HVAC goes to defrost on acceleration

[N8N_99xj]

I'm assuming it's a vacuum leak. Been doing this since I bought it two years ago. Finally to the point now that once I get the rear springs changed the next job is either mount my CB f'realz or else chase this issue. Before I get started with the mityvac, smoke machine, etc. I figured I'd ask for shortcuts. Is there a usual failure point? Is there a check valve anywhere that may have failed?

thanks!

 

[WB9YZU]

Vacuum canister isn't working. Cracked line, plugged line, and or missing canister.

Do you have an aftermarket bumper? If so the PO may not have realized how important that little ball was and deleted it.

 

[Pelican]

One common point of failure is where the vac line passes under or around the battery, at least on earlier years -- not sure about the '99. A place to start looking.

Could be a check valve but I don't know how often they fail on later models.. I had this exact problem on my '85 with a failed check valve that is built into each of the little "soup can" reservoirs under the hood.

 

[StevieRT]

Also check on the firewall near the center/passenger side. A small line comes through the firewall and into the engine compartment with a fairly sharp curve in it. This is where my 98's had a vacuum leak and caused the same systems.

 

[N8N_99xj]

well I turned into a big woosy boy and decided not to troubleshoot in the snow... went over to my buddy's shop and hooked it up to the smoke machine through the port for the cruise actuator. Found two cracks in the line under the battery tray which doesn't surprise me at all as the battery was leaking like a sieve when I bought the vehicle. Patched up that line, resmoked, no smoke evident anywhere. However when I try to pull a vacuum on it with a Mityvac I can't. Vacuum diagram is no help as it does not address that system at all. Is there a check valve somewhere that needs to be replaced?

 

[Sidewinder CC]

well I turned into a big woosy boy and decided not to troubleshoot in the snow... went over to my buddy's shop and hooked it up to the smoke machine through the port for the cruise actuator. Found two cracks in the line under the battery tray which doesn't surprise me at all as the battery was leaking like a sieve when I bought the vehicle. Patched up that line, resmoked, no smoke evident anywhere. However when I try to pull a vacuum on it with a Mityvac I can't. Vacuum diagram is no help as it does not address that system at all. Is there a check valve somewhere that needs to be replaced?

Yes, there is a vacuum check valve at the intake manifold. You can remove it and try blowing through it both ways. It should allow flow one way and not the other. If it doesn't allow flow either way, try squirting some Liquid Wrench Silicone Spray in it to see if it loosens up. If it doesn't, replace it.

Where are you hookng up your Mityvac?

 

[N8N_99xj]

Hooking it up at the cruise actuator connection. So check valve is not working or else I've got another leak that the smoke machine didn't find. Thanks for the info, I'll look at that tomorrow.

 

[N8N_99xj]

OK, was back at it today, found check valve, removed & cleaned, now I can pull vacuum on system but it'll slowly fall off over a minute or two. Don't know if it's good enough to keep heat on floor or not as Heep is still at shop; got both rear springs replaced today but didn't get chance to use alignment rack nor was I able to paint any of the new parts after install. Thanks for pointing me in the right direction, you all saved me loads of searching

 

[ParadiseXJ]

If you found leaks, you can expect more. Especially if you're just "patching" lines. Vacuum line (bulk) is dirt cheap. I recommend replacing the entire length of any compromised vac lines.

The emissions side of the vac system has little if anything to do with the HVAC system so all the vac lines are replaceable with a couple different sizes of tubing. The emissions vac lines can be a tad pricey.

 

[N8N_99xj]

Well I finally brought it home last night and it was DAMN COLD (by even native yunzer standards, not wussy NoVA standards) and I didn't notice any weirdness, but it was all flatland driving... we'll see... replacing all looks like a project as this is all that hard plastic tubing not hose, and if I replaced with hose I'd need to buy a whole mess of fittings too

 

[GrimmJeeper]

if you're going to replace the plastic lines with hose personally I would use silicone hose, it will hold up MUCH better to extreme cold than rubber will, and it won't get soft or brittle with age.

 

[Sidewinder CC]

OK, was back at it today, found check valve, removed & cleaned, now I can pull vacuum on system but it'll slowly fall off over a minute or two. Don't know if it's good enough to keep heat on floor or not as Heep is still at shop; got both rear springs replaced today but didn't get chance to use alignment rack nor was I able to paint any of the new parts after install. Thanks for pointing me in the right direction, you all saved me loads of searching

Try hooking your vacuum checker up at the rubber elbow fitting at the intake manifold then check the system.

 

[MT Mike]

if you're going to replace the plastic lines with hose personally I would use silicone hose, it will hold up MUCH better to extreme cold than rubber will, and it won't get soft or brittle with age.

What size vacuum hose Grimm?

 

[GrimmJeeper]

What size vacuum hose Grimm?

oh sure put me on the spot Mike, lol.

1/32"? i can't remember but its smaller than 1/8" and the silicone stuff stretches great.

alternatively the stock black nylon tubing is available through NAPA but you have to find a parts guy worth his salt to dig up a part number in the books for you, and i believe it only comes in 6 foot straight lengths.

 

[MT Mike]

Thanks. I thought maybe one of your parts books would list the size.

 

[GrimmJeeper]

Thanks. I thought maybe one of your parts books would list the size.

unfortunately we don't get that deep into part listings, same reason we don't have bolt size and thread listings or trim clips, etc... the catalogs would quadruple in size immediately.

 

[MT Mike]

unfortunately we don't get that deep into part listings, same reason we don't have bolt size and thread listings or trim clips, etc... the catalogs would quadruple in size immediately.

:thumbup: