Vacuum leak of death

[ChevyGuy1022]

My car started acting funny at idle speeds; the engine wouldn't stay idling, instead would rev and drop, sputter, and rev back to life (higher than normal idle speeds I might add). I kept hearing EGR from everyone. So I dropped 170 on a new digital egr valve (factory) and still the problem persists. My dad took the car to a guy he knows and from his diagnostic tools concluded it was a vacuum leak. So a vacuum leak. Where to look to fix that?

Here's what I've done to try to fix it:
Throttle Position Sensor
Idle Air Control Sensor
EGR valve gasket
EGR valve recirculation tube gaskets (2)
Intake plenum gaskets (left and right)
Upper intake manifold gasket
Throttle body gasket

Unrelated to vacuum, the fuel injection system was also flushed out, leaky freeze plugs replaced, and PCV valves were also replaced.


I also checked all the vacuum hoses I could see and replaced ones that were cracking (the original ones, now almost 14 years old). Still the problem persists.

What have I missed???

[ July 22, 2004, 10:21 PM: Message edited by: ChevyGuy1022 ]

[3.4 [deployed]]

missed: worn timing chain.

ALso, when was the last time the distributor was rebuilt? The evap cannister also has a filter under it that needs replacing. Do a search in the forum for TomP tuneup, it's very comprehensive & usually weilds great results.

edit: Welcome to the board [img]graemlins/wavey.gif[/img] [img]graemlins/rock.gif[/img] [img]graemlins/banana.gif[/img] [img]graemlins/dj.gif[/img] :D

edit 2: idle valve solinoid could need cleaning, I think it's covered in the tune up thread.

[ChevyGuy1022]

As far as I know the distributor has never been rebuilt. I don't understand how that could cause a vacuum-ish problem at idle speeds.. Could you explain that theory please?

And just as a note, it needs to be known that the problem is not consistent. It's quite intermittent. My dad and I keep thinking it's something electrical that causes the MAP sensor (also replaced by mechanic) to read low vacuum, then the engine sputters, then it revs to rectify the problem, cycle repeat.

[Jerriko]

That's definitely a load sensor (MAP, MAF) related problem. A couple posibilities are either something is making supply vacuum to the MAP change. Check the vacuum circuit and see if it T's off to other component. If it does, reroute the vacuum lines so the MAP sensor is isolated from anything else. If that is the case, rerouting the lines will let you know if the problem is the sensor or another component that shares the vacuum circuit. Another posibility is damage to the wiring harness for the MAP. Any time the wiring harness gets extremely heat, high resistance and insulation decay are the results. Changing the resistance value of a wire will change the voltage that the computer reads from the sensor. Look for discolored wires or melted wire looms in the vicinity of the sensor.
And on a side note, I have seen cars that where the power steering pressure switch causes surges in the idle. Anytime you do anything that increases the load on the engine will raise the idle. If the a/c system, power steering pressure switch, etc was faulty, that could also cause your problem.

[ChevyGuy1022]

I took it back to the shop this morning. The car barely stays running without giving it gas. Every time it stutters and dies I see smoke (exhaust?) near the EGR valve area. Given the pressure readings and temperature of the cat (692*) he thinks the cat's clogged. Makes sense to me... Anyone else?