Trying to figure out what to replace next. I have a 1997 4.6l 180,000 miles. The idle drops low after letting of throttle. Will drop below 500 and rebound. Cleaned throttlebody butterfly and I've replace iac valve, pcv valve and both rubber boots. Replaced egr valve, pressure switch and vacuum regulator. All aftermarket standard motor parts from rock auto. I haven't been able to pinpoint anymore vacuum leaks. Battery might be suspect but it starts right up. My next guess is intake gasket and will probably do fuel injectors and plugs while I'm in there. Also driver side cats are shot ( broken internally ) so i plan on deleting them. Looking for any guidance before I go and spend more $. no CEL or codes.

 

willing to bet plugs will solve your issue since you've addressed the other potential issues.

 

Let’s figure out why those cat converters disintegrated.

If I was a betting man, I’d bet a dead cylinder (or more) is the reason for that. That rough idle has me itching.

Check compression and read the spark plugs, proceed from there.

 

I got under the truck today and the noise from the cats on driver side ended up being heat shield. I ripped off the crusty shield that was loose and the noise went away.

 

Just pulled plug boots and there was water or coolant in my #4 plug well

 

With all the repairs you’ve done and still having the issue, I’d definitely do your spark plugs and injectors. Those usually are a big culprit for rough idle. But you know this you’re obviously very smart with these things, but I just wanna help where I can!

 

I blew the coolant out of the plug today and packed the boot full of dielectric grease and she ran great. The truck sat for 4 years after my grandpa passed and now I'm trying to bring it back to it's former glory. We will see if the coolant returns now that it will be getting driven more. If it does I'll peel off the intake and replace the gasket. Thanks for the input so far.