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Thread: What is this smoke?

  1. #1
    Senior Member DonTom's Avatar
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    What is this smoke?

    While I was checking things with the doghouse removed on my RV (2000 Chevy 7.4L), while the engine was running, I noticed some smoke under the exhaust manifold, near the number 7 cylinder on the bottom (of course where I cannot see it!). Not when I start the engine, but after it warms up for about five minutes. I leave it running and after 20 minutes or so, the smoke completely stops.

    I let the engine cool down for more than an hour thinking it could be just a drop of oil that burnt away, but when I restarted it, it was all the same all over again. Smoke in five minutes and stopped smoking in about 20 minutes. What could cause that? I cannot really tell where the smoke is coming from other than it looks like it's under the exhaust manifold by cylinder number 7, just below the spark plug. But what would cause the smoke to happen only between five and 20 minutes after starting?

    -Don- Auburn, CA

  2. #2
    Senior Member Ken's Avatar
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    Sounds heat related, Don. A slight leak when cold that seals itself off when it gets hot.
    Takes a little time for the oil/whatever to get hot enough to smoke. Only thing i can
    think of in the area you describe is a cylinder head gasket. grouch may have a better
    idea if he reads this post.

    Ken.
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    Ken.
    Wolds Bikers, Lincolnshire, England.

  3. #3
    Senior Member DonTom's Avatar
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    Quote Originally Posted by Ken View Post
    Sounds heat related, Don. A slight leak when cold that seals itself off when it gets hot.
    Takes a little time for the oil/whatever to get hot enough to smoke. Only thing i can
    think of in the area you describe is a cylinder head gasket. grouch may have a better
    idea if he reads this post.

    Ken.
    I see edit is still not working, as I was going to try to edit the last post because it's cylinder number 8, not seven. The cylinder in the right rear from the driver's seat. I don't know why I said number seven, but it was on the even side, so its number eight, not that I expect that to make any difference with this type of problem.

    Anyway, I was thinking about the same. Either the head gasket or perhaps a small crack on the bottom of the exhaust manifold under the number 8 cylinder.

    There is perhaps one more possibility, a leaky valve cover gasket. I see some oil above the number 8 spark plug and I was wondering if it could be dripping down there somehow, but I don't think that would explain why the smoke stops. But I dried it off the best I could with a paper towel when cool and it still happens around five minutes after I start the engine. None of the other seven cylinders have any signs of oil in that area, so I am wondering if this is related. I would rather deal with a valve cover gasket than a head gasket! I did check for loose valve cover bolts, but they are all tight.

    If it seals itself when hot, and is the head gasket, how concerned should I be? Is it a sign of a much more serious problem coming down the road? Such as a blown head gasket soon to happen?

    For all I know, I could have had this problem for years without even knowing, as I cannot remember ever running the engine before with the dog house removed.

    -Don- Auburn, CA

  4. #4
    Senior Member Ken's Avatar
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    Hi, Don.

    Re; 'Edit', I've sent Eric two emails - one general and one private,
    but so far have not had a reply from him. Usually he replies very
    quickly so I am wondering if he away for a while.

    A leaky cylinder head gasket is something you should check out
    before it blows and leaves you stranded. First thing I would
    try is check the torque of the cylinder head studs/bolts. If any
    are below the required figure torque 'em up. I would also
    change the rocker cover gasket at the same time. If that
    stops the problem, fine. If not then my advice would be
    try and get a positive location of the leak and, if necessary,
    replace the cylinder head gasket.

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  5. #5
    Senior Member grouch's Avatar
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    The most common cause for something like this is a leak at the valve cover gasket. It seeps a bit when cool. If your sense of smell is still good (I may be deaf as a post and losing sight in one eye but my smeller is still good.) start the engine and let it run until the smoke starts. Sniff the smoke and see what it smells like. Hot oil, probably a valve cover gasket. An acrid/sickly sweet smell, it might be coolant. Coolant could be a head gasket or just a leaky hose. Generally, head gaskets blow but don't seep. They can, but usually they leak internally. RV's usually have pretty good maintenance as shown by your current activity. This means the coolant is changed regularly. If a previous owner didn't change it every 5 years or so, you might have an expansion plug (freeze plug) starting to leak. If this is the case, that can fail quick.

