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  1. #1
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    Default engine knock when warm

    I know this has been discussed in a thread a long time ago, but I wanted to start my own thread.

    One of my 86 capris that I bought last spring has an engine knock. It ONLY does this when it gets warm and comes up to temperature. I have had enough exhaust leaks, etc. to know this is a real engine knock. I have done some homework and will run it by you all.

    - I have been told to pull each individual plug wire and see if it disappears. I have been told this may indicate a rod knock problem? Any validity to this claim ?

    - I have not popped the valve covers off yet. May get to that this weekend or next weekend. If I do not see anything out of the ordinary with the valve springs, retainers or lifters, how the heck do I tell if a lifter is bad ? And - can a lifter cause 'pump-up' problems when warm ? (Remember - I do not have this knock when the car is cold). I have already drained and changed the oil.

    - I haven't stuck a screw driver (as a stethescope) on the valve covers yet to determine if it is coming from a particular side of the engine yet. Will definitely do that this weekend.

    I know I don't have a lot to go on yet since I have not really done any proper trouble shooting as we speak, but I thought maybe you guys could give me some other things to check before and when I actually start trouble-shooting this weekend. This car is a bone-stock 86 capri (at least from what I can tell and hear). Factory intake, heads, gears, speed density, etc. I think someone has 'cracked' open the motor in the past as the head gaskets, valve cover gaskets, etc., look newer than this 86 should be. But - I strongly do not feel any mods have been made to this car (only a gasket kit replacement) as it has factory heads, and factory cam sound sound.

    Thanks,
    Jason

  2. #2
    FEP Super Member bwguardian's Avatar
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    Default

    Miles?
    HAD
    '82 GT monochromatic (red)...black cloth

    HAVE
    '85 GT vert two tone (white on charcoal)...white leather
    '00 F350 two tone (white on silver)...gray cloth
    '00 Excursion Limited two tone (white on tan)...tan leather...wifes ride

  3. #3
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    Default

    I am guessing about 120,000 (could have 220,000 - don't really know). Motor is clean. I never had a main engine failure in a 5.0, but of course I know it happens - especially since I believe it was opened up before to repair some gaskets. I don't know what else was or wasn't done internally, but from the outside - it is still stock.

    I would think if someone tore it down, they would have changed the intake, cam, or something to that effect, but this thing is totally stock from the the outside part #s.

    Jason

  4. #4

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    have you tried running any Marvel Mystery Oil through it? Doubt that would get rid of a knock but never know.

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  5. #5
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    Haven't tried yet. May give that a try as well....

    I could see certain things helping if it knocked all the time, but when cold - It doesn't knock at all. Once warmed up (and the oil gets warm/thin) - it knocks. Unfortunately - there may be a connection between the oil getting thin once warm and the knock that starts....

  6. #6

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    Have you checked your oil pressure with a mechanical gauge. I think I'd do that before anything at this point and see how your pressure is. Knocking could be caused by a variety of things.

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  7. #7
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    I'll check that. One of the other tools I do not have in my 'toolkit'. I need to buy one though. I'll run to advance this week and see if I can pick one up.

    I was looking for a guage like my fuel pressure guage tester. Do they make a direct mechanical guage for oil pressure ?

    Jason
    Last edited by 85mcLaren; 10-06-2010 at 09:38 AM.

  8. #8

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    You can use any kind of pressure gauge that measures up to around 70 PSI. I've got one
    from a hardware store, along with a high-pressure extension hose that I made up with the
    supplies they had there. You can also just buy one of those underdash oil pressure gauges
    and temporarily plumb it in.

    Need to narrow down the kind of knock you're hearing. Does it happen at idle? Or at a
    specific engine speed? Does it happen when the engine is under load? Or specifically when
    it's just holding speed?

    The one knock you mentioned has a very distinct characteristic. Rod knocks usually only
    happen when the engine is cruising. If you're accelerating, even gently, or decelerating, a
    rod knock will usually go away. Rod knocks usually have a much more metallic sound
    than other bottom-end knocks such as mains or piston slap.

