87-300D Turbo (603.96) (185K miles. owned since 10/96)

Over the last year or so, the thrust so to speak has been dwindling on the high end, I could drive 100mph+ sustained or pass with rocket like performance with no trouble, and as time has past over the last 12 months or so, I notice a "missing" like behavior early on during high load passing where acceleration was very brisk, and then I felt a missing like hesitation, though very slight as rpm and load climbed. This has given way to deteriorating performance on the high to midrange now, were 65 mph is a chore.

Interesting symptoms,

If you hold the throttle to the floor, it will go to the governor stop at 5250 rpm for about 3-5 seconds and decrease to around 3500-rpm and thats it. The colder the engine, the quicker the fade away is. Initial breakaway is brisk, but gives way to an underpowered situation.

I have boost. So far tested items inclued; system vacuum tight at about 20 inches, the EGR removed and checked, air recirculation valve on the turbo checked, turbo spins free, the mass airflow door hinges freely against it's spring, both switch/change over valves (Y29 & Y30 functionally tested and swapped) over boost switch checks open and has no effect on performance if it is connected or disconnected, as i have no way of testing it's operating pressure and with it disconnect, this would presume normal function. All pressure lines tested tight with no drop in pressure, some rubber connections replaced.

So, I have three guesses left, first one:

Has anyone had a main fuel filter (spin-on) gradually clog as it seems that given enough "recovery", initial throttle responses is normal and gives way to the underpowered situation? The clear filter has good flow through it. I would have changed it already but I'm out of stock and World Pac is closed till the 26th.

Along these lines, would a tired primer/lift/lowpressure fuel pump exhibit these symptoms?

Lastly, does the ALDA ever fail/stick/require a once over, etc? I am reluctant to mutts with this as it was working normally and as I understand it's function, it provides reference infromation and it works or it doesn't. Or does it???

Feedback would be really appreciated, I feel like Doc and Johnny on the tonight show with stump the band as I have beaten this to death and I'm still missing the fix.
-Mark

Sorry Brian, good question the trap has been gone since 98 and I have the new catalyst style installed parts free of charge by MB along with a new turbo as well. The head failed at 103K, in '97 trap still installed, overhauled replacement installed and followed by the complete exhaust and turbo one year later. Post exhaust replacement, performance was perfect until 12 months back as i mention above.

Does this vehicle have a catalyst in the exhaust system?

Does it still have the trap oxidizer in place, by chance?

I believe it has a catalyst ahead of the front muffler.

Yep, that's what I thought.

You have the signs of an exhaust restriction. Disconnect the downpipe from the turbo exit and run the engine without the exhaust connected. See if the problem is eliminated.

Report back, either way.

Edited:

If the catalyst is only seven years old, it's less likely to be plugged, but not impossible.

Definitely change the secondary fuel filter before bothering with the exhaust. That's a simple item that you can immediately rule out.

Sounds like it might be the spin on fuel filter that is clogging up and causing starvation. Also look at your primary filter for the black crap that tells of tank critters growing.

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1984 300D
1985 300CD
1980 300SD
1993 SL500

The primary fuel screen is clean and flowing nicely, which also frustrates me in that if the spin on or secondary is clogging, how did it get past the primary, unless of course we are talking about fine particulate contamination obviously sub 100 micron sized particles will flow through the screen and not the filter.

With regard to catalyst being clogged, and I don't doubt it could be, I would think the cold to warm transition in operation temperature and improvement in performance, i would guess that an exhaust restriction issue change regardless of operating temp but fuel would as a cold engine requires more fuel than a warm one, no?? Also, is the catalyst a "burn clean" device as the trap was?

Be glad if it is the secondary filter.
Bad diesel is becoming more common every day.
I have learned the hard way.
My secondary filter is averaging seven to ten months, before performance loss alerts me to change it.

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Exactly. Fine particles go right by the primary and clog the secondary.

Change the secondary and let's see what happens. It's cheap and fast.

[QUOTE=markg612]87-300D Turbo (603.96) (185K miles. owned since 10/96)

Over the last year or so, the thrust so to speak has been dwindling on the high end, I could drive 100mph+ sustained or pass with rocket like performance with no trouble, and as time has past over the last 12 months or so, I notice a "missing" like behavior early on during high load passing where acceleration was very brisk, and then I felt a missing like hesitation, though very slight as rpm and load climbed. This has given way to deteriorating performance on the high to midrange now, were 65 mph is a chore.

QUOTE]


Mark,

'Had similar experience recently. The problem was the secondary filter - and at only 20K miles since replacement. The pre-filter was clear.

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Tyler, TX(Anchorage, AK summers)
'90 560 SEC (original owner)
'82 300 SD (original owner)