(Previously posted in 1987-1996 F-150 forum)

1989 Bronco 5.0L auto California emissions

Runs perfectly strong when cold, but when it gets warm (10 mins driving, and the temp needle just moves off the peg), it sputters/stumbles under moderate acceleration and at idle. The idle stumble is not regular but occasional, like every five seconds or so and knocks rpms down a couple of hundred for a second.
I replaced rotor, cap, and plugs less than two years/10,000 miles ago; plug wires 5 years ago 40,000 mi. But I can’t figure how they would act differently when warm.

Had codes 31 and 33; replaced the EVR, EVP, and EGR Valve and O2 Sensor.
Still have code 33 (no signal that EGR valve is opening) .
Checked vacuum line to EGR from EVR- holds vacuum.

I did check the volts to the EGR valve position sensor (EVP), and it was within limits, so the sensor has power- and I have checked the sensor itself by applying vacuum to the EGR valve and reading the variable ohms resistance put out by the EVP, which varied according to valve position, but the code that is set seems to indicate that that info is not being sent to the computer.

Question:
1) what changes in the EEC-IV system at warm-up?
What "turns on" or opens up that may be causing the problem?
2) Is there a "shop" procedure for the 33 code?

Thanks, Chris

 

Have you had any luck in figuring it out? I'm having the same problem with an 88 with a 302. I'm only getting a code 33 and the EGR tests good also, there isn't any vacuum coming into the EGR so it looks like the Solenoid isn't working correctly?? Thoughts?