Just gonna add in here.
Okay, So since no one answered, I will start. I have been thinking about this more overnight.
Start
A car’s ECU does not need any vacuum information to start a car, a car just needs Air, Fuel, Spark. Therefore a car that has a somehow ‘overtime’ broken vacuum line should realistically have no problem starting. UNLESS the vacuum leak causes it to flood on the turn off.
When the engine is starting it looks only at 2 things, coolant temp (how much extra startup fuel to add), and TPS (the TPS is tuned so that it aids starting almost similar to how a carburator works - full throttle adds a bit more fuel). Note this is RIGHT up until the point the engine comes to life and shoots up to idle rpm. Obviously that is RUNNING, so i’ll move on to there. Here vaccum leaks do not matter at all. I do not belive the ECU even looks at MAFS readings for startup since it has BASE calibrations for timing and injector cycle, which are filtered by the coolant temp and TPS settings - however it may take the MAFS into effect SOMEWHAT. Even if you have a huge intercooler piping leak here the engine will STILL start. You could have no vaccum hoses on and the car would start. We’ve started cars with NO vaccum lines before - the engine STARTED haha ok lets move onto running.
Running
Lets look at the nature of vacuum lines and what they are used for in the car. Please feel free to correct me or add on to this thread at any time. A regular NA car uses vacuum lines for EGR, Cruise control (referenced in order to activate a vacuum pump, so that it can function properly), Brake booster, Secondary butterflies in the intake manifold.
So when you have a vacuum leak, it will affect the whole system, however a tiny leak would be very inperceptiable.
Okay, so the car is now running, how does a vacuum leak affect it? Well, for idling the vacuum leak can create lumpy idling, not very smooth (Even a giant leak would not create big run issues). When you rev however, the car will do weird things because of EGR and AIV (emissions stuff). You would feel loss of power and it will feel like it’s running lumpy. (from my experience with SRs). Alright, so in reality vacuum leaksmake your car run like crap, but only if they are severe, i.e. Brake booster vacuum line broken.
Ok basically lets break it down to 4 sections:
Idle: here if you have a massive vaccum leak (we’re talking like no brake booster line on the car) the engine will run lean and the idle will be very loppy (almost like you have cams) or even stall after it pops to life as soon as you apply any throttle. This is because the MAFS readings are totally off. The engine will also rev up higher since it can suck air through these open ports with ease. It will only rev to the maximum power it can produce with the amount of fuel being injected (remember the fuel in the engine is the same as idle since the throttle plate is closed, only the idle air valve is opening, so it limits how much fuel it will inject. So you can see here you would need pretty big vaccum leaks for the engine not to be able to continue running at all since most leaks can balance themselves out.
0 Load: When the TPS is reading 0% and the throttle plate is closed the injectors go to a duty cycle, i belive its 0% until around 2000-3000rpm when they start coming online to catch the idle. I belive the timing is pretty well the same as the engine running at that RPM under low load, possibly slightly more advanced. Here there are pretty much no readings from the MAFS, and the only time you would get the reading from the MAFS is when the injectors were starting to come online to catch the idle, or when there was a movement in the TPS.
Intake manifold vaccum: Ok lets assume you’re cruising along in vaccum. Not boost yet, however there is a load on the engine and it does need to be running pretty close AFRs (between 11.0:1 - 16.0:1 to feel like nothing major is wrong i’d say). So assuming you should be at 14.5:1 or so in cruising (thats just a guess), lets imagine what would happen if you had an open vaccum port. There are different sized holes in the throttle body, some are about 0.4mm in diameter (very small) and i belive the norm is around 1.5mm in diameter (just from memory here bare with me). So lets assume one of the 1.5mm diameter hoses broke entirely. You would be sucking in slightly more air that was unmeasured. The engine would run lean in this condition. If the vaccum leak was too small (or the throttle position was so little that this vaccum leak was a huge % of airflow into the engine) then the car would start to run very poorly (basically become un-burnable because there simply is not enough fuel). After saying that, you’d have to be a pretty big idiot to not see one of your vaccum lines was entirely missing a hose, usually its just a very small crack, which doens’t alter the engine too much here, infact in some instances you may gain power (since you’re leaning it out to just the perfect AFR for cruising)
In boost: ok when you are in boost, the entire system is pressurized. usually only up to the throttle plate is pressurized. This means a vaccum leak would infact be pushing air out, causing the AFR to go rich (since now there is less air in the system than the MAFs measured). Again, like while cruising the leak can be large or small, and the size will determine how much it affects how the engine runs.
A huge leak such as an intercooler pipe that has come off entirely, will sometimes allow the engine to continue if you hold it at 0 atmospheric pressure, however the mafs would basically be reading the air the turbo was sucking in, instead of the air the engine was sucking in. There is a “happy spot” in the throttle where the amount of air you are letting into the engine matches the amount of air the turbo is sucking from the MAFS, however when you want less power the engine floods and dies (since the turbo is still sucking a lot of air, resulting in more fuel, yet you are cutting off air supply to the engine). Or when you want more throttle/power you allow more airflow to come in. Theoretically if you COULD produce a LITTLE more power the turbo would speed up a little bit more and the mafs would suck a little bit more and you could progress… but it has to be so smooth and perfect, because as soon as you bog down you’re back to square one - sometimes even lower. Usually too much throttle is applied and the engine leans out exhaust pressure goes down and the turbo slows down. In a nutshell, the engine is driveable if you coast or are an angel to it, usually enough to just get it off the road, if the leak isn’t too bad then the car can still be driveable.
Now everythign I just said about intercooler piping coming off relates to vaccum hoses except on a much much smaller scale. Any leak will result in the engine running rich. Sometimes you cannot feel the leak but theoretically, fixing it could result in more power. A leak enough to cause a 10.5:1 AFR would be about 25hp less than 12.5:1 in a 200rwhp engine. Ok i think ive beaten that one to death enough - in boost vaccum leaks make a big difference thats all i really needed to say haha sorry.
Turning the car off
I am really not too sure about this one, i really can’t see how the car would flood the engine when you turn it off. In order to flood the car has to be off, but the injectors still have to be firing for a certain period of time, and when the power is off, i just can’t see how it would still happen unless you have a wiring issue. WHICH YOU WILL NOT HAVE UNLESS YOU MESSED WITH THE WIRING.
When you turn the car off your injectors close. Its simple as that. you could rip the intake off entirely and shut the car of it should turn off just the same. Fuel pump turns off, injectors stop opening, im pretty sure spark dies right away, however I could understand why it would continue for a couple moments. These aren’t carburated engines where you get like run on and stuff like that, turning the key off means turning fuel off entirely… so im not too sure why a vaccum leak would alter anything.
Remember guys, vaccum leak just means the reading the MAFS is getting is different from the reading the engine is getting. I guess that means any time you think there is a vaccum leak it could also be ap roblem with the MAFS, however I’m pretty sure thats kinda rare. If you’re asking qusetions try to list every single thing you can that the car is doing out of the ordinary, even stuff you dont think is worth mentioning… i think thats really the only way to diagnose what could be wrong.
Guys who answer, maybe its best to just not reply at all unless there is sufficient info? It would show the person asking that there isn’t enough detail and the problem is… well unsolvable in the current state. There are too many posibilities. Thats an idea… Because you shouldn’t have to waste your time trying to list off every single thing that could alter the way the engine runs, and i’m sure noobs hate to hear a 40 part list of things they have to check. Usually its possible to rule things down to 3-4 problems i’d say, and usually 2 of them are obvious (like compression or battery terminals connected for example)
