1982 240D Glow Plug Questions - PeachParts Mercedes-Benz Forum

mjquillici · #1 12-01-2008, 11:33 PM

I just followed the step-by-step instructions on the DieselGiant website on how to check the glow plugs. I removed the large plug from the glow plug relay and tested the resistance of each plug/wire by sticking the positive probe of the multimeter in each socket. According to DieselGiant, each plug/wire should have a resistance of less than 1 ohm (I hear normal is around 0.6 ohms). However, after testing all four plugs/wires, each one measured exactly 1.3 ohms. They are all equal, but this seems a little high. The rest of the system checks out okay (battery is about 12.7 volts when engine is off, and the relay works properly). Does anyone think there may be a problem, or could this be normal? I did not remove the glow plugs to check each one individually. I am having a hard time cold starting when the temperature is in the 40s, so I figured I'd start with the glow plugs. . .Any ideas???

tangofox007 · #2 12-01-2008, 11:47 PM

How long does the glow cycle last? (From the time the key is moved to the glow position until the timer opens the relay, assuming the engine is not cranked.)

dieseldan44 · #3 12-01-2008, 11:57 PM

inconsistency with very low resistance readings is normal for multimeters, especially inexpensive ones. maybe grab a new plug from a store and test it to verify?

have these gp's served you through a winter before?

the next thing to check is if each plug gets 12 volts.

dd

mjquillici · #4 12-01-2008, 11:59 PM

Roughly 30 seconds, give or take. I didn't actually time it, but that's a close estimate.

mjquillici · #5 12-02-2008, 12:05 AM

No. They've only been in for 10 months, or so. But I had the shop install them, and I'm not sure which brand they are. To check the voltage at each plug, I would touch the positive probe of the multimeter to the exposed end of the glow plug, right?

Diesel911 · #6 12-02-2008, 12:12 AM

The Glow Plugs have to be Cold when you test them.
You could also buy an extra new Glow Plug made by the same company and test it with the Ohm Meter to compare it to the old ones.
Also If you think you are picking up resistance in the wires you can leave the Glow Plugs in the Engine and put one of the probes on the connector end of the Glow Plug and the other to ground.

Do the newer types of Glow Plugs test at the same resistance as the 617 Pencil types are are they different?

mjquillici · #7 12-02-2008, 12:19 AM

Got it. I didn't know that they had to be cold, but now that you mention it, that makes sense. Thanks for the heads up. . .

Assuming that the glow plugs are okay, now I just have to figure out why I'm having a hard time cold starting with temps in the 40s. Valves were adjusted about 9,000 miles ago, so they shouldn't be too out of whack. I do know that the primer pump needs to be replaced, it leaks like a sieve when I pump it. . .I've got one on order. . .a job for this weekend.

barry123400 · #8 12-02-2008, 12:48 AM

Anything that impedes the injection pump elements getting good clean fuel loading such as air present is going to be especially noticable on a cold engine startup. I would wait for the new primer pump before going further.

mjquillici · #9 12-02-2008, 01:29 AM

Got it. But is it possible for air to be introduced into the system through a bad primer pump, even if the pump is screwed tight when not being used. It only leaks when attempting to use it. Incidentally, what is the path that the fuel travels when being pumped into the engine (i.e. flows through in-line filter, enters injection pump, is pumped to the injectors, and then what)? As always, thanks to everyone for sharing your endless knowledge.

vox_incognita · #10 12-02-2008, 01:13 PM

Fuel is sucked by the lift pump in the following order:the lobe of the IP camshaft moves away from the roller follower of the lift pump and fuel from the tank passes through the plastic pre-filter and the via the inlet valve of the lift pump into the suction space of the lift pump.As the lobe of the IP camshaft engages the roller follower again(called pre-stroke),the plunger of lift pump pressurizes the fuel in the suction space and forces it through the outlet valve in the pressure space behind the plunger.As the camshaft lobe relieves the tension on the roller follower,the massive spring behind the big bolt pushes the plunger back,forcing the fuel behind it to the main fuel filter and at the same time sucking fuel from the tank...Note that the suction and pressure stroke occur simultaneously!(clearly visible in the attachments diagrams).
The pressurized fuel goes into the fine(big)fuel filter and then into the IP.The IP elements are saturized with clean fuel at a given pressure(regulated by an overflow valve) and then at given pressure via the delivery valves through the metal lines reach the injectors...The excess fuel from the IP, the excess fuel from the injectors and the excess fuel delivered by the lift pump(as it always delivers quantities times more than the engine needs) unite at one point to be used again or to flow back into the tank,this constantly purifying and warming the fuel...
The fuel is delivered at a given pressure,regulated by the overflw valve on the IP(the side towards the engine block,or on the fuel filter cannister on pre-1965 models)...
Note that the succion/delivery stroke of the lift pump is determined by the overflow valve,which is constructed to maintain an appropriate pressure...thus making the system pressure-proof(say in cold weather ,when the fuel has "frozen",there will be zero sucction/delivery )...
The hand primer pump is connected only to the sucction side and while priming by hand,fuel flows through the inlet valve into the sucction side and then via the outlet valve towards the fine fuel filter. So I `d think that unhermetical primer pump would compromize the fuel system performace even without apparent fuel leaks,as it is more easier for the pump to force air/fuel mixture through the outlet valve into the pressure side than up the primer pump internals.Same applies to the o-rings of the hollow bolts` threaded nipples on the lift pump(green now,Bosch,were black).Anyway,the new type primer pumps don`t leak when operated and are cheap.A leak-proof sytem will drip minimum or zero fuel back to the tank,even after months of non-operation.

P.S. the sub-ohm glow plug resistance numbers are temp.dependant and are usually ginven at room temperature in the manuals(20 celsius).