This is regarding my 1990 fs bronco w/ 351w and E4OD. In 1998 I swapped the engine w/ a ford motorsport 'Sportsman' block [ M-6009-C58 ], added a crane 2020 compucam, gt40heads, edelbrock upper / lower intake manifolds, jba shortie headers, cobra 1.6 roller rockers, ford motorsports lightning upgrade kit [ M9000-L58 ] that includes ECU, 30lb injectors, harness, airbox w/ 80mm MAF. so I went from speed density to a EEC-IV Mass air system.

The original mileage is 193k. 1998 upgrade was at 127k miles. The components on the engine are 12 years old except for the parts I mention in the upgrade.

Last few weeks it has developed unstable idle that wanders ~100 rpm and is on the high side of 950+ rpm. If is was not warmed up, it would die upon release of the gas pedal.

I though it might be a TFI ignition module going out. Replaced it, and problem still not found.

Read in my Haynes manual that it could be the IAC valve on the TB or the EGR valve and or the [regulator] EVR for the EGR. Tried all three, some partial relief, it doesn't die on pedal release, just a very rough idle.

When I was testing it, I had the air hose disconnected from the TB's, engine would not run, put hoses back on and it ran. Did I do any harm to the MAF sensor?

For about two days it ran normal, I jiggled the harness on the air hoses upon disconnecting from the TB's and the problem came back. If there is a harness interrupt near the MAF sensor, could it cause the rough idle?

I get normal mpg and the usual performance as I test it going up a steep long hill and it doesn't bog down.

BTW, Haynes manual was wrong, I believe in their test of the EVR they say to do a vacuum test of the supply port of the valve and it should hold vacuum, it doesn't and neither did a new part from the dealer. How do you check a EVR?

Is the pintle in the IAC tracking valve suppose to completely close or bottom out on the seat or completely block off one chamber from the other? New ones don't. The original one looks like it does but is 12 years old and the spring is weak?

When I disconnect the vacuum hose from the EGR valve, the rough high Idle goes away and when I push throttle it sounds good. Does that prove anything? Also, putting a vacuum gage on the EGR vac. hose show zero on idle, but indicates 5+ in. Hg when throttle is pushed. Is that a good test of the EVR?

The dealer said that if I disconnect the SPOUT connector it would isolate whether the rough idle problem was in the ignition or a vacuum problem. By disconnecting the spout it smooth out all the roughness indicating that there must be something wrong in the ignition system? Is that true?

Any help on these issues, especially from pros, would be especially grateful to.

Ron