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Thread: Vacuum leaks

  1. #76
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    Tried the steam clean today. Does seem to have made a difference in the idle quality. Checked the manifold vacuum and that's up to 17. Still dieseling. I'll pull a few plugs tomorrow and check the insides out again. But could still just be the old plugs too.

    Have noticed one strange problem now though. The oil pressure gauge never drops below 40 and is at times pegged well past 80. Could just be a poor connection though I guess. Hopefully that's all it is and it's not correctly reporting those pressures.

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    Quote Originally Posted by Antz81 View Post
    Have noticed one strange problem now though. The oil pressure gauge never drops below 40 and is at times pegged well past 80. Could just be a poor connection though I guess. Hopefully that's all it is and it's not correctly reporting those pressures.
    Poor connection it was. at the plug that goes through the firewall. The bolt that holds the two together seemed a bit loose, but gave the connectors a bit of a clean too.

    I had it out for a short run this weekend, after fitting the AC delco plugs. I would have had it out longer but it started to intermittently miss fire; and it seemed to be on the primary side of the coil not the secondary. This is what led me to check the plug at the firewall, and I found the hot wire was loose in the connector and was pushing out the back of the plug instead of going into the terminal. Hopefully I'll get it out for a good run soon.

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    Dieseling issue seems to be sorted. I've made a few small changes since it last happened so not sure which one it was. I suspect it was changing to the AC Delco plugs; but I did find a couple of small issues in the distributor as well.
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    Been A while since I updated this. (took a little bit of a break from it for winter and holidays + just doing other things instead)
    I've been doing a few small changes over this weekend trying to get the idle issue finally sorted. (It would get rough if left idling for more than about 30 seconds often less) I was thinking about when this would happen and it occurred to me that it seemed to be temperature related. This led me to replace the O2 sensor with a heated sensor. No more rough idle now. even when idling for a few minutes. Vacuum is still around 15-16 (I have checked my gauge on my brothers C10 when helping him with the timing and it read around 19) but I'm not sure why. I'm wondering if it's not a valve timing issue or a aftermarket cam.

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    Quote Originally Posted by Antz81 View Post
    Been A while since I updated this. (took a little bit of a break from it for winter and holidays + just doing other things instead)
    I've been doing a few small changes over this weekend trying to get the idle issue finally sorted. (It would get rough if left idling for more than about 30 seconds often less) I was thinking about when this would happen and it occurred to me that it seemed to be temperature related. This led me to replace the O2 sensor with a heated sensor. No more rough idle now. even when idling for a few minutes. Vacuum is still around 15-16 (I have checked my gauge on my brothers C10 when helping him with the timing and it read around 19) but I'm not sure why. I'm wondering if it's not a valve timing issue or a aftermarket cam.

    Sorry, I don't have time to read the whole post but 15-16" vacuum at idle is too low for a mild or stock cam. If you have a quadrajet, then sunk floats or leaking well plugs come to mind but that should also cause sooty plugs. For vacuum leaks I like to use propane, with a small hose attached to the end of the bottle. With a small enough hose you can really pinpoint the leak, plus it is safe to use, easy to modulate how much to dispense, and propane is heavier than air and goes down not up. Any time you spray an area with a propellant you never know for sure if the residual just got sucked up and into the throttle opening or that there actually is a leak, plus most propellants are flammable.
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    Quote Originally Posted by LLC5 View Post
    Sorry, I don't have time to read the whole post but 15-16" vacuum at idle is too low for a mild or stock cam. If you have a quadrajet, then sunk floats or leaking well plugs come to mind but that should also cause sooty plugs. For vacuum leaks I like to use propane, with a small hose attached to the end of the bottle. With a small enough hose you can really pinpoint the leak, plus it is safe to use, easy to modulate how much to dispense, and propane is heavier than air and goes down not up. Any time you spray an area with a propellant you never know for sure if the residual just got sucked up and into the throttle opening or that there actually is a leak, plus most propellants are flammable.
    Thats what i suspected. It is the original e4me quadrajet. Float level is fine, and still has fuel in the float bowl after sitting for over a week, plus plugs seem fine.
    Have checked for vacuum leaks a few times and not found anything, but not using propane. I do have some so wouldn't be hard to try.

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    Ok, Just finished the check with propane, seems to be all clear of vacuum leaks. Must be something else causing it to be low.

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    Decided to do a leak down test to see what that shows up. Unfortunately the tester I used was a piece of crap so the results probably aren't worth the paper they are written on. Highest leakage was 26%, so if correct no problem there. But I did notice that the intake valve seem to close very close (IMO) to TDC. This lead me to do a few other checks.

    1st: I checked how far a can rotate the crankshaft before I see any movement at the rotor in the distributor, This was About 7* (+/-1/2*). Is this too high?

    2nd: I checked the valve positions at exhaust TDC and found the the intake valve was starting to open, but the exhaust valve had already closed. That to me also seems wrong. Am I correct in my assumption? (I thought both should be open partially at this point)

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    Quote Originally Posted by Antz81 View Post
    Decided to do a leak down test to see what that shows up. Unfortunately the tester I used was a piece of crap so the results probably aren't worth the paper they are written on. Highest leakage was 26%, so if correct no problem there. But I did notice that the intake valve seem to close very close (IMO) to TDC. This lead me to do a few other checks.

    1st: I checked how far a can rotate the crankshaft before I see any movement at the rotor in the distributor, This was About 7* (+/-1/2*). Is this too high?

    2nd: I checked the valve positions at exhaust TDC and found the the intake valve was starting to open, but the exhaust valve had already closed. That to me also seems wrong. Am I correct in my assumption? (I thought both should be open partially at this point)


    26% is high in my opinion, anything over 15-20% and I try to figure out why. Did you lightly tap the valves while they were fully closed to loosen any carbon deposits while testing? Did you test with a hot engine?

    7* is at my limit for timing chain stretch, 8* and I would consider replacing it, but I don't think this is your vacuum issue.

    I have never checked valve position as you have so I cannot answer that, but a flat cam could cause low vacuum.
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    Quote Originally Posted by LLC5 View Post
    26% is high in my opinion, anything over 15-20% and I try to figure out why. Did you lightly tap the valves while they were fully closed to loosen any carbon deposits while testing? Did you test with a hot engine?

    7* is at my limit for timing chain stretch, 8* and I would consider replacing it, but I don't think this is your vacuum issue.

    I have never checked valve position as you have so I cannot answer that, but a flat cam could cause low vacuum.
    The tester seemed unreliable so I don't really trust the results it gave me anyway. I didn't tap the valves. But the only place any leakage could be heard was at the oil cap, so I don't see what difference tapping the valves would have made.

    so if the chain is at it's limit then I may as well go ahead and replace it. Worst that could happen is it won't solve the vacuum problem.

    from what what I can tell there is not a lot of overlap (if any) but not sure if this position is correct. I have checked for flat cams, while I have not measured lift there was no valve the had obviously less than any other, but still could be some wear on all of them.

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    OK, new chain, water pump, and fan clutch. Also shimmed the distributor to removed some play there. No change in vacuum. the new chain sure was a lot tighter than the old chain though. It was good to get it running again after so long though.

    Just one thing I noticed, Does anyone know what the timing should be at idle with the 4 pin plug connected? mine appeared to be about 18* I would have thought there should be more advance than this at idle. Or am I overthinking this?

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