    Removing the plug wire is normally a test for a piston pin knock, which is pretty rare on
    Fords, or it can also help find a cracked piston that's rocking in the bore when that cylinder
    fires.
    Cheers,
    Jeff Cook

    '79 Mont StaWag, 5.0, 58K original miles ... '81 Futura Coupe, 3.3, 62K original miles ...
    '85 GT 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT CFI/AOD Vert ...
    '65 notch, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ... '68 Vert 307 4-speed...
    (And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune)

  9. #9
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    Knock is at idle and at very light acceleration while in neutral. I do not hear it when I am driving. Maybe because of the noise of the motor, wind, etc., but I do not notice it while driving. Only at idle and under acceleration while in neutral and it is RPM related (increases as rpm increases, but goes away at high RPM - but engine noise might be noisy and drowning the knock out at high rpm.)

    Unfortunately - it does sound metallic.

    Jason

  10. #10

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    I have a knock just like that in my 86 Tbird with the 3.8. Exact symptoms. I was lead to believe that it may in fact be a broken left side catalytic converter. I haven't got to that yet. It sounds reasonable and it does sound metallic and it is not the heat shield. Broken pre-cats are apparently common.
    1986 Tbird 3.8
    1992 Ford TBird LX 3.8
    1992 Ranger XLT 2.3

  11. #11

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    I have a 86 5 liter in my 91 convertible that does the samething its not a very load knock but it still knocks, I think its a wrest pin. It carriers 60 psi of oil pressure and i also replace the rod and mains and the oil pump. Its been doing for a year now and it hasn't blown up yet. I'm going to pull it and put new piston and heads on it this winter hopefully that fixes it.

  12. #12

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    Definitely could be catalyst, especially if the car has aftermarket ones. The bricks get loose
    in the shell and rattle back and forth as the exhaust pulses. Got one of those on my '85 T-Top
    right now, and it sounds just like a lifter hammering, 'ceptinur you can hear it coming out the
    tailpipes.
    Cheers,
    Jeff Cook

    '79 Mont StaWag, 5.0, 58K original miles ... '81 Futura Coupe, 3.3, 62K original miles ...
    '85 GT 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT CFI/AOD Vert ...
    '65 notch, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ... '68 Vert 307 4-speed...
    (And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune)

  13. #13
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    The cat could explain a lot, but unfortunately - she has an off road motorsport pipe. I will crawl under it and check it out to make sure it is an offroad pipe, but I think that is one of the only things that was changed.

    I should have remembered that and stated that earlier, but forgot.

    At least you guys are making me think.

    Jason

  14. #14
    FEP Power Member 306gt's Avatar
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    Check the oil pressure when warm with a mechanical guage. If you pull off the plug wires 1 by 1 and it goes away it could be a worn wrist pin or a cracked piston skirt or a cracked or worn wrist pin boss in the piston. Like JACook said. A connecting rod knock will usually get worse and louder as time goes by. If the oil pressure drops real low when warm it could be loose bearing clearances from worn bearings. Main bearing knocks can usually be heard at a steady cruise with and very light load applied from the throttle. (It will sound like growling).
    85 G.T. All motor
    337 c.i.d 11.44-120 mph

    88 L.X. Hatch 5.0L H.O.
    5-speed. 13.41-102 mph on E.T. Streets winter beater

    1984 1/2 G.T. 350
    306, E-7's, Air gap, 750 edelbrock carb, 5-speed, 3.73's
    13.10-106 mph on radial street tires. Kid neighbor driving. Summer car

  15. #15

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    Quote Originally Posted by 306gt View Post
    Check the oil pressure when warm with a mechanical guage. If you pull off the plug wires 1 by 1 and it goes away it could be a worn wrist pin or a cracked piston skirt or a cracked or worn wrist pin boss in the piston. Like JACook said. A connecting rod knock will usually get worse and louder as time goes by. If the oil pressure drops real low when warm it could be loose bearing clearances from worn bearings. Main bearing knocks can usually be heard at a steady cruise with and very light load applied from the throttle. (It will sound like growling).
    Thanks for the info

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