Slight idle drop when in drive or reverse

BRANDON28425

MEMBER
2012 CHEVROLET CRUZE
1.8L
4 CYL
2WD
AUTOMATIC
108,000 MILES

I noticed RPM drop at a consistent rate while sitting idle when in drive or reverse. When it's in neutral and no transmission load it isn't enough to be noticeable any more. Its only a slight RPM drop (maybe 50 to 100 rpm's) but enough to shutter the car. I replaced the plugs and replaced the coil about 10,000 miles ago. In the attached video you can hear a possible slight pressure build and when it hisses with the release of the pressure is when the idle drops. I checked the PCV valve which is a common fail item on these vehicles but don't have any noticeable leak from it. Any help is appreciated.

Saturday, June 27th, 2020 AT 6:32 AM

4 Replies

BRANDON28425

MEMBER
3 POSTS

Here's the video mentioned.

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Saturday, June 27th, 2020 AT 6:34 AM

JACOBANDNICKOLAS

MECHANIC
110,055 POSTS

Hi,

I do hear it. It sounds like a release of pressure rather than a vacuum. Can you tell approximately where it's coming from?

I want you to check fuel pressure. I have a feeling the pressure relief valve is functioning as a result of a faulty fuel pump flow control module. There is also a fuel pressure switch that can go bad. See pic 1 If the module fails, the relief valve takes it's place. However, we need to check to see if the fuel pressure is building until you hear that noise. If it drops, then we'll have a plan of attack.

Here is a link that explains how in general to check fuel pressure:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

If you look at pic 2, it shows where the schrader to attach the fuel pressure gauge is located. I realize most people don't own a gauge, but most parts stores will lend you one.

Let me know what you find.

Joe

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Saturday, June 27th, 2020 AT 8:04 PM

BRANDON28425

MEMBER
3 POSTS

Okay, so I grabbed a fuel pressure tester and followed the testing procedure above and in the Haynes manual for my car. I also got the pressure specs for my specific car and this is what I got:

1. Pressure when the engine is off but the key is set to run I get a spike to 60 psi then a gradual drop and stabilized pressure at around 50 on my first run and 56 on my second run which is within the specs for my car. (Specs are 50-94 with engine off)
2. After trying the test with the engine off, I checked pressure with the key off which my Haynes manual states I shouldn't see more than a 5 psi drop after 1 minute. (Which I did not see. I seen a drop of about 3-4 after 2 minutes and it stopped dropping.)
3. I tested pressure with engine idle and got a consistent 58 psi which is good according to the Haynes manual (43-58 psi is the specs). I still heard the pressure release but did not see any change at all in fuel pressure

However, I did buy an automotive stethoscope and tested around on the top left of the engine where I am hearing the sound. I tested injectors, around fuel rail and not the sound. But I am hearing it faintly through the throttle body and it gets more prominent when it touch the black hose/line in the attached photo circled in red. Any ideas?

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Sunday, June 28th, 2020 AT 12:46 PM

JACOBANDNICKOLAS

MECHANIC
110,055 POSTS

I looked at the pic, and it appears to be the PCV tube. That makes sense if it is plugged or the PCV valve is plugged. See the attached pic and let me know if what I circled looks like what you are seeing on the vehicle.

I want you to try something. Open the oil fill cap and see if the pressure release sound stops. With the oil cap off and engine running, some oil could mist out, so be careful.

Let me know.

Joe

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