Hello, all. I have a 1997 Dakota SLT with a 5 speed stick. The truck runs fine. I suspect I have vacuum problem because when I accelerate, the air stops flowing through the vents. When I release the accelerator, it comes back. Has anyone ever experienced this? Could it be a bad vacuum hose? Does anyone know how this hose can be traced?
Thanks,
Richard.

 

Does the fan speed seem to change? Could be a blend door losing vacuum. The hose goes from inside the dash to the upper firewall. It then runs along the firewall to a couple T's and then one of those heads to the intake. You might have a hose that is cracked but only opens when there is extra strain on the hose. Press on the hose and gently flex them to look for cracks. If there isn't anything in the engine bay it could be in the dash side. That is a very long and tedious job that might not be something you want to do.

 

Yes, the fan speed does change when the selector is moved. I will check for vacuum lines. I cringe when I think about having to have the dash pulled apart.

 

I meant when the air speed changes. So you open up the throttle and the fan speed decreases? Or does it stay the same? Listen for the noise to get an idea of the fan RPM.

 

At first I thought it was the switch because in Max/AC, it seemed like the air was fine even when driving. In vent position is where I first noticed the problem. Last night I had the windows open so now I can't be sure if the blower speed changed with acceleration. I will check next time I drive it. 108 here today so I might wait until it cools down. Thanks.

 

It's not the fan. It IS vacuum related. All the blend doors work by vacuum in your '97.

There is a check valve in the line to the reservoir which should, not positive on a '97, be under the cowl. The check valve can fail. I've attached a picture of the one on my truck. It could also be a leak in a line or a damaged reservoir.

 

Hi. Thanks for the picture it helped me notice where mine is. Basically the same place. I pulled it off, it was easy on the brake booster side but real tough on the opposite side. I blew and sucked through it both direction and it stops and allows the air flow in both directions respectively. I put it back on but on the hoses, I didn't push them in all the way so that it would be in a slightly different and tighter position. I drove it around, went up some hills and it seems to be ok now. I can't believe it would be that sensitive to "perhaps" a possible easy to remove hose. I also wonder how does a check valve fail? It seems like there's just a ball in there that plugs a hole depending on the direction of air flow. Anyway, I will test it out on the freeway during the weekend. For now, Thanks for all the info.

 

All it takes is a tiny bit of debris, dust, or plastic to make them fail. Thankfully its also just as easy to clean them. Most PCV valves are the same and can be cleaned instead of replace for the same reasons.

 

Well, I thought it was fixed but no. It also seems as the symptoms get worse as the truck gets hotter. I wonder if the check valve can seem fine when you blow through it but not fine under certain less forceful pressures? I connected a vacuum gauge at the booster and drove the truck. Also connected it after the check valve and drove. It's the same either way. When I climb hills or get on the freeway, it can drop from 20 to 5 lbs. So, should the check valve stop the pressure from dropping so low or does it need something sucking the other direction to activate the valve?

Anyway, I called the Dodge dealer and explained where the check valve is. They only show one that snaps into brake booster housing and nothing after that. Mine looks like the one pictured above and my neighbor's Dakota looks just like mine. I'm taking it in later today so they can see it. I'll just replace it in case it makes a difference.

Does anyone know if my truck has a vacuum reservoir? Other automakers diagrams show it but I can't see one anywhere in my engine compartment. If it does exist, I thought I should check the vacuum lines to it.

Thanks.

 

There should be a vacuum res, maybe under the cowl with the wiper transmission and such.

Vacuum will bleed off slowly if there isn't a sufficient source.... the reservoir is supposed to take care of that,,,, but, it doesn't always. Could install an additional res, to make the time it takes to bleed off longer..... Replacing the check valve sure won't hurt.