats. John Kennedy can help with this (he has them, and I can't find them anywhere else that I trust).

[DennisG01]

Thanks, will do. I can't remember off hand how old the stats are (but they are Shaws from Kennedy). I'm thinking I probably replaced them when I did the engine.

I'll check on the restrictions and post back. Might take a few days... until this cold front passes. It got down to 6* last night. Only about 19* right now - should be in the mid-40's by the weekend.

[DmaxMaverick]

Just had a thought.......

Maybe an easy test for the stats and flow function...... Start cold, then shut down as soon as you see the coolant temp begin to rise (before it's warm enough the stats should begin to open, but enough you should have vent heat). Clamp/pinch the upper radiator hose. I use a large pair of channel lock handles and a zip tie to hold them closed. If they don't pinch closed enough, you can slip sections of PVC pipe over the handles. Start and watch the coolant temp, and monitor the heater output. If it's remarkably improved, you can bet the stats aren't closing enough when cold.

Also, if you are using any selection of "defog", or "recirc", the A/C compressor will engage, creating a colder air situation. This "feature" is intended to do 2 things: help dry the air in the cabin, which reduces internal fogging; and "maintains" the A/C system when it wouldn't normally be in use (circulates the oil, rather than letting it pool for months). This is normally a good feature, but will also reduce the effectiveness of the heat available to the core.

[rustyk]

I couldn't see where you mentioned the rear heat - whether or not it's normal.

If not, you may have an arrangement like my Ford van; there's a solenoid valve to the heater core supply (from the 'stat). It cuts off water flow to the heater cores when the A/C is on max.

If this is sticking, it could be the source of the issue.

[DennisG01]

I couldn't see where you mentioned the rear heat - whether or not it's normal.

If not, you may have an arrangement like my Ford van; there's a solenoid valve to the heater core supply (from the 'stat). It cuts off water flow to the heater cores when the A/C is on max.

If this is sticking, it could be the source of the issue.

Thanks, Rusty. I don't think there is one - at least not to my knowledge, but I could be wrong. The rear heat has been a problem for years. I just figure the system isn't balancing (flow) properly to get adequate flow to the rear. The front has always been good, though, up until recently. Now they are both problematic.

[DennisG01]

Just had a thought.......

Maybe an easy test for the stats and flow function...... Start cold, then shut down as soon as you see the coolant temp begin to rise (before it's warm enough the stats should begin to open, but enough you should have vent heat). Clamp/pinch the upper radiator hose. I use a large pair of channel lock handles and a zip tie to hold them closed. If they don't pinch closed enough, you can slip sections of PVC pipe over the handles. Start and watch the coolant temp, and monitor the heater output. If it's remarkably improved, you can bet the stats aren't closing enough when cold.

Also, if you are using any selection of "defog", or "recirc", the A/C compressor will engage, creating a colder air situation. This "feature" is intended to do 2 things: help dry the air in the cabin, which reduces internal fogging; and "maintains" the A/C system when it wouldn't normally be in use (circulates the oil, rather than letting it pool for months). This is normally a good feature, but will also reduce the effectiveness of the heat available to the core.

If I understand correctly... clamping the hose will keep the coolant from circulating (or at least not entering the radiator)? OK - that's easy enough to try.

I have been using the recirc button since the air blows out harder. I experimented with the button (on and off) on the way to work today. I didn't notice much, if any, difference - at least by using the back of my hand as the temp sensor, anyways.

[DmaxMaverick]

Yes. Clamping the hose will simulate closed thermostats. If there is no difference, then they were already closed.

Selecting "recirc" is the same as selecting "Max A/C". It circulates air from inside the cabin, through the HVAC stack, with only a little outside fresh air mixed in. You should have noticed occasional A/C compressor cycling with it selected. This is also why there is more air blowing from the vents. However, if the air isn't being heated, more volume isn't helping any.

