1987 300D SLUG after 40mph
 

Seems fine off the line but anything above 40mph and it takes FOREVER to get anywhere. Checked the banjo fitting on the intake manifold and it wasn't that gunked up. Fuel at the prefilter is fine. Ran dieselkleen thru the tank. It does smoke white constantly and smells like diesel not too bad though. Gets decent fuel economy.

Compression? Timing? White smoke and diesel fuel smell isn't normal. Low compression and/or late timing would certainly cause both smoke AND low power. Checked boost pressure?

I think that's the next step is to check compression and timing. I'm worried that it's going to be low. Turbo seems very healthy. Not a lot of play and spools right up.

I'm also not sure where I'd get a compression tester that would work on this car.

What's weird is the car runs very well besides that. It does start up no problem. Coughs a second or two cause of a bad plug but idles real well and is very responsive. I was looking at a HF kit. I barely test compression, so I think for $30 it will work. I'll post back results. This engine I think has a lot of miles. It looks well maintained though. No oil leaks and not an extreme amount of blowby

My car has a constant whiff of white smoke too. Smells of diesel as well. One day I will probably buy a leakdown tester and get to the bottom of it. No blowby in the engine, so it could just be bad timing. But I really won't know till then.

There's no banjo fitting on that intake manifold, are you sure you checked the right thing?

If the manifold boost sense fitting is clear, make sure that you have a clear air path all the way to the alda. The switchover valve or a line might be clogged.

Manifold boost sensor is what I should have said. Where is the switchover valve? I will check that line with the clamp and see if it is clogged.

I bought the compression tester. I'm hoping it fits the flow glow plug hole correctly.

Could the alda issue cause power issues over 40? I'd always heard that it's a loss of compression with power issues like that

Pull the turbo crossover tube...

Look inside...

Cry

The manifold sensor is directly above the sense port. You need to disconnect the hose from the sense port and remove the fitting. Make sure it's clear. then remove the hose from the alda and apply some air to the hose that you pulled from the manifold, make sure the air reaches the alda. If that's the case, the switchover is fine. The switchover should be somewhere on the left inner fender, it's been a while since I've worked on a 124. The photos show the sense port and two switchovers on my 190DT. One of the switchovers is controlled by the overboost sensor and shunts off the boost sense line when an overboost condition is present. In all normal circumstances it should be off and there should be pressure continuity between the manifold and the alda. The switchover sometimes becomes packed with soot and that could cause the problem.

The previous owner of my 87 300D opened the hood and pointed at the switchover valve and said "You will have to remove the hoses from this, spray WD-40 in them occasionally and make sure they and the valve are free of soot" if the car looses power. A few months later my car lost power and I did what he said. It helped.

Now to address what another poster said about the cross over tube, looking inside and crying...When these higher mileage diesels start experiencing blowby, said blowby mixes with the exhaust from the EGR and creates a thick coating inside the crossover and intake manifold runners and on the backside of the intake valves. I removed the intake, soaked it in mineral spirits, lye (oven cleaner), brushed it out ...this took a couple of days of soaking.. and had it as clean as the day it left the factory...did the same with the crossover. I then fixed the egr valve so it could never again allow exhaust into the intake. Now 4+ years later, my intake is still clean. And I don't have to spray WD-40 into the switchover valve anymore.

My high mileage diesel has poor compression, some cylinders are in the low 300 psi level and lots of blowby that I run through a catch can to remove the oil on its way back to the inlet side of the turbo. Yet it still cranks fine (with all 6 glow plugs working) gets reasonable fuel economy and has decent power.

Monitor the fuel pressure to the inlet of the injection pump, I think the lift pump makes around 10 to 12 psi, the car will run fine down to around a couple of psi, but that would be an indication of a fuel delivery problem. As another poster said, check your boost pressure as well.

Your problem is that your car doesnt have a om617

And now you have a car that emits more NOx than ten VW’s and is illegal in all 50 states and Europe. Every 200k miles, clean the manifold and forget about it. Gasoline can be used (with care) to wash that stuff out.

Little known or noticed: your engine has a PCV valve. In another example of brilliant German engineering, it’s built right into the valve cover and is non-replaceable. I’m pretty sure this accounts for more than it’s fair share of manifold sooting. Search for my thread on “valve cover, dissected” to see how this gem works. I’m still looking for a way to repair it.

When the car was new, yes, the NOx was reduced by the EGR valve. After 30+ years and likely being gunked closed anyway, I doubt there's much of a difference in its performance. The CO and carbon emissions from the tailpipe of a car with an extremely clogged intake manifold are FAR WORSE for the environment than a higher NOx production.

The PCV valve may or may not be contributing to the oil buildup in the intake. My PCV valve is known not to work (even rattles) and I did an experiment with an oil catch can with some paper towel in the bottom. Drove the car for nearly a month and came back to check it. One single solitary drop of oil.

Since I do have a fairly large amount of oil in the intake, there's only 1 other place it can come from - turbocharger. On a higher mileage engine, I'd blame the turbo for oil in the manifold over the PCV system. It doesn't take much of a leak for it to aerosolize the oil and coat the entire intake with it. With a still functional PCV system the oil and soot team up to make a sort of glue that's truly awful to remove.

There is a flapper on the fresh air side of your air filter, under the air filter box. Were something to be lodging that flapper valve mostly closed, it could limit the amount of air through the filter and on into the engine. But checking the manifold pressure during the experiencing of your issue, would let you know if you are getting a good supply of air to the engine.

Got 350psi in compression. I do get a puff of blue smoke, but I'd guess that's from the turbo seals.