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Old 03-13-2010, 03:04 PM
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Join Date: Mar 2006
Posts: 545
Intruiging low power/surging power in "mid" range

I have an interesting low power issue that's emerged on my 1983 30SD with about 100 K miles on it.

Symptoms -

1. starts from cold and idles either cold or warm happily
2. Goes like a bomb from a standing start to 60mph - can keep rpms around 4,000, lots of power, no issues
3. BUT - when trying to accelerate when rpms around 1800 to 2000 or when driving on a level road at about 45 to 50 mph car feels very low on power and get "surging power" feeling. Exactly same feeling as when trying to accelerate hard with a gunked up secondary fuel filter.

The LAST time I had issues like this the problem got worse and worse as the power demand went up and it was easily solved with a new secondary fuel filter. This time I'm much less certain what the problems are.

Here's the state of the car

- valve lash adjusted about 3K miles ago
- rebuilt injectors installed about 5K miles ago
- new primary and secondary fuel filters installed about 3K miles ago
- throttle linkage never adjusted to my knowledge and probably needs it
- very very slight vacuum in diesel tank when you open cap when the car is cold
- original primer pump, spews diesel when you pump it
- banjo bolt in manifold and vacuum switchover valve on bulkhead cleaned about 10 K miles ago
- air filter is clean
- no apparent transmission slip or issues

What I'm planning to do

1. Take it for a drive with the fuel cap off and see if the issue goes away. If it does clean breather on fuel tank.

2. Follow conventional wisdom and do diesel purge, replace both fuel filters and primer pump at same time.

3. Check engine compartment for any obvious disconnected vacuum lines

4. Put her up on ramps and check fuel line clamps and connections underneath for possible problems

5. Adjust throttle linkage

The fact that I have power from a standing start worries me. I think I'm on the wrong track. Anyone got any throughts?

Thanks guys

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Old 03-13-2010, 04:24 PM
Diesel911's Avatar
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Join Date: Sep 2007
Location: Long Beach,CA
Posts: 46,202
Most likely not the problem but your Hand Primer is obviously "on the way out".

Since the problem is happening about the time the Turbo kicks in I would check/clean the banjo bolt in manifold and vacuum switchover valve on bulkhead again.

If you have an EGR on your Car you night try disconnecting and plugging off the Vacuum line and see if itthat changes anything.
84 300D, 82 Volvo 244Gl Diesel
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Old 03-13-2010, 04:41 PM
junqueyardjim's Avatar
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Join Date: Apr 2005
Location: Cicero, Hamilton County, Indiana about 30 miles north of downtown Indianapolis
Posts: 2,623
Has good power at 60

Well, that does not say too much. Will it power on up past 60 mph. Don't want you to get a ticket, but 60 is just easy cruising. Sounds to me like it might be sucking a little air into the fuel system, and you indicated the pump was leaking a little. Instead of throwing everything at it, like you said you were going to, why don't you just replace that hand pump and the primary right by it and then tell us how it runs.
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1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

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Old 03-13-2010, 06:29 PM
tbomachines's Avatar
Join Date: Mar 2009
Location: Philadelphia
Posts: 7,145
X3 on the primer pump replacement. Even if it isn't the source of THIS problem, you'll need to replace it at some point anyways so why not now. If your EGR is hooked up you should clean out the banjo bolt again since the EGR will gunk it right back up easily in 10k miles.
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Old 03-13-2010, 11:14 PM
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Join Date: Mar 2006
Posts: 545
Status report - progress :)

It was a beautiful afternoon in the Okanagan today so I ignored the 25cm of new snow on our local ski hill, Silver Star, and tackled the car for a couple of hours. This is what I did/fixed:

- removed primary fuel filter, inspected it - clean as a whistle so re-used it
- replaced the old primer pump with one of the new black Bosch ones
- replaced secondary (canister) fuel filter and did a lazy purge by filling the filter with Diesel Purge
- removed banjo bolt - it was half gunked up - so cleaned it
- removed vac switchover valve between ALDA and banjo bolt - confirmed it was clear and replaced it
- the big "aha" was I found the vac line that heads from the cam valve assy on top of the valve cover to the EGR (via a sensor on the block) was pushed into the wrong hole so one of the vac lines in the cam assy was open to atmosphere.
- threw in the spring oil & filter change as well

Bottom line - vastly improved power and smoother shifting as well, no doubt the vac correction helped there. Got the car up to 145km/h or 90 mph on a nearby straight - still some power variation but it seems to be smoothing out.

It's had a lot of short trips in the past couple of months. I'll take it for a much longer run and report back.

And I have ordered an EGR blanking plate kit from Brian Carlton so I can get rid of the thing altogether!

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