1979 300CD Fluctuating Idle

[WVOQuine]

Hello,

Apologies if this has already been addressed elsewhere. I haven't been able to find much. Long time forum user, first time poster. Thank you to everyone who keeps these up. They're hugely useful.

My issue is as follows:

Recently bought a 1979 300CD. I've owned a number of Mercedes Diesels and this one is in pretty rough shape. Basically original everything except glow plug setup and hoses used for (possibly) running WVO from a tank in the trunk. (Was given story by P/O about how it was not a WVO tank...)

Work done: replaced fluids, filers, oil, replaced hand primer pump. Diesel Purge, oiled linkage.

The car smokes quite a bit on startup, then seems to idle well for about 5 minutes, and then usually starteds surging or fluctuating. the surging is usually consistent. up for a few seconds and then back down again. sometimes after i drive it around for a while it stops doing this. generally it seems to get better after the car has been driven on the highway, although sometimes it does not and will still do it when very hot. One of the injectors also nails intermittently, but this seems not to be directly correlated with the idle issue. I.e., often the injector will stop knocking and the idle will continue to do strange things. Also, sometimes at idle it will be idling consistently (not rapidly fluctuating), but will have apparently increased so that when i shift it from park to drive the transmission clunks into gear because it is idling too high.

Wondering if this is a compression issue, or if there is some vacuum issue or fuel issue that could cause these symptoms, or all of them combined. Sorry if this is too vague, just hoping to find some direction before i start spending money trying to randomly troubleshoot.

Thank you for any help in advance.

- Eric

[WVOQuine]

Should also add that I'm going to do a valve adjustment soon, but have never had that take care of something like this.

[Stretch]

If it is evident that the vehicle hasn't been used too much recently I'd opt to keep on using it and see if that helps or alternatively it develops into another more obvious problem.

From the list of things that you've done you've done most of what's to be expected.

If the surging is really causing trouble and the revs keep on rising (!) think about things that could cause this to happen such as

1) Messed up cruise control?
2) The ADA
3) Sticking rack linkage / poor accelerator spring / floor mat stuck under the accelerator pedal (<= now that's a classic)
4) If the surge ONLY happens with a gear selected (automatic gearbox) then you need to consider something's up with that...
5) Air conditioning - even if it doesn't work is the mechanical connection to the compressor still there => remove the belt
6) SLS pumps - does the surging coincide with pressure fluctuations in that system?
7) Whilst power steering doesn't soak up too much power from an engine the "free-running-ness" of all pulleys ought to be checked

[Mxfrank]

The fuel return on this car should be a hose shaped like a cigar...fat in the middle. The odd shape is supposed to smooth out fluctuations in fuel pressure. It's worth a shot to replace this.

[WVOQuine]

Thanks for the advice. This is all very helpful.

To update, i ran another can of diesel purge directly through the injectors and it came out all black. The car runs a lot better now, but the idle fluctuation persists, although now it only happens when the car is not in gear. So, it will fluctuate while idling in neutral or park, but will smooth out and be almost perfect as soon as i shift into drive. This makes me think transmission vacuum issue.

Another thought was that it might be that there's a small head or head gasket leak and that coolant is getting into one of the cylinders at certain RPM. There are some bubbles in the radiator when i fill it all the way to the top, ie when it's overfilled and the water is just below overflowing. When it's at normal level there are no bubbles. If it is a head gasket, say, then it seems strange that it would be worse in different gears. it also smokes at idle intermittently, but today after i warmed it up a little bit it ran with no visible smoke at all for the rest of the day.

Very strange indeed.

[WVOQuine]

to clarify: by "worse in different gears" i mean the idle is worse at different gear settings. ie, worse when out of gear, in park or neutral; and then fine when in drive and reverse.

Also, seems as though PO did basically nothing to the car. oil was dirty, radiator level was very low, checked the power steering fluid today and it's black with what looks like the original filter in the housing still. filter looks like a literal piece of wood, a wood cylinder. really odd.

all original hoses, suspension components, cv joints, exhaust, etc. basically looks like the car was parked in 1985 and never worked on.

[WVOQuine]

except of course for the extra tank. PO said the car was owned by a missionary couple in mexico before he bought it and that they installed the tank. probably BS, but no other evidence of wvo conversion, and it is from California, so who knows. anyway, seems mostly original, for better or worse.

Also seems quite fast. Much faster than any other MD diesels I've owned. Seems to have about as much power as a 73 280c I had once which had an M110. Also has 4 wheel disc brakes. Are all of these 300CDs this cool?

[Desert Panther]

Originally Posted by WVOQuine

except of course for the extra tank. PO said the car was owned by a missionary couple in mexico before he bought it and that they installed the tank. probably BS, but no other evidence of wvo conversion, and it is from California, so who knows. anyway, seems mostly original, for better or worse.

Also seems quite fast. Much faster than any other MD diesels I've owned. Seems to have about as much power as a 73 280c I had once which had an M110. Also has 4 wheel disc brakes. Are all of these 300CDs this cool?

I have heard that when these diesel cars were sold new, that there were plenty of dealers who installed an auxiliary tank in the trunk as a convenience to the customers, as back in the 70's & 80's, diesel fuel was a lot harder to find, often requiring out of the way trips to truckstops. I have no facts to back this up though, it is just what I had heard.

