Vacuum Leak!

[rean1mator]

Need some input on tracking down a vacuum leak. The only vacuum connection to the intake manifold I can find is on the passenger side behind the carb. I've traced all the hoses coming off it and they are all connected and there doesn't appear to be any leaks.

I'm getting a reading of only about 3 on the vacuum gauge.

I've looked everywhere around the manifold and cannot find any other vacuum connections going into the manifold.

It runs fine once fully warmed up but it does want to stumble a little when coming to a stop even when I set the idle rpms to 800.

Completely stumped and don't know where else to look.

 

[savage]

What engine, automatic or stick,carb or fuel injection? What year is the engine and bronco?

 

[rean1mator]

'74, auto, carb, 302

What engine, automatic or stick,carb or fuel injection? What year is the engine and bronco?

 

[savage]

Did you check the line going to the distributor? How about any vacuum nipple on the carb.Are you using all the connection on the vacuum tree in the back or off the carb?The hose that goes to the transmission modual.You can take caned air and spray air around the base of carb and around the intake when the engine is running, to see if it speeds up, do to a leaky carb or intake gasket.

 

[broncnaz]

With vacuum that low plus it seemingly getting better when warmed up. Id suspect the intake manifold gasket is leaking. Have you tried spraying carb cleaner around the edges of the intake? Base of the carb? if the engine speeds up then you've found your leak. But sometimes they do leak on the inside and you cant detect them. While I dont think it will help you might try retorquing the bolts on your intake. See if it helps but usually the gasket ends up damaged and it wont help. Hopefully the intake is not warped to.

Never heard of the compressed air trick I dont really see how it would work as its still air. carb cleaner or something similar will do 2 things it will slightly block off the leak and add something burnable to the air mixture which will increase Idle speed.

 

[savage]

I fixed it,broncnaz. I've done it using carb cleaner, I meant to say caned air, the caned air uses butane or similar gases to release the air which changes the engine speed if it's has a leak.

 

[half cab]

X3 on probably intake gasket.

 

[rean1mator]

Yeah, there was one nipple that had the vacuum plugged torn on it, one of the vacuum hoses was disconnected which I put back and I traced every other vacuumhose on both ends to ensure that they are connected.

Haven't checked modulator hose recently but I did replace the modulator recently with an adjustable version. will double check that.

Did you check the line going to the distributor? How about any vacuum nipple on the carb.Are you using all the connection on the vacuum tree in the back or off the carb?The hose that goes to the transmission modual.You can take caned air and spray air around the base of carb and around the intake when the engine is running, to see if it speeds up, do to a leaky carb or intake gasket.

 

[rean1mator]

I will check the intak manifold this weekend. other than pulling the carb, is there any special instructions in replacing the gasket/pulling teh intake manifold off?

I'm planning on dropping in a new engine sometime next year or the year after but would like to the vacuum sorted until then, so don't want to do any major repairs on the engine.

With vacuum that low plus it seemingly getting better when warmed up. Id suspect the intake manifold gasket is leaking. Have you tried spraying carb cleaner around the edges of the intake? Base of the carb? if the engine speeds up then you've found your leak. But sometimes they do leak on the inside and you cant detect them. While I dont think it will help you might try retorquing the bolts on your intake. See if it helps but usually the gasket ends up damaged and it wont help. Hopefully the intake is not warped to.

Never heard of the compressed air trick I dont really see how it would work as its still air. carb cleaner or something similar will do 2 things it will slightly block off the leak and add something burnable to the air mixture which will increase Idle speed.

 

[67RT]

Do you have a power brake booster and if so, are the brakes functioning well? Check that hose and/or the check valve...

 

[Slowleak]

I fought that problem on my 77 for months and it turned out to be a bad vacuum advance module on the side of the distributor. The diaphragm had dried out and had a hole on it.

 

[rean1mator]

No brake booster.

Do you have a power brake booster and if so, are the brakes functioning well? Check that hose and/or the check valve...

 

[broncogt]

You have to check the diagrams the lines run to. The ported vacuum switches can leak too if they are old.

 

[bronco t]

Mine was acting like this, and as it turned out the EGR vacuum line was hooked to a dead vacuum port. The smog shop teed in to my distributor vac advance hose and it ran like a top.

 

[broncnaz]

I will check the intak manifold this weekend. other than pulling the carb, is there any special instructions in replacing the gasket/pulling teh intake manifold off?

I'm planning on dropping in a new engine sometime next year or the year after but would like to the vacuum sorted until then, so don't want to do any major repairs on the engine.

Just drain the coolent. You may want to pull the distributor as well it makes things easier. so you will want to make sure the timing marks are at TDC and the rotor is pointed to the #1 plug wire so you can restab it easily. Also dont used the cork or rubber end pieces they give you with the intake gaskets RTV works way better on the end rails. I usually put a thing coat of RTV on the gaskets around the water passages as well.

With only 3in of vacuum it has to be a large leak. I think you could unhooked the PCV valve hose and still get over 10 in of vaccum at idle on a engine that doesnt have a vacuum leak.

 

[rean1mator]

So I picked up some carb cleaner and started spraying around the intake.

It appears the leak is coming from the connector at the intake manifold where all the hoses connect to as well as the connections at a multi- T fitting where a bunch of hoses connect to as well.

I'm thinking I need to just replace all the hoses as well as the connector at the intake manifold. what is that part called that screws into the manifold where the hoses connect to?

But when I spray the carb cleaner into those areas the engine actually speeds up instead of stumbling. I thought the engine was supposed to stumble?

 

[half cab]

Speed up not stumbles.

 

[rean1mator]

Looks like I have multiple leaks. I swapped out the old hoses going to the ported vacuum switch at the manifold and it no longer speeds up when I spray carb cleaner at it.

Vacuume level did not change however and started spraying around again and now there seems to be a leak here:

It sits directly behind the carb and is mounted on to the intake manifold at rear edge.
The leak is not at the vacuum hose as I swapped that out with a new one as well. seems to be isolated underneat it where it mounts to the manifold.
Anyone know what this is?

 

[broncnaz]

EGR valve. Or actually its probably the spacer plate that its mounted on thats actually leaking. Id gt some gaskets and remove the carb and spacer plate. Clean everything up and reinstall.

 

[rean1mator]

Why would I have to remove the carb to replace the egr gasket?

EGR valve. Or actually its probably the spacer plate that its mounted on thats actually leaking. Id gt some gaskets and remove the carb and spacer plate. Clean everything up and reinstall.