could this be the cause of my problem? is it safe?

[91bbc]

1991 454 tbi

I am having what I believe to be a vacuum leak (1k rpm idle, rough and hunting idle, very rough idle when in gear, surging when going up steep hills, low power). The idle does not seem to change when brake is applied in park. Brakes seem to work ok. The truck is new to me though, and I don't know what it feels like when working properly.

It looks like brake fluid has leaked from where the master cylinder meets the booster at some point in time. If it did, could this be causing my vacuum leak now? Is it safe to drive like this? What else can I do to rule the booster out of my vacuum issue or prevent an accident?

View media item 32719View media item 32720

 

[Erik the Awful]

The idle does not seem to change when brake is applied in park. Brakes seem to work ok.

Probably not the problem.

You can put vice grips on the brake booster hose to clamp it off and see if it fixes the vacuum leak, but please don't drive it on the roads like that.

 

[sntrym]

The booster doesn't have any fluid in it, it simply has a rod that pushes the piston on the master cylinder. If there was a fluid leak, then it would be the piston seal in the MC, not the booster. If the booster goes bad, you could have a vacuum leak but that's not very common. I'd focus on the hoses around the engine or TBI base gaskets, those are the most common points for vacuum leaks. Spray starting fluid around those points and see if the idle changes. When it does, you've found the problem area.

 

[east302]

Any codes stored? Does the check engine light come on and go off when started (is the bulb missing in other words).

 

[someotherguy]

I'd focus on the hoses around the engine or TBI base gaskets

Seconded on the TBI base gasket(s) - plural being if the 454 has the carb style intake with the adapter plate for the TBI.

Richard

 

[91bbc]

Probably not the problem.

You can put vice grips on the brake booster hose to clamp it off and see if it fixes the vacuum leak, but please don't drive it on the roads like that.

I tried holding down the pedal before starting to see if pedal goes down, it does not. I tried holding pedal 30 seconds after shutting off engine, and it stayed the same. I tried pumping the pedal while engine not running, and it built pressure. I pulled the vacuum line off the booster after sitting for a few days and it hissed. I also unplugged the vacuum line from the booster while running and the idle dropped but slowly came back up to 1k and smoothed out a little but. It was loud and sounded like it had plenty of vacuum.

The booster doesn't have any fluid in it, it simply has a rod that pushes the piston on the master cylinder. If there was a fluid leak, then it would be the piston seal in the MC, not the booster. If the booster goes bad, you could have a vacuum leak but that's not very common. I'd focus on the hoses around the engine or TBI base gaskets, those are the most common points for vacuum leaks. Spray starting fluid around those points and see if the idle changes. When it does, you've found the problem area.

I was thinking the mastery could have had a leak at one time and the brake fluid could have leaked in to the booster. Its holding pressure just fine though. I rebuild the tbi with all new gaskets, and don't think that is the issue. All the vacuum lines around it seem fine, even when I wiggle on them. I do hear some hissing, but I think it's just the tbi. Not positive on that one though. I'm a little nervous about using the starting fluid.

Any codes stored? Does the check engine light come on and go off when started (is the bulb missing in other words).

No cel showing, and I just get code 12 when jumping a and b.

Seconded on the TBI base gasket(s) - plural being if the 454 has the carb style intake with the adapter plate for the TBI.

Richard

Yeah I just replaced that and rebuilt the tbi. I replaced a bunch of sensors. Not the iac though, they kept giving me the wrong one so I gave up. I can see the pintle moving though, and it drops idle after warming up a little.

I'm starting to wonder if it's the intake manifold gasket. Any tips on checking that?

 

[91bbc]

The pvc valve grommet is definitely leaking. I can see oil coming out of it, and when I plug the pvc valve with my finger the idle drops about 100-200rpm. I'll order a new one today.

Also, I did the iac relearn (bridge a+b pin, wait, disconnect iac, adjust butterflies to about 500 rpm, shut off, reconnect) and the idle went back up to 1000k rpm after a few minutes. It made an awful saxophone like sound that changed in pitch when I adjust the butterflies, but when away once I plugged the iac back in. It still ides rough. Feels like it misfiring randomly. Has new plugs, wires, cap, and rotor.

I should also note the truck on has 145k miles on it so it definitely sat for long periods of time.

I'm leaning towards intake manifold leak.

 

[someotherguy]

I'm starting to wonder if it's the intake manifold gasket. Any tips on checking that?

UN-lit propane torch with a nozzle tip, turn it on and move all around the intake gasket area, if it sucks any propane in you will notice a rise in idle speed.

Richard

 

[91bbc]

UN-lit propane torch with a nozzle tip, turn it on and move all around the intake gasket area, if it sucks any propane in you will notice a rise in idle speed.

Richard

is there any risk of starting a fire with that?

 

[someotherguy]

is there any risk of starting a fire with that?

Yes, but only if you're really careless with it. You aren't going to be running the bottle full blast, so what propane you do put out will dissipate with the airflow from the engine fan. Obviously you would be best to do this outdoors, not in an enclosed garage. It's pretty much standard practice for checking vacuum/intake leaks; some use carb cleaner or other flammable sprays that will behave the same way but I find propane is a lot less messy and easier to direct if you have the correct tip on the bottle.

There's probably some youtube videos you could search that would demonstrate it.

Richard