    I'd see what it smells like. If it's oil, I'd replace the valve cover gaskets and use some RTV when putting them on. If it's coolant, I'd put some UV dye in the cooling system and use a test light (be sure to use the yellow safety glasses because of the UV) to track the spot of the leak.
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  6. #6
    Senior Member grouch's Avatar
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    Oops, I forgot to add and the edit still isn't working, shut the engine off when it starts to smoke so the smoke is more concentrated.
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  7. #7
    Senior Member Ken's Avatar
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    This 'Edit' problem is a damn nuisance. I have emailed Eric
    twice and also sent a PM to 'Oooorgle' our software guy but
    so far have had no response from either. I've checked all
    the settings and permissions and they all seem to be OK.
    It may be that a software change the Forum designers
    have made somewhere has caused a bug. In the meantime
    I guess we will just have to either make fault free posts or
    just keep adding riders to our original posts.

    Ken.
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    Ken.
    Wolds Bikers, Lincolnshire, England.

  8. #8
    Senior Member DonTom's Avatar
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    Quote Originally Posted by grouch View Post
    The most common cause for something like this is a leak at the valve cover gasket. It seeps a bit when cool. If your sense of smell is still good (I may be deaf as a post and losing sight in one eye but my smeller is still good.) start the engine and let it run until the smoke starts. Sniff the smoke and see what it smells like. Hot oil, probably a valve cover gasket. An acrid/sickly sweet smell, it might be coolant. Coolant could be a head gasket or just a leaky hose. Generally, head gaskets blow but don't seep. They can, but usually they leak internally. RV's usually have pretty good maintenance as shown by your current activity. This means the coolant is changed regularly. If a previous owner didn't change it every 5 years or so, you might have an expansion plug (freeze plug) starting to leak. If this is the case, that can fail quick.

    I'd see what it smells like. If it's oil, I'd replace the valve cover gaskets and use some RTV when putting them on. If it's coolant, I'd put some UV dye in the cooling system and use a test light (be sure to use the yellow safety glasses because of the UV) to track the spot of the leak.
    Thanks, and I do see evidence of the valve cover gasket being the problem as there is noticeable oil under the valve cover closest to the number 8 cylinder. I could not tell if it was dripping down to the exhaust. Now I am now convinced that is the problem.

    I hate to try to replace the valve cover gasket now that I have the RV all back together, but I assume something like this can wait until I have a need to take the doghouse off again. IAC, I will buy the valve cover gaskets today. Looks like a hassle as there is a lot of stuff in the way. OTOH, I could start doing it today now that I am fairly sure that is the problem.

    Thanks.

    -Don- Auburn, CA

  9. #9
    Senior Member DonTom's Avatar
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    I went to the local auto parts store. Seems they have to order everything for a 7.4L engine unless it is also used elsewhere. I did get the RTV, the blue stuff. I noticed they had several different types as well as colors Also for different temps. Are some types of RTV better than others for valve cover gaskets?

    I will get the new valve cover gaskets in a couple of days.

    -Don- Auburn, CA

  10. #10
    Senior Member grouch's Avatar
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    Quote Originally Posted by DonTom View Post
    I went to the local auto parts store. Seems they have to order everything for a 7.4L engine unless it is also used elsewhere. I did get the RTV, the blue stuff. I noticed they had several different types as well as colors Also for different temps. Are some types of RTV better than others for valve cover gaskets?

    I will get the new valve cover gaskets in a couple of days.

    -Don- Auburn, CA

    I generally use RTV black. You can use any color you want. Just make sure it's marked "oil resistant". I'm not real up on metric. Is the 7.4 a 454? It hasn't been used for a bit so I can see parts not being on the shelf. I'm starting to find a shortage of parts for a 350. (5.7)
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  11. #11
    Senior Member Ken's Avatar
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    7.4 L is 451.56 cubic inches. Assuming the 7.4 L is actually
    slightly under a full '7.4' it is probably a 450.

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  12. #12
    Senior Member Ken's Avatar
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    Looking it up it seems the 7.4 Liter GM engine was called a
    454 which is actually works out at 7.44 L. Looking up the
    specs for the 454 there is quite a wide range of different
    bores and strokes quoted so it appears that the CU block
    size is just a nominal figure. Do you know which actual
    spec your engine is, Don? From the date it might be
    an L21 or L29 Vortek.

    Ken.

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  13. #13
    Senior Member DonTom's Avatar
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    Quote Originally Posted by Ken View Post
    Looking it up it seems the 7.4 Liter GM engine was called a
    454 which is actually works out at 7.44 L. Looking up the
    specs for the 454 there is quite a wide range of different
    bores and strokes quoted so it appears that the CU block
    size is just a nominal figure. Do you know which actual
    spec your engine is, Don? From the date it might be
    an L21 or L29 Vortek.

