84 300SD power lunging/ lack of power - PeachParts Mercedes-Benz Forum

axm1955 · #1 03-08-2009, 09:48 PM

OK Guys,
Sorry for the bad title, but here is the issue. My beloved 84 300Sd about two weeks ago developed a bad, bad power surging/lunging behavior. What I mean by that is that as you go down the road at a steady throttle position the engine will throttle up or down all by itself without any changes in the throttle linkage position. Additionally it has no power to speak of during take off.

So being a smart fellow I presumed: a) water in fuel and or b) clogged filters. Well Saturday morning I replaced both primary and secondary filters. No change.
Today I pulled teh tank, checked the fuel in the tank, cleaned the strainer (not very dirty at all for almost 600K miles) flushed the tank. Still no change....?????

What could possibly be my issue?

Any pointers are greatly appreciated.

Adriano

Ian White · #2 03-08-2009, 09:51 PM

Mine was doing basically the same thing, so I changed filters, cleaned strainer, and still no change. I never have figured this out. My prblem is not bad at all just a tiny annoyance.

tankowner · #3 03-08-2009, 09:59 PM

Does the engine ever throttle up without having throttled down first? Asked another way, the "throttling up" isn't just the resumption of power after it has "throttled down" is it? Are you actually accelerating beyond the holding speed? Or is it possible that you might just be getting air in the system?

axm1955 · #4 03-08-2009, 10:11 PM

Thanks guys.
No I hold the throddle steady and it goes up and down by itself. Tell more about air in system. Hoses from tank to primary and seconday show no leak what ever......

Would air in fuel system cause lack of power and said surging?

Thanks.

tankowner · #5 03-08-2009, 10:55 PM

Quote:

Originally Posted by axm1955

Thanks guys.
No I hold the throddle steady and it goes up and down by itself. Tell more about air in system. Hoses from tank to primary and seconday show no leak what ever......

Would air in fuel system cause lack of power and said surging?

Thanks.

If air is leaking into your fuel system somewhere, then it is just going to interupt the continual flow of fuel - so the engine will hesitate according to the amount of air that is being drawn in. A jerky pattern of hesitation followed by resumed normal engine output suggests that the engine is being starved of fuel - if your filters are good and tank strainer clean, air in the lines is certainly a possibility. Check your fuel lines and o-rings over good. However, if your engine is accelerating or surging beyond where you are holding it, then this is not your problem.

#6 03-08-2009, 11:36 PM

As a simple first advice, maybe your air filter is clogged? I know you may think "DUH!", but in my years in the Army, that seemed to ALWAYS be a Diesel problem, "Man, my Hmmwv is dog slow, or MAAN, my Bradley wont get up to speed, then all the sudden it surges" These problems were always remedied with a new air filter, or fuel filter. I was a Diesel Mech. for 3 yrs in the Army, working on Duece and a halfs, hummers, Bradleys, M88's, 5 tons, APC's.. so on, anyways, they only other thing I could think of is your governor is acting up?

bgkast · #7 03-09-2009, 12:07 AM

Try bypassing the switchover valve (it is mounted on the fire wall and a small line runs from the rear of the intake manifold to it, then to the ALDA on top of the injection pump.)

If this solves the problem your turbo is either overboosting or the pressure switch on the top of th intake manifold that controls the switchover valve is bad. Tee a boost gauge into the line and watch it under heavy acceleration. It should top out at 12-13 psi.

cmbdiesel · #8 03-09-2009, 12:29 AM

Quote:

Originally Posted by bgkast

Try bypassing the switchover valve (it is mounted on the fire wall and a small line runs from the rear of the intake manifold to it, then to the ALDA on top of the injection pump.)

If this solves the problem your turbo is either overboosting or the pressure switch on the top of th intake manifold that controls the switchover valve is bad. Tee a boost gauge into the line and watch it under heavy acceleration. It should top out at 12-13 psi.

Hopefully it is not overboosting, otherwise bypassing the overboost solenoid could be dangerous.
If you have a boost gauge, tee it in before you bypass the solenoid on the firewall.
My 83 was doing the same thing, and it seems like it was a couple things all conspiring. Have you cleaned the banjo bolt lately? How about the pressure sensor? I would do both of those things while I was under there bypassing the solenoid and teeing in my boost gauge.

bgkast · #9 03-09-2009, 12:38 AM

If the turbo is indeed overboosting bypassing the switchover valve for testing purposes fo a few miles will not cause any harm.

tankowner · #10 03-09-2009, 12:52 AM

You guys and your turbos - just one more thing to fix. (Actually I am a little jealous, my next diesel will be a turbo.)