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Thread: Smart 450 Cdi - How to change glow plugs & measure compression

  1. #11
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    Thanks for your explanations!
    Now preheat light stays on about 15 secs. also I check old plugs and 2 are out (infinite Ohm) , then I think ok now :-)

    Are the glow plugs just preheat or also postheat on Smart ???

  2. #12
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    The glow plugs are preheat, stay on whilst engine is being cranked and started and post heat for up to approximately 30 seconds depending on coolant temperature.

    Mjolinor has reminded me that the resistance measured in a glow plug varies with temperature. Glow plug resistance at room temperature is about 0.6 Ohm. At 100 C resistance is about 5 Ohm. Just mentioning this so nobody will change their glow plugs just because they do not measure 0.6 Ohm.

    An infinite resistance means surely that the glow plug is fubarred regardless of temperature.
    Drives a Smart Cdi - 65 to 85 MPG

  3. #13
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    Thanks Tolsen!!!

    Will fuel consumption decrease after glow plug changed? How much?

  4. #14
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    Quote Originally Posted by PhilLu View Post
    Thanks Tolsen!!!

    Will fuel consumption decrease after glow plug changed? How much?
    No reduction in fuel consumption. Glow plugs only make it possible to start engine when cold.
    Drives a Smart Cdi - 65 to 85 MPG

  5. #15
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    Thanks for all your fast and detailled answers!!!
    Have a nice Sunday evening!!!

  6. #16
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    Thanks for all your fast and detailled answers!!!
    Have a nice Sunday evening!!!

  7. #17
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    Here is why one should always remove glow plugs when engine is at operating temperature:

    Steel and aluminium expand at different rates. There coefficients of linear expansion α are:

    Aluminium αal = 0.000023 [mm/mmK]

    Steel αst = 0.000012 [mm/mmK]

    Compression of the glow plug body in cylinder head socket is relaxed by "Δx" mm when the glow plug is at engine operating temperature as opposed to room temperature.

    Δx = (αal - αst) x glow plug clamping distance x temperature difference

    "Glow plug clamping distance" is distance measured on glow plug from centre of threads to end of glow plug body, i.e. the part of the glow plug that is compressed when it is tightened up in cylinder head.

    Δx = (0.000023 - 0.000012) x 32 x 50 = 0.018 mm. This may appear tiny so perhaps better viewed in terms of stress in glow plug body using Hooke's law:

    σ = E x ε, where E is modulus of elasticity of steel 210000 N/mm2 and ε = Δx/32

    hence,

    σ = E x Δx/32 = 210000 x 0.018/ 32 = 118 N/mm2. This is a theoretical relaxation stress in glow plug body. In reality the stress will be much less, perhaps no more than one half and no less than one third of above since elastic deformation of the mating aluminium socket was not considered.

    The obvious conclusion is that there is a significant reduction in clamping load on glow plug body when engine is up to operating temperature. Less clamping load means much easier to unscrew with less risk of snapping off the plug.
    There will be a similar reduction in torque required to turn the business end of glow plug, which is stuck in hard carbon deposit. The hard deposit does not transfer any compression onto the glow plug cylindrical tip at operating temperature. Compression is applied as the aluminium head cools down. That is why torque required to undo a glow plug on a cold engine can be quite considerable, often much higher than torque causing glow plug to snap off.

    It follows from above that attempting glow plug replacement in Arctic sub zero conditions with cold engine will result in tears and failure.

    Did this make any sense I wonder?
    Drives a Smart Cdi - 65 to 85 MPG

  8. #18
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    LaserTools 5455.

    A low-torque air-powered impact vibration wrench set specifically designed for removing stuck and seized diesel engine glow plugs. Wrench has four pre-set torque settings and is reversible.
    Set includes three CR MO steel impact sockets (12mm, 10mm and 8mm) and a 1/4"D universal joint for use in restricted access.
    Can be used with other impact sockets to loosen and undo low-torque fixings where conditions (corrosion or carbon build-up) have led to tightness and seizure.

    The set includes:
    • Low-torque air impact wrench (1/4"D)
    • 1/4"D universal joint
    • Impact glow plug socket (12mm)
    • Impact glow plug socket (10mm)
    • Impact glow plug socket (8mm)
    • Air-line connector
    Set in foam for security and supplied in steel box.

    Link to manufacturer's web site:
    http://www.lasertools.co.uk/item.asp...1156&item=8046

    Replacing glow plugs when engine is at or near to operating temperature is advisable as per previous post.

    This is the 1/4" LaserTools 5455 impact driver with 10 mm socket fitted:

    Fantastic professional tool. Recommended!

    I made up an aluminium socket so I could test the tool breaking loose glow plugs at various ambient temperatures.


    Torqued up glow plug in socket to 2 x 30 degrees angle turn to simulate a nearly seized glow plug (i.e. torqued to twice the recommended angle turn).

    Let the assembly heat for a couple of minutes in boiling hot water and then attempted to unscrew glow plug using the impact driver at lowest torque setting. Glow plug came loose almost instantly.

    Repeated same test having left socket with glow plug fitted overnight in a freezer at minus 20 degrees C. Had to turn impact driver up to highest torque setting to loosen glow plug.

    Conclusion: Heat does it. Therefore never attempt removing glow plugs on a cold engine.
    Last edited by tolsen; 01-11-2017 at 12:44 PM. Reason: Corrected image links following Photobucket fiasco.
    Drives a Smart Cdi - 65 to 85 MPG

  9. #19
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    Have just corrected the cocked up image links caused by Photobucket greed and policy change regarding third party image hosting.

    Adding a wee note regarding the posted compression readings in post no 1.

    My initial readings:
    No 1 - 24.5 bar
    No 2 - 25.0 bar
    No 3 - 25.0 bar

    Much later, when soap testing around the injectors, I found a small leak around injector 1. Pulled injector, removed fire washer, cleaned seat and refitted with new fire washer. Repeated compression test and all cylinders were 25.0 bar.
    Drives a Smart Cdi - 65 to 85 MPG

  10. #20
    Join Date
    Jun 2015
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    Somewhere on the Wirral
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    Certainly makes for positive reading that your engine has even compression at 143000km (at the time the readings were taken), gives me some hope my 128000km 450 CDI engine will have reasonable readings when it is running again.

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