'96 4.0L OHV with Hot Start problem

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Just_Steve - 26 Aug 2005 01:45 GMT

Greetings all,

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...

Symptoms:
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
tailpipe).
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
as usual.

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.

My guesses:
Vacuum leak
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
rpm)

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.

TIA,
Steve

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Just_Steve - 26 Aug 2005 04:29 GMT

Meant to say the MIL bulb lights when ignition is turned on *prior to
starting engine*. It does not come on while engine is running, even during
the periods of uneven idle.

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Just_Steve - 30 Aug 2005 05:35 GMT

Well I checked the EGR (it does cause RPM to drop when vacuum applied to the
valve at idle) and the IAT resistance is within spec. EVR test was
inconclusive, couldn't tell a difference on or off, but may not have been
"testing" it properly.

More driving has pretty much confirmed the main symptoms, which are: have to
push accelerator wide open get it to start when the engine is warm (acts as
if the engine is flooded), higher than normal idle speed after warm-up, and
intermittent disengagement of torque converter lockup after engine reaches
operating temperature. Still no check engine light.

Any suggestions about what could be the problem will be appreciated.

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Just_Steve - 02 Sep 2005 06:45 GMT

In case anyone digs this one up in the future, the shop replaced the MAF and
ECT. Per the tech, they scoped the MAF and it was sending back very
unstable readings, and the ECT was out of spec. He commented that previous
use of a K&N air filter could have contributed to the MAF problem - per the
tech the MAF gets contaminated by the oil in the filter and sometimes can't
be cleaned well enough to fix it. According to my "eyeball" check of the
MAF, both before and after cleaning, it looked like it was pretty clean, and
I have to wonder if the main problem was the bad ECT. Really wish I'd taken
the time to measure the resistance on the ECT before I took it in since it
was one of my suspect sensors - if the EEC thought the engine was still cold
(based on what the ECT was telling it) I could see how that would cause the
overly rich restart problem, the torque converter lockup disengagement, and
possibly even the too high idle. Oh well, it's running great now, will have
to see if the mpg goes up now that the EEC realizes the engine is actually
warmed up and the MAF should be providing better info too.

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