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Thread: Over Revved and no compression

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    Default Over Revved and no compression

    The other day I had my 260 out and the pedal got stuck under the floor mat. In any case it revved pretty high before I could release it. Now I have a loud valve tap and the # 5 cylinder seems to have lost compression. I did not do a compression test but rather a simple finger over the spark plug hole.
    Could I have bent a valve? It over revved for maybe 20 to 30 seconds. Can these Z engines typically have piston to valve contact. The head was rebuilt 500 miles ago, timing chain etc all new.


    Any ideas or suggestions would be appreciated

    Thanks
    Glenn

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    Valve float is a possibility if you pegged it past redline for that long.
    -Bo

    1972 240z - Not original and still not done.
    "Something wicked this way comes...."

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    Registered User doradox's Avatar
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    Quote Originally Posted by zdude1967 View Post
    The other day I had my 260 out and the pedal got stuck under the floor mat. In any case it revved pretty high before I could release it. Now I have a loud valve tap and the # 5 cylinder seems to have lost compression. I did not do a compression test but rather a simple finger over the spark plug hole.
    Could I have bent a valve? It over revved for maybe 20 to 30 seconds. Can these Z engines typically have piston to valve contact. The head was rebuilt 500 miles ago, timing chain etc all new.


    Any ideas or suggestions would be appreciated

    Thanks
    Glenn
    A lash pad could be cocked and holding a valve open. I'd pull the valve cover and take a peek.

    Steve

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    Registered User LeonV's Avatar
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    Yep, pull the valve cover and inspect the valvetrain for loose parts. I'm not getting where "valve float" comes into play here.

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    Pull the cover, I will bet a cup of coffee that you popped a rocker off, maybe even two. I routinely take my Z to 7k rpm, the rev limiter is set to 7100 and occasionally I pop a rocker off. FWIW, at my last race I spent most of the time between 6200-7000 in 3rd or 4th gear. While it is certainly possible you may have damaged something, it is unlikely.

    If you haven't driven it, the lash pad(s) is probably lying in the head.

    Re-adjust your valves, factory settings, and I will bet you will be fine. I ran 13 laps of a race on 5 cylinders, I popped #5's intake and exhaust rockers off. They were lying in the head and the lash pads made it to the pan. Replaced everything, re-adjusted the valves Compression test and leakdowns were good enough to race the same engine another 6 months.

    YMMV,
    Paul

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    Quote Originally Posted by zdude1967 View Post
    The other day I had my 260 out and the pedal got stuck under the floor mat. In any case it revved pretty high before I could release it. Now I have a loud valve tap and the # 5 cylinder seems to have lost compression. I did not do a compression test but rather a simple finger over the spark plug hole.
    Could I have bent a valve? It over revved for maybe 20 to 30 seconds.
    I said valve float could be a possibility. Pedal stuck, revved pretty high for 20 to 30 seconds, etc.

    A popped rocker would not cause a loud tap; a cocked rocker could but if the rocker is lying in top of the head if would not tap. If a rocker popped off the cylinder would build pressure as well as the valve would always be closed.

    Pull the valve cover and take a look. It could be a loose rocker, cocked rocker, etc. I would also run a compression test to verify your assumption. If #5 is low, run a leak down (or just run a leak down test first).
    Last edited by ktm; 10-09-2010 at 07:26 PM.
    -Bo

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    "Something wicked this way comes...."

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    The rockers are on... two were very loose and the lock nut was loose as well. Seems like the lash pads are in and correct. The retaining spring (not the valve spring) seems ok as well.

    Any suggestions

    Thanks
    Glenn

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    Which two were loose?

    You really need to run a compression test or preferably a leak down test.
    -Bo

    1972 240z - Not original and still not done.
    "Something wicked this way comes...."

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    Registered User Gary in NJ's Avatar
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    A leak down test (aka diffential compression) is the tell-all method. Harbor Freight has a tester for around $30. Not professional grade mind you, but it is something you will not often use.
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    The number 4 intake and both # 5 were loose.
    I guess what I was/am hoping for is some issue in the head that can be corrected with the head on...bound springs or something. The lash pads seem fine, the rockers do not seem to be bent, I would have to remove them to make sure. My understanding is the leak down test will determine where the compression is going, exhaust, intake or rings/piston. My goal at this point is to hopefully determine the cause to be in the head and able to repair without head removal.Can these engines make piston to valve contact without snapping the timing chain.

    Glenn

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    Quote Originally Posted by zdude1967 View Post
    Can these engines make piston to valve contact without snapping the timing chain.

    Glenn
    Yes, which is why I said valve float. Google valve float for a better understanding.

    However, I am not saying you experienced valve float only that valve float can cause piston to valve contact. If the rockers were loose then that could explain the louder tap. When I had a hydraulic lifter collapse on my L28ET it was a very loud tap.

    Do a compression test or preferably a leak down test. We are only guessing at this point. A finger over the hole is not really a valid test.
    -Bo

    1972 240z - Not original and still not done.
    "Something wicked this way comes...."

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    Supporting Member Diseazd's Avatar
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    You probably did bend a valve if you've got stock valve springs. Just run a compression check......if you bent a valve, you'll read "0" in that cylinder. If you put your finger over the plug hole and it didn't push compressed air against your finger, my guess is you're running "0" compression in that cylinder.
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    I am heading to Harbor Freight now for the compression tester.
    If I do find it is a bent valve should I have concerns about any damage to the bottom end.
    If so is there a way to check it for damage. I am not sure how many miles are even on this engine, it did run weel prior to the over rev.

    Glenn

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    Don't get overworked yet. Just do a compression test and see what it says. Post the numbers once your done and go from there.
    -Bo

    1972 240z - Not original and still not done.
    "Something wicked this way comes...."

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    KTM...Thanks.

    Short delay due to family matters. I hope to do the compression test by the weekend. I will post the numbers then. Thanks for your help

    Glenn

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    Glenn,

    When I conduct a differential compression test I put the piston in the cylinder to be checked at TDC, then place the transmission in 1st gear (with the e-brake on and the wheels chocked). If you have a helper. have them sit in the car with their foot on the brake.

    I then open the valve on the tester to allow 80 pounds into the cylinder. 78 pounds is the magic hold number, but if the cylinder holds within 10% (72 pounds), you have a good/OK cylinder. Below 72 pounds listen to the exhaust, carb or crankcase breather for escaping air. Based on where air is escaping from will tell you where you have a problem, i.e air at the carb is the intake valve, air at the exhaust is the exhaust valve and air at the breather are the rings. If you don't hear air from any of those and can not maintain at least 72 pounds of pressure, spray soapy water around the head gasket.
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    same think happened to me. over revved and the lash pads jump. same noise after that and very low compression. check this first. good luck

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    Supporting Member Diseazd's Avatar
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    I don't think he's ever going to come back and let us know what his compression tests results were...........guess we'll have to wait for next weeks episode!!!
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    Hey sorry- it was valve float. Two valves made contact with two piston tops. i had the engine totally rebuilt. Sorry I did not keep you guys in the loop-thanks everyone for there help

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