I am having a curious issue with the heat in my 70 cutlass. When I got the car nothing worked with the climate control. Turned out the blower fan wasn't hooked up so I took care of that. The control wouldn't change from heat to vent to defrost etc. Took care of that, it was just a vacuum line. I also replaced the heater core. So now everything works except when I turn the temp to warm the air is hot for about 10-15 seconds and then goes cold. If I throw back to cool and than back to warm it does the same thing. You can hear the door moving so I know that is working. The coolant is full and the car runs steady at 190 degrees. Any ideas what would be causing this? I am stumped. Thanks for any help

 

Radiator, and thermostat are new as well. Not sure if that would cause anything since the car does run at proper temperature

 

My firstguess is a vacuum leak.
It couldcause a few issues.

Do you havea Vacuum valve on the Heater water hoses? My 71 has a vacuum controlled valve on the PS rear of the engine. This value connects the hot water return into the engine. If the value is open it allows the water to pass through the heater core. If the valve is closed, the water through the heater core is minimal. The Heater core water comes from the water pump, into the heater core. Then from the core into the rear of the engine block through the vacuum water value.



When you setthe temperature lever to Cold, you build up a vacuum in the line and the value is Off. When the lever goes back to HOT,the vacuum in the line opens the value and allows the water to flow for a few seconds. However there is a leak and the vacuum drops and the valve closes (that’s my guess).

 

Yes I do have that valve it is also brand new. I had this same issue before replacing any of the parts. Where would I begin looking for a leak that would cause that issue?

 

If you have a hand held vacuum pump you can use it hooked to the line coming threw the firewall to the engine and then you should be able to hear if anything under the dash is leaking while somebody keeps the vacuum up.

 

It seems the problem is 100% reproducible.
Set temperature control to COLD, wait 10 seconds, then move to HOT and it will work for a few seconds (Hot air until the water cools).
If this is true, then we are getting close.
Do you have a hand vacuum pump? If so, use a long house (3-4 feet) and connect to the block water valve.
Run the engine and with Zero vacuum to the value, check your air vent temperature (ill guess and assume No vacuum is COLD?).
Then pump the vacuum to 10 to 15 inches of vacuum on the Block water valve. Now check your heat vent.
You should now have HOT air?
Monitor the vacuum gauge and make sure it Holds for 5 minutes.
If the vacuum stays at 10-15 for the duration of the test, then you know the valve is working OK.
Verify you still have Warm air on heat vent after 5 minutes.
This also points to a leak in the Hose to the under dash Manifold switching area.
It’s either the Valve hose is leaking at one end or the other, or a problem with the manifold switch.

 

Ok guys I will dig into this and check it out

 

Yeah, my thought is that maybe you have the wrong heat valve on the engine - one that works the opposite way (they do exist). If the valve is closed when it should be open, you would feel some heat until the heat in the core was exhausted, then it would run cold.
When you switched to temperature switch back to Cold (which should close the valve), the valve would open and you'd let more hot water into the core, which would provide heat again for another minute, etc.

- Eric

 

If that were the case wouldn't the temp control work but opposite? Meaning wouldn't I get steady hot air when set to cool and cold air when set to warm? Just wondering cause I didn't try that and that would give me my answer.

 

Nope. The overall temperature is controlled by a cable that moves a flapper from the cold to the hot air supply.

The heat valve shuts off with the temperature lever all the way over at COLD (you can feel a bump), and when the selector is set to Max AC. It's open all other times.

- Eric

 

I agree with MDchanic's diagnosis. I just wish I could remember which model years used which water valve.

I have heard, but am not certain, that the two different heater control valves had different size threaded bases. So when Olds switched to the new valve, they also machined the intake manifold differently. What this means is, if you find you have the wrong valve (meaning it's not matched with your dash control unit), then you likely have the wrong intake manifold as well. I can easily see this situation developing if someone drops a newer engine into an older car.

You can "fix" this situation by figuring out whether your valve needs vacuum or doesn't need vacuum to let water flow. Then either disconnect the hose or connect the hose directly to manifold vacuum accordingly so that water is permantly flowing.

 

I never looked into this possibility, but I do know that when I installed a '68 engine in my '73, I found that the '73 valve I'd been dragging around in the trunk for a couple of years didn't fit - if I recall, the '68 had a ½" NPT thread, while the '73 had a ¾" NPT thread (which is bloody huge for the purpose, by the way). I'd be damned if I was going to put aside a perfectly good valve I had paid $25 for, and then pay $50 for a new valve (!!!), so I bought a brass adapter fitting and used the '73 valve in the '68 manifold.

Maybe that was a smarter idea than I realized.

