Question: Why 5 Minute Ignition-Off When Engine is Warm?

[WoddyBoyd]

My '99 GMC van recently exhibited presumed PMD problems. Under normal operating temps the engine would intermittently die as though it were fuel-starved. The warmer the ambient temperature the more problematic the issue. Simply waiting several minutes would permit me to restart the vehicle, but the next failure was spurious and not far behind. The rate of instantaneously cutting-out appeared to be directly proportional to outside air temp and the length of time from the first instance; though not entirely repeatable or predictable as to when.

These symptoms closely paralleled previous instances of PMD failure.

Hoping to minimize engine heat build-up and thermal-runaway on the PMD, the PMD was relocated to the passenger cabin, mounted on an oversized heatsink with dual computer cooling fans, and the resistor value changed from the default #5 to a #9 for easier winter starts.

Well, several observations can be conveyed:

1) The matched-pair of Motorola transistors on current PMD showed no deformation as a result of thermal overheating and cooling. Have other defective PMD's whose transistors have deep concave indentations due to thermal run-away and subsequent cooling. Decided to reuse existing PMD since it showed little visible evidence of heat related stress.

2) Simple static (parked in driveway) testing with quick ignition on-off-on cycles while the engine was cold initially indicated swifter and smoother starts.

3a) A new problem arose when the vehicle was placed under normal use and brought to typical operating temp. While the engine is hot (normal op-temp) and the ignition is turned off for a short duration it will not immediately restart. In fact it will sputter, misfire and smoke like a clogged chimney.

3b) Given roughly a 5 minute ignition-off duration the vehicle will restart. At this point it will restart 100% of the time, I fear the surprised of when it won't.

3c) The previously presumed intermittent PMD issue vanished with relocation and/or the #5 to #9 resistor mod. No instances of engine cut-out occurred during normal use despite a 30 degree increase in outside temp.

4) A new PMD was purchased prior to cabin relocation and performed exactly as the current PMD (see items 3a to 3c).

5) The only similarities between relocated and original PMD mounting is that restarts require a sufficient inactive ignition dwell time.

6) No current OBD trouble codes, or least no idiot-light enable on the IP.

*** SIDE NOTE *** When placed under a deliberate linear load, slow and steady acceleration beneath the max governed RPM, the engine performs smoothly. High or sudden accelerations result in skips or misfiring more commonly seen on mistimed gassers. This symptom existed prior to the engine cutting out and still exists with #9 resistor and relocated PMD. This has also made cold-weather starts increasingly difficult.

QUESTION -- Should I look to the lift pump or fuel filter to see if inline pressure drops dramatically when the ignition is killed while the engine is warm or is the vehicle experiencing another electro-mechanical issue?

Please advise and best regards.

[JMJNet]

Welcome!!!

Your observation is good but you need to do what you are advising yourself:
QUESTION -- Should I look to the lift pump or fuel filter to see if inline pressure drops

Do that while the engine is running not engine off.
Make sure that LP is working and the fuel filter is not clogged.

I am not sure on your geographic location but if you are in TX or anywhere south, the inside cab ambient temperature can be hotter than outside the hood. The cooling fan is really not necessary even if it is outside the hood.

Unfortunately, to know if the PMD is the problem is to try a KNOWN WORKING PMD.

Also, make sure that the extension cable connecting the PMD is good.

As with any other diagnosis, make sure all the electricals are good: connectors, grounds, battery, cables, etc.

[WoddyBoyd]

JMJ,

Appreciate the swiftness of your response. My van is a situational work vehicle used just twice weekly or in emergency. The aforementioned symptoms have been superseded by today's events.

After sitting untouched for nearly 86 hours the engine fired on the first or second crank. This hasn't happened in 6 years. However, I could not reliably maintain consistent engine firing.

By allowing the throttle to fall back to idle firing was rough and eventually died. I noticed it would start again if I primed the fuel system with bursts of ignition on-off-on cycles. Lift pump activation can be audibly detected at ignition 'ON' and intermittent starting was virtually eliminated. Hence, the lift pump seems to be working to a large extent (repressurizing the fuel system).

Nonetheless, I could only get it to achieve a steady flow to the injection pump once after 30 minutes of exhaustive attempts. Not reliable enough to take upon the road.

I will seize upon a new fuel filter and will eagerly share the outcome.



Best regards.

[WoddyBoyd]

Replaced fuel filer and the following occurred:

1) Engine fires evenly at mid to high RPM, no skips or misfires as before: IMPROVEMENT.

2) Idle skips, misses and is uneven and will die at times, unlike prior behavior; DEGRADATION.

3) Performing IGNITION on-off-on cycles to prime fuel pressure insures nearly 100% engine start; IMPROVEMENT.

4) When fuel pressure is sufficiently primed, engine will start on minimal crank. Unlike prior start conditions: IMPROVEMENT.

What more can I tweak or adjust, short of taking it to a diesel mechanic, to remedy the unstable idle?

[JMJNet]

Did you bleed the air from top of fuel filter?

Seems like you may want to try a KNOWN WORKING PMD?

[JMJNet]

You have to bleed first and see if you are still having the rough start.

Make sure that LP is pumping not just purring.

[PrivatePilot]

Like JMJ is saying, you need to try a new PMD. Any PMD that was abused or exhibited any symptoms of failure previously can NEVER be trusted again, much less during diagnostic testing.

As for the relocation, where did you buy the cable, or Did you make it yourself?

And lastly, the lift pump simply making Noise does not mean it's working - my last one that died still "sounded" perfect but was pumping absolutely nothing. Pefrorm the lift pump tests and confirm if you are getting a strong steady flow of fuel at engine idle.