Engine Intermittent Hot Idle Surge Driving Me Crazy

[$ilverback]

All,
I need some assistance on further troubleshooting. I purchased a beautiful light titanium 1990 GT Hatchback 5-Speed a few months ago. Upon initial inspection the brake master cylinder was relaxed to to pedal sinking when depressed. It also was hard starting and after driving it for a bit was able to determine the fuel pump was failing. I replaced fuel pump, filter and cleaned system and the car now runs incredible.
There is only one issue that I cannot sort out and that is intermittently the idle will surge widely and at times if I don't assist the throttle at a stop light, it would drop so low on the down side that the car would die. I have done the following:
- cleaned MAF with MAF cleaner
-cleaned IAC
-cleaned throttle body
-replaced IAC
-reset base idle
-TPS currently at .89V
-fuel pressure tested and found to be at 34 psi at cold and hot idle
-visually inspected for vacuum leaks
-WD-40 atomized around intake and potential vacuum leaks with running engine at idle (not surging)
-ran KOEO codes and was clear (11)
-ran KOER codes and had 12, 41, 91 (12 is for high idle but had to adjust to about 850-900 during this test or car would die at the low idle portion of test)
-at times was able to coax system into surging by turning on AC but not always

What should be my best next steps? Is there a more methodical vacuum leak check? Should I suspect the TPS or EGR valve? Are there additional tests I can run to help narrow down the issue?

I don't think it's vacuum related due to it being intermittent and seemingly random in nature. I am leaning towards TPS but am unsure what effects each component has at idle. The codes 41/91 show both intake banks being run lean as far as I understand.

Thanks in advance to all who read this post and kudos to anyone with suggestions. Please help!

 

[jrichker]

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 159,000 hits, which indicates it does help fix idle problems quickly and inexpensively.