I've got 1996 Ford Superduty F-450 with a 7.5L gasoline engine. It's an old Hiway Dept. 4 yard dump truck.

When first started, the engine will fast idle for a minute or so and then settle down to a smooth idle. Opening the throttle to 2500-3000 rpm is smooth as well.

After 5 minutes, the engine will no longer run smoothly at 2500-3000, it begins to sputter and cough at around 2000 rpm.

After 10 minutes, any rev up over 1500 rpm results in coughing and sputtering.

After 15 minutes, any rev over the base idle is rough.

After 20 minutes the engine will idle smoothly for 10 seconds or so and then have an 'episode' of rough running. The episodes become more and more common and last longer as time goes on.

By 25 minutes all smooth idling is gone and the engine will sputter and cough until it dies. Engine will not restart.

This entire scenario can be repeated if you leave everything to settle and cool for a couple of hours. Very reproducible.

Full disclosure: The power steering / hyrdoboost pump seems to be on its last leg as well, probably not related but I felt compelled to mention it just in case.

Help?

 

Moved to the correct forum. Welcome to FTE, by the way!

Sounds like a fuel issue to me. When was the fuel filter last changed? If not recently, that's your first step. If that doesn't fix it, you're going to need to pressure test the fuel system.

 

Sounds like to me you have a temp sensor problem.
This could be the air (IAT) or the water (ECT) temp sensor (not the gauge sender).


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I'll replace those and see what happens.

NICE graphic BTW!

 

The fuel filter was the first thing I changed.

The ECT and IAT sensors have now been replaced and the issue is unchanged.

Here is a 53sec video of it having several 'episodes' near the end of the 25 minute run time. It died about 2 minutes after this video ended. You can tell when it is having an episode by the exhaust sound and how much the engine shakes.


On the next run I will attach a fuel pressure gauge and see what is happening there.

 

Definitely check the fuel pressure. Have you tried checking code codes yet?

 

I went ahead and replaced the fuel pressure regulator and the coil (thinking the coil could be arcing when hot). No change in the issue.

Fuel pressure is around 35 and jumps to over 40 when I unplug the new regulator.

I shorted the ODBI connector and the only code it flashes is 654, which seems to be something about the tranny not being in Park? (it is definitely in Park)

 

The pickup coil in the distributor or the iCM is bad. Heat soak causes them to fail. Pull the codes I have had distributor codes stored and not turning on the cel. My bet would be the pick up coil in distributor.

 

That might mean your MLPS (manual lever position sensor) on the transmission is not adjusted properly, or is faulty.

I'm not sure the KOER tests will even run if the computer thinks the transmission is not in park.

 

Was this when it was parked and running bad?
Was this when it was on the road and running bad?

 

My guess is heat soak to some electrical component. Mine had a similar issue that I traced back to the PIP sensor in the distributor.

 

Just replaced the distributor and everything inside of it with a new one from AutoZone... no change in behavior.

Fuel pressure is about 32 when idling and at 40 when not running (no vacuum). It changes little (if at all) during the rough running episodes.

What else could I try?

Again, the issue is that the top RPM at which the engine will run without sputtering/coughing/dying starts off above 3000 rpm and steadily drops until after 25 minutes it drops below the base idle speed, at which point the engine dies and will not restart for a while.

 

When the engine dies and does not re-start check for spark. If it has none unplug the PCM computer and see if spark returns. If it does replace the PCM computer.

 

Hooked a timing light to the coil -> distributor lead. Ran until it died... turned the key to start and the timing light indicated steady spark.

I could swear the thing detonates occasionally while having an episode.

For the first time ever I got a new CEL blink code: 327 (EGR?) I know an EGR can make an engine run rough, but would it stop an engine?

 

Now you need to put a fuel pressure gauge on it and run it until it dies and see what happens to the fuel pressure when it dies.
When it dies short pin #6 of the self-test plug to ground and see if the fuel pressure comes back if it had a lack of fuel pressure when it died.