High stft's. Need advice...

[HoustonGT]

Update: Took it to a local shop for more diagnostics. Fuel pressure was ok, compression was good and no vacuum leaks. Turns out it was the tune.

2015 mustang with Whipple gen 2 stage 2. My stft's bounce all around, up to 40% and all the way back to single digits while cruising at 55 mph. Idle is a little rough, not super bad. Power feels way down.
My tuner wants me to do a compression test. No engine codes. No recent mods. What else should i be looking at to help diagnose the issue? Thanks!

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[SheepDog]

How long has it been doing this? do you have a data log of the issue? Does it go lean at WOT? Check for vacuum leaks, dirty MAF sensor, Fuel delivery issues.

 

[HoustonGT]

How long has it been doing this? do you have a data log of the issue? Does it go lean at WOT? Check for vacuum leaks, dirty MAF sensor, Fuel delivery issues.

Started yesterday and headed right home taking it easy. Did a data log on ngauge on way home. Stft is normal at idle. Sluggish throttle response from a stop and runs a bit choppy when cruising at easy throttle. Didn't want to WOT. No obvious vacuum leak.

 

[SheepDog]

Do you have a return style Fuel system or BAP? Any way to monitor fuel pressure?

 

[HoustonGT]

Do you have a return style Fuel system or BAP? Any way to monitor fuel pressure?

Runs the same with and without JMS BAP. No return fuel system, on 93 octane. Only have ngauge, no fuel pressure gauge.

 

[SheepDog]

Without being able to look at it in person, my first guess would be that your BAP smoked your Fuel pump. Double check to make sure you don't have a leak post MAF. Even if your STFT's are just a few points positive at idle, it may indicate a leak. Let the car idle and carefully spray short bursts of brake cleaner/carb cleaner around the intake and the blower and see if the engine responds. It would also change your STFT's if you can look at a scan tool while spraying. Remove your MAF sensor and make sure there isn't a moth stuck to it or something.

 

[HoustonGT]

Without being able to look at it in person, my first guess would be that your BAP smoked your Fuel pump. Double check to make sure you don't have a leak post MAF. Even if your STFT's are just a few points positive at idle, it may indicate a leak. Let the car idle and carefully spray short bursts of brake cleaner/carb cleaner around the intake and the blower and see if the engine responds. It would also change your STFT's if you can look at a scan tool while spraying. Remove your MAF sensor and make sure there isn't a moth stuck to it or something.

Thanks! Will report back

 

[Bluelightning]

There was a thread on here somewhere from a guy with his car doing something similar. He was NA, but it turned out to be a broken valve spring. I think a compression test picked it up. Check all the easy stuff first though.

 

[SheepDog]

There was a thread on here somewhere from a guy with his car doing something similar. He was NA, but it turned out to be a broken valve spring. I think a compression test picked it up. Check all the easy stuff first though.

Could be, however generally a broken valve spring will have miss fires associated with that cylinder, as well as a "huffing" sound through the intake or exhaust depending on which valve is effected, and you will see your vacuum/boost gauge flutter (if equipped).

 

[Bluelightning]

Could be, however generally a broken valve spring will have miss fires associated with that cylinder, as well as a "huffing" sound through the intake or exhaust depending on which valve is effected, and you will see your vacuum/boost gauge flutter (if equipped).

Agreed. Here's the thread I was thinking of.
https://www.mustang6g.com/forums/threads/short-term-fuel-trims.111443/

 

[HoustonGT]

Agreed. Here's the thread I was thinking of.
https://www.mustang6g.com/forums/threads/short-term-fuel-trims.111443/

I'll check my fuel pressure first tomorrow, been wanting to add a fuel pressure gauge for a while. If fuel pump checks out ok, i'll move to a compression test.

I have the same strong exhaust odor as the OP in that thread. No huffing sound or missfire codes. MAF is clean.

Only boost gauge is the OEM premium digital gauge. Here's a pic at idle. Anyone know what vacuum should be?

 

[SheepDog]

Vacuum at Idle, at your elevation should be around 20in/hg, so your gauge is fairly accurate. However, that OEM "gauge" isn't actually getting a manifold vacuum/boost source. The data is inferred based on things like throttle position, and RPM and probably other inputs. Basically the PCM is guessing what the manifold vacuum is. The more modifications you make to the motor, the less accurate I would think that gauge is. Also, I wouldn't expect it to react fast enough to register a rapid flutter in pressure, like a mechanical gauge will. For sure though, a compression test would be definitive. I think you would be able to hear a broken valve spring rattling or scraping, especially with a stethoscope.

When you say "seeing 40% STFT's," do you mean +40% or -40%? Do both banks read the same, or just one side?

Unfortunately, one other thing I could think of that would cause your symptoms would be a bent rod, but look for the simple stuff first.

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