Diagnosing a vacuum leak
Vacuum leaks are a very common problem on cars, they're sometimes tough to find, so here are some tips.
First off lets go over the simple bits
Rough idle, lots of causes, dirty/bad maf , airflow, dirty/bad throttle body, bad fuel, bad 02 sensors, exhaust leaks, bad spark plugs, wrong gap, bad injectors.
High idle, air's usually getting somewhere it isn't supposed too, bad throttle body, bad iac , intake leakes, injector leaks.
Stalling or misfire on acceleration, bad spark plugs, big leak, coil problems.
there are lots of things that cause it, even more than i've listed but those are the common ones, the most common are maf sensor,iac, throttle body and leaks.
for the idle problem leaks are the easiest to check for,
1. Get an OBD II reader that can show you the short term fuel trims (STFT) in realtime, ie it has to be able to show it updating, not just a code reader.
2. Read the STFT normal range is -4 +4 but can be as much as 8.
3. If the STFT reads over 8, a lot, and swings around this is the first sign.
4. Raise the idle by increasing the throttle to 1500/2000 RPM, check the STFT's again, if they're back in the normal range , its very likely its a small leak. Give the engine time to sort itself out after raising the speed, its not super fast to react.
5. If its still reading in double figures and swinging around, time to take a look at the MAF, IAC and TB.
We're assuming the O2 sensors are working here, and there are no obvious issues, the primary O2 sensor is very important to make sure its all running right.
Small vacuum leaks only tend to affect the idle because of the relatively small volume of air passing, when the engine speed is raised, the air increases enough so a small leak doesn't make much of a difference.
Spraying the engine bay with non residue leaving contact or maf cleaner is good way of finding leaks, but they can be tough and expect to use a case of cans for the tough ones, make sure its well ventilated area, i do not recommend use of carb cleaner, you might damage the engine.
A hosepipe with a very small stream of water is also a good way to pinpoint small leaks, just be careful of the electronics.
Spray one part at a time, give it some time to settle in, if you hear a change, wait for it to clear out and try it a few more times, eventually you ought to be able to cause the engine speed to change or stall on demand.
Any tubes, o rings, gaskets etc are targets, the pipes on the IC, the throttle body are the typical suspects.
The long term fuel trims are also valuable to look at, large positive values in here, double figures, show a problem in manifold leaks, misfires and fuel pressure , injectors ( the car is running lean, so the computer is adding a lot of fuel )
Negative numbers means the car is running richer than its supposed too, large numbers in the LTFT mean that the computer is subtracting large amounts of fuel to maintain where it wants to be, this suggests fuel pressure problems, defective injector, air blockages, you'll be looking for largish STFT and LTFT for big problems.
The computer will throw a code after about 17%, this isn't to be ignored it means somethings up.
So large positive value STFT and/or LTFT means its adding fuel because of a lean condition, not enough fuel or too much air is being delivered.
Large negative values mean too much fuel is being delivered, or not enough air.
I'll add more stuff to this thread including traces of some of the sensors and so on so you have reference to them,but you will need something capable of viewing a trace, so an oscilloscope or good OBDII reader that can graph stuff out, scopes are very useful and not that complicated, don't be put off by their apparent complexity, you can get them cheap off fleabay or new ones at $150+ but the money is in the accessories, i'll go into that later..
Black Exige S / Elan M100. Don't run a smaller pulley without an upgraded fuel pump! http://www.goth.am
ecu stuff.. New reflash box coming soon!