Finding vacumn leaks - how ?

[Bigbartguy]

My 1985 has refused to idle correctly after sitting for a couple of days.

It will fire, run for a second, then die. I have to 'goose' the throttle to keep it from dying. After the car is warmed up (5 min) it seems to idle correctly but I still have a sputtering/stumble under heavy acceleration (sometime light) at about 1500 rpm.

If I am at idle and rev hard, I sometimes get a backfire at around 1500 rpm as well.

fyi- replaced DME relay - no change. Ran injector cleaner thru - no change.

considering: vacumn leak / bad spark wire / bad injector.

I'm thinking that I may want to start looking for a vacumn leak but am unsure how to find one. Is there an easy way to find a vacumn leak ??
thanks
Tim
ps- Bentley manual has been ordered (finally).

[Chuck Moreland]

Get a can of starter fluid (ether). While the engine is idling, spray it on all the hoses and manifold gaskets. If you hear the idle change, you have found a leak. Just be sure it isn't being sucked into the air box.

[Gunter]

We tried a little propane or azetylene and found the leak coming from the brakeline connection. But it could be anywhere. Once the nozzle from whatever combustible you use comes near the leak, the idle improves noticeably. Hope this helps.

[Ned nyna 11]

Tim,
My plan to chase down the vacuum leak in my Carrera is as follows.

1 Listen using a piece of garden hose as a stethoscope.

2 Squirt starter fluid around the leak possibilities and watch for the revs to increase when the fluid is sucked in the leak.

3 Cork the exhaust and with the engine off, get a few psi of air into the system and listen for the hiss of air out the leak.

4 Was also thinking of using a little vacuum pump to pull a vacuum in the inlet manifold and listen for the hiss in but don't think the pump will suck enough.

Thats it, Will post the results.

Ned Monaghan

[ChrisBennet]

It doesn't sound like symptoms I would associate with an air leak but...

If you want to check for leaks at idle, you should disable the idle stabilizer valve. On '84-89 Carrera's the ISV attempts to keep the idle at 800/880rpm. Remove the oil filler cap (making a big air leak in the process) and you'll notice that the idle doesn't change much if the ISV is working and the idle is set correctly (and there aren't huge air leaks). To disable the idle stabilizer, put a jumper between "B" and "C" of the test socket located in the left side of the engine compartment under the plastic cover.
-Chris

[Bigbartguy]

Chris
I was thinking that the idle/sputtering/slight backfiring issue may be related to a vacumn leak since the issue seems to disapear once RPMS are higher. It runs smooth as silk over 2000 RPM.

I've heard that vacumn leaks can cause idle/low RPM issues.

any other thoughts about causes?

[grant lyon]

be careful poking around in the dark. if you see Christmas lights -- bad wires.

you also might want to look over all your vacuum hoses. too obvious - sometimes.

[ChrisBennet]

Originally posted by Bigbartguy
Chris
I was thinking that the idle/sputtering/slight backfiring issue may be related to a vacumn leak since the issue seems to disapear once RPMS are higher. It runs smooth as silk over 2000 RPM.

I've heard that vacumn leaks can cause idle/low RPM issues.

any other thoughts about causes?

First understand that I'm not an expert on these things. I'm more of a student. When I've had back fires at low rpms it was ignition related - wires crossed, distributor timing off, that sort of thing. While you may be right about the vacuum leaks, it isn't what I personally would be looking at based on my limited experience.

On DME cars, only the DME can get the distributor timing off. Bad reference or speed sensors can confuse the DME. Reference signal is weakest at low RPMs.
gotta go now but your get the idea,
Chris

[Superman]

I'll go ya one better. Before ANY troubleshooting, the engine's ignition system must be made perfect. The good news is that ignition systems are kind of simple in that you're making sure you have the right spark at the right time, with no exceptions. The bad news is that ignition systems are a bit persnickity in that if there is anything wrong, they're not working right. But again, before trying to solve a fuel/air problem the ingition system must be running perfectly.

Oh, and as far as finding vacuum leaks go, I admit that can be difficult. When I have used stuff like carb cleaner and starting fluid, I almost never see the engine's idle increase. Even though those things are flammable, when they get into an intake system, they tend to make the engine stumble, or the idle decrease. One quick way to check is to begin by just spraying it into the air intake horn. See what happens.

[nhromyak]

Check the normal the stuff...

You already did a basic tune-up...right?

Plugs would be my #1 culprit... what brand are you running? Champions have a softer material and therefor will not last as long, but will fire the best in poor conditions. NGK and many others have a good long life, but do not do so well in poor engines (oil). Most agree the Bosch Coppers are the way to go.

Valve lash too tight can do the symptoms you describe, but I don't think this happened over sitting for a couple days.

Check for loose intake seals (common on the Carreras), check the ground wires on intake runner #1, and their associated body grounds.

You already did the DME relay. Now you have a spare!

Check the ISV as stated above.

Could also be the infamous single wire Head Temp Cylinder. This should be updated to the two wire one.

Another check is the O2 sensor.

Good luck.