Heater core recommendation needed + another HVAC issue
 

I just got back from feeding the beast at a local gas station and realized that my heater core is most likely leaky. Symptoms are fog build up at the windshield after stopping the engine and moisture in passenger footwell. So... Seems obvious, no?

Rockauto offers a variety of cores. Can anyone recommend any of these? Or are they all the same cheap Chinese crap?

Second issue I have is with the HVAC. Here I found 2 symptoms. First: sometimes, usually upon starting the car, the system sort of "resets" itself to 24 degree celsius, all automatic mode and in few cases also turns on the rear window defogger. Last weekend this happened while I was driving with the system turned off (targa roof was off, so no airflow needed).
Second: with the system in "Off" mode under hard acceleration (not sure if that is relevant) the fan started blowing full power, outlet air was maximum hot. As I don't expect an emergency engine cool down function I suppose my HVAC controller module (??) has an issue.

No LED lights are flashing, no codes present either. What do you think?

Re: Heater core recommendation needed + another HVAC issue
 

Matt I believe they are all Chinese crap however you are correct your heater core is toast. The passenger footwell wetness and the moisture out of the defroster vents is the symptoms of a failed heater core indeed.

No idea on the HVAC a problem but the controller is indeed suspect

Re: Heater core recommendation needed + another HVAC issue
 

I concur as always with GC :thumbsup:

Many Heater Cores for C4 Corvettes on Ebay.

Post 135 - Replacing the Heater Core and Heater Hose Connection

Re: Heater core recommendation needed + another HVAC issue
 

Thanks for sharing this! The guide looks very helpful and thoroughly made!
I'll also check Ebay though it's not as easy for me to deal with as many sellers don't ship to Europe or at least charge a fortune for it. Rockauto doesn't...

2 more questions: is an aluminum core to be preferred over a brass version? In my understanding aluminium is first and foremost a CHEAP solution. But is also a GOOD (in terms of fit and durability) solution?
I remember from replacing my Lincoln Mark III heater core that the replacement part (aluminum, too) was about 1/2 inch thinner the the original and doesn't properly seal against the walls of the housing. I had to add a lot of alien material and also do some modifications to the housing to ensure proper fit and air blending.

Second, when reading your guide - is it really that much easier than just pulling the dash? I removed my CDM twice and I know that even with all the skills gathered during the first attempt working down there is a PITA due to the absolute lack of space.
Due to the total lack of personal experience: is this really a shortcut that saves significant working time (which is not paid off with bleeding hands and a mental breakdown ;) )?

Re: Heater core recommendation needed + another HVAC issue
 

Matt, one word of advice, whatever heater core you get test it first before install to be sure it does not have any small leaks. We tested mine with air pressure and it was good. It took several of us almost 8 hours during Wazoo maintenance day to do mine. We pulled the dash. As the guys took out screws, I would bag and tag them so we put them back in the right spot. Pulling the dash was stressful, I remember it would not come out and Nelson said, "just yank on it". On the count of three, pop, it came out about the same time my heart jumped out of my chest. Still cannot believe it all went back together without any broken plastic clips and without any rattles. No left over parts is good!

Re: Heater core recommendation needed + another HVAC issue
 

8 hours by professional corvette guys... That sort of scares me (and answers the question whether doing the job without pulling the dash might be more time efficient...)

Pressure testing is likewise a good advice! Thanks, I'll definitely do.

For the moment I'm considering to bypass the core and fix it later that year. We have STUNNING weather and it's a shame to not run the targa ZR because of a broken heater core she definitely doesn't need...

With that I could also invest some time to go through all the pesky noises my dash produces...

Re: Heater core recommendation needed + another HVAC issue
 

Matt,

Having done this work on dredgeguy's 92, to remove and install a new heater core in one piece will require dash removal.
I think a short cut would be to cut the protruding supply and return line at the outside bulkhead for easier removal. New heater core supply and return line can be cut just enough for splicing back on after installation. This strategy might negate wholesale disassembly of the dash.

Re: Heater core recommendation needed + another HVAC issue
 

Thanks, Yun. I'll definitely take this into consideration. I already started today by taking out the drenched carpets. I ordered a "TYC" replacement core from Rockauto. Most likely it's china stuff but I will have my OEM piece fixed by a specialized radiator welder so I can put it back in at a later stage (if ever needed).

So later this week I will pull the dash... Get s**t done :)

Re: Heater core recommendation needed + another HVAC issue
 

So, upper dash is out. Went surprisingly well, less than 1 hour incl. permanent cross checking the shop manual and searching for tools :D

Now that I'm in there... What can I do to improve my rattling, squeaky little diva? I remember that someone mentioned a GM Service Bulletin that covers squeak and rattle in early ZR-1. Can anyone share this? Thanks!

Re: Heater core recommendation needed + another HVAC issue
 

I am not aware of the bulletin. My experience with 91 and 94 has been squeaks and rattles originated from places other than under the dash.
I would examine and look for suspects since there already. Combination of foam wrap, zip ties might help.
Also, check the condition of vacuum connections at the HVAC controller. Again, since you already have the dash out.

