Do bad glow plugs effect mileage?

spiderb_151

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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.
 

Tickbait

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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.
 

bugablue

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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.
 

Joe_Meehan

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bugablue said:
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.
 

bugablue

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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??
 

Slave2school

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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.
 

Tickbait

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Joe_Meehan said:
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.
 

Slave2school

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Tickbait said:
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.
 

spiderb_151

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Slave2school said:
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.
 

whitedog

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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.
 

Slave2school

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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-clean-diesel/what-is-clean-diesel/new-technologies/glow-plugs/
 

spiderb_151

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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...
 

weedeater

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spiderb_151 said:
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.
 

spiderb_151

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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.
 

whitedog

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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.
 

spiderb_151

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Update: More fun with the car this week. It started having a very hard time starting and would kill on the highway, then restart if I coasted in gear long enough. Turns out the ECT (engine coolant temp sensor) is bad and it's running very very rich as if the engine were cold. This explains the bad mileage and smoke I noticed recently.

The part is only available from the dealer so my car is stuck in a nursing home parking lot for now. I can't justify a tow for a $25 part... ugh.

Thanks again everyone for your suggestions. I hope this is it!
 

dspencer

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Just to reinforce the thought, a diesel needs no glo plug to run properly or fire. My Dodge uses a grid heater which heats the air in the intake to the engine. No glo plugs at all.
 

PDJetta

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spiderb_151 said:
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.
Just heed the warnings about the ceramic glow plugs that the PDs use. You may want to sort on this. Long story short is that you have to be careful installing them or they could break off later in the cylinder head and destroy the engine.

--Nate
 

McBrew

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The part is only available from the dealer
Yeah... did the dealership tell you that? I grabbed a few on sale at Impex for $5/each. I think they are normally about $15 or so. Also, check tdiparts.com.
 

dieselTQrules

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Where are you filling up? I haven't had the best of mileage either, and I am using the station by the 5 & Main Street near the Navy base, I think it's called National Gas. Maybe there's some winter blend in there because we've been feeling quite a cold spell. This is damn cold for San Diego. For cryin out loud, we're fifteen minutes from Mexico!
 

Huweth

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spiderb_151 said:
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.
Not the case. No matter what you put in there, it can only get as hot as the rest of the chamber (unless of course there is current passing through the coil still). For example, if you put a thermometer in hot water, whether the thermometer is alcohol or mercury it can only ever be as hot as the water itself. If you put a piece of metal in a fire, just because it's metal doesn't mean it gets hotter than the wood it's being heated by.
This all means that once an engine is up to temp, the chamber walls, piston head, valve heads etc... are all at the same temp. While the temp of the glow plug is the same as the rest, the fact that it protrudes slightly might mean that it has some effect, though I expect not.

Glad you got the problem sorted.
 

McBrew

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My tractor doesn't even have glowplugs. If it is cold outside, you can hold the key in the pre-start position and it turns on a heating element in the air cleaner to pre-heat the air. You only need to do this if it is 50F or below. It is a very simple engine, compared to our TDIs!
 

Brock_from_WI

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On my 03 the regular glow plugs are only on when I am starting cold, they do stay on about half as long as the glow plug light was on. So if it was lit for 10 seconds the plugs actually stay on for another 5 seconds or 15 seconds total. They shut of after the engine is started and I have never seen them on once the engine was started. However once the engine starts my three coolant glow come on and stay on until my coolant hits about 105F (even in summer), then they shut off. This may be different for autos since they don't have coolant glow plugs, only manual transmissions have the coolant glow plugs. The PD's could be different.
 
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spiderb_151

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I'll reply to the last round of posts here...

I know the part is available online but I couldn't wait for shipping so it was only availble from the dealer on short notice. FWIW, the install is as simple as they get. Make sure you have a little bit of coolant to refill with. According to the dealer they replace 5 or 6 of these a week. They stock 50 and reorder at 10 to 15!! I now carry a spare sensor, o-ring, clip and coolant and added the tools to my fuel filter swap kit.

I'm done talking about the need for glow plugs, I think we all got it by now. I understand (and did before this) very well they are not needed but rather help. That doesn't mean they don't have some effect on operation.

I'm filling up mostly on B100 but use name brand diesel when I need it. I don't have a favorite other than high volume stations. Before I get asked - the difference in MPG between D100 and B100 is about 1.5.

Huweth, the metal making up the chamber doesn't get as hot as the actual fluids IN the combustion. The coil in the plugs can get hotter than the surrounding metal since it requires less energy, due to less mass of course, to change temperature. A great example of this is engines used in model aircract/boats/cars etc. The element in the glow plug is much hotter than the surrounding metal of the chamber, piston and head. It glows a very bright red hot. If the rest of the material followed suit there would be hell to pay! Anyway, points made and we can't measure it so I consider it closed.... unless someone can actuall measure it. I wonder if we can measure resistance in the coil to determine temperature.... probably....:) Maybe it isn't a closed topic!

Thanks for the input everybody. FYI, the sensor didn't end up being the whole problem. The pickup line in the fuel tank was closed off since the flapper valve got jammed.... It ran better after the sensor was replaced but didn't run well until I pulled the fuel level sending unit.
 

DPM

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Bosch tell us that steady-state temperature of the GP's tip is around 1000C, and that the required temperature to initiate combustion is 850C...
 

TornadoRed

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dieselTQrules said:
Where are you filling up? I haven't had the best of mileage either, and I am using the station by the 5 & Main Street near the Navy base, I think it's called National Gas. Maybe there's some winter blend in there because we've been feeling quite a cold spell. This is damn cold for San Diego. For cryin out loud, we're fifteen minutes from Mexico!
It can get pretty damn cold in Mexico, too.

I don't think we are getting winterized fuel here -- there doesn't seem much point. Truckers who plan on driving north or east typically won't be filling up in SD -- they will wait until Flagstaff at least if they are going east on I-40, or until southern Utah if they are headed up I-15. Or they will just put in enough to last them until they reach those colder-weather stations.

My own fuel numbers for the last couple weeks have been very normal for this time of the year -- withing 0.5 mpg of the same period in both 2005 and 2006.

So dieselTQrules, if you are seeing lower mileage, either your driving pattern has changed or there is some other unknown reason. It's not the fuel, IMO.
 

jettawreck

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151-I don't think you do quite get it but good work getting those problems solved. Thanks to this site my first mod was to remove the check valve in my fuel pickup unit as it can be a real problem up here in the cold. Any restriction to fuel flowing at below 0F is not good.
 

spiderb_151

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If you believe I don't understand how these things work, please tell me why.

I do know that plugs are not required. If temps drop to much some form of warmed air helps and may be required. Other than that, what is there? My questioning of whether they have any effect on general operation is fair, chances are they do but it's very very minor and may be limited to nothing more than emmisions not power or obvious efficiency.

If there's something beyond that, explain.
 

jettawreck

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Above 40F aprox. they do nothing-no hot spot effect-nothing. Even though the indicator light comes on briefly.. still nothing. Below 40F aprox they come on for various length of time to warm air inside cylinder to help starting (this amount of time can be manipulated w/VAG-Com). They can also remain on for a short time after starting (mostly to speed warm-up/emissions). If you have a manual trans only there are 3 coolant glowplugs that heat/cycle below set temps and rpms until water temp is up to about 120 IIRC. Didn't really intend to be rude, just in a hurry to close up shop here for the day and head home. Time to turn on some glowplugs and fire up the TDI, it's about 10F outside. Sorry and goodevening.
 
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