This is more of a general question than a specific problem but I've been trying to figure out for the sake of our Honda engines.

I love my vacuum gauge since it can tell me a lot about what I don't see. But I'm wondering about when it comes to vacuum leaks, can the vacuum gauge really show that there is a leak (when the in/hg are lower than normal) in a MAP sensor engine?

The MAP sensor would read the vacuum and adjust the engine accordingly so according to the vacuum gauge there would not be any signs of a leak? As opposed to a MAF sensor engine that can see how much air is going through and will respond more sensitively to a vacuum leak.

As well as with the FITV and IACV that purposely create vacuum leaks.

Or am I not seeing this correctly?

This would be for reference when being discriminative while diagnosing MAF vs MAP sensor engines with a vacuum gauge.

Thanks

 

In a general way, that's right. The MAP sensor can't tell the difference between a vacuum leak vs. the throttle being open a bit, or the FITV or IACV allowing some air through.

However, the ECU is somehow "aware" of those other things. The TPS signal says what the throttle is doing. The ECU controls the IACV directly. The ECT sensor says what the water temperature is, which results in the FITV position. A vacuum leak is something that isn't designed into the system.

 

Yeah. Thats when the ECU throws a lean code or something if it reaches that threshold. But for mild vacuum leaks, wouldn't a MAP sensor engine not show it on a vacuum gauge reading as much as a MAF sensor engine?

Or do the two types respond the same when a small vacuum leak is introduced causing a drop in vacuum pressure?

 

They should respond about the same, where the result is just an increase in RPM. Whether it's a MAP, MAF, or (more likely on newer engines) a hybrid of the 2; all those sensors are doing is enabling the EFI to give the correct fuel rate for the amount of air.

Using a vacuum gauge to understand engine operation should be the same regardless. MAF engines have their own versions of FITV & IACV. The sensors are set up to measure what's needed to calculate the fuel rate. But the pistons & valves still pump air the same way as a carburated engine.

 

Ok. That answers my question. Thanks