[mammadovn]

Hello

Chevy Caprice 1993, 305.

Setting the heater to “heat” position, which blows to foot and when pressing gas pedal to “kick down” / hard acceleration, the heat start to come from upper vents. Heater speed is also increasing. It looks like the system loose vacuum somewhere, but cannot identify. Checked for vacuum leak at engine compartment with smoke - no leak identified.
Any thoughts?

 

[Fred Kiehl]

Check the port on the TBI for a partial clog. Another thing to check is the check valve at the TBI.

 

[mammadovn]

Thanks a lot for response!
Just to be more specific, which port of tbi to check and on which line is the check valve ?

 

[mammadovn]

Btw, when checking for the leak with smoke, observed just small amount coming from inside of tbi. Is that ok or it’s time to change tbi gaskets and could it be the reason?

 

[Fred Kiehl]

There is only one port with a check valve on the front of the TBI. It allows vacuum to port into the hose, and traps it there. If you are pushing smoke into the line from the interior, you can get smoke from the TBI. If the line does not hold a vacuum from the inside, the check valve is not working.

 

[mammadovn]

I’ve checked the hose in front of tbi, could’t see any check valve. The narrow hose connects directly to the port and goes into the harness batch.
The smoke test I’ve done from engine compartment. through valves ventilation hose.

 

[Fred Kiehl]

You need to install a check valve.

 

[mammadovn]

Ok.
will any kind of check valve from ebay do the job?
And the last one - could this issue cause slight power loss durning hard acceleration?

 

[Fred Kiehl]

Make sure it is fuel proof. There is usually a small length of vacuum hose between the TBI and valve.
There are a number of things that can cause power losses, but I don't think you would notice it.

 

[91ss]

Might want to check if the HVAC controls can hold vacuum too.

 

[mammadovn]

Thank you All for your solutions! Will do all the recommendations. Hope it will be sorted out.

 

[Fred Kiehl]

Might want to check if the HVAC controls can hold vacuum too.

The HVAC is supposed to hold vacuum. The check valve is what keeps the vacuum in the system. The OP is having a vacuum issue, so, he needs to find the leak under increasing throttle.

 

[91ss]

'Supposed' to is the key word. I've seen a few controls where the seals in them or the vacuum lines to the various diaphrams no longer have perfect seals. Then they lose vacuum even with a perfect check valve.

 

[mammadovn]

Happy New Year to all!
Some updates.
Indeed there was a check valve close to the tbi port, as it was mentioned and that valve didn’t work. Air passing from both sides. Replaced it with the new one (with bigger body than the original). Result - the slight power loss on hard acceleration almost disappeared. 👍 However the issue with HVAC vents still remains.

 

[mammadovn]

Does it mean I have to remove dashboard to check leakage ? 🤦🏼‍♂️

 

[Fred Kiehl]

The hoses can be checked with minimal disassembly. Remove the P/S bottom panel to access the hoses. Get a vacuum tester (borrow from Autozone), isolate every line, and check each line.

 

[mammadovn]

Ok.
Thanks.
i’ll check them.

 

[jetfire88]

I have had, and have, many '91-'93 B Wagons, RMW/OCC/CC.

This is a common trait on all of them I have owned, regardless of brand, year, or mileage. I can't really recall if there is a difference twixt manual or auto HVAC controls.
Right now my automatic HVAC ones are put away for the winter so I can't run out an try it, but it will do this on all the manual ones.
I usually don't drive the auto ones in the winter, its decision about my comfort is usually not the same as mine, and in Wisconsin, at -30, that is a priority. The auto ones invariably start with heat going to the floor, but as soon as there can be a little heat felt on my hands, it switches to the center vents. My feet stay cold and it does not melt the ice/snow on my boots.
I can force it to the floor for a while, but as soon as the cabin fully warms up it forces the flow back to the vents and will not allow an override.
Because it has been common on all I have owned, I have never looked into it. I will pursue Freds' suggestions. If anyone has a pic of an original TBI check valve installation, that may help. Were they in the same place on all TBI's?

Thanks for the info, Fred.

 

[mammadovn]

If you take out air filter box and standing in front of the tbi, it’s on right hand. In fact you will not miss it. There are two small OD lines, the one in the middle goes to EGR’s solenoid. But HVAC’s vacuum line disappears into the general cable channel to the car interior. The check valve is very close to the tbi port.

 

[mammadovn]

Btw, I didn’t see check valve on the line to EGR solenoid, not sure if there must be?…