    Ken.

    Ken.
    My 7.4L is a Vortec, but it's an aluminum (uses 5W30 engine oil) MPFI engine and has a lot more guts than the older 454. The engine size is about the same, but that is all. It has normal long ignition wires and a distributor, so I don't think it is the L21 or the L29 as mine has just one ignition coil for all 8 cylinders. Even though it has a distributor, I don't think it can be adjusted. This engine uses a CPS.

    -Don- Auburn, CA

  14. #14
    Senior Member grouch's Avatar
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    Quote Originally Posted by DonTom View Post
    My 7.4L is a Vortec, but it's an aluminum (uses 5W30 engine oil) MPFI engine and has a lot more guts than the older 454. The engine size is about the same, but that is all. It has normal long ignition wires and a distributor, so I don't think it is the L21 or the L29 as mine has just one ignition coil for all 8 cylinders. Even though it has a distributor, I don't think it can be adjusted. This engine uses a CPS.

    -Don- Auburn, CA
    That's why I drove 200 miles to buy a barn find pickup that has sat for 5 years. It's a '93. In '96 you have to use a computer to adjust or even install a distributor.
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  15. #15
    Senior Member DonTom's Avatar
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    Quote Originally Posted by grouch View Post
    That's why I drove 200 miles to buy a barn find pickup that has sat for 5 years. It's a '93. In '96 you have to use a computer to adjust or even install a distributor.
    Can it be done with a laptop or does it require special equipment that only dealers and shops have?

    Anyway, I removed both my valve covers today. On the right (leaky) side it was somewhat of a hassle, but I got it out. I am not sure if I can get it back in without the new gasket touching stuff in the very tight fit. And that is after I loosened the bolts on both sides to the tranny and engine dipstick (runs right on top of the valve cover) and totally removed the throttle body (in the way of one VC bolt).

    The new gaskets are those blue rubber gaskets. They match the color of my RTV! Does the RVT only go on top of the head where it makes contact with the rubber VC gasket?

    -Don- Auburn, CA

  16. #16
    Senior Member Ken's Avatar
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    Generally over here it was normal to use RTV on cork
    VC gaskets but not with new silicone rubber gaskets.

    Mating surfaces should be spotlessly clean and dry
    before fitting the gasket whether or not RTV is
    used..

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  17. #17
    Senior Member Ken's Avatar
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    I'll be glad when the 'Edit' function gets fixed!

    Re. cork gaskets, it was a sealant we used on cork gaskets
    usually some thing like Hermetite Red or Hylomar, not
    silicone RTV. (Blame fingers typing quicker than brain
    can think.)

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  18. #18
    Senior Member DonTom's Avatar
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    Quote Originally Posted by Ken View Post
    Generally over here it was normal to use RTV on cork
    VC gaskets but not with new silicone rubber gaskets.

    Mating surfaces should be spotlessly clean and dry
    before fitting the gasket whether or not RTV is
    used..

    Ken.
    Would it be best for me just to use the new rubber gaskets without anything?

    -Don- Auburn, CA

  19. #19
    Senior Member Ken's Avatar
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    Quote Originally Posted by DonTom View Post
    Would it be best for me just to use the new rubber gaskets without anything?

    -Don- Auburn, CA
    Hi, Don.

    The main consensus of opinion suggests that there is no need
    to use sealant with silicone rubber gaskets. A couple of sources
    say there is no need but it won't do any harm. Was there any
    advice on the package they came in?

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  20. #20
    Senior Member DonTom's Avatar
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    Quote Originally Posted by Ken View Post
    Hi, Don.

    The main consensus of opinion suggests that there is no need
    to use sealant with silicone rubber gaskets. A couple of sources
    say there is no need but it won't do any harm. Was there any
    advice on the package they came in?

    Ken.
    Read number 10 here.

    I replaced both VC gaskets yesterday, but I have not put it all back together. Many things had to be removed to get clearance (tranny dipstick tube, throttle body, air intake, plug wires and more). And even after all that is removed, it is still a very tight fit.

    Yesterday, I didn't start working on it until kinda late in the afternoon.

    After I wake up a bit more (I just now got up) and work another 20 minutes or so putting the rest of it back together, I will be able start the engine and check for leaks.

    One more cup of coffee first.

    -Don- Auburn, CA

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