96 7.3 idles rough at times and "shuts down hard"

[jemorgan3]

just bought my first diesel and have been catching up on routine maintenance the previous owner didn't do.. its a 96 7.3 5 speed.

first problem, stumbling at idle: the truck will idle fine for a few minutes, then it will start stumbling and act like its going to stall, but it never does. it will correct itself, then start stumbling again at random intervals. if you touch the gas it smooths out and picks up rpms without skipping a beat and never stumbles when you place a load on it.

second problem: when you shut the engine off, it is a lot louder than any other diesel ive heard, ending with a loud bang, and sounds like its discharging air or sucking air at some point under the hood. i know a diesel engine can,, well, "diesel" on shutdown but this is a little exaggerated.

we chased a fuel leak around the valley of the engine, replacing all the short fuel lines, the mechanical fuel pump and changing the fuel filter in the process. the leak was finally found to be the banjo bolt on the back of the fuel pump, fixed.

[Patrick Feeley]

Many times those symptoms are due to a bad Dual Mass Flywheel (DMFW). They were problematic on the OBS trucks. You can check yours (to see if it still has one too) by removing the flywheel inspection cover. If the flywheel has dampening springs in the flywheel itself, and not the more "normal" type in the clutch disk, its probably a dual mass. See if all the sprngs and plastic retainers are there. You should be able to counter-rotate the two masses against each other slightly. Measure the distance of the counter-rotation ... anything more than about 5/8" measured at the flywheel rim and it's bad. You can also run it with the cover off and check to see if both masses are running true and not wobbling. If it is bad, you can opt to replace it with a Single Mass FlyWheel and Clutch kit which is actually a lot cheaper ($500-$600) than just the stock flywheel (used to be about $1100-$1200 just for that!). The SMFW will have a lighter pedal action, you can resurface it, and its more reliable. The only drawback is some increased noise (gear rollover noise) when lugging it under decelleration and slow speeds and RPMs. Most folks are OK with that. Cheers!

[catlinB]

do a search for OBS chat and post your problem in there.... TONS of knowledge in there.

[jemorgan3]

i dont think its the clutch, it only happens when its in nuetral and idling and it is on and off, one second its smooth and one second its rough as hell, then smooth again a few minutes later. the truck shifts really really well.

[Patrick Feeley]

Notice I really didn't mention the clutch. Trust me on this and go check yours out so at least you know what you have (DMFW or SMFW conversion) and its condition. If you do have a factory DMFW you will deal with it someday. A bad one can cause the truck to fail injection contribution tests, mimick a miss, and if ignored for too long can lead to a cracked transmission case. I've only got 29 posts on this forum, but probably over 10,000 combined on the other ones I'm on (so it's not like I haven't been around these things for a bit ). Cheers!

[jemorgan3]

checked my clutch as you recommended and it already has a single mass clutch and it looks to be in good condition. It seems that it is a problem within the fuel system, rough stumbling idle that comes and goes when there is no load on the engine. even the smallest hint of pressing the throttle and the stumble goes away every time and the engine rev's without hesitation. never stumbles at any time other than idling. depressing the clutch, releasing the clutch, leaving it in gear or taking it out of gear has no effect on the stumble. if it is stumbling i can press and release the clutch and it does not change the stumble at all, the only thing that clears it is pressing the gas slightly. someone mentioned an IPR... what is that?

[jemorgan3]

to add one more thing,,, i bought the truck last january right before being deployed to iraq. didnt drive it before leaving, and it wasnt driven regularly until i came home last week. just this week we have replaced the fuel filter, lift pump and all the fuel lines in the engine valley chasing a fuel leak (ended up being the lift pump).

[greasyF250]

Check codes? Throttle position sensor?

[jemorgan3]

No codes at all...

[Patrick Feeley]

The IPR is the Injection Pressure Regulator control valve (solenoid operated valve). Based on what the Injection Control Pressure (ICP) Sensor is telling the computer, the IPR is controlled to get the High Pressure Oil to be in the correct range for operation. It is located on the rear of the High Pressure Oil Pump (HPOP), which is located on the back of the HPOP Reservoir on the front top of the engine. The IPR is down towards the engine valley in front of the fuel bowl. It has a two-wire connection. Make sure that the "tin" nut that holds the IPR solenoid coil on the IPR body is snug. Also make a thorough inspection of the IPR wiring (it is part of the fuel bowl wiring harness and can get messed up due to leaking diesel fuel ... the wiring insulation turns to goo causing shorts and other issues). Cheers!