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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old January 9th, 2007, 09:55   #1
spiderb_151
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Default Do bad glow plugs effect mileage?

I have a '04 Jetta and recently got a cel for the glow plug on #3. At the same time I took a hit of 3 MPG but it was at the same time the weather got a lot colder. Weather hasn't changed mileage much if any in the past, can the glow plug? It still starts perfectly.
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Old January 9th, 2007, 10:01   #2
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I believe the short answer here will probably be 'Yes'. A quick search should reveal results of previous threads on this.
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Old January 9th, 2007, 10:27   #3
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how can a glow plug which has no use other than easier starting of the engine, cause worse fuel economy. Once it is running you don't need glow plugs at all. If they work or not, your engine will run the same once its fired up.
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Old January 9th, 2007, 10:49   #4
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Quote:
Originally Posted by bugablue
how can a glow plug which has no use other than easier starting of the engine, cause worse fuel economy. Once it is running you don't need glow plugs at all. If they work or not, your engine will run the same once its fired up.
I am sure someone will correct me if my memory is wrong here, but as I recall, the glow plug has two related functions. It provides an electrically heated spot in the cylinder to provide the "match" to start the burning of fuel and once the engine is up to temperature, it still provides that hot spot, but no longer needs the electrical power to provide outside heat.

Again, my memory on this one is shaky.
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Old January 9th, 2007, 10:54   #5
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I was under the impression that the heat created from the compressed air ignited the fuel. Heavy duty diesels don't have glow plugs.
Maybe this is the case on these smaller diesels though.
any other thoughts??
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Old January 9th, 2007, 11:12   #6
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Our cars can run (starting below 5C is another story though) just fine with no glow plugs provided the HOLE is plugged lol. Winter diesel and longer warm up do to the colder weather eats up 3mpg.
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Old January 9th, 2007, 15:25   #7
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Quote:
Originally Posted by Joe_Meehan
I am sure someone will correct me if my memory is wrong here, but as I recall, the glow plug has two related functions. It provides an electrically heated spot in the cylinder to provide the "match" to start the burning of fuel and once the engine is up to temperature, it still provides that hot spot, but no longer needs the electrical power to provide outside heat.

Again, my memory on this one is shaky.
This was my thinking too.
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Old January 9th, 2007, 15:46   #8
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Quote:
Originally Posted by Tickbait
This was my thinking too.
Are you clear on how a diesel operates now? (I'm not trying to be insulting). It has nothing to do with the glow plug except to start it, though granted ours keep them on after they start to help smooth them out and for emmissions purposes but it isn't a requirement.
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Old January 9th, 2007, 18:30   #9
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Quote:
Originally Posted by Slave2school
Our cars can run (starting below 5C is another story though) just fine with no glow plugs provided the HOLE is plugged lol. Winter diesel and longer warm up do to the colder weather eats up 3mpg.

I hear what you're saying but I live in San Diego... it isn't cold here to speak of and the change was sudden. I have records for the last year with very consistant fuel consumption, hence my concern for this having an issue. The timing was right on.

From what I understand plugs aren't used after cold starting anyway as others have mentioned. I do know they create a hot spot once the engine is running similar to glow (alcohol) engines for R/C toys.

Thanks for the input everybody, I'll change it out and see what happens.
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Old January 9th, 2007, 18:37   #10
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Do you know that the glow plug is bad? It may just be a connection. Check the glow plug before changing it.

Then if you do actually need to change it, be very careful with the new plug as it is ceramic and very fragile.
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Old January 9th, 2007, 19:04   #11
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Really not tying to be difficult here but the glow plugs do not create a "hot spot" once the engine is running. If fuel was to come in contact with the glow plugs you'd quickly find little bits of ceramic blowing out the exhaust. They serve only to heat the air in the cylinder to aid starting and in our case stay on for a while after for whatever reasons the vw engineers decided. You could put bolts in the glow plug holes and the heat required for combustion (after it has been started) will happen via compression of the air inside the cylinder.

Here's an explaination of the glow plug's purpose in 99% of the cases out there http://www.dieselforum.org/meet-clea...es/glow-plugs/
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Old January 10th, 2007, 09:16   #12
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I understand that glow plugs are not required. However, they are little more than a tube with a platinum coil in it. After combustion occurs they get very hot and the coils will glow red hot in the plugs. The metal making up the coil is so thin it heats instantly and gets hotter than any other metal in the chamber. It will create a hot spot, can't avoid it. That's all I'm getting at. This is a side effect and not required for operation or starting when it's warm out - understood, a critical point no doubt.

I'm really curious the impact it might have and how much the VW engineers rely on that for the expected emissions and performance of the engine. Does this have any impact on regular combustion? Probably not much, if any. We'll see if I gain any MPG back once it's replaced as I don't see a problem in the wiring harness...
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Old January 10th, 2007, 09:44   #13
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Quote:
Originally Posted by spiderb_151
I have a '04 Jetta and recently got a cel for the glow plug on #3. At the same time I took a hit of 3 MPG but it was at the same time the weather got a lot colder. Weather hasn't changed mileage much if any in the past, can the glow plug? It still starts perfectly.
No. Won't affect mileage.

GPs are used after startup for a few minutes in order to keep the cylinder hot for emissions purposes. If you lived in a climate that never got below freezing you could permanently disconnect them and live a much happier life.
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Old January 10th, 2007, 14:02   #14
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Thanks everyone. We all seem to agree that it shouldn't be an issue for normal operation, just emissions. Now I need to figure out what's really going on. I did have a helluva time starting this morning and the lift pump was really churning hard, normally I can't hear it at all. It suddenly caught and has been running much better. If that contiues we'll see how the MPGs end up.


Slave2school - nice link aboug plugs, thanks.
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Old January 10th, 2007, 17:37   #15
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If the lift pump is making noise, maybe that is your problem. There is a port to check fuel pressure to the rail.
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