I still can't find the source of low power under load

[jbach36]

1991 300d, 257k

The car doesn't have the proper amount of power to get up hills or to even accelerate from say 15 mph to 35 mph on a flat area. Before and after that, the car does ok.

Previously when this problem would come up every 7 years or so, the fix was to run Liqui Molly diesel purge to clean the injectors. I just did a diesel purge maybe a year ago and was surprised that I needed another one so soon, but this time doing another diesel purge hasn't really helped. It helped just a touch, but not much.

After testing the car for a day after assessing the effectiveness or lack thereof of the diesel purge, the next day I replaced the pre-filter and main fuel filter, and that hasn't helped either. I'm going to get some of that blue stuff you put in the fuel tank today, to clear out any potential algae in the tank filter but I highly doubt that's the problem.

Any other thoughts of anything I'm missing? I don't think the crossover tube is clogged with any gel; I checked that a year ago and it was as clean as a whistle.

I'm thinking maybe the injectors aren't popping at the right psi or aren't spraying well enough. Would doing a pop test on them and watching the spray pattern help? I had the injectors taken out and cleaned maybe just 4 years ago.

[Diesel911]

I don't know what 'blue stuff' is. But Startron has an enzyme in it that it claims breaks up particles in the tank small enough that it can pass through even the spin on fuel filter. Perhaps cleaning off the fuel tank screen of crap.

When there is stuff growing in the fuel tank typically the pre-filter has visible greenish black or rusty stuff in it.

Some members had had to rebuild their fuel supply/lift pump, and they had similar symptoms.

[jdulle]

I remember some people saying that they switched the two fuel hoses coming from the tank, in order to check if that filter is the problem. How is your turbo boosting? Are your pressure lines in good shape to the Alda/injection pump?

[rcnrichmond]

Before you decide to remove your fuel tank for an intense cleaning or replace your fuel lines, try a fuel can up front.

Also when blowing out the fuel line with your air hose, remember to blow from both directions. I remember a Nissan truck that had a fuel issue when accelerating, more accurately lack of. When blowing back to front nothing exciting. But front to back a surprising amount of silt came out. And that solved the problems.

[Diesel911]

Originally Posted by jdulle

I remember some people saying that they switched the two fuel hoses coming from the tank, in order to check if that filter is the problem. How is your turbo boosting? Are your pressure lines in good shape to the Alda/injection pump?

That works on a W123 not sure on other models.

[Diesel911]

Don't do this with a full tank. Try driving without the fuel fill cap. If that makes a difference you have an issue with your fuel tank venting system.

[JHZR2]

Have you ever serviced injectors besides running diesel purge? If not, I’d have Greazzer service them.

Regardless of cause, it’s prudent.

Do you know your compression? Any issues ever with cold starting?

Are you sure your vacuum actuated wastegate is working correct? It’s unlike other engines. And if you don’t get boost you won’t get fuel, then you’ll lack performance.

[Diesel911]

What is unfortunate is that while the factory service manual would tell the specs on fuel pressure or even the vacuum on the fuel supply lift pump. In particular there is no easy way to hook up a fuel pressure gauge and on mine when I did that even a liquid filled and dampened gauge needle osculates to fast to get an accurate reading.

I have never tried a check on the fuel inlet vacuum.

Over time I have seen the trend from me members wanting repair manuals to them totally relying on information on the forums. The fact is to trouble shoot some things you need information from both.

That makes it difficult to accurately troubleshoot.

[Mxfrank]

Originally Posted by jbach36

1991 300d, 257k

The car doesn't have the proper amount of power to get up hills or to even accelerate from say 15 mph to 35 mph on a flat area. Before and after that, the car does ok.

I'm pretty sure the '91 still had the same overboost control as my '87. Try this:




https://www.peachparts.com/shopforum/diesel-discussion/385395-sluggish-87-300d.html#8

[JHZR2]

Originally Posted by Mxfrank

I'm pretty sure the '91 still had the same overboost control as my '87. Try this:




https://www.peachparts.com/shopforum/diesel-discussion/385395-sluggish-87-300d.html#8

1991 has vacuum operated wastegate. Which needs a bunch of permissives to allow boost. OP needs to validate all the underlying bits.

If if doesn’t see changes in airflow due to egr, etc. it won’t allow boost.

There is lots to scrutinize…

[vstech]

Trap ox clogged?

On my 87, i had an issue, zero power to speak of, i pulled the brass plug from the manifold allowing exhaust somewhere to go, and the car ran great, pulled the cat? Whatever it was and never looked back.

[jbach36]

Originally Posted by JHZR2

Have you ever serviced injectors besides running diesel purge? If not, I’d have Greazzer service them.

Regardless of cause, it’s prudent.

Do you know your compression? Any issues ever with cold starting?

Are you sure your vacuum actuated wastegate is working correct? It’s unlike other engines. And if you don’t get boost you won’t get fuel, then you’ll lack performance.

I have no idea about the compression. No issues with cold starting.

I don't know anything about the wastegate; what it does or how to test or fix it if it's messed up.

[jbach36]

Originally Posted by jdulle

I remember some people saying that they switched the two fuel hoses coming from the tank, in order to check if that filter is the problem. How is your turbo boosting? Are your pressure lines in good shape to the Alda/injection pump?

What you're talking about is over my head, switching the hoses. I already replaced the pre-filter and main fuel filter.

As to the pressure lines and Alda and injection pump, all that too is over my head. I'll have to take it to a mechanic.

Thanks for the tips though.

[JHZR2]

Originally Posted by jbach36

I have no idea about the compression. No issues with cold starting.

I don't know anything about the wastegate; what it does or how to test or fix it if it's messed up.

These cars are rapidly losing experienced folks who know them and can work on them well, despite their general simplicity and elegant design.

If you cont know how to verify boost pressure, learn how. A “Y” vacuum fitting, a bit of hard line into the cabin, and a small 0-15 psi pressure gauge is what you’ll need. If you can’t check boost you’re going to have a hard time doing any other diagnosis, and frankly o wouldn’t trust most shops on these anymore unless they’re well proven to know the engines and their control systems.

Checking for the existence of boost pressure will inform if there is a wastegate issue. My gut feel is that the wastegate is not closing, so boost can’t be formed, so fuel can’t be augmented, so there isn’t much power. But the easiest determinant to my theory is to see if you get boost.

[jbach36]

Originally Posted by JHZR2

These cars are rapidly losing experienced folks who know them and can work on them well, despite their general simplicity and elegant design.

If you cont know how to verify boost pressure, learn how. A “Y” vacuum fitting, a bit of hard line into the cabin, and a small 0-15 psi pressure gauge is what you’ll need. If you can’t check boost you’re going to have a hard time doing any other diagnosis, and frankly o wouldn’t trust most shops on these anymore unless they’re well proven to know the engines and their control systems.

Checking for the existence of boost pressure will inform if there is a wastegate issue. My gut feel is that the wastegate is not closing, so boost can’t be formed, so fuel can’t be augmented, so there isn’t much power. But the easiest determinant to my theory is to see if you get boost.

I have a vac pump. But how do I test the boost? Get a Y fitting and what then?