I’m stumped and could really use the forums brain trust on this one…I have a 1993 TBI 7.4L 4L80E Truck with about 68,000 miles and it has the 16147060 – BAMM ECM. The issue is high idle (900 RPM) once in closed loop, warm. On cold start engine high idles at 1200-1500 RPM and idle drops with increasing coolant temp just as it should. Typical RPM in closed loop, warm is 875-900 RPM in park and 725 in Drive. Vacuum at idle is steady at 21 in Hg. I have read every forum I could find on this issue but have not found a definitive answer...
Additional background details;
- Thoroughly checked for vacuum leaks, used a hand vacuum pump to check each component including the brake booster diaphragm, and checked all vacuum hoses for leaks, all were good.
- Recently had a valve job due to worn valve guides, so new intake manifold gaskets have been installed along with new TBI base gasket.
- TBI throttle shaft has been rebushed, thoroughly cleaned and reassembled TBI with new gaskets, O-Rings and injector screens.
- Cleaned all ground wire connections with the exception of the one behind the passenger side head. I ran a second 10 Gage wire direct from the thermostat housing to the firewall ground just to ensure I had good grounds for test purposes.
- I have tried setting minimum idle speed two different ways;
- 1.) With IAC electrical connector and IAC removed and IAC air passages blocked, set minimum idle to 550-600 in drive. Engine idles very smooth with IAC passage blocked and will idle down to 350RPM easily
- 2.) With IAC connected set idle RPM screw to achieve .49 Volts at TPS and with IAC counts at 27 in Park.
- I can set the minimum idle lower, resulting in .43 TPS Volts but the ECM will just increase the IAC counts to end up with 900 RPM at idle
- I’ve removed and checked the EGR, it seals tight.
- New O2 Sensor, MAP and IAC and fuel pump. Did not replace PCV valve and hose but I believe the PCV vacuum is through a “fixed orifice” so it should be a designed in vacuum leak correct?
I connected TunerPro RT with the following readings at idle:
Coolant Temp 200.3 F
MAP 32 – 33
FP Volts 13.60
Spark Adv Relative to TDC 20.12
Spark Adv Actual 16.24
Target AFR 14.7
Inj Duty Cycle 3.37
BLM Enable Yes
IAC Postion 27
O2 Sensor (Rapidly Jumps all over (132-800)
Knock Counter 01
Alternates Between Rich/Lean
Idle Status Yes
TPS Volts .49
RPM in Drive 725, IAC counts 58 in Drive
If I shut off the engine wait 10 sec restart IAC will drop to about 10 then increase and hold steady at 23 counts and idle at 725 RPM. Until I place the transmission in drive then back to park which results in 875-900 RPM at idle and IAC counts back up anywhere from 27 to 40. The only odd thing I've noticed is the in dash tach seems to jump around a bit which makes me a bit suspicious of the distributor.
Thanks for any thoughts or suggestions you guys can offer....