This past summer the A/C Compressor bearing on my 89 C4 cooked the bearing, I replaced and converted to R-134. Shortly after the conversion the A/C fan control switch went and the system was not cooling at an idle and was running rough. I replaced the switch and the car was cooling properly and once again running smooth.

A few weeks ago the car started running rough at idle unless it had a load on it, ex. if the A/C is compressor is on. Actually when the A/C is on my idle is at 700 RPM and the idle is smooth.

My engine goes lean to rich with the idle swinging from 550 to 790 RPM creating the surging, the ECM wants to see 625 RPM.

I had tuned the car up at the beginning of the summer (Cap, Rotor, Bosch Platnum plugs, and new Delco heavy duty carbon ignition wires.

Here's what I have done since the rough idle started:
Cleaned the Throtle body
New fuel filter, new distributer cap and rotor
Pulled Plug 1 & 3 (the easy one's) and they are burning clean
New Air Hog air cleaner

I scaned the sensors the with my Auto Scan at an Idle/upper radiator hose hot/closed throttle/part/closed loop/Acc. off with these results:
MAF 7 grams also borrowed my neighbors from his Firebird with same results
Injector Pulse 1.6ms
Air Flow 7 grams
BPW 1.6
O2 sensor 1 - 800 varying
TPS set at .56
IAC 32 counts see it stepping
Block Learn 120
Eng. load 50 idle 170 when kicked
MAT 161.68F degrees
Knock sensor 6
Fuel Pump 13.5 volts
INT 130 counts
Knock Signal No
Water Temp up to 220 degrees, fan on about 221 deg.

My next plan of attack would be fuel pressure (need to buy a gauge)
distributor module, change/replace plugs (take compression test), check ignition wire resistance.

Any ideas or past experience, i'm running out of things to check, HELP!

DMC89

 

Have you tested the EGR circuit
Have the tested canister purge valve

 

Tapped it with a large screw driver at the base, plus isn't it suppose to be closed at an idle. My Auto Scan reads off.
DMC90

 

I uploaded some info from my 86 manual
http://members.shaw.ca/agent86/EGR%20System.pdf
http://members.shaw.ca/agent86/Canis...ge%20Valve.pdf

 

ECM may say its off but it could be stuck or not working properly.
It costs nothing to test the EGR and canister

 

Do your IAC counts coincide with the fluctuating RPM ? Or are they steady even when the RPM's are not ?

 

Indeed the EGR may be leaking which might not be enough to throw a code. Check manifold vacuum to see it there are any vacuum leaks.

Also, pull the IAC, clean it & its bore under the TB and reset base idle to 450rpm. There is a method on my site, just ignore the part about setting the TPS as yours is not adjustable.

Also, you could put a timing light onto each plug wire to see if there are any missed flashes corresponding to engine misses.

 

IAC seems steady and not fluctuating with RPM's, I will also all the sugestions especilly with the EGR which I hope doesn't have to be changed (big job). I was also thinking maybe I should actually take the throttle body off and clean it up really good. I'm heading down to FLA this afternoon so won't be able to get back to the car for another week. Thank You to All!
DMC89