Diagnosing a rich condition on a 3100/3400 using OBD-I ECM

1)    Check all vacuum, electrical and ground connections

2)    Pull vacuum line from fuel pressure regulator and check for the presence of gas
If gas is present, diaphram is most likely leaking and regulator should be replaced

3)    Check fuel pressure with engine off and key on
1999 & earlier = 43.5 psi
2000 & up = 55 psi
Excessive fuel pressure can be caused by bad regulator or restricted fuel return line
Fuel pressure should be 5~10 lower with engine running and vacuum line connected

4)    Check injectors for leakdown
Fuel system should maintain pressure for at least 30 seconds after fuel pump shuts off
and should slowly leak down - a really good system will hold pressure for many hours
Fast leakdown is an indicator of a leaking injector or o-ring
Excessive fouling of a plug will point to which injector is leaking

6)    Using ALDL cable, Scanning software and a laptop, scan ECM with key on and engine not running
** Disconnect power to ECM for 5 minutes to reset all data before testing **
MAP should read 85~100 Kpa depending on altitude and weather conditions
TPS should read less than 2% when throttle is closed and smoothly go to 90+% as throttle is opened
Coolant Temp should read same as ambient temp if engine is cold
Inlet Air Temp should read same as ambient temp if engine is cold
Be aware that Temps may be dispayed in Celcius and need converted to Farenheit - ((C x 1.8) + 32) = F

7)    Start engine and scan ECM
MAP should read aound 20~30 Kpa depending on speed, throttle position and other variables
Coolant Temp should equal engine temp
IAT readings will vary somewhat depending on engine bay heat, IAT location and IAT heat soak
Abnormally low Temp readings will cause the ECM to think the engine is cold and supply extra fuel

An ALDL cable can be purchased at: www.aldlcables.com

There is a list of downloadable software on the ALDLcables site.

Tunerpro RT can be downloaded for free at www.tunerpro.net - I use Tunerpro RT but it may not be the easiest one since its primary function is to edit BIN files.