Vacuum Related Problems (I Think)

[mjtr21]

I have a wierd problem when I start thecar up. My engine is 1999 3.4 GT. When I start car up sometimes it will hesitate and when it starts the RPM bounce between about 1200 and 500. It will do this and sometimes smooth out or other times stall. I am getting a feeling this is vacuum related. Also I tried checking for vacuum with power brakes. I sat with car idling at about 600 and I pumped the brake pedal about five times quickly. The engine raced a little then the RPMS jumped very quickly between 1000 and 1100. Is there something wrong with the vacuum line going to the power brake booster? Thanks in advance for suggestions.

[Aki334]

This is something that I was trying to explain to some people, boosters can make a lot of problems. They usually do not go bad very often, but if you see any oil leaks from between your master cylinder end seal and power booster you probably have some brake oil in your booster. And that brake oil will swell up the rubber vacuum diaphragm and create a small holle which can also get bigger later on. What you did is one way to test for bad booster, but I would not change booster yet unless your braking ability is singificantly reduced. Before you do anything make sure that thick hose is tight at both ends. Check that plastic L shape part that connects plenum hose with booster you should be able to suck air from the side of the hose and not be able to blow air from the opposite side. Again braking ability would be so bad, and you should hear hissing air sound under your dash if booster is bad. Also make sure your pcv line is tight pcv should rattle if you shake it, also MAF sensor the best way to test them is to use screw driwer handle and lightly hit your MAF sensor see if engine RPM changes. Good luck man, I hate vacuum leaks, the best way to find them is with that expensive smoke machine. I do not want to pay $ 1800 for it. If someone has any idea how to make smoke machine let me know... I was thinking something with a smoke Bmb.

[xeroinfinity]

I wouldnt listen to Aki334 :
Maybe he can hit and shake his MAF sensor but I dont suggest it.

You rpms are supposed to rise when the brakes are pressed like that. If not the idle would fall and engine would die.
Vacum leaks are pretty easy to find.
Thier are only so many places, the intake area, the PCV.
One behind the coils(O2 perge),
which is prone to pop off under the right conditions.

And if its cold where you are the vacum will fluctuate when you first start the car and its not warmed up. ANd its probly done this all the time, you just didnt notice it.

[Aki334]

bouncing RPM between 500 rpm and 1200 rpm and stalling is not normal it does not matter how cold it is RPM should be around 900-1000 RPM when cold and should gradually start to decrease to about 675 rpm.

[xeroinfinity]

Originally Posted by Aki334

bouncing RPM between 500 rpm and 1200 rpm and stalling is not normal it does not matter how cold it is RPM should be around 900-1000 RPM when cold and should gradually start to decrease to about 675 rpm.

FYI- when its below 30deg it very well can & will bounce!
Idle on the 99+ GA's is 650rpms warm, 950 when cold.

Bad plugs & wires can also contribute to the cold stalling.

[mjtr21]

xero: i know it's normal for the rpms to rise when i pumped the brake pedal but after a second or two the rpms jump fast between 1000 and 1100. but where is the one you were talking about behind the coils. is it right behind them. i will also check pcv and possibly change it. i just got new spark plugs (iridium)

[mjtr21]

okay i just wen out and checked any vacuum lines and i listened al over. i didn't hear any hissing. i looked for that thing behind the coils and i think i found it. on the drivers side of it there is an electrical connecter hooked on and right underneath that thers a vacuum line going over to the brake booster. i leaned underneath the steering wheel and i didnt hear any hissing inside the car either. i listened under the passengers side and i heard a wierd noise that sounds almost like a hiss but it doesnt really. i tried the brake cavuum check again and it still jumps. i pumped about 10 times quickly and i watched rpms and they bounced between 400 and 700 kinda fast. then the rpms dropped and it almost stalled but it didnt. i dont know where else to check.

[xeroinfinity]

The brake booster requuires a vacum, under extreme conditions it can pull enough vacum to stall the engine.

The vacum lines behind the coils, you will see a red connector. The vacum leads up over the TB and to the PCV .
Its the Evap purge not the o2, but the O2 does connect there .

I'll see if I can locate a picture.

[mjtr21]

ill try to take a picture of what i see because on one end theres a connector and a vacuum line i think and on the other side theres nothing and i feel a small hole. its a small black box shape.

[mjtr21]

okay the thing i seen was the EVAP purge valve. Is there supposed to be anything hooked to the passenger side of it. If thats not the problem then i will do what my haynes book says to check my brake booster. but would the brake booster affect the jumping rpms at startup? Im very confused at this.

[xeroinfinity]

I think thiers just a wire connector onthe pass side of the evap purge valve. The vacum connectors to it on the drivers side.

If your foot is on the brake when you start the car i cuold affect it. But if you havent had new plugs & wires in a long time then a good tune up will do it wonders.

Good Hunting

[mjtr21]

no i dont put my foot on the brake when I start the car. And i already said i got new iridium plugs 2 months ago.

[Aki334]

disconect your MAF see if anything changes if no you know what to do
test it

[xeroinfinity]

Oh, did wires come with those plugs ?

Only thing I can say is,
dont hit the brakes 10X real fast

But on the stalling it could be your IAC, wet or damaged ignition system components.
You emissions system could have something faulty.
And you could have loose intake bolts/nuts leaking.

Good Hunting!

[Aki334]

XI I think we should open one electricla diagram subtitle where we woud have all the diagrams scanned for different model years that way we can tell people easier where to focus their attention by telling them part name, circuit number wire color etc. I can help you with diagrams from 1985-1998