Glow Plugs test fine, but horrible start - PeachParts Mercedes-Benz Forum

TurboDMercy · #1 02-27-2018, 03:23 AM

I have an '82 300D with the OM617.

In the mornings I find myself cranking for 45-60 seconds (as the owners manual suggests). I just hold the key and let the starter go until she starts up. I thought I had bad glow plugs until I tested them with my multi-meter, all read .8 ohms. Once I finally get it started, it doesn't idle rough at all. All the forums I read point to this being the glowplugs still, and I'm wondering if my glowplugs can still be bad even though they all get the same reading of .8 ohms.

Thank you for all suggestions.

Junkman · #2 02-27-2018, 08:07 AM

Test the glow system to be sure they're getting current. Dieselgiant has destructions. It still sounds like glow plugs.

Were this a Cummins, I'd suspect the fuel system was losing prime but that isn't a frequent problem with these engines.

Edit: http://dieselgiant.com/glowplugrepair.htm

vstech · #3 02-27-2018, 08:21 AM

Yes.
An ohm reading is only a go no go test...

To know if you have good plugs you have to pull them and look at them heat up.
Tip glow instant is good...
Anyplace else heating is useless.
Carbon buildup in chamber keeps plugs from performing when hey are good, and kills good plugs.

tangofox007 · #4 02-27-2018, 08:37 AM

Anaother possibility is that the glow plugs are operating properly, but the glow timer is not. I would recommend timing the timer.

vstech · #5 02-27-2018, 09:36 AM

Quote:

Originally Posted by tangofox007

Anaother possibility is that the glow plugs are operating properly, but the glow timer is not. I would recommend timing the timer.

Understanding how the timer works is important as well.

The plugs are energized (when the timer works) when you turn the key, but they stay energized well after the light goes out.
If your dome light works, the glow plugs should dim the dome light when the key is turned. You keep watching the light and when it brightens, the glows are off.
Usually around 35 seconds...

On cold (sub freezing) starts, I wait for the brightening, the cycle the key again and wait... then start.
Usually instant starts then.

Diseasel300 · #6 02-27-2018, 10:02 AM

Let's also not rule out the possibility of air entering the fuel system...

barry12345 · #7 02-27-2018, 11:03 AM

Start as already mentioned by observation of the cabin light. It should dim for the glow plug period if the system is working.

All you really establish by reading the ohms on the glow plugs is that one or more are not burnt out. Not a bad thing in itself. Yet does not indicate they are actually operating when needed.

The reason the dome light dims somewhat of course is because the current the glow plugs use is very high. Another alternative test is to read for the presence of glow plug voltage when it should be there at the glow plugs.

The glow plug indicator in the dash does not present an absolute certainty that the glow plugs are actually on. At least you are on the right track. The glow plugs proper operation in situations like yours is job #1 to find out. You also have a meter and that is a good thing.

Remember if there is no voltage at the glow plugs when there should be. I believe your year has a fuse link in the glow plug relay. Also it is possible for the heavy amperage contacts in the relay to be bad or not activating with the glow plug cycle.

We all have different levels of electrical knowledge and skills. Sometimes just sitting down and drawing the circuit out as we would expect to find it can help. Or looking it up.

The more time that passes now the more I like these 123 model cars fundamental simplicity. An average guy can deal with almost all the issues that arise with them that I can think of. Also welcome to the forum. It may help you really respect and appreciate these cars for what they are. Almost if not the last fairly simple cars built.

Diesel911 · #8 02-27-2018, 11:41 AM

I you think air is getting into the system before you start do a lot of pumping on the Hand primer. I mean like 60 pumps.
If you start normally after that you I would believe that was a sign you have air leaks that are causeing that loss of prime mentioned by the other member.

Diesel911 · #9 02-27-2018, 11:43 AM

This is a pic of a Glow Plug that ohmed out OK. The problem with the Plug is that it is the tip of the Glow Plug that sticks out into the precombustion chamber and it is the tip that needs to be hot.

Check the Strip Fuse on the Relay to see if it is cracked or the screws are loose onit.

When I had a cracked Strip Fuse sometimes the Glow Plugs worked but mostly not.

dtf · #10 02-27-2018, 01:18 PM

here's another idea: does it have a block heater? plug it in over night then start the car in the morning without waiting for the glow plugs. if it still runs rough then it is not the glow plugs it is something else.

my glow plug wiring harness has been shot for 2 years now. I plug it in every night in the cold weather and it fires up like a gasser in the morning. I also plug it in a work in the garage

BillGrissom · #11 02-27-2018, 01:22 PM

Clip your red MM lead to the nut on one of the glow plugs, w/ blk lead on BAT- (or engine block). You should measure ~12 VDC during the 20 sec the glow plug relay is ON. That is a fixed ON time, regardless of temperature. The dash bulb turns off based on temperature, but that is just a prediction: "glowed long enough to start", which is most applicable to a new engine.

If you haven't changed the hand primer from the original white plastic one to the smaller black one, do so. A new pump is ~$10 and the white ones are notorious for letting air into the fuel system.

Best way to insure the plugs are glowing correctly is to peer down the hole where the injectors were removed. You should see them glow at the tip. But, the pre-chambers vary. Early ones have a narrow neck, which can also trap carbon below, and perhaps obscure the view. Anyway, removing the injectors for a compression check and pop/spray test will likely be the next step. Member Greaser here checks and rebuilds injectors, or a local diesel shop can.

TurboDMercy · #12 02-27-2018, 02:07 PM

It does have the block heater, but I seriously doubt that it works. I have left it for a few hours and not felt any heat from the engine, with the usual rough start.

TurboDMercy · #13 02-27-2018, 02:11 PM

Quote:

Originally Posted by Diseasel300

Let's also not rule out the possibility of air entering the fuel system...

It starts wonderfully once warm, could this still have something to do with air in the fuel system?

dtf · #14 02-27-2018, 02:48 PM

test the block heater with DMM. you should have 30-40 ohm resistance or something like that across the plug spades. my block heater on the '95 is not a coil attached by magnet it is one little plug that sticks into the freeze plug in the block (I think).

I don't think you'll feel heat from the engine - that would be a huge block heater

Maxbumpo · #15 02-27-2018, 04:10 PM

Quote:

Originally Posted by TurboDMercy

It does have the block heater, but I seriously doubt that it works. I have left it for a few hours and not felt any heat from the engine, with the usual rough start.

It is very common for the block heater cord to fail, and that can be replaced for about $20 or so with an aftermarket cord. Remove the cord from the heater, and test the heater for resistance. Test the cord separately for continuity on all three conductors.