Re: Heater core recommendation needed + another HVAC issue
 

Leak is absolutely heater core.

HVAC is losing Vacuum.
It goes into default mode, when vacuum drops, sending air to windshield and floor vents. If vacuum supply is good to the programmer, then perhaps the programmer, but more likely the hose that troubles most everyone that you can inspect when doing the heater core.
:cheers:
Marty

Re: Heater core recommendation needed + another HVAC issue
 

Interesting comment about the vacuum, I'll definitely check that.
Are there any "usual suspects"?

Also, I already was intended to ask you gentlemen about the following - maybe it relates to each other? When I run her at part throttle, I'd say somewhere between 5 and 20% throttle, I hear a hissing noise pretty much from the center of the dash, I'd locate it below the front window defroster outlet.
It's frequency doesn't vary by engine revs neither by how much throttle I apply, amount of throttle only changes the intensity of the noise.
It's pretty loud, very easy to hear. I already checked the engine vacuum connections and fixed the dual PCV grommet. Noise is still there.

As I said, the malfunction of hot air being blown into the car with maximum fan speed (and HVAC in "OFF" mode) happened during extended WOT conditions in a mountain run.

You think these things are the same animal?

Re: Heater core recommendation needed + another HVAC issue
 

VERY Likely the same animal.
Use the Factory Service Manual to identify vacuum hoses and routing for the HVAC supply.
A smoke machine will assist, although if the leak is big, it will quickly envelope the behind dash area.
It is either a, hose, connector, or the programmer itself.
It is there, just search from origination to the programmer.
Others correctly cite the courtesy light area under passenger side dash.
If the hose has been disturbed, and now rests on the bulb, it can melt it.
Most of the time we encounter previous rodent damage.
Good luck, you will find it!
:cheers:
Marty

Re: Heater core recommendation needed + another HVAC issue
 

Thanks Marty! Very helpful. It's 2 p.m. here so I'll go and check it this immediately. I assume there is a troubleshooting guide in the shop manual.

Now that the dash is out it should be (relatively...) easy to work on that topic.

I'll give an update later on.

Re: Heater core recommendation needed + another HVAC issue
 

Okay, I just got back from my shop. I pulled the vacuum connector off the HVAC controller (that little retainer disc is AWESOME!!! *lol*) and tested all lines positive. Each holds vacuum as it should, actuators are moving nicely.
Obviously I could not test the vacuum source as engine is not running. Where exactly is the vacuum drawn from the engine? Id like to test the source line for integrity but have no idea where it ends on engine side.

So... No potential noise source found yet.

The heater core housing is nicely accessible now, next step will be to remove the coolant hoses and pull it out 👍

Well... IMMEDIATE next step will be to prepare a home made pizza and empty some bottles of beer. Cheers! :cheers:

Re: Heater core recommendation needed + another HVAC issue
 

Hello Marty,

Off-topic a little, but how do you apply smoke into the vacuum line? I have problems with my windshield wiper cover opening on my 1970 LT1. I use a refrigerant vacuum to check the different devices and listen to the hoses, but I am unable to find the leak. I think applying smoke might be my solution.

Thanks in advance Marty.

Nelson

Re: Heater core recommendation needed + another HVAC issue
 

Nelson,
Using any of the available smoke EVAP testing machines ( the more you spend the better they work) simply go to your initial source of vacuum for the combined headlight / wiper system and hook the machine up to that hose and if needed, use vice grips or a clamp to shunt out the working headlight system although at first I would check all as multiple small leaks spell trouble for that darn door.
The machine pushes the smoke through the system.
Then just look for the escaping smoke and there is your problem.

If you find no leaks, first remember to look in the passenger compartment, as that is often the culprit, i.e. headlight switch, overrides etc.

If no leaks are found, start at the manifold and work outwards through each device both in the engine and passenger compartments, until you again find the culprit.
Make sure your hard to find a replacement door mechanism is not binding.
Also double check that the hoses are properly connected. There was a long production run of wiper control valves that had the colored dots on the wrong ports causing the system to work against itself.
Patience and starting at the beginning always seems to find the problem.

Or send it up and we will take care of it.

Go get em!

:cheers:
Marty

Re: Heater core recommendation needed + another HVAC issue
 

Nelson, ROAD TRIP! I call shotgun!

Re: Heater core recommendation needed + another HVAC issue
 

Sounds like a smoke machine is a must-have. I'll check my local shops if someone sells them. Old cars are never ending vacuum stories...

Talking about vacuum... I'm still looking for the exact source of the HVAC vacuum. Does it have a vacuum canister? If so that's a potential issue...

Re: Heater core recommendation needed + another HVAC issue
 

Thanks Marty,
Will see you at Carlisle.
Nelson

Re: Heater core recommendation needed + another HVAC issue
 

After spending the evening in my shop manual I finally found the relevant section under General Engine Mechanical. More on-car research tonight!

