Faolan

Picked up a mint 90 v6 firebird. At idle temps climb to around 230-240 and has very little heat coming into the cab. Driving temps drop to about halfway between 100-220. Blows cold air. I got coolant flowing through the heater core. The only really hot coolant lines are the two radiator lines. All of the others a practically cold. Kind of stumped here. I replaced the heater control valve and it the climate control switch is working I believe. Where should I start? Was thinking weak water pump maybe?

Handy_man91

Have you checked to make sure there's a thermostat in the housing? Just throwing ideas out there

Faolan

No, but you can see when it opens on the gauge

this is all in 20-40f weather fwiw

Handy_man91

You have a good gauge, or good eyes lol. when mine hits norm operating temp it's rock solid.

Water pumps aren't too pricy, I guess that would be the next step provided you already checked your blend doors over good. And you said you already verified heater core flow.

Faolan

Yeah, guess it time to at least pull and check it. It pretty noticeable when the tstat opens on this one, it’ll drop a lot Then go two or three notches over 220 then drop two or three notches under 220. Going to try burping the system today in case there’s any air bubbles. It was too cold to risk running water through the heater core yesterday so I checked it with compressed air. Might not have got it all out

Handy_man91

Might consider checking proper water pump rotation. Could have a belt routed wrong or maybe wrong style pump. Just something else to look at.

Faolan

Checked that last night and it’s going the right way. Noticed on my 91 305 there’s a line going off the heater control valve straight to the radiator. And the 90 3.1 they have a line going to the water pump..

sofakingdom

iTrader: (1)

Sounds to me kinda like a blocked heater core. You could try backflushing it with a garden hose but the hose you use needs to be able to deliver AHELLUVALOT of water.

Could also be an issue with the heater control valve, if that car has one.

Faolan

Just replaced the heater control valve. I can blow air through the heater core, haven’t tried water through it yet, too cold for that.

FRMULA88

The heater control valve works with a vacuum line... check for a missing/broken/leaking vacuum line.
If you have a pump you can apply vacuum to the valve to see if you can get heat.

.

Faolan

Mine has the line attached. It gets vacuum with a/c on. With it in the heat position it get warm but only when the car gets to 220. My 91 gets heat the second the gauge goes over the 100 mark..

The only HOT coolant line are the two radiator ones, the heater lines and the one Corning directly off the water pump to the control valve get lukewarm at best.

think I’m going to pull the water pump tomorrow and make sure the fins aren’t wore down. It’s got 220k on it

OrangeBird

iTrader: (1)

This is incorrect .

The operation of the valve is that with vacuum applied , the heat is off , and with no vacuum applied the heat is on . As Faolan noted "It gets vacuum with the A/C on" , this is proper since you would want the heater core cold when trying to cool the car with the A/C .

The reason why it's designed that way is to ensure the heater core gets hot in the event of a vacuum system failure . This coincides with the operation of the rest of the HVAC system , a total loss of system vacuum causes a default condition of all air being sent to defroster , and with the hot heater core this ensures a fog free windshield .

Gotta love those GM engineers , always lookin out for us

sofakingdom

iTrader: (1)

There are 2 different valves. One has the logic Orange refers to; the other the opposite logic. Needless to say, it's important that the CORRECT valve be installed, not just that it be "replaced", without any further details.

Eeeeeeeeezzzzzzzzzzy enough to verify... stick yer finger over the end of the vac line that drives it, first with the temp all the way cold and system off, second with temp all the way hot and system in heat or defrost. If you have vac in the former but not the latter, then you need a valve with the logic Orange talks about. If you have NO vac in the former but DO in the latter, you need the opposite logic. If you have vac in BOTH modes, or NO vac in EITHER, you have a control head or vac line problem.

Break the system down into its simplest components and test them individually.

OrangeBird

iTrader: (1)

With all due respect to Sofa , and I'm not trying to be a know it all here ;

All of the V6 cars like the OP's used the same valve as I described . The listings for 1987 through 1992 V6 all call for the same valve (A/C part # 155342 / Four seasons 74805) and the logic for that valve is no vac = heat on , vac applied = heat off .

It's very possible that the V8 cars may have had two different valves , I'll freely admit to knowing more about the V6 than the V8 because my last V8 third gen was over 20 years ago (Had a sweet 87 T/A back in the early 90s) ....

Faolan

Right. Yes the heater valve works properly and is the same as the one I replaced. Heat on no vac, ac on vac. Ac used to work until the compressor started leaking. All is well and correct with the valve. And the blend door.
things I’ve checked
heater valve
blend door
heater core flow
Thermostat
proper water pump rotation
vac line to the valve
coolant level
burped the coolant system

there may be a few others I checked but that’s all I can think of off the top of my head

OrangeBird

iTrader: (1)

A side thought here for you ;
If the way you determined you have coolant flowing through the heater core is only that it would pass air , from the rest of your description I'll bet that heater core very likely is partially (mostly ?) clogged . Air is way thinner than antifreeze and i'll bet you can force all kinds of air through a heater core that is clogged enough to not produce heat . The two big hoses being hot means you've got hot coolant flowing through them , the two lukewarm heater hoses are telling you that a restriction (likely that mostly clogged heater core ) is keeping hot coolant from flowing through them .

FRMULA88

try flushing the cooling system; add the jug of flushing chemical to the system and drive the car normally for a day with the heater on so the treated coolant can work on the heater core too.

Drain, enjoy what you see, add new coolant mix with another jug of flushing chemical and repeat the process.
If the you have more heat now and the heater lines are hot, success.
Drain, enjoy what you see, add new coolant mix and happy motoring.

Unless the heater core is leaking leave it alone and try to unclog it by flushing the system. Replacing it on 3rd gen; or any late model car a major PIA.

FRMULA88

If your heater core is producing some heat flushing the system should clean it out so it works properly, winter may not be the best time for DIY project if you don't have a garage.

Any of those quick change oil places can do a system flush and re-fill in about 30 minutes to get you thru this winter. (just don't get sucked into an "up sale")
Come spring/summer time you can do it yourself and replace the heater core if you feel the need so you are ready for next season.