STFT/LTFT Way Off No Codes

98GTAZ

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So I am pretty sure I have a bad vacuum leak, but I am having trouble isolating it. It is also possible I could have a different problem that presents as a vacuum leak. I am stumped on how to proceed.

The car is a 1998 Mustang GT with the 2v 4.6L. It was driving fine, but then had two problems happen at the same time. I went out for a drive and the car had no power. It could barely accelerate, and the engine felt like it was resisting going to higher RPMs almost like it was blocked or clogged. I returned home and it blew a heater hose and barfed coolant all over the driveway. I shut it off right away and the car did not overheat.

I fixed the heater hoses and ever since then the car idles terribly, almost stalls, and has a slight backfire. It will accelerate to higher RPMs without issue, however.

My code reader live data stream shows the both O2 sensors reporting a lean condition. STFT and LTFT are consistently positive.

At first, STFT read between 0.8 to 5-7% (positive) on both banks with Bank1 always being higher than bank 2. LTFT never changed and was consistently pegged at 6.3% for Bank 1 and 1.6% for Bank2. LTFT never went to zero or negative. It literally never moved from 6.3/1.6. All numbers improve when the engine accelerates up past 2000 RPM. At higher engine speed, STFT goes to alternating between -3% and +3% and LTFT drops to near zero.

The problem seems to be getting worse. The last readings I took the numbers made massive jumps:
STFT: 25-30% (both banks)
LTFT: 23.4% on bank 1, 16.4% on bank 2

After getting these readings, STFT shifted again. This time it went negative (-6.3% bank 1, -7.8% bank 2). LTFT stayed the same at positive 23.4/16.4.

At this point car is running very rough and stumbling at idle. Engine shakes badly while idling.

So far, I have removed and closely inspected all vacuum hoses. All check out with no holes, cracks, etc. I replaced the PCV valve. I tested the O2 sensors and they check out. I did a fuel pressure check and got a consistent 30 or slightly more PSI (Car has a return system so this is at spec). I sprayed some carb cleaner on hoses and gaskets with no change to idle. I did the test where you pour some water on the engine to induce a temporary gasket seal with no observed change. I tried doing a smoke test with a homemade smoke tester. It didn't work very well, but I did get some smoke flowing so it could be an intake gasket leak, but it could also just be escaped smoke from my coffee can smoke tester as it was hard to tell where it came from.

I am stumped on where to go next. It seems like it could be an intake gasket leak, but I didn't want to tear into the engine without more definitive proof. since my smoke test didn't really work. Any ideas on other tests to run, or how to do a better smoke test are appreciated.

Also, if I am missing something, or perhaps ignoring another cause of this issue, I'd appreciate other systems components to check for problems.
 
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98GTAZ

98GTAZ

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I should add that the PCM has never logged any codes related to this issue. It has no codes logged at all. The car also recently passed an emissions test last month
 

lwarrior1016

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The way your fuel trims are rich at idle and level out with rpm is really saying you’ve got a vacuum leak. When you blew that hose, did you find the cause? Did you pull the spark plugs to see if there is any coolant in the plug wells?
 
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98GTAZ

98GTAZ

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When I did the smoke test I ran the hose into the throttle body inlet where the pcv valve hose connects. I wrapped the end with some duct tape to seal it to the port.

It looked like some smoke was coming up from the intake manifold. However, it was near the throttle body plenum kind of towards the passenger side but mainly in the middle so it could just be leakage where I attached the hose.

I have not pulled plugs.
 

07GtS197

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Don’t rule out a hairline crack in the intake. It’ll keep expanding with each heat cycle and make a vacuum leak worse.
 
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98GTAZ

98GTAZ

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The intake is a stock non-pi intake. It’s the newer one with the aluminum crossover so a previous owner must have replaced it at some point.

Checked all plug wells, no coolant in any of them. I pulled the number 1 and 2 plugs. Not sure how to judge them, but I attached a picture of them.
 

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96blak54

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Ill check at work, but im thinking ford never made a npi with aluminum crossover....ive been wrong before. The company i work for has made and still makes all the plastic ford modular intakes. We no longer make the npi but we still have all the examples ever made. Is it to much to ask to snap a picture of your intake manifold? A good clear engine pic will do.

I will say this. The 4.6l pi intakes can have bad spots that will pass our leak testers, but over time will fail. This is assuming your intake is a pi ford.

Their also are issues with the use of wrong intake gaskets. Only factory stock and felpro specific 4.6l gaskets do the job correctly
 
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98GTAZ

98GTAZ

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Here is a picture of the intake manifold/top of engine.
 

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07GtS197

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That’s not a Ford intake but probably a Dorman. They are junk and cause nothing but issues. It’s likely there is a leak somewhere.
 
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98GTAZ

98GTAZ

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Did a smoke test again. Was able to get a good seal at two ports and some good smoke going. Ran it in under very light air pressure from my compressor. Both times I saw no smoke at any vacuum lines or coming from under manifold. Eventually smoke wafted out from the CAI filter in front of MAF.

Took new readings. Definitely getting worse.

At idle: STFT +26-32% B1, +24-28 B2
At 3000 rpm: +24-32% B1, +7-14% B2

LTFT on both banks at zero even after engine warmed up, never changed.
 

apsmith49921

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That 2nd plug looks pretty discolored as well like oil or its running rich. Which would also indicate a possible vacuum leak
 
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98GTAZ

98GTAZ

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Car has no special tuning. Stock PCM running stock program. Plugs were changed 6 months ago. Only have a few thousand miles on them.
 
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98GTAZ

98GTAZ

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MAF sensor was cleaned with no change to lean condition. CAI looks to be an ebay special installed by a previous owner. There’s no brand and it looks hand fabricated.
 
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98GTAZ

98GTAZ

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Here's some other data from my latest readings:

B1 O2 S1 Voltage: 0.081-0.85
B2 O2 S1 Voltage: 0.065-0.81

MAF Air Flow Rate: 0.7lb/min
MAF (Gallons per second): 5.72
MAF (volts): 0.87

Ignition timing advance: 12 degrees

Fuel Rail Pressure (volts): 1.35

HEOS Bank 1: Lean
HEOS Bank 2: Lean
 

07GtS197

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What does your iat read? What about coolant temp sensor? What are you using to datalog? Forscan is my go to app for datalogging.
 

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