1997 Ford F150

I've been taking this forum apart looking for solutions on how to fix my vent/ a/c problem. I posted earlier about my vents changing to the default when I accellerate. I received feed back on a possible blend door problem, but mostly about vacuum leaks.

I have found a lot of this same problem being asked, but I never can find the solution. I'm looking for a way to find a leaky vacuum hose or whatever. Tonight I took the battery out and checked the vacuum hoses and if there was any doubt I changed out a hose. I checked the vacuum box behind the battery, but it did not have any cracks. I also have a vacuum gauge, but I do not know the normal range for vacuum.

Any help will be appreciated.

I'm Leroy a crazed Bass fisherman and I approved this message.

 

Leroy,

You should see at least about 20 inches of vacuum at idle in neutral. Anything less than this and you have either a vacuum leak, incorrect timing, leaking intake, or other engine problems. The fact that your defaulting to the defrost position under heavy throttle is definitely vacuum related. Is their a check valve in the vacuum line to the vacuum canister? To check the vacuum canister you need to be able to apply a vacuum to the canister and it should hold the vacuum. A vacuum pump is the ideal way but if you tee your vacuum gauge into a hose connected directly to the canister and suck on the hose to pull as much vacuum as possible and then see if the canister holds. Also, what I do to find a vacuum leak is to isolate each vacuum circuit and with the vacuum gauge take readings at idle. All should hold the highest reading that you can get and that should be at least about 20 inches at idle in neutral. You may have a vacuum motor that is leaking so you need to check them as well. Some vehicles have a vacuum distribution block on the firewall that is a godd point to isolate. Also, not sure if the 97 F-150 uses hydroboost for the power barkes or vacuum, but if it's vacuum make sure that the large vacuum line going over to the power brake booster isn't leaking also. Best way to isolate problems is to plug your vaccum gauge into a line right off the intake where the vacuum originates and then plug each vacuum line one at a time at idle and see if the vacuum stays the same, drops or rises. That will help pinpoint a problem.

John

 

Thanks for that post, I'm off tomorrow and I will be doing battle with my trusty truck. 97' with 170,000 miles and still going strong.


Leroy

 

I found a vacuum check valve on the passenger side where there is a connection of multiple vacuum lines (between the battery and firewall). A group goes into the fender well where I found a large black plastick (in my words 'a holding tank'). downstream from the connections I found 2 vacuum lines that sort-of T-off, one going to the smaller vacuum holding tank behind the battery and the other (a black line) going through the firewall.

I checked the vacuum at the check valve and found it to be a steady 17in at idle spead. When I rev up the engine it does creap towards 20in, but never find that mark. I also tried to follow some of the lines that are upstreatm and I disconnected them one at a time and found it (still at the same check valve) lost 1in of vacuum.

I found a larger black line that is connected to a part of the engine, Is this where the vacuum starts ? I'm going to be checking that origine to see what it reads.

I did run the a/c (parked at idle) and pulled the smaller black vacuum line that disappeared into the firewall and found that, after pulling that vacuum line, the a/c switched to the defrost. So does this mean that I'm probably looking for a leak in the engine side ?

 

Make sure the check valve works. With 170,000 mile I would not worry if you had 17 or 20 inches. 17 is OK. The check valve is to hold vacuum in the can during full throttle operation...Don

 

That larger black line from the intake is probably a main vacuum source. Is there a line to the power brake booster? When you disconnected that line going through the firewall and the A/C defaulted to the defrosters you removed the vacuum source for the HVAC controls. I would do the vacuum test on each of those canisters to make sure they are not leaking. Also, when you make that disconnect of the hose through the firewall, plug the source end off and see if your vacuum goes up. If it does then you have a leak in the dash, probably at the HVAC controls or a HVAC component. If the vacuum doesn't go up then your vacuum leak is on the engine side. Are you sure the engine timing is correct? The reason I ask is that if the timing is retarded it can reduce vacuum even if idle speed is correct. Best way to check out vacuum problems is with a vacuum pump. You can get a hand pump version from NAPA, not sure how much they are now. I have one that is over 20 years old and I have used it many times to help find vacuum problems. Be sure as you disconnect various vacuum circuits that you make sure to plug the end of the source vacuum line. Golf tees are really good to use for this. Also, pay attention to each of the check valves and note the marking on each of them. They should be marked with "manifold", "source" or something like that. If there isn't any marking on the valve pay attention to the two colors of the check valve. If memory serves me black was the side towards the intake and the other color was towards the accessory. Best way to check is to remove the check valve and see which side will allow you to pull a vacuum through the valve and which side is closed or blocks pulling a vacuum. Let me know what you find out.

