Stumble returning to idle

[djustin973]

I would first like to congratulate my car for making it through an entire winter without making me do any repairs! She's finally learned how much I hate doing repairs in sub-zero temperatures!

That aside I'm trying to hunt down the cause of an ongoing annoyance. My idle is very smooth and higher rpms are also fine, however when I let off the gas the car stumbles for a few seconds before returning to idle. At any stop light or even in traffic letting off the gas causes the rpms to drop below idle almost stalling the engine and then it rises back up and idles normally. To give a place to start the following have already been done:

• Periodic injector cleaning
• New injector tips (calibrated)
• Overboost cleaned (banjo fitting and line)
• Clean air filter
• Clean fuel filters (Including tank screen)
• Injection pump o-rings and copper washers replaced

Any help is as always greatly appreciated as I would really like to see her running as intended!

Thanks!

[vstech]

air leak into the fuel system.
I'd start by pressurizing the fuel system, and crawling every inch of the lines, and finding where fuel is seeping out.
I use a shop vac sealed into the fuel filler neck to do this.

[djustin973]

Wouldn't an air leak cause a constant rough idle?

[vstech]

air leaks do all kinds of weird things. depends on where the leak is, and how bad...

[rscurtis]

The rack damper could need adjustment also.

[Manual Life]

Originally Posted by rscurtis

The rack damper could need adjustment also.

Agreed, and Vstech, the shop vac idea is excellent. It would never have occurred to me.

[Biodiesel300TD]

Also check, clean, and lube all your throttle linkages. If something is worn or stiff it could be causing the linkages to catch in funny places and make the engine stumble.

[barry123400]

This is one I really did not want to post on. It could be the injection pump. You are not running wvo or are you?

Most of the posts that the owners suspect it may be never usually are. Things I would look for first.

Fuel pressure is adaquate and sustained in the base of the injection pump when returning to idle. Takes a meter to verify. A rough guess would be if the relief valve is constantly overflowing but then you have to assume the valve is functioning properly. Good place to look for air ingressing into the system earlier as well. The relief overflow line should only be releasing fuel.

If those things check out well I would internally soak the injection pump next. Using a good solvent as some crud may be interfering with the govenor.
Again it is semi important to mention if you are using wvo or not. I would also check for water in the fuel tank before going very far. This is unlikely but something unusual is going on. As mentioned earlier air in the fuel can do strange things as well so it may be the best place to start.

One other last thought was if the shutoff on the injection pump might be contributing but perhaps you could tie it open temporarily to eliminate it being partially activated somehow. Or remove the vacuum line from it. Another long shot was if the engine was dropping a cylinder when it drops to idle and picks it up again. I assume the valves clearances have been checked reciently. The hot valve stem would tend to start to cool and shrink as soon as you were at idle again. This might enable the valve to seal well enough to let the cylinder come back on line again.

[djustin973]

Originally Posted by rscurtis

The rack damper could need adjustment also.

My understanding was that the 603 doesn't have a rack damper? Am I wrong?

[aaa]

Cat-related?

[vollpreuss]

The 603 doesn't have a rack dampner like the 617. Check the trimming plug by the fuse box to see what's it's set at. You may be able to mitigate the problem by bumping the rpm up. If that doesn't work there is a mechanical adjustment on the IP. Here's the link that includes the adjustment procedure - setting the idel on 300SDL?. The word idle is spelled idel so this is tough to find via search. Fortunately I often type in a dyslexic manner.