Re: Heater core recommendation needed + another HVAC issue
 

So, I checked the vacuum system. I pulled 15 in hg from the canister/secondary air valve/cruise control branch. Absolutely no issues here.
Next I pulled vacuum directly at the plenum connector into the HVAC branch. Also super tight, no issues. Also the check valve seems to be fully operational.
So... I can't spot any unexpected behavior along with the HVAC vacuum circuit.

I will try to pull the modulator tomorrow and check for a potentially defect valve. But I doubt that this is the source of the noise I mentioned earlier.

Also the new heater core arrived today. Big day tomorrow...

Re: Heater core recommendation needed + another HVAC issue
 

So, while I was in there... I decided to take the chance and fix my CDM holder. Both tabs were broken. Inspired by one of our brothers who did a similar job (just don't remember who...) I shaped two new tabs from a strip of aluminum and used pop rivets to attach them to the carrier.
Also I reinforced the rivets with washers to avoid that the brittle plastic breaks again.

I added some more washers to the cruise control module screws to gain clearance and compensate the rivets heads.

Built to last!
Attachment 18466Attachment 18467Attachment 18468Attachment 18469

Re: Heater core recommendation needed + another HVAC issue
 

Looks like a nicely done job Matt. --Bob

Re: Heater core recommendation needed + another HVAC issue
 

Thanks, Bob!

However, now I'm stuck and need advise. I removed everything to gain access to the core. But at the end the shop manual isn't very precise...
My understanding is that I don't remove the entire heater core housing but only the rear cover. Is that correct?

If so, there are 2 things in my way which I can't remove/separate.
First is the hose that feeds the passenger door defrost. I removed the 2 screws that attach it to the heater core cover. But I don't see how I remove the hose. Do I just have to pull it towards the passenger door?

Second, there is a kind of cylindrical shape riveted to the heater core rear cover. Towards the passenger it connects to the interior temperature sensor hose. Towards the engine it connects to another hose that disappears behind the temperature control motor. There is little to no room to grab those hoses where they connect and separate them.
I tried to take a picture but it'll be hard to tell from that one...

Long story short: how do I finally remove the core when everything else has been taken out?Attachment 18470Attachment 18471

Re: Heater core recommendation needed + another HVAC issue
 

Matt,

it has been a while since I did this work. And, secondly, I don't have very good memory.
What I do recall is fighting to get some pieces out and in some case, locating and removing 7mm hex purely by feeling it with my fingertips.
Also, I believe you are right that you need to remove the housing cover and not the entire housing.
I just kept removing pieces until I can pull and remove the heater core.

Re: Heater core recommendation needed + another HVAC issue
 

Thanks, Yun. Sounds exactly like what I did - work on until there's nothing left to remove!

I found and removed all 7mm hex keys and can actually easily MOVE the cover. But I can't REMOVE it. Maybe I need to apply more brute force to get the clearance I need.
But I don't want to break anything that requires the whole center console and lower dash to be removed in order to fix it...

Re: Heater core recommendation needed + another HVAC issue
 

Us Zr1 owners (and C4 I presume) refer to the heater core as part #0000001 - the very first piece to be installed on the chassis.

Re: Heater core recommendation needed + another HVAC issue
 

Hahaha, oh my God... That doesn't sound very promising.
But you may be right. It seems that EVERYTHING is in the way here...

Re: Heater core recommendation needed + another HVAC issue
 

Finally!!! Not sure who designed this, but... SHAME!!! [emoji1787]

Seriously, this is a really nasty piece of work. If another brother has to go down that road I STRONGLY recommend to pull the dash, etc. It causes additional work but pays off easily the further you dig in.

I again studied the shop manual and realized that the horizontal tube runs all the way through the lower dash into the drivers footwell. There is another screw hidden behind the main wiring harness.
With that screw removed the tube first has to be pulled away from the rear heater core cover for about 2 to 3 inches to clear the connector between the two and then towards the passenger door.
What a tricky surprise at the end of the disassembly story...

Now everything is set and ready for reassembly. Attachment 18472

Re: Heater core recommendation needed + another HVAC issue
 

I'm nearly ready with putting everything back into place. What a job...
I spent about 2 full days with it, whereas plenty of time went into research as neither the shop manual nor the YouTube videos were very precise in giving instructions.

The only things that were painstaking or complex was to remove the heater hoses and removal and installation of the horizontal tube that is attached to the heater core rear cover. This thing is a nice example of what happens when engineers go to work after a long party night.

Summarizing I can say that the job isn't too complicated but requires patience and a bullet proof way to keep your overview.

Also, I can only recommend to pull the upper dash. It's extra work... But it quickly pays off the further you dig into the guts of your dash.
I can't imagine who the job can be done PROPERLY with the upper dash in place...

Re: Heater core recommendation needed + another HVAC issue
 

Matt B,

Thanks for taking the time to share your trials and tribulations of this ordeal. If it only affects heat I might just do a bypass. I don?t know if I have your patience. Congrats and well done.


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