The 300SDL just repeated a symptom that I had previously only been able to reproduce on extremely hot ambient summer days, so I thought I'd ask again in a non-related-to-summer-heat context and see if it produced any new theories.

I cranked the SDL up at school last Friday to drive home for the weekend. Had spent two nights below 5 degrees, and was 15 ambient when I cranked. Started fairly effortlessly, shook and rattled and missed cylinders for a minute or two, and was perfectly fine. None of this is out of the ordinary for this car or that temperature, so I'm writing it off.

Left school and drove a good solid three and a half hours. The last... I'd say very easily the last two hours were done at a constant 65-75 mph, constant 3k, 3.2k rpm, nice long medium-hard uninterrupted interstate run for the better part of 150 miles or so. Had absolutely no suspicion that anything was wrong.

Following this 3-hour constant road-speed drive, I took my exit, and rolled up to the stop light at the end of the ramp... with no engine. No fireworks, no noises, nothing unusual - it's just that as the road speed (and engine rpm) slowed from road 3k rpm, it slowed steadily down, coasting down... bypassed idle and just went straight to a smooth "completely shut down." It didn't stutter or stumble or carry on idling roughly or anything... it's just the first time I came down from road rpm, instead of idling the engine just went straight to zero at the light. Dash lit up like a christmas tree, OP was down to zero, and tach was... well, stuck at 1500 where it always is.

Threw it in neutral, panicked slightly since I was in traffic and driving a friend home, in front of whom I didn't want embarrassing car trouble... reset the ignition switch, hit the starter - didn't wait for the glow plugs because I wanted it running ASAP... had to lean on the starter for about 2.5 or 3.0 very full seconds, at the end of which it cranked up... and idled as if nothing was wrong.

Rest of the trip, including the 2 hour drive back home was completely uneventful.

I've had complete failures to idle before. But, previously, it was ONLY happening on the hottest of the hot summer days. As in, a 95 degree day, plus road heat, plus me making an interstate run and then getting stuck in stop and go traffic. I would accelerate off idle in the stopped traffic, get moving a bit, take my foot off when it was time to stop again, and the car would die instantly instead of stopping at idle. Same story, however, even then - it would immediately restart itself and continue working until the next time the idle "stop" was supposed to "catch" the descending-speed engine and failed.

Any ideas what's going on? It's been months since I had this trouble.

EDITING: forgot to mention that ambient temp for that entire 3 hour trip was in the range 15 degrees F to 25 degrees F.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Some extra possibly relevant information:

* I don't think it was a low-voltage situation (electronic idle control) because the car's battery was replaced with a brand new, properly sized battery this November, and I have verified with my voltmeter that the alternator is charging properly.

* The ELR thingie, electric idle computer or whatever it is, has never been worked on, replaced, or anything. No idea if it could be failing or not. Or if it would cause this problem.

* Previously, since the problem only occurred on the hottest of days, I was blaming it on something overheating, possibly causing fuel vaporization, due to a non-functioning fan clutch. Just verified that the fan is clutching in and out properly, and also can't believe that this could be a problem on a 25 degree day. This is the first "cold" failure, so I just thought I'd mention it.

* No anomalies in diesel fuel quality, dirt, water levels, etc, should have been present. Was running the same fuel from the same pump I usually buy from. Filters are clear.

* The only way I know of to shut this engine down is by stopping its fuel supply, with either vacuum to the shutoff mechanism, with a finger on the shutoff mechanism, or by causing the above scenario by turning the ignition switch off. If it were a problem with a flakey ignition switch, it seems to me that it would have died at some point in history while at cruising speed, not JUST exclusively when attempting to hit idle from higher RPM. Similarly, I can't imagine how stray vacuum could get to the shutoff vacuum lines, even if I had a misrouted hose, in such a way that it only was a problem at idle.

* The shutoff lever on the side of the IP does bob up and down a little when I'm just looking at the engine idling under the hood. When I use a finger on the accelerator linkage and "rev" the engine, the shutoff lever does bob distinctly downward as the engine HITS idle, but then bobs back up when the engine's idle stabilizes. It has demonstrated this behavior for a long time, but I can't say for sure if it demonstrated this behavior prior to the first ever episode of this problem (which was two summers ago).

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

A bobbing shut-off lever is definitely not normal. There should be spring tension keeping it in the vertical position. Disconnect the vacuum line to the shut-off actuator. There should be no vacuum at the line while the engine is running (key in run or start position). If the shut-off lever stops bobbing, it's a vacuum problem, most likely at the key switch. If the shut-off lever continues bobbing, save up for a rebuild or replacement IP :/

If there is a next time, pop the hood as soon as you can without touching the key and check the position of the shut-off lever. If it's down, it's more likely a bad key vacuum switch than fuel supply problem. An alternative is to temporarily tee a vacuum gauge to the shut-off actuator line and monitor from the driver seat.

In my experience the shut-off lever is not a very effective fuel cut device. If you press down on it slowly, the engine might not shut off. I'll confirm when I remember to do so but I suspect it takes more vacuum to shut off the engine at 3000 rpm than at idle. If so, this lends more credence to the scenario of vacuum leaking past the key switch.

