What to look for with scantool to confirm vacuum leak?

[Fordiesel69]

This is a 2007 F-150 with 4.6L V8. I have forscan installed on my laptop, and am well versed in using it for basic functions.

What values should I be monitoring and what should I look for to determine if I have a vacuum leak?

P0171
P0174

These are both bank lean codes, and 9 out of 10 times this is a vacuum leak per internet research. I will not be able to get a smoke machine in time to do the diagnostics, so I want to use forscan to confirm thru data that I have a vac leak, instead of a fuel delivery or o2 sensor issue.

 

[eljefino]

Well you're going to see fuel trims far from zero, but you already know that from the codes.

I'd run the thing and try to enrich the air around various things like throttle bodies and intake gaskets. Spray brake cleaner or whatever while it's idling and see if the idle smooths out.

 

[D60]

Both banks, so I'd look at MAF values and inspect everything from MAF to intake. I'd definitely start by just cleaning the MAF if it hasn't been done, but for giggles get values before and then after.

LMK what you find, I'm at 240k mi on my '07 F150 4.6 but no problems like this.

 

[Fordiesel69]

Thats the thing i dont know know what to look for with MAF readings on the scantool.

 

[bdcardinal]

I seem to remember the PCV hoses were a good source of vacuum leaks on these.

 

[Dave9]

These are both bank lean codes, and 9 out of 10 times this is a vacuum leak per internet research. I will not be able to get a smoke machine in time to do the diagnostics, so I want to use forscan to confirm thru data that I have a vac leak, instead of a fuel delivery or o2 sensor issue.

Yes a vac leak is arguably one of the most common causes of both banks lean, but you can't really determine that it's not a fuel delivery issue, unless Forscan picks up a fuel pressure reading (which it might on an '07?), or the O2 sensor doesn't seem to be fluctuating normally, in which case it might have triggered an O2 sensor code.

MAF, you can pull and clean it. If you can't get a smoke machine and can't visually see the leak area, and spraying suspect areas with a combustible fluid doesn't change idle, then how do you propose to find a vac leak? I mean that doing a process-of-elimination thought process to decide it's a vac leak, isn't really proof.

 

[wtd]

One way to possibly verify a vacuum leak is to monitor fuel trims. Say for instance that your fuel trims are 17%. If you rev the engine and the fuel trims start dropping, that is a good indication that you have a vacuum leak.

 

[Marcozi]

I have the OBDFusion app. You can monitor a wide range of values. As noted, check the short and long term fuel trims.

 

[Fordiesel69]

So the IWE'S are not leaking on the 4wd, and I checked a bunch or vacuum hoses and their is nothing I could find with starting fluid. Here is a screenshot at idle. Would there be any indication of a vac leak?

 

[Fordiesel69]

 

[clinebarger]

Powerbrake it just a little....Don't spin the tires & be safe about it.

Do the trims correct or even go negative on the short term?....If so, It likely has a vacuum leak.

 

[Nukeman7]

According to this video, you have a significant vacuum leak. Ford 4.6 liter engines often have degraded rubber nipples/elbows that are used to couple the vacuum hoses to the vacuum ports and/or PCV valve, especially those located at the back of the engine near the firewall.

 

[Chris142]

Check the fuel pressure first.

 

[ripcord]

If it's a vacuum leak, you should see bigger fuel trims at idle than at higher RPMs.

 

[thescreensavers]

One way to possibly verify a vacuum leak is to monitor fuel trims. Say for instance that your fuel trims are 17%. If you rev the engine and the fuel trims start dropping, that is a good indication that you have a vacuum leak.

Plot the short term fuel trim over time. Then while it's plotting you can spray brake cleaner or starter fluid which ever. The leak is found once the short term fuel trim goes bonkers.