I've had this car since 90K miles (since 1995) and now it has 315K and I LOVE IT. Am 2nd owner. Actually, I drove it for about 40K before my wife took it over and now she loves it. It's had all it's scheduled maintenance religiously done since new.

For the last 10 years it's had a very intermittent turbo "cutting out" problem. The turbo will work great for weeks on end, then all of a sudden in the middle of a trip, it just doesn't kick in when you mash the pedal. Let it sit and cool and it works again. Sometimes it'll work for 30 minutes before cutting out again, and sometimes it'll work for 3 months without cutting out again. We've lived with this problem after spending $2.5K to track this down about 5 years ago at the dealership while they replaced everything including the ECU. It worked just long enough (a few months) that when the problem happened again, I didn't bother taking it back. They had a helluva time even reproducing the problem until I let the service manager commute in it for a week.

Now I'm writing this post because my wife reports that it now does it on almost any trip over 30 minutes and sometimes even on shorter trips. Bless her she says she can still drive it without the turbo just that's it's "annoying." (It would drive me nuts!).

One thing I've noticed about this problem over the years, that might be a clue, is that the car has always run like crap at high altitude. Wouldn't even start at 6000 ft. outside Las Vegas one time and had it towed down to 3000 feet where it started, but belched white smoke. At 3000-4000 feet it always starts rough and belches some white smoke for the first little while until warmed up. And, the turbo always goes out at high altitude and stays out, even when it cools off. When we used to drive it from Dallas to go skiing in New Mexico, the turbo has always gone out on the highway about half-way there, or at about 3000 feet altitude. We just don't take the car on road trips anymore and keep the car at 500 ft. altitude (Dallas) which is ok for a daily driver. I don't know how this altitude issue may related to the intermittent turbo, but it is worse at high altitude.

It'd be nice to get this resolved once and for all. I read a number of posts here about banjo bolts, ALDA's (is that like in "Alan?"), wastegate controllers, switchover valves, and various lines clogged with oil and/or soot. However most of these threads seem to relate to either total permanent loss of boost or reduced boost. My car works perfectly great most of the time and then mysteriously looses all boost in the middle driving for no apparent reason. My local mechanic who is otherwise awesome and has maintained this car the last 225K miles has not been able find the problem.

Oh, and we had the turbo replaced completely at 280K miles after it went out from my wife overheating the car with low coolant (no other damage, though).

Since this is purely intermittent and seems to happen when the car has been running awhile, is there something in particular I should be looking for? I was thinking that one of the oil lines to the turbo might be partially clogged causing an overheat shutdown of the turbo, but I'm just guessing.

Did I mention we love this car? We could buy something else, but see no reason why just yet.

In conventional applications, the turbo pressurizes the intake manifold and the fuel system provides fuel to match. The turbo is lmited in how much pressure it can produce. MB turned the tables in this car - the ECU decides how much pressure the turbo puts out. You can get into what inputs influence the ECU, how the ECU rationalizes these inputs and how the ECU effects its decisions. Many here have effectively cut the ECU out of the loop by retrofitting a conventional boost operated wastegate in place of the ECU controlled vacuum operated wastegate. This modification will cost you about two hours of dealer diagnostics.

My best guess from the information you provided is the temp(?) sensor that informs the ECU that the EGR system is operating is clogged with soot from 300K miles of proper EGR function. When the ECU thinks the EGR system is not functioning as it should, it penalizes the driver with reduced or no boost.

Take a look inside the engine at what EGR has done over the years and determine whether the environment is better servied by another engine or entire car in a wrecking yard and the resources expended to afford you a replacement, or a properly functioning car with EGR disabled.

Sixto
87 300D

Sixto is Correct

Please read this post:

Wastegate actuator swap for OM602.962?

It details how to free yourself FOREVER from the Tyranny of the EDS allowing
"NO BOOST".

