'96 4.0L OHV with Hot Start problem
My '96 XLT V6 4.0L OHV 4X4 Auto with 102k miles has started having a hot
start problem. At first I thought it was 'cause of the tank of ethanol/unl
blend I had in it (when will I learn it does NOT like ethanol in hot
weather), but I've since refilled with regular unleaded and am still having
problems. Acronym defs included for future readers...
No MIL (malfunction indicator lamp - check engine) light and per
AutoZone no DTC's (diagnostic trouble codes) stored on EEC (electronic
engine control) (MIL bulb is good, displays on ignition on)
Starts fine when cold. Initial idle is normal, smooth, quickly settles
down to about 700-800 rpm.
Idle can start to get unstable as engine warms.
No start when hot. Engine cranks fine, have to floor accelerator to get
engine to catch, acts as if it is flooded (with nice blast of smoke out the
Often idles faster than normal when at operating temp, often around 1000
rpm, sometimes as high as 1500 rpm (in neutral).
Sometimes will not engage torque converter lockup, yet other times will.
Can't seem to find any pattern to this, other than lockup usually works fine
until engine reaches operating temperature. Lockup kicks out during braking
Other possible symptoms (or just my paranoia)
Engine seems to surge at times
Seems quicker to downshift than usual
Gas mileage is down, but could be due to previously unnoticed lack of
torque converter lockup
Checked so far:
No obvious vacuum leaks.
No obvious unplugged devices/sensors
Air filter fine (changed, brand new). Fuel filter about year old.
Cleaned MAF (mass airflow sensor) (looked ok to start)
IAC (idle air control) is only about year old, seems to be working fine
(when idling hot or cold, unplugging IAC will stall the engine)
TPS (throttle position sensor) is couple years old (was replaced when
attempting to fix cold start problem that cleaning previous IAC fixed)
Engine temp per dash gauge appears normal. No loss of coolant.
Unplugging EGR during hot non-start does not help.
PCV is only year old and rattles.
Bad ECT (engine coolant temp sensor)
Bad DPFE (EGR backpressure sensor)
Will try to check tonight:
IAT (intake air temp sensor) resistance
ECT sensor resistance
EVR (EGR Vacuum Regulator) solenoid (blow through test)
DPFE sensor (no way to test? if I unplug, will EEC go into FMEM [failure
mode effects mgt] mode, and possibly run better than with a bad DPFE?)
EGR function (will apply hand vacuum to EGR at idle, see if it drops
Suggestions of possible cause/repair welcome. If I can't figure this out
soon it'll have to be a trip to the shop for the ol' exploder.