My truck had been running great since the fuel filter change. Today it started hesitating again. Very strange. I’m going to have to get busy and clean the contacts on the sensors as outlined in the cut and paste above.

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1986 300SDL, 362K
1984 300D, 138K

Yet still not "actual number"

Any chance you got a bunch of crud in your tank and/or lines? Long ago I noticed some crud coming out of the line when I disconnected the filter.

I put a pail on the line and bumped the pump ( key on for second) . unreal grey sludge came out in the first cup of gas. then clear.

Try it and let us know.

THIS - it never ceases to amaze me how bad fuel from the pump can be ~

There was a time when fuel full of debris wasn't common but those days are long gone .

This is why I insist upon adding clear / translucent filters some where in the system, you cannot do this with modern fuel injection systems .

TWO YEARS AGO I cautioned my brother to change the intake screen in his 1987 300SDL, it packed up tight and stalled the car last weekend.....

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I’m starting to wonder if the engine is past his pull date. It’s been putting out some gray smoke at start up for some time. When I first had the trouble two or three weeks ago, I limped into an O’Reillys by the skin of my teeth, it was barely moving. I put in plugs, cap and rotor, cables and air filter and it ran way better, but still not 100% great. The symptoms came back, I put in a coil, again it ran a lot better but not ideal. There was some improvement with the new fuel filter, but again not perfect. In my experience, a clogged filter reduces performance at the high end, does not make for hesitation and missing at the low end.

Now the symptoms are back, I put in a new EGR plus solenoid a few days ago, very little improvement.

My mechanic pulled off the vacuum line and played with it and said it seemed the EGR was stuck open. There’s a little ring of felt-like substance inside of it, it was falling out as I took it out. So I suspect it was indeed bad.

I’m wondering if the recurring symptoms are basically the plugs becoming fouled again by the blow by. About three years ago I ran at way low on oil one time accidentally, barely showed on the dipstick. Who knows, maybe I did some damage, and it does have 175K, and is 33 years old.

A new motor is not out of the question, one decision would be to stick with 5.0 or go to 5.7. I understand it usually doesn’t have much effect on MPG.

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1986 300SDL, 362K
1984 300D, 138K

Have a good mechanic check it out before considering changing the engine. If you work at it every day. You can often identify problems you have seen a lot of before.

Sounds like you did and he did find one problem. He might locate the real one since that is repaired. What did he think the ignition spark looked like?
Good mechanics develop intuitive skills. Even more so in the old days where they were not paid for diagnostic time.

This guy is pretty good. He hasn’t had it in his possession to look at it yet. We’re talking about doing that on Monday. But yes, you’re absolutely right, I’m not going to replace the engine on a hunch.

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1986 300SDL, 362K
1984 300D, 138K

It almost behaves like that’s what’s going on. But some of the evidence contradicts that. After I gave it the tuneup - plugs, cap n rotor, cables, air filter - it ran much better for a week. And it barely ran before that. Why would the fuel supply suddenly be better for a week?

But one thing is certain, it did run better the evening I put in the fuel filter.

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1986 300SDL, 362K
1984 300D, 138K

If a fuel filter changed it for a bit I wouldnt change the engine. Get a compression check first. Im leaning towards a bad fuel pump.

I'd remove the EGR valve proper and hand clean it ~ so often they get gunked or carboned up and cause weird running problems .

If there's a bunch of crud in the fuel tank you may have allowed it to run better until the new filter began to get blocked .

I know removing the tank isn't fun but sometimes the fuel intake screen gets slowly restricted.....

Sounds like you're close to getting it done, don't be in any hurry to increase the engine displacement, gasoline isn't going to drop in price any time soon .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Have you tested the O2 sensors? Might even be cheap enough to just replace.

Plenty of diagnostics you can do...test fuel pressure, test O2 voltage, check vacuum at idle, pull the plugs and check for fouling, test for vac leaks, etc. Not sure if you can bench test those old TBI injectors bur st 175k they might be giving up too.

I did a quick skim of the thread, does this issue happen all the time or only cold/warm?

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Yes, a compression check of course before even thinking of changing it. I’m speculating wildly. The symptoms in the way it’s reacted to various improvements is just mind boggling, it doesn’t make any sense. I also wonder about the fuel pump. I’ve Red speculation that Bad valve seats could be leaving to the smoke on start up, not sure how much all of these things would foul the plugs.

I’m going to ask the mechanic about a fuel pressure check. This is where my shadetree mechanic ass is limited. I’ve never done a fuel pressure check, I don’t know how, and I don’t have the equipment. I’m sure it’s not that tough, but the learning curve just gets massively steep sometimes.

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1986 300SDL, 362K
1984 300D, 138K

Get a quote on valve seal replacement as well.

The old in for a penny, in for a pound thing is rearing it’s head. I’m wondering if a new fuel pump and cleaning the tank while it’s out would be good. I don’t have a good facility to do a lot of the stuff at present. I’ve got enough work on the table such that it makes more sense for me to spend my time on my speciality and pay the mechanic to fix the rig. He knows so much more than I do and has the shop for it. He rebuilt the tranny on this thing and it runs fantastic.

Like I said, in for a penny, in for a pound. I heard an account of one of these things totally cherried out, asking price $60K. This one is still halfway solid. I get purchase offers all the time.

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1986 300SDL, 362K
1984 300D, 138K

Yup. If the compression is good, my guess is doing the heads is all that’s needed in the rebuild category.

My former neighbor, the outstanding welder, thinks the intake manifold gaskets might be aged out as well. I tried the old brake cleaner spray test, didn’t get any action from that.

Could will be the complete renewal of the TBI is called for as well.

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1986 300SDL, 362K
1984 300D, 138K