Gentlemen,

I am completely baffled by this one. I have a 1985 300D non-turbo that has always run extreemely well. I take very good care of the car and do all the maintenance myself. It has 265,000 miles on the clock and is my daily driver.

All of a sudden (late last week) the car became very difficult to start cold (ambient temperatures above 60 deg F). It was acting similar to a very cold start (below 10 deg F) or after opening the fuel system. The car cranks and smokes while cranking; it will start after about 10-15 seconds cranking. Runs fine thereafter and starts fine warm.

Fuel filters were replaced a year ago. The valves were lashed last FEB and are not past the 12.5k mi interval. Glowplugs were all replaced about four years ago with Bosch plugs. 1 had failed early this year and replaced with a Beru plug (since I was annoyed with the Bosch plug failing prematurely).

One thing I had noticed was that the glow plug indicator light would cycle for less than 2 seconds or NOT AT ALL and was inconsistent. After repeated testing, the glow plug light would only come on intermittently thereafter. Diagnostics with the power and signal connection to the glow plug timer (12vdc battery, gnd, 12vdc ign, crank signal, and light) all checked OK so I replaced the relay this morning. Light now functions correctly but the starting problem remains...

Fuel solenoid appears functional.

ALL fuel injectors (and spill hoses) were replaced to correct a nailing problem fall 2007. Fuel filters replaced also.

This car ALWAYS started immediately above 40 deg F. Now, all of a sudden it acts this way. The car is about at the mileage for the next oil change which I will complete this weekend. I plan to also change the fuel filters and possibly investigate valve lash but I am baffled about the immediate onset of this problem. I would suspect the preglow system but it appears to be working now.

Any thoughts?

Thanks,

-Adam

You didn't mention Air Filter. Did you OHM out the glow plugs? Timing chain stretch? Battery Condition? Slow turning starter? Bad fuel?

Just some thoughts that come to mind.

Air, Compress, Fuel, BANG...

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92 300D 2.5L OM602 OBK #59

The air filter is replaced yearly, or every 15,000 miles. I inspected it but it looked alright. I will replace it in 5000 miles.

The starter sounds normal, albeit I'm not used to listening it crank for more than about 1 second.

The battery is less than a year old. The battery is a MB battery; alternator is good as well; the regulator was replaced 3 mo ago and system voltage is about 14.2 running.

As for ohm testing the glowplugs, no I have not done this yet. I thought that as long as the glow plug light illuminates, there is continuity to each plug. I will resistance check tonight and check voltage drop.

I am baffled by this. It must be something that affects ALL cylinders simultaneously...

It's almost surely GP related. There is a 80A strip fuse in the GP system which, even if it looks good can be cracked...replace that first. Check for voltage at the GPs when the light is on. Report back on findings.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Dumb question, but did you replace the GP strip fuse?

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I agree that it sounds like a glow plug issue. I think Diesel Giant has a write up about testing this system.

Yep here it is
http://www.dieselgiant.com/glowplugrepair.htm

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1983 300SD
200000miles

Not a dumb question at all as I was not aware of fuse protection for the GP circuit. If this is the fuse on the glow plug relay itself, the relay was replaced so I assume the fuse would be good on the new relay. However, I don't recall looking at the relay when I installed it so it's possible it didn't come with a fuse...

Does anyone know if the Hella glowplug relay comes equipped with the in-line fuse or if it needs to be swapped from the original relay?

Thanks,

Adam

The preglow time relay is located on the left inner fender well. The relay is protected by an 80A fusible link, mounted outside on the cover of the relay.

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92 300D 2.5L OM602 OBK #59

I replaced the relay this morning with a new one so I know where it's located. My question is: does a new relay come with a fuse installed or do you swap the fuse from the old relay? If the latter, this would explain my problem as I didn't swap any fuses.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2DA0XXP7Z2DA0XXY23&year=1985&make=MB&model=300-DT-001&category=All&part=Preglow+Time+Relay

Is this what you replaced? It comes with a new fuse so thats not the problem.


Still it sure sounds like a GP problem. I'd put a meter on them and see if your getting juice.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Put a voltmeter on the glow plugs There should be circa 10 volts when first energized and raising to circa 11 volts when plugs get hot. The voltages are approximate and perhaps = to + or - 1 volt from where I have stated.

Do this first. If this passes it is time to do a wet and dry compression test and post the results for our analysis. If you have inadequate compression it will never start. One post (Janet in LA) spent four plus months troubleshooting only to find that she had inadequate compression. Also, contrary to popular opinion, from my experience and if you read that Janet thread a compression can take a car from start and run normal to 'will not start' overnight. I would not have believed it myself but I now have experience.

You can do the test yourself. Get a Tester at Harbor Freight for circa $30. This is IMHO a necessary investment. I am seeing far too much troubleshooting going on without this vital and most fundamental information on hand.

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Dionysius

I checked the car over lunch and the fuse is in place. The car required about 2 seconds of cranking before starting for my lunch drive (longer than normal) but the car had been driven fiver hours earlier so some heat would have been retained in the block (72 deg F and sunny).

Yes Hatterasguy, that is the relay I replaced. It did correct the intermittent indication problem that developed at the same EXACT time as the starting problem; I can't see these two failures being just coincidental, but replacing the relay did correct the indication part. I will do further testing tonight. Compression tests will follow if the fuel and preglow systems check out.

I don't think low compression is the case since the car started on cold days fine (live in Philly PA, keep car outside and never have trouble starting down to 10-15 deg F without block heater); then suddenly it developed this problem "overnight", yet on a warmer day. If compression was at fault, I couldn't see it affecting all cylinders simultaneously without first causing missing etc first.

Electrical testing will commence tonight.

-Adam

I hope beyond hope that your problem is not compression. But please reread what I said about "overnight" once again. It could be compression.

In the meantime follow through on these items:

First verify the GP system (I stated the voltmeter test in my post). Next validate the fuel supply by running it from a bottle of diesel at the engine compartment and doing the chirp test with the primer pump. Next check the hi pressure supply by cracking the nuts at the injectors.

These three checks are needed first. I am assuming a good air supply and prechambers. These are less likely candidates from your descriptions.

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Dionysius

Dionysius. I realized that's what you said; that's why I said "overnight" with quotes. I too really hope compression is not the cause.

Actually the car had been driven once and just for a few minutes the day before this problem occured. It was moved in my driveway last Thursday evening (I had another car out that day) by my in-law, it started and ran fine, probably just longer than a minute. It also shut down fine. The next morning the symptoms began. That fact is one of the reasons I can't percieve compression as the cause.

At least the good part of this situation is that I can blame my in-laws....

Testing results to follow this pm.

Adam

I bet it's the glow plugs... and I bet you need to ream the holes... if you have a large layer of carbon covering the plug, it will absorb all the heat, and prevent the prechamber from warming up... seriously hindering starting.
even with perfect ohm test on the plugs, pull them, verify they heat up, and then ream out the carbon from the holes... should start back like new.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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