Ever have a hard to find vacuum leak?

[PineRidge]

AlanB the heating and cooling vents have ideas of their own. IE: you can be driving down the road with the AC on and if you accelerate a bit the dash vents close off even though the fan is running. Once the speed comes up and the vacuum stabilizes the vents once again open as they should. At times it idles a bit rough but runs real nice at road speeds.

 

[mojoinco]

That's exactly the area I was thinking I would start first. I have had lines not quite make it back on after working on stereos, or other dash area work. I'd plug off all the non-essential stuff and work with a vacuum gauge (use a mityvac in line. Monitor it while the engine is cold and monitor it to see if any particular combination of events seems to make it change. Another fluid that can be used is WD40. The carrier that is used to deliver the penetrant is moderately flammable (maybe more than moderate -- hold a match in a pair of pliers and briefly spray towards it). Anyhow, another time tested method checking for a leak. You might want to spray around the intake manifold, and the air hose from the mass air sensor on in as well. Any engine codes?

 

[PineRidge]

That's exactly the area I was thinking I would start first. I have had lines not quite make it back on after working on stereos, or other dash area work. I'd plug off all the non-essential stuff and work with a vacuum gauge (use a mityvac in line. Monitor it while the engine is cold and monitor it to see if any particular combination of events seems to make it change. Another fluid that can be used is WD40. The carrier that is used to deliver the penetrant is moderately flammable (maybe more than moderate -- hold a match in a pair of pliers and briefly spray towards it). Anyhow, another time tested method checking for a leak. You might want to spray around the intake manifold, and the air hose from the mass air sensor on in as well. Any engine codes?

Mojoinco there have been no fault codes displayed by the computer and to this point the obvious things have been checked including the vacuum tank located under the van. The air cleaner/mass air sensor has been on and off the van many times while trying to locate the leak. The vacuum tube from the MAF looks solid and so does the grommet where it enters the valve cover. Does anyone know if there is a check valve located in the vacuum brake assist, I need to rule out a leaking diaphram at that point.

Think the next game plan is to try the propane 1st and if that doesn't pan out then switch to isolating individual circuits and use low air pressure with bubbles to see if the leak can be found.

 

[TCBoomer]

For safety reasons, all vehicles should have a check valve at the brake booster. This will allow 1 or 2 power assisted brake applications in case the engine shuts down. The check valve should be the plastic fitting where the hose connects to the brake booster. One way you can check it is to run the engine and then shut it off. Next pull the hose off the fitting. No leak should be heard. Now pull the fitting out and see if the vac releases from the booster...it should.

As for your closing vent problem, I think you have a bad check valve for the HVAC vac supply. It also could be a bad storage tank, but I think? you said you checked it already. Starting at the intake manifold, find the vac hose that feeds the cruise control (if equipped) or is routed to the interior of the van. Usually it is attached to a main wire harness bundle. Pay close attention to any vac hose that enter the cab near the HVAC module. If you follow that vac line and any branch lines, you should be able to locate the check valve and storage tank. I'm fairly certain they both will be located under the hood somewhere and not inside the van.

Your symptoms describe a leaking or missing check valve. ie...vents close under heavy throttle due to loss of vac.

 

[PineRidge]

TCBoomer the plastic vacuum storage tank on this van is located under, on the frame rail, and behind the passenger front tire. I removed it from the van and pressurized it with air before giving it a dunk test in water. It doesn't leak but I saw no check valve in the line or at the tank. I'm aware there should be one somewhere and will need to keep looking until it's found. I'm also assuming that since the vacuum lines are smaller (in the range of 3/16") this plastic vacuum tank is meant to assist the heat/AC circuits only?

I'll take your advice for checking the brake booster check valve. Knew there had to be some way to maintain braking should the engine stall unexpectedly.

There's been lots of good advice been given here, thanks for all the help guys!

 

[TCBoomer]

Just an FYI...I worked on a Ford many yrs ago that had the check valve built into the vac tank. I don't remember the yr or model, but I know it was a Ford. Maybe that is where yours is?

If all else fails and you can't find the valve, just find the source vac for the HVAC and cruise control at the intake manifold and install one there. A dealer or parts store will have one.

I'd offer to look up the location of that valve in AllData, but I'm on vacation for the next couple weeks. If you can wait that long, post the yr and model of your van and I'll look it up when I return to work.

 

[TCBoomer]

I'm also assuming that since the vacuum lines are smaller (in the range of 3/16") this plastic vacuum tank is meant to assist the heat/AC circuits only?

You are correct and if equipped, it's also used to assist cruise control

 

[AlanB]

Mike my truck does the same thing with the vents cycling or whatever you want to call it, I have not fixed mine, and I may be looking at the wrong problem, but I know my plastic lines are going to crud, I am assuming (which often gets me in trouble) that the fluctuation in the vents will quit when I get the plastic lines straightened out.

 

[PineRidge]

AlanB replacing your deteriorating vacuum lines should put you back in business. In my case all the lines look good and are still flexible. (may still be leaking however)