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Old 01-06-2004, 09:51 PM
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Join Date: Feb 2003
Location: New York
Posts: 161
Question Highway cruise fluctuations 85 300TDT

I've been noticing a uneveness when cruising at 55-60mph. It seems like I have a faulty filter. I changed both fuel filters 3k ago. Does anyone have any experience with the tank filter on a 85300td. Could that be clogged? Also could it be my fuel pump or maybe the injector pump? Is there any test? Just don't want to suddenly die on the road. Approaching 400k miles probably by summer so I have some concerns. Any input appreciated. Thanks
85 300TDT 456,000 orig. miles
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Old 01-06-2004, 09:57 PM
Registered Diesel Burner
Join Date: Aug 2001
Location: Northern Virginia
Posts: 2,911
Also take the fuel primer pump into consideration if you have an original. Check to see that the pump handle is screwed down tight as a minimum. Look for diesel weepage in that area.

Look for air bubbles entering the plastic fuel pre-filter - that indicates a leaking supply line between it and the tank, possibly the one right there at the pre-filter. Also look for foreign matter like specs of algae/fungus. Your spin-on filter is probably not clogged with tank scum unless the pre-filter is getting clogged too.

I did die on the road once so far and it was the fuel primer pump leaking air into the system - the handle had come loose running down the road.

1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles
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Old 01-06-2004, 10:37 PM
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Join Date: Feb 2003
Location: New York
Posts: 161
I'll take a look at the pre filter. As far as the primer pump, that was changed along with the folter 3k ago. Thanks, I'll also look for algae.
85 300TDT 456,000 orig. miles
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Old 01-07-2004, 12:54 AM
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Join Date: Dec 2000
Location: Republique de Banana
Posts: 3,496
When mine had cruise fluctuations, I found that the shaft on top of the valve cover where the linkage from the cruise actuator pivots was loose and wobling in it's bracket. Fixed with JB weld.

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg

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