John

 

Thanks again for your replies, I'm applying them as we type.

Since I'm not a mechanic, but I would rather try to tackle this than to pay out the booty for this fix I have some questions.

I was calling a capped off fitting a vacuum check valve, now I saw you say to check the 'check valve' I must be talking about something else. Are you saying that the plastic black containers on the passenger side are the check valves ?

This hand pump for vacuum checks, does it pull air/vacuum and when done pulling, does this then check to see if it holds or loses air/vacuum ?

On to the timing, I could not tell you if it is off, is there a simple way to check or will a timing light have to be used ?

I'm sorry for the amount of questions, but I love this forum. I have already fixed two other problems that have been bugging me through use of past posts.

The internet is your friend

 

I tried unhooking the black line going into the firewall and the pressure stays the same. So hopefully I wont have to pull the dash.

I finishing for the day, but this time I'm putting silicone on the repaired hoses when I put them back together to make sure there is no minimal leakage from those spots. I had not done that before.

Thanks again for the help

 

Those black plastic containers on the passenger side are probably vacuum canisters/reservoirs. They are used to complement the engine vacuum whenever the engine is under heavy load and vacuum is low because of the amount of opening of the carburetor or in your case the air valve in the intake to atmosphere. That is why your vents are defaulting back to the windshield, which is what the HVAC system will due by design, is because of low vacuum. Your vacuum reservoirs aren't providing the extra that the system needs under these engine load conditions. Check valves are in the vacuum line and are used to keep vacuum in a vacuum canister when the egine is off. When the engine is running the check valve is opened by the engine vacuum making the vacuum system totally as one. Most vacuum pumps have an integrated gauge so that you can see how much vacuum you are pulling as well as to be able to see if the vacuum is holding or if there is a leak. It is ideal to use to pull a vacuum on different components. It won't work trying to pull a vacuum on the entire system since without the engine running the intake is just open to atmosphere making it impossible to pull a vacuum. When you say putting silicone on the reapaired hoses I'm not quite sure what you mean. Do you have cracked or split hoses that your trying to repaire with silicone? You should just replace the length of hose if it is cracked or split since that indicates it is drying out from age and engine heat. Hope this helps.

John

 

I had a battery last year that leaked acid on some of the vacuum hoses so I repaired them with 1/4 rubber hosing, but I did not use silicone when I placed the rubber over the existing lines.

I don't know how long it was like that since I only found it after a check engine light came on and it showed no vacuum to another part. I'm pretty sure the problem started long before that, but I never wanted to go to a mechanic and give them money I could be using on fishing

can the canisters be checked with a vacuum pump to make sure they are holding ?

and where are the check valves ? I wish there was an online site that showed vacuum lines and associated areas where there could be problems. Kinda like a fixyourtruck site

After doing battle with it yesterday and not really finding something that I could say was the culprit I'm still wanting to fix it. After putting everything back together last night I took it for a drive and it seemed to not divert to the windshield when excelerating, but today it definitely did the same ole thing.

jtmerritt thanks for your assistance, but dont stop I feel that we are neighbors. oh well I will be trying again. I did buy a haynes manual, but it mainly describes how to charge your system, nothing really on vacuum.

Thanks again

Leroy, a bassmaster in my own mind

 

Yes you can check the vacuum canisters with a vacuum pump and that is one of the advantages for using one. The check valve(s) should be in the line between the canister and the accessory. You might want to surf the internet using the search words "vacuum hose routing" or something like that. Might get lucky and find some descriptions of vacuum components along with pictures.

John