I have a 1978 CD, 1 year older & slower than yours, haha! Really these cars are slugs off the line, even with their advantageous gearing compared to the turbo equipped cars. The turbo 300 CD's will leave a non turbo one for dead in a drag race. I do enjoy the heck out of driving mine though, it still is fun even with its meager 77 horsepower.

[vwnate1]

You're still not yet done the valve adjust so no way to be sure just yet .

Maybe another Diesel Purge , I use a clear glass gallon jug and fit a clear plastic micronic fuel filter in the return line to help remove the black stuff that comes out .

Until you can do a Diesel Purge that comes out clear fuel in the return line , you're not done purging . remember to you must vary the engine speed to fully clean everything out .

My '78 300CD wasn't quick off the line but it zipped right along , much to my surprise .

Keep at it , you're not doing things in the proper order , when you do , it'll sort out .

[WVOQuine]

Thanks again for the info. As for the proper order, my thought was just that I would start with the cheapest and easiest fixes and work my way up from there. Good point about doing diesel purge until the lines are clear. Ran another can through today and it's still black, although it's a slightly clearer black this time.

The weird thing about this car is that it is actually sort of fast off the line. It throws me back in the seat if i floor it at any speed under about 60.

I'll try to post a video.

the minimum horsepower for the 280c was something around 160, and i'm wondering how it's possible that this car can feel close to that at 77hp.

[WVOQuine]

https://vid.me/cuwZ

not sure if you can hear it very well, but it sounds like it's loping.

[vwnate1]

Always , ALWAYS do things in the proper order and make sure one job is finished before moving on to the next .

Failure to follow this simple logic is why Junk Yards , used car lots and auctions are full of perfectly good vehicles that " can't be fixed no matter what I do " .

The car looks o.K. , what's the grille badge ? .

Maybe an under hood video when asking about engine issues ? .

Be sure to include a decent pic of the clear plastic fuel intake screen .

[WVOQuine]

[QUOTE=vwnate1;3546610]Always , ALWAYS do things in the proper order and make sure one job is finished before moving on to the next .

Failure to follow this simple logic is why Junk Yards , used car lots and auctions are full of perfectly good vehicles that " can't be fixed no matter what I do " .

The car looks o.K. , what's the grille badge ? .

Maybe an under hood video when asking about engine issues ? .




Hello, thanks again. The grille badge says "Westfalenpokal" it seems to be original, looks like it's from the 70s. Only thing i can find on that is that there's an autocross there, probably the original owner was a fan?

As for the proper order, what would you suggest other than to do the valve adjustment?

Generally when I buy these cars the first thing i do is to change all the fluids and filters. The second thing i do is to run diesel purge and techron in the gas tank. Usually I'll drive the car for a while after doing this and see if idle issues clear. Generally they get better. If not, I check the injectors by cracking the nuts at the line and seeing if idle changes. After I've driven the car enough to determine that there aren't any serious issues with it (usually i'm looking to make sure the rings aren't hopelessly worn out, that the transmission is good, that it's not burning coolant, see how much oil it's using over the course of a couple weeks of light driving, etc.) then I'll do a valve adjustment.

I tend to do valve adjustments as the last procedure before actually putting any real mileage on a car because they're slightly more involved and because in my experience they usually make it run more smoothly, but if there's some minor head gasket issue I couldn't catch when test driving, for example, or if it turns out the rings are really worn, then a VA isn't not going to help much.

A lot of problems can be masked by the sellers using shady measures. Ring issues will be counteracted with motor honey or super thick oil, additives in fuel tank mask injector problems, etc. Generally I try to buy these things dirt cheap (like 500-100 dollars) so there's often a lot of sketchy stuff going on, but equally often it turns out that the previous owner just didn't know what they were doing and most if not all of the issues can be pretty easily cleared. So that's my rationale behind this ordering. Any advice on a better order of operations would be much appreciated.


I should also add that I cracked the injectors on this car and there was discernible change in idle for each one, which seems to indicate that it's not an injector issue?

Lots of white smoke on startup, (although it did just get cold out) still periodic loping idle that is much more noticeable when not in gear. Also, there's on and off white smoke from tail pipe which seems to have gotten worse since running diesel purge. Perhaps the injectors are just majorly worn out and leaking / spraying really off pattern.

Still can't help but wonder if someone built or swapped the engine. As mentioned before, it seems to have a whole lot more power than any of these that I've owned previously. The engine compartment sticker says 77hp, but there's just no possible way that's accurate. Will check block numbers and post a more detailed video soon.

Thanks again for everyone's help!

[vwnate1]

Valve Adjust before anything else as they got tight not loose so a valve with zero clearance makes a HUGE impact on every aspect of how the engine starts , runs , idles and so on .

Anything else is just flubbing your dub .

Good on ya for fining the cheap and saving them ! that's what happened to most of the nice rust and dent free California ones ~ they never get service or maintained and once the valves got to zero last they're hard to start so the dimwitted yuppie d*ckwads who were buying them all up 10 + years ago simply scrapped them en masse when most were still full of life .

Remember : valve gaps affect cam timing so it's critical to do them first and very carefully . once you get the hang of it , it's a 1/2 hour job in the morning when the engine is stone cold .

[Dr. Benz]

Surging or loping at idle can be caused by a weak or broken idle stop. This thing is spring loaded and helps to keep the idle speed leveled out.
Look down along the IP and the cylinder block and you will see where the end of the throttle arm comes up against this idle stop. The stop can be moved in or out to change idle speed after you undo the lock nut or removed completely if replacement is needed.