2018 Mustang GT vacuum gauge fluctuating when accelerating

[nj888]

Hi,

New to this forum. So, my mustang gt 2018 vacuum gauge shows around 21 at idle, which I think is fine. But when I am cruising and then I accelerate I get some hesitation and noticed that the gauge reads like 5 then stays at 10 to 20 afterward. Is this normal, do you guys have any idea if this could be a vacuum leak or exhaust leak? Or is this normal behavior.

Thanks!

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[Grimace427]

Both a vacuum leak and/or exhaust leak(that would affect performance) would cause a check engine light long before you noticed it on a vacuum gauge.

 

[Bluemustang]

Probably normal. The PCM is always making adjustments. Usually when you accelerate part throttle, the vacuum will drop more and will then climb back up. If you WOT you should see about 0 inHG.

The accelerator pedal is not connected to the throttle body in a mechanical sense. The PCM commands a certain amount of torque and the throttle body will open/close the necessary amount to achieve whatever is commanded.

 

[Jay-rod427]

Not to mention that gauge is nothing more than eye candy. It's not an actual vacuum guage. It is inferred value. Even if you had a vacuum leak the gauge wouldn't tell you anything.

 

[smoke_wagon_6g]

Not to mention that gauge is nothing more than eye candy. It's not an actual vacuum guage. It is inferred value. Even if you had a vacuum leak the gauge wouldn't tell you anything.

It's wouldn't be hard to install an actual vacuum sending unit. You need load, RPM, throttle position to do the inference, no? Seems like that's the hard way.

I know the oil temp is just a calculated value but why would this be the case for vacuum?

 

[smoke_wagon_6g]

Hi,

New to this forum. So, my mustang gt 2018 vacuum gauge shows around 21 at idle, which I think is fine. But when I am cruising and then I accelerate I get some hesitation and noticed that the gauge reads like 5 then stays at 10 to 20 afterward. Is this normal, do you guys have any idea if this could be a vacuum leak or exhaust leak? Or is this normal behavior.

Thanks!

Stick or automatic? The hesitation is odd. Try keeping launch RPM up over 2,000. Mine chokes on hard launches at low RPM.

Anyway, you've have described typical vacuum behavior. Manifold vacuum is related to the throttle plate opening. Given a constant load, more throttle equals less vacuum available and vice versa. A fully open throttle should create close to zero PSI. As the plate closes (by your foot or the computer) it will rise back up. Of course part throttle creates partial vacuum, midrange values.

Since the pedal isn't directly connected to the throttle in these cars, and direct injection engines can vary RPM independent of the throttle by controlling fuel delivery, the manifold vacuum number may not always be the exact value but should still be close to how cars have worked in years past.

I could be wrong, someone will tell me, since cars have changed so much since I began, but watching my vacuum gauge there seems to be valuable information and no surprises. If I were troubleshooting actual vacuum leaks I would have to use proper gauges connected mechanically under the hood.

 

[Kong76]

fwiw on my 16 manual when I floor it the vacuum drops to 0 and after a second goes up to 5-6 and when I let off it spikes to 22 and settles back down during cruising to 15-16. At idle I am at 20.

 

[nj888]

Stick or automatic? The hesitation is odd. Try keeping launch RPM up over 2,000. Mine chokes on hard launches at low RPM.

Anyway, you've have described typical vacuum behavior. Manifold vacuum is related to the throttle plate opening. Given a constant load, more throttle equals less vacuum available and vice versa. A fully open throttle should create close to zero PSI. As the plate closes (by your foot or the computer) it will rise back up. Of course part throttle creates partial vacuum, midrange values.

Since the pedal isn't directly connected to the throttle in these cars, and direct injection engines can vary RPM independent of the throttle by controlling fuel delivery, the manifold vacuum number may not always be the exact value but should still be close to how cars have worked in years past.

I could be wrong, someone will tell me, since cars have changed so much since I began, but watching my vacuum gauge there seems to be valuable information and no surprises. If I were troubleshooting actual vacuum leaks I would have to use proper gauges connected mechanically under the hood.

It is an automatic. So, the hesitation is on from 1500 to 2000 rpm. It is odd and it only happens some time. Also when I drive on wet/snow I see more of hesitation. On Sport mode this does not happen at all. I went to the dealer they re-flashed the PCM. Still some issue while driving today.

 

[nj888]

fwiw on my 16 manual when I floor it the vacuum drops to 0 and after a second goes up to 5-6 and when I let off it spikes to 22 and settles back down during cruising to 15-16. At idle I am at 20.

Ya it is almost like this on my car

 

[nj888]

Also, had check engine light code. The code P0300, random misfire. The dealer reflashed PCM and cleared the code. the code hasn't popped again but had some acceleration issue.

 

[Hardmoney07]

Also, had check engine light code. The code P0300, random misfire. The dealer reflashed PCM and cleared the code. the code hasn't popped again but had some acceleration issue.

Check your plugs you might have a bad one or gapped wrong

 

[nj888]

Apparently, my ride is much better now, after I dumped sea foam and 93 gas. But occasionally get minor hickups.

 

[Hardmoney07]

Apparently, my ride is much better now, after I dumped sea foam and 93 gas. But occasionally get minor hickups.

So a clogged enjector maybe? Or plug

 

[nj888]

So a clogged enjector maybe? Or plug

Ya highly possible.

 

[Hardmoney07]

Ya highly possible.

At least you narrowed it down obviously I’d take the plugs out first change them for new ones that’s easy and cheap and go from there.

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