__________________
'84 300SD sold
124.128

Could the EGR system partially clogged with soot also adversely affect the engine's ability to compensate for altitude? Or is that likely a whole separate problem? And can I just take it in to my mechanic and say "please clean out the soot and other glunk out of the EGR system"? It sounds like that would probably be advisable anyway after 313K miles. Also, do you think it might be a sticking wastegate, after all the turbo is all or nothing, it's not like it degrades. Maybe the heat is causing the wastegate to somehow stick open.

Thanks for the info on the wastegate conversion. Looks like a pretty complicated conversion, but maybe there's some more info out there now on the necessary bolt-on parts.

I've always hated excessive electronics on cars. Wish I still had my old Chevy Luv with the aftermarket Weber carb and NO emissions controls. I could do a full tuneup on that truck in about an hour flat and get that thing singing. Ran perfectly with nary a miss and I never had to bring it in for any service I couldn't do myself in over 10 years of owning it. Driving up to the mountains in New Mexico from the flatlands of Texas, altitude compensation was done by pulling over to the side of the road when the engine would start running a little rough and manually leaning the carb. But I digress....

Thanks for the help.

CI and Sixto are dead-on. I had intermittent boost in my 92 300D also, and this mod cured my problem entirely. It's not that difficult to do at all... well worth the effort. Good luck!

__________________
1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

What makes the mod appealing is while my guess cost you nothing, a shop will require a few hours to diagnose the problem then more hours to fix the problem. These engines have some built in diagnostics but they're a far cry from modern OBD. Having fixed that problem, the car continues to age and something else confuses the ECU. I'm sorry if this is a non-answer but it's likely to be the only answer you'll get unless you catch the attention of Steve, Donnie or one of the bonafide pros.

Sixto
87 300D

Yet another perspective: the mod Sixto refers to will cost you a hundred bucks or so, and will eliminate perhaps the greatest source of boost problems under your hood. This mod fixes problems that will occur eventually anyway, and you will have a much cleaner intake to show for it.

__________________
1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

Intermittent boost on a high mileage 2.5 is most likely bad throttle linkage microswitches. Located on the bracket that the throttle cable attaches to, the small grey microswitches tell the computer when the linkages are in the idle and full load positions. When they have a ton of miles the little roller followers that are part of the switches get flat spots and then don't actuate the switch. If the switch doesn't actuate, the computer still thinks its at idle and doesn't allow boost. Before you go modding everything give fixing the problem a try. Visually inspect the little rollers for flat spots and check the switches with an ohm meter.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

I just made it through all 8 pages of the mod post. That looks like the answer to my problems. Hooray! I'm not an advanced mechanic, but I think I might be able to do this mod.

So in the end, what were the parts required? Just the pressure actuated wastegate controller and the EGR cutoff plate and the various fittings and tubing, from what I can tell. Two questions: What was the final verdict on the best actuator to order? and Where was the best place to connect the actuator vacuum line? I think it was the predrilled hole in the intake next to the cruise control unit?

I am really grateful for this information. I love any solution that makes a system simpler and less prone to failure!

__________________
1972 280SE 4.5
1984 300SD, 250K
1991 300SE, 89K
1992 300D (330K miles when KIA'd by mother-in-law 8/10/09 - still sitting in my barn)
1994 E320, 109K
1995 E420, 60K
Proud Member of the Mercedes Benz Club of America - Idaho Chapter

Good point. In fact you can try something simpler if you have the discipline - route a vacuum line directly from the vacuum pump to the wastegate actuator. This will shut the wastegate flap when the engine is running. There will be no ability to regulate boost! If you go easy on the pedal as I imagine you do in the course of everyday driving, this will simulate the effect of cutting the ECU out of the equation.

Don't get me wrong, duxthe1 makes an excellent point. However, I've driven a 2.5 and I don't see any reason to apply more than half throttle in normal driving. The IP would never be under full load. Such being the case, I don't know how effective fixing bad microswitches can be at sorting your intermittent boost situation as I understand it. It's quite possible I don't have the facts straight.

Sixto
87 300D