1998 E300 will not pass CA smog because of bad glow plug. What to do? - Page 2 - PeachParts Mercedes-Benz Forum

ESchwab · #1 01-25-2017, 10:34 PM

Quote:

Originally Posted by ROLLGUY

The wires are very short, and can't be removed without removing the manifold. The soup can is a great idea though, I might give that a try.

He may not have to take the old wire out. He could unplug from the glow plug relay the wire to the bad glow plug and just use a different wire in its place that would go from the relay to the dummy glow plug or resistor.

On second thought, I don't remember if one of the wires can be separated from the six wires that plug into the relay. It's worth checking.

ah-kay · #2 01-25-2017, 08:30 PM

Are you sure it is the glow plug? I would wire a glow plug to dummy out the bad one and give it a shot. I really cannot see how the computer can see the GP is inside the cylinder or outside. Just put it somewhere secure. A resistor would not cut it. The resistance of the GP would increase with temperature, a 0.5 resistor would not. It is cheaper to put a GP in than a 50W 0.5 ohm resistor in any event.

ROLLGUY · #3 01-25-2017, 09:03 PM

Quote:

Originally Posted by ah-kay

Are you sure it is the glow plug? I would wire a glow plug to dummy out the bad one and give it a shot. I really cannot see how the computer can see the GP is inside the cylinder or outside. Just put it somewhere secure. A resistor would not cut it. The resistance of the GP would increase with temperature, a 0.5 resistor would not. It is cheaper to put a GP in than a 50W 0.5 ohm resistor in any event.

Yes I'm sure it has at least one bad glow plug, and that is why the CEL is lit.

Frank Reiner · #4 01-25-2017, 09:22 PM

Just as a thought on current, if each of six glow plugs drew 25A, and as they are in parallel, the total draw would be 150A, which would require a cable of 1/0 or 2/0 feeding the relay. Does that correspond with what is installed?

ah-kay · #5 01-25-2017, 10:13 PM

Quote:

Originally Posted by Frank Reiner

Just as a thought on current, if each of six glow plugs drew 25A, and as they are in parallel, the total draw would be 150A, which would require a cable of 1/0 or 2/0 feeding the relay. Does that correspond with what is installed?

GP would draw current in the order of 10-12A when hot.

Mxfrank · #6 01-26-2017, 11:56 AM

Quote:

Originally Posted by ah-kay

GP would draw current in the order of 10-12A when hot.

So one ohm. The resistance is very important, not just to calculate the current draw of a resistor, but also because the relay compares the resistance of the GP's and will only turn off the CEL if they are nearly equal.

tjts1 · #7 01-25-2017, 10:44 PM

This BS is the only reason I bought a 97 instead of a 98 or 99. Pre 98 diesels are exempt from smog.

ah-kay · #8 01-26-2017, 01:28 AM

Quote:

Originally Posted by tjts1

This BS is the only reason I bought a 97 instead of a 98 or 99. Pre 98 diesels are exempt from smog.

What year is your car doesn't help the OP. You are stating the obvious.

tjts1 · #9 01-26-2017, 04:02 AM

Quote:

Originally Posted by ah-kay

What year is your car doesn't help the OP. You are stating the obvious.

See page 1

compu_85 · #10 01-26-2017, 08:33 AM

Not that I'd recommend this, but I'd think that if you bridge all the glow plug outputs in the glow plug relay it would think everything was fine. Note, if you do this you'll lose the circuit breaker protection on each glow plug circuit.

-J

pimpernell · #11 01-26-2017, 10:33 AM

Quote:

Originally Posted by ROLLGUY

My friends E300 is past due on the registration because it won't pass CA smog test. The CEL is on because of one or more bad glow plugs, and the head will have to come off in order to replace them (stripped threads in the head). In the meantime, the car needs to be registered so he can legally drive it. My question is- can a resistor or something else be put in th glow plug circuit to fake out the computer to turn off the CEL? The smog tech will not ignore the light, even though it is only on because of a bad glow plug (or two). Any ideas would be greatly appreciated. Also, he wants me to buy the car from him (I used to own it), as he needs an SUV. Either way, the head has to eventually come off in order to fix the glow plugs. But until then, I would like to be able to get it smogged. Thanks, Rich

I have read a number of posts on this forum where buyers have purchased E300ds with no cel showing on the dash. Then after doing some work on the car, they found that one of the glow plugs was stuck in the head, and had a dummy one tack welded to the side of the head with the plug wire attached to it. It usually turned out to be the number 6 plug that was buried in the back of the engine and was not easily seen.

BillGrissom · #12 01-26-2017, 10:47 AM

I agree that an external glow plug is the best load. Any resistor that fakes it would need to draw similar power and thus also glow. Screwing it into a metal tube is the best idea, to manage the heat. But, the suggestion to bridge all the glowplug wire pairs together might be simplest.

First time I heard of this big problem that glow-plugs get stuck in the head (aluminum?). Perhaps M-B installed them w/o anti-seize on the threads. Stuck spark plugs in aluminum heads is a similar problem. Sometimes removing one shears the threads in the head. One reason to remove and anti-seize them before their 100k mile lifetime. I can see smaller glow-plugs being a bigger problem. The 1990's M-B diesels look nicer and perform better than my 1980's clunkers, but I read more horror stories about serious maintenance issues.

ah-kay · #13 01-26-2017, 12:40 PM

The GP current has to be 15 amp max at steady state ( when hot ) as the fuse is rated 80A or there about for 6 GPs. That is why putting a 0.5 ohm 50W resistor is a BAD idea as the resistance of GP varies with temperature.

ROLLGUY · #14 01-30-2017, 01:59 PM

I got the car today and checked all the glow plugs, and they all seem to be working. I would have thought that th glow plug was the problem, but it may be a bad relay instead. I took off the manifold so I could isolate each wire for testing, and also each plug. I tested each wire between the connector and each plug, and there are no shorts or open leads. I then tested each plug with my meter on the 200 ohm scale, and I got between 1.4 & 1.9 ohms on all of them. The glow plug light is still lit is still lit, and the "check engine electronics" message still appears. I am not sure what to do now, other than grab the GP relay off of my friends parts car for testing. The car I am working on is three months past due on the tags, so I would like to help him get the car smogged before he get pulled over for expired tags.

funola · #15 01-30-2017, 02:14 PM

You cannot measure accurately the resistance of a glow plug with an ohmmeter. Easiest way to check while installed in an engine is measure the power they draw with an ammeter. Put the ammeter on the 20 A DC scale, connect one lead to each glow plug, other lead to battery + terminal. If the glow plug is good, it should surge to 18 - 20 A, then steadily drop to 8 -10 A as the glow plug heats up. The amps drop because the Nichrome wire resistance increases when heated.

Edit: If the 1.4 and 1.9 ohm reading is accurate, it indicates a bad glow plug. A good one should be around 0.7 ohm.