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  #1  
Old 11-08-2013, 10:24 PM
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Join Date: Jul 2007
Location: Cambridge, MA
Posts: 56
'87 300TD manifold sensor craziness

Hi All,

This is way beyond my skills, I thank you in advance for your input. My '87 300TD (#14 head, yes!) runs well except for one big problem - lately when on the interstate (for example) and I push down hard on the accelerator, the vehicle won't really accelerate - seems like it wants to, then backs off, then feeds more diesel, than backs off, over and over again until I back off the gas pedal. Here's the interesting part: if I yank the manifold sensor wire from the manifold and leave it dangling, the car accelerates normally, even quickly. Sometimes a little white cloud of smoke (even when warm) from tailpipe when flooring it at, say, 75 mph (kickdown switch triggers downshift, cue small amount of white smoke and the car accelerates quickly). When I reconnect the manifold sensor wire - problem resumes. What???

Everything else largely normal, all filters new, did a diesel purge a while back, getting solid 30mpg @ 75 mph on the interstate, probably about 23-26 mpg around town depending.

I do occasionally notice the dome light doing a 2-flicker, 1-flicker (on the seatbelt warning light above), repeat, while my (new) glow plugs are heating, but apart from that I don't notice anything out of the ordinary.

Thanks!

Josh

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Cantabridgian

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1987 300TD Black/Palomino 120k on donor engine (thank you CS), 305k on everything else

MISSED 1985 300CD Blue/Palomino 250k lovely but had to part with it; now wife has Jetta TDI, you know how it goes
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  #2  
Old 11-09-2013, 04:19 AM
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smoke gets in your eyes
 
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,806
What is a manifold sensor wire? If you mean the overboost switch, hook up a boost gauge to see how much boost registers before the engine backs off. If the engine backs off at or above 15 psi, overboost is performing as expected and you have to adjust the wastegate and your expectations. If it's backing off at lower boost, you might need a new overboost switch.

Sixto
87 300D
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  #3  
Old 11-09-2013, 09:18 AM
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Join Date: Jul 2007
Location: Cambridge, MA
Posts: 56
that's already tremendously helpful, Sixto, thank you. Is the overboost sensor that giant knot sticking out of my intake manifold on the very top of the engine? That's the sensor I was trying to describe (poorly) in my post. It has one single wire connector sticking out pointing toward the driver's side of the car.
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Cantabridgian

------------------------------------------------------------

1987 300TD Black/Palomino 120k on donor engine (thank you CS), 305k on everything else

MISSED 1985 300CD Blue/Palomino 250k lovely but had to part with it; now wife has Jetta TDI, you know how it goes
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  #4  
Old 11-09-2013, 09:55 AM
Rockyriver's Avatar
1985 300TD
 
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Location: Harrisburg, N.C.
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A picture is worth a thousand words m
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  #5  
Old 11-09-2013, 10:27 AM
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The dome light module are a known issue. Mine used to strobe in the door open on position.
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  #6  
Old 11-09-2013, 02:17 PM
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smoke gets in your eyes
 
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,806
The overboost switch the hexagonal thing with the only wire attached to the manifold.

Sixto
87 300D
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  #7  
Old 11-09-2013, 02:26 PM
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Join Date: Jul 2007
Location: Cambridge, MA
Posts: 56
perfect...yeah, got the hexagonal thing attached to the manifold with the wire sticking out. apologies for my lack of good terminology! I'll take it off and clean it out before ordering a new one, in case it's just all sooty.

So...what was the consensus about the disco dome light effect, in fact? I think mine (during a full glow plug cycle, I wait until I hear the relay "thunk") does two blinks on the dome, then one blink on the seatbelt warning, then repeat until I actually engage the starter motor (after I hear the "thunk"). And it doesn't do it every single time...

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Cantabridgian

------------------------------------------------------------

1987 300TD Black/Palomino 120k on donor engine (thank you CS), 305k on everything else

MISSED 1985 300CD Blue/Palomino 250k lovely but had to part with it; now wife has Jetta TDI, you know how it goes
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