ELR only matters at idle. You can disconnect the alternator and battery and the engine will run normally. Idle will be low if it idles at all. Disconnect ELR from the round plastic thing at the tail end of the IP and see for yourself.

The typical fuel supply and clogged oxidation catalyst symptoms are inability to rev the engine. The fact that you can restart immediately and get going again suggests the problem is at or downstream of the IP. The fact that it only happens at or near idle suggests it's not a bad return line check valve.

Getting into exotic scenarios, maybe the IP timer gets stuck in some advanced position at which idle cannot be sustained. The jolt of the engine shutting off resets the timer so it's ready for a restart. Seems far fetched to me.

edit - I was speaking theoretically about disconnecting the alternator and battery. You can damage the alternator by disconnecting the battery while the engine's running.

Sixto
87 300D

This is the first time I heard that the shutoff levers weren't supposed to move a little while the thing is running. I'll keep a closer eye on that, but I suspect it may be related. I already have a vacuum line routed from the hood compartment through the ashtray to the inside, I think we were going to use it for a boost gauge one time. Assuming it's long enough, I'll put the Mityvac in line with the shutoff line and watch while I'm driving. This is a sporadic occurence, but when it happens I should notice the gauge move.

Mine is VERY effective. Doesn't matter what speed you press mine down, when it's about 2/3 of the way down the engine starts struggling and very shortly dies completely and instantly when you press harder.


When my battery died completely last month, the car refused to idle but would run just fine. So I think you're right, but I'll check just to be sure it's working.


Here's hoping it's something cheaper than the IP.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I had the same thing happen. It is most likely the vacuum switch in the dash leaking. Pull off the line to the shut off valve and stick your vacuum gauge on the line. If it pulls any vacuum you have a bad switch. Once the car shuts off, the vacuum is released letting you start again. The shut off lever should not be moving while the engine is running. It should move when you shut the key off.

__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
BrownHilda '79 280SL
FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
Krystal 2004 Volvo S60
Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
Fritzi '84 BMW
'92 Subaru
'96 Impala SS
'71 Buick GS conv
'67 GTO conv
'63 Corvair conv
'57 Nomad

I like this theory. Can somebody please clarify for me why this NEVER happens when the engine is running at speeds higher than idle?

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

It could be that when you are running above idle, you have moved the rack and are holding it against the vacuum of the shut off valve.

__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
BrownHilda '79 280SL
FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
Krystal 2004 Volvo S60
Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
Fritzi '84 BMW
'92 Subaru
'96 Impala SS
'71 Buick GS conv
'67 GTO conv
'63 Corvair conv
'57 Nomad

Sounds good to me. I just went out and did some vacuum rigging... lacking a "t" and not feeling like going out to find one that fits, I just disconnected the shutoff valve and ran a line from the shutoff vacuum line into the cabin, and set the MityVac up where I can watch it going down the road. Just got back from a test drive on which I was unable to reproduce the problem.

I'll run it that way for a few weeks and see if I can get it to goof up again.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Just drove another several-hour hard run just like the one that shut it down the other day, and had no incidents. MityVac on the vacuum shutoff did not show any fluctuations in vacuum at any point during the trip (until obviously it spiked when I turned the ignition off).

I'll keep experimenting, but if it IS stray vacuum getting to the shutoff mechanism, it's very specific conditions that induce it. There's definitely zero vacuum under normal operating conditions, just like there should be.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Is the stop lever bobbing with the vacuum line disconnected?

It occurred to me that maybe the shut-off actuator diaphragm is good but the internal spring is bad.

Sixto
87 300D

I'll take a look next time there's enough daylight to mess with it. I ran a vacuum line out the hood up by the windshield so I could shut it off without having to pop the hood each time, so I haven't looked.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

When I was having vacuum problems in my 300d, I perfected the technique of leaning over and sucking on the vacuum lines with my lips while appearing to check the condition of the wipers, so as not to embarrass the wife.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

You didn't send the wife?
(this one's too easy!)

__________________

Gone to the dark side

- Jeff

Okay, folks, some new information on this.

I was able to successfully reproduce the problem with the vacuum supply to the shutoff mechanism COMPLETELY disconnected. Was making a trip home this weekend; 2 straight hours on the interstate at ~75 mph. Took the off ramp and threw it into Neutral while doing about 60. Watched it drop like a rock straight to zero. Cycled the ignition (still rolling), hit the starter, it cranked within 1 second and ran normally the rest of the day. For this experiment, the vacuum line that normally supplies the pump shutoff was instead supplying the MityVac gauge - which didn't budge.

So... whatever's causing this isn't related to the vacuum shutoff.

Next experiment I need to try? Could a dying ELR produce such symptoms? I've never looked at the OVP, is there any chance it's related?

I'm desperately fishing for theories here before this thing decides to do that to me in traffic one day and gets me stuck under a changing light cranking on the thing...

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Not sure it's smart to slip the transmission into N at 60mph. The pumps might not move enough fluid to keep things lubricated.

I suppose it could be a bad OVP or faulty ELR signal but it can't be a coincidence it only manifests when coasting to a stop. You might mask the problem by increasing base idle.

Sixto
87 300D