Engine Sudden lean idle/deacceleration issue, backfire, loss of power, etc

So I have hit a wall lately when it comes to figuring out what is going on, I have a 91 LX with an edlebrock intake and heads, e-cam, shorty headers, CAI, and the other general bolt ons for your engine and has been road tuned. These past couple weeks I have had a rather big loss of power, a misfire or stumble most noticeably in 3rd and 4th gear, a lean idle and lean deacceleration when letting off the gas to the point its backfiring alot when it has not before.

FPR, fuel pump and fuel filter have been replaced very recently and even checked fuel pressure while driving and that seems to be okay from what I can tell so it does not seem to be a fuel issue I dont think. Ive tried swapping out the TPS, IAC, plugs, wires, distributor cap, and coil with ones I had sitting around with no effect. Now after my last battery reset the idle seems to not respond to the TB adjustment screw which makes no sense and idles very low and lean on a cold start while when it is warm it will sometimes idle at 1k and the AFR is fine but there is still a loss of power. Sorry for the long post but just wanted to try and convey all that is going on and I'm hoping someone else has had a similar experience and found a solution

Noobz347 said:

Sounds like a vacuum leak. A Wild Ass Guess (WAG) is that the sealant in the gap between the aftermarket head surface and deck surface on the inboard side has opened up.

What codes are you getting?

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Thats the crazy part is that it is not throwing any codes besides 15 which it always has for the tune, I'm going to do a thorough check of the hoses tonight to make sure but all the hoses are newer for the most part so there should not be any brittle ones

Noobz347 said:

Carefully spray starting fluid in and around suspected areas (engine running) to include the very back of the lower intake manifold.

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I'll try that, typically in the past I've used a mechanic stethoscope with the audio attachment but I assume the starting fluid will cover more area

Noobz347 said:

If it's leaking where I suspect it's leaking, you will likely never target/hear it with a scope.

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That makes sense, it will be nice to find out what it is if that is the issue but removing the heads is never a cheap or fun time

Noobz347 said:

You shouldn't need to remove the heads. You'll need to take it apart down to and including the lower intake manifold. From there you can clean it up and reseal it.


Make sure that this is your issue before ripping into it.

Generally, we like for folks to complete the Surging Idle Checklist before throwing parts or man-hours into the pit:

Help me create the "Surging Idle Checklist"

Updated 26-Oct-2019 to clarify creeping idle RPM increase as the car is being driven. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of...

www.stangnet.com

The top post is kept up-to-date and will run you through troubleshooting each component.

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Oh okay I gotcha, that makes a lot more sense, I thought you mean’t the head gasket for some reason, but oh I know about that checklist all too well unfortunately, I have it bookmarked on the ready, had to use it a lot when I first got the car to fix a surging idle and such