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  #1  
Old 11-06-2016, 07:00 PM
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83 300td large power loss ???

car has been running great for years all at once last week I got a massive power drop off. car still starts, runs, idles perfect just its like I'm only getting half throttle.

I change 2 fuel filters and cleaned the k&n air filter. also I installed a boost gauge to see whats going on with that. showing 9 psi at full throttle 3000 rpm. I think it should show 12? car idles and runs very smooth just takes forever to accelerate. like half or so throttle.

anyone have any advice on where to start diagnosing? also is there a way to do a compression test?

thanks as always for any help guys!!

you guys have been a HUGE help in my travels with these cars

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  #2  
Old 11-06-2016, 07:27 PM
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Are you sure you're not only getting half throttle? If the bearing in the firewall took a dump or you have a linkage that's worked loose, you really could only have half-throttle with the pedal to the floor. Have a helper push the pedal to the floor (engine off please) and make sure the linkage is maxing out the fuel rack lever on the injection pump.

If you aren't getting to RPM, you won't be getting to boost pressure either.
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Old 11-06-2016, 07:42 PM
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I just did. its not going quite to full its looks like its going 7/8s or so almost full. if she floors it I can pull the linkage and get another 1/8 inch or so think that could be it?
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Old 11-06-2016, 07:43 PM
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also turbo seems to spin freely and does not really jiggle around like the bearings are pretty good
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  #5  
Old 11-06-2016, 08:34 PM
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Check the small vacuum hose on the top of the injection pump toward the back. My 85 TD did this because the hose was porous.
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Old 11-06-2016, 09:21 PM
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Ill check in the morning
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Old 11-06-2016, 09:29 PM
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Not the one for the shutoff on the back of the pump; the one on the top near the back.

Good luck!!!
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Old 11-06-2016, 09:52 PM
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Does the TD have a switchover valve like my '81 300SD? Sounds like the classical bad switchover valve problem.
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  #9  
Old 11-06-2016, 10:47 PM
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I don't know what the switchover is. ??
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Old 11-06-2016, 11:05 PM
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It is a gizmo that works with the ALDA and prevents it from overboost. It sits on the driver's side firewall and has plastic hoses connected with one nipple vented to the air. If it is bad and you have lost power, it is often the problem.

My 81 300SD lost all performance and would not go over about 30mph. I replaced it and the power magically reappeared. Search for switchover valve on the forum.

Will try to find a picture. Found one. This is the new style; note one nipple does not have a plastic hose, is vented to the air. (If you get the hoses mixed up, it will not give your performance back so take a pic before you remove the hoses.) As I remember this new one was installed completely upside down from my original switchover valve; not sure why they changed the design. But it really works great now and I have all the original pep.
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83 300td large power loss ???-switchover-valve-1981-300sd_2016.11.06.jpg  

Last edited by tyl604; 11-06-2016 at 11:22 PM.
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Old 11-07-2016, 09:58 AM
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Is there a way to test the switchover?

Or service it?
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  #12  
Old 11-07-2016, 11:03 AM
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changed a couple of vacuum lines that looked so so. not really thinking they were bad tho. car has a lot of new lines on it. I pulled the switchover off the fire wall and found the line on the bottom blocked with a bolt stuck in the rubber. what is this supposed to be hooded up to?
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  #13  
Old 11-07-2016, 11:48 AM
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On mine, the way you're holding it, the tube right next to the bolt is open, the line with the bolt in it goes to the back of the intake manifold.

So that's interesting. If the ALDA is still operational then that would definitely kill your acceleration. However, that's not something that would happen suddenly, unless someone pranked you or a bolt bounced up just right and lodged itself in the line. Seems strange.

You can easily test this by removing the ALDA from the top of the injection pump (takes some big wrenches is all), and going for a drive. It should damn near peel out (though it won't, sadly).

-Rog
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Old 11-07-2016, 11:52 AM
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Some say the switchover gets clogged with carbon and it can be cleaned out; I do not know. However it is only about $25-30, at least when I bought it some five years ago.

Since this problem just occurred, I assume that you did not recently stick that stopper in the hose. That said I am not sure where it goes; may be part of the EGR system which many folks have stoppered off long ago. Not sure.

Cannot tell from your picture but the new switchover valve has one hose and the elec connection on the bottom. On the top is an L set of nipples and the nipple pointed straight up is to be left open.

Suggest you buy a new switchover valve and make sure you match up the proper hoses. I got mine mixed up and had zero power; then when I got them matched up properly, all the power came right back. Very noticeable difference when I drove the SD.

Good luck.

Looking again I think you have the new generation switchover valve. Make sure the top nipple is open. It looks to have a one inch black rubber hose on it but that could be open. Since this is a new problem, I must assume you did nothing to the old switchover valve before this loss of power happened and that none of the hoses fell off.
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  #15  
Old 11-07-2016, 11:55 AM
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that cap that's on the end of the switchover seams to be some kind of vented cover. ill try taking it off also but yes this has been set up like this for some time I assume

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