Since I don't know which years and models suck for On versus for Off, and I'm a bit too busy to research it in my CSM collection, I'll leave it to a more knowledgeable member to enlighten us all.

- Eric

 

I love it when that happens! Better to be lucky than good, eh?

More knowledgable -- or simply someone who has the time to go research the Chassis Service Manuals at WildAboutCars. I'm guessing the change was made in 1971 or 1972, if anyone wants to look. The opening Theory of Operation in the A/C section should tell.

Or maybe I can find the differenent HCV listings in the an Olds Parts Catalog then look up the thread size in a Dole catalog.

 

Here is the thing, I had an engine with a 72 intake on it before I dropped in my rebuilt numbers matching engine. I currently have an edelbrock rpm intake, the new valve I bought was the proper thread size. I had the exact same issue before i switched engines. I don't know if that tells you anything or not. I haven't had a chance to dig into this yet. Maybe I had the wrong valve on both engines?

 

Try hooking up a vacuum line directly from your intake to the water valve. If it is the correct water valve, it should pull it open and give you continuous heat (when your motor has warmed up). Pull off the vacuum line and it should close the valve and you would lose the heat.
This would at least determine if you have the correct valve. That's how my 1970 442 w/AC works.
Once you determine if it is the correct valve, you probably have a leak under the dash, possibly the vacuum switch, like this one.
http://www.ebay.com/itm/160995686885...84.m1438.l2649 or the line itself.

 

Thank you, lots of good info guys. Hopefully I can pin point the issue

 

So I researched a little more. I should get a life.

The dividing line between old and new heater water control valves was between the 1970/1971 model years. 1970 and earlier (at least back to 1968) used a valve which flowed coolant when vacuum was applied. 1971 and later blocked the flow when vacuum was applied.

The 1970 Chassis Service Manual states that vacuum is sent to the valve (to open it) dependant only on the position of the temperature lever. As soon as you move the lever off of "cold", it opens the valve. Doesn't matter which mode (heat, vent, A/C, Max A/C) you're in. I don't know if later years operated the same.

When I googled the part numbers, I was able to find some pictures. This isn't scientific, but it appears to me that the later valve (1971+) has the larger pipe threads. It also has the hose exiting vertically instead of horizontally like the 1970 valve, though the 1971 picture I found was an aftermarket unit, not NOS GM, so I'm not certain it's identical.

As was suggested, since you dash control head is a 1970, you need to purchase a valve for a 1970 (I would think NAPA has a replacement) and then adapt it to fit your manifold.

 

Thanks for checking that out Brian. My new valve I did get from napa and is the horizontal style. The reason I actually came back on here was to give an update on how its working. I took the car for the first decent cruise last night. As I said when the car is up to temp and I turn the heat to warm the air is hot for a few seconds and then goes cold. Last night I had the heat on and turned to warm, when I step on the gas and get the rpms up the heat gets hot again and then when I am back to cruising rpm's it goes cool again. So it seems the increased vacuum does open it. So, I would assume that would mean there is a leak somewhere or it is the vacuum switch that 442dale suggested? I still haven't had the chance to dig into it yet just thought I would share that bit of info.

 

You need to check the function of the valve and the way that the vacuum signal works.

Turn on car.
Put Temp on Cold.
Pull hose off of water valve.
Check for vacuum.
Put Temp on Hot.
Check for vacuum.
Write down results.
Turn off car.

Attach 3 foot length of vacuum hose to valve.
Remove heater hose from valve.
Stick pencil or screwdriver into hole heater hose was connected to.
Suck on vacuum hose.
See which direction valve moves inside (with that valve, should pull inward, away from heater hose fitting and toward vacuum hose, which opens valve).
Write down results.
Reassemble.
Compare written results: Does valve open or close when Temp is set to Cold?

The action you describe in your post immediately above sounds more like air in the lines than like a valve problem

- Eric

 

Ok, I figured that would shed a little more light on the subject. Thanks for the info

 

Yeah, I mean, we can all blow a lot of hot air around discussing which valve went to which year, and which year worked which way, but the bottom line is what your car actually does and what your valve actually does, especially since your car has components from different years, and an aftermarket valve could have been mislabeled or mis-listed.

This discussion has been going on for days now, when it would take ten minutes to just check what's actually going on.

- Eric

 

Yeah, I mean, we can all blow a lot of hot air around discussing which valve went to which year, and which year worked which way, but the bottom line is what your car actually does and what your valve actually does, especially since your car has components from different years, and an aftermarket valve could have been mislabeled or mis-listed.

This discussion has been going on for days now, when it would take ten minutes to just check what's actually going on.

- Eric

X2 on that....