With the engine up to operating temp and idling, feel the heater hoses. If they're hot, then the problem isn't flow. The problem will be with the temp gate/servo not fully diverting the through the heater core. A lot of this isn't making sense, and I think this is what it will lead to.

If it were a front/rear balance problem, one or the other should be getting hot, not neither. The rear may have a separate issue.

There is no flow control solenoid in your system. Coolant flow to the heater core(s) is continuous, or supposed to be, anyway. Older Ford heater coolant flow systems were either mechanical (push-pull) or electric.

[DennisG01]

The problem will be with the temp gate/servo not fully diverting the through the heater core.

I did play around with the temperature selector dial yesterday, wondering the same thing you are stating. I moved the dial to about 4 or 5 different positions (from full hot to full cold) and I was able to feel a difference in the temperature at each position. However, maybe I'm not actually getting a full swing from cold to hot (0% heat to 100% heat, for lack of a better term)? Maybe the door isn't completing it's movement so I'm only getting 0% to 75%.

I'll be able to check on the other stuff tomorrow evening.

[Robyn]

The Blend door servo motors on these have been known to take a poop.

Under hard running the heat output from the core is sufficient to give "WARM" air but at idle or lower speeds the heat will fall off.

This is due to the air blend door motor not moving the door properly.

The recirc door motor is a BITCH to get to and IIRC you almost have to remove the HVAC unit to do so.

Now the recirc door is not really needed for heat or AC, just used mostly during HOT weather to get the max cooling from the system.

The Blend door servo I believe is accessable.

There is a special sequence for setting these things too.

The HVAC system I believe is tied in with the computer ??????
If you can gain access or see under the dash, move the heat control from cold to hot and watch whats happening.

The servos are a little black box. There are three of them, one for the blend door, one for the mode (defrost, heat, etc) and one for the recirc

If you can access the servo motor, remove it so you can see what its doing and then run the thing to see if it makes its full sweep.

These little servo motors have plastic gears and can/do wear out.

The way they work is fairly simple, when they reach the end points the motor runs against the stops and then the amp draw increases and the electronioc controls senses this and shut off the power. Then when the selector is moved back to say the mid point, the controller allows the motor to move the door a predetermined distance to approximate the heat level desired.

When the selctor is moved to the opposite end say cold from hot the process reverses and the motor will run aqainst the stops and shut off.

The doors can bind due to crud finding its way in through the cowl vents and or plastic parts wearing out.

If you can get the servo off the shaft, you may be able to manually move the door shaft/door and see if the issue is the servo or the door itself.

If you can move the door manually and get HOT air then the issue is far easier.

My guess is that the issue is either the Dash controller or the servo motor.

My first hunch is that the servo has/is dieing.

Now depending oon whatcha find, the dash may have to be loosened, lifted and or removed to gain access.

IIRC the blend door servo is accessable (sort of)

Good luck and keep us posted

[Robyn]

OK

Dug out my 94 service manual and took a peek.

The HVAC system should be very similar.

The servo's (actuators) are located as follows

The temperature/defrost servo is loacted on the end of the HVAC case close to the throttle pedal. It will have a small bundle of wores pluged into it.

The Mode servo (acutuator) is located on the front of the HVAC case in about the middle between the drives right foot and the RH side of HVAC case.

The recirc servo is on the far RH end IIRC (could not find reference to it in the book right off)

You should be able to watch the servo on the LH end (defrost/temp) and see what its doing.

Missy

[Robyn]

I"M BACK

Here is a link to a well put together You tube video on this very subject.

http://www.youtube.com/watch?v=Ow0OOfIFtlU

This gives a fairly good idea of whats up and where stuff is.

Good luck

Missy

[DennisG01]

Thank you, Robyn! I can certainly check on that. Is the blend door the same thing as the "temp/defrost" door? I assume yes, just wanted to double check.

[QUOTE=Robyn;287303] It will have a small bundle of wores pluged into it.

ummmm... Is that "o" supposed to be an "i", or did you forget an "h"?