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Old 09-29-2004, 17:20   #1 (permalink)
351 XE on LPG
 
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Please help me diagnose a sporadic knocking sound

Well my dramas are continuing and I'm getting a bit discouraged.

I have a knocking sound that comes and goes and causes the motor to run rough when it's knocking, then smoothes out when it's not knocking.

I've systematically removed each plug lead to try and identify which pot is dropping, but without success. On a test drive the motor backfired a few times when the knocking occurred.

I've heard that pre-detonation (pinging) can be misleading and sound a bit metallic.

I've recently changed my lifters and the feet to my YT roller rockers as a result of having too much lifter preload for the last 2 years. I've also had to remove about 30 thou from a damaged valve tip, (see the following thread) . . .

Warn valve tip.

After making these changes, the motor was smoother and ran the best it ever has, but this happy state only lasted 2 days before the weird knocking sound and associated roughness developed.

The 351 has 302 heads with flat tops and runs straight gas thru an Impco 425. The initial timing is set at 10 degrees and I've had the vacuum advance removed. The dissy has also been re-curved to suit gas.

PS - I'm not sure if this is related, but since I've had these problems, the motor is a bit harder to turn over when starting, as if there?s a spot where it is harder to turn, instead of turning over smooth like it used to be. (The battery is only 1 month old and cranks over 600A)
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Old 09-29-2004, 18:20   #2 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

Rhett pinging is fairly distinctive as opposed to knocking because of bearing clearances. Pinging to me always sounds like a chain rattling, like a tinkling sound, and is much more higher pitched than knocking.

Were all the plugs looking good and equal? Any sign that something is happening to only one pot?

Maybe it's not a big issue, I once had a bad knocking sound that I thought was fatal and it turned out to be the fuel reg vibrating on top of the rocker cover when the motor ran rough. So check for the obvious before you jump to any conclusions.

But you say the performance has dropped off after 2 days of running. I would look for the reason why the rough running first and then investigate the knocking sound because I would guess that is secondary. ie A motor with symptoms of knocking would not necessarily run rough.

So look at ignition and fuel again, torque up your inlet manifold again as after a few days running this is a good idea when it's been removed.

Is the roughness all the time, under load, cruising? More info would help.
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Old 09-30-2004, 18:00   #3 (permalink)
351 XE on LPG
 
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Re: Please help me diagnose a sporadic knocking sound

It runs rough when it knocks, so I presume whatever is causing the knocking sound must be causing the roughness. I'll swap my plugs and re-torque the manifold and see if that makes a difference. Thanks StrokeXD.
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Old 09-30-2004, 18:37   #4 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

I'm with STROKEXD, the knocking could be caused by the engine running rough rather than the other way around.

I didn't realise you'd changed the lifters, did you coat the cam lobes and lifters in molly and run the engine in?
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Old 10-02-2004, 17:03   #5 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

The 351 has 302 heads with flat tops and runs straight gas thru an Impco 425. The initial timing is set at 10 degrees and I’ve had the vacuum advance removed. The dissy has also been re-curved to suit gas.

hey there, but I wa reading away and saw this and was a lil concerned. ur running gas, higer octane than fuel and running at 10deg?? I dunno bout u guys but that seems very low and u say it's slow to turn ova? y not try to advance ur timing a lil, maybe 2 more deg and see wat happens? i run on premium and mine runs at 13deg and vac advance is still connected. just watch the temp gauge as u make adjustments.

as far as the other stuff goes, knocking and running rough do wat the other guys said obvious and easy stuff first!!!
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Old 10-02-2004, 20:12   #6 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

You might have a valve hanging open (valve guide issue). You knocking maybe the piston hitting the valve. Once the valve does go up/seal you don't have a vac leak and it runs smooth with no knock.

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Old 10-03-2004, 05:46   #7 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

Quote:
Originally Posted by brenx
You might have a valve hanging open (valve guide issue). You knocking maybe the piston hitting the valve. Once the valve does go up/seal you don't have a vac leak and it runs smooth with no knock.

Brenden
Yea Brenden, I'm also starting to think it's something like a sticky valve. I just can't understand why. I use Mobil 5/50 synthetic and I had stellite guides installed to cope with the higher temps of gas. It must be coincidence that I'm having this problem now, because I can't see how my rocker and preload problems would cause a sticky valve. The motor also seems harder to turn over. I'm not sure why. I pulled all the plugs to do a compression test, and that was even hard to turn over. 4 pots scored 160, and the others were 150, 165, 175 and 180. The knocking sound occurs even when the motor is cold or warmed up.

Do you think a sticky valve could cause it to backfire?
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Old 10-04-2004, 02:30   #8 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

Have you checked the valve springs? Are they old? Can one be broken or not to height specs,Or can you have a valve lifter issue thats farted out giving rough idle at certain points of your delema. A faulty hyd lifter can give a dull knock at various times of engine rotation They are sometimes the most overlooked items which can play circus ozz with a good cleveland.

Good luck and hope you get good rythm going!

By the way without vacuum advance it will give you poor fuel economy and make the old girl work like one trying to "climax" at a peak.
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Old 10-04-2004, 02:35   #9 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

Yes it'll cause a backfire if it's an exhaust valve unburnt fuel in the mufflers is fun. I had that before my tune. Shooting flames was fun. My issue was a fouled plug though It was obvious as the plug looked perfectly clean (new) and it'd just done a 500k trip.

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Old 10-04-2004, 18:12   #10 (permalink)
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Re: Please help me diagnose a sporadic knocking sound

Plugs are new. Lifters are new. Valve spring are 2 years old and were the correct ones for my cam.

ToughOne,
I've had the mechanical advance customised so the dissy doesn't require a vacuum advance. This is quite common with gas conversions, because LPG requires a totally different advance curve to petrol. The 351 has run very well for 2 years in this tune - so I don't think it's an issue.

Brenden,
I think you hit the nail on the head.
It's looking more and more like the behaviour of a sticky valve.

CONFESSIONS OF A PARANOID

I think it's my own fault, because in the two years since I built the motor, I've treated it twice to a Nulon E30 (PTFE) treatment, and I think I've over-done it (as usual) and stuffed up some critical clearances. This was the main reason I started using a very thin synthetic oil in the first place, because I realised back then that the motor wasn?t as happy as it used to be.

I suppose I've played it too safe and gone over-board. I think I applied the first treatment after about 10,000 kms from new, then the second one about 17,000 kms. I've treated many cars over the years to Slick 50 and swear by it, (Nulon is the Aussie equivalent), but I've never done it to a new motor, only ones that were a bit long in the tooth. I should have waited till I had done at least 50,000 before applying the first treatment, then maybe the second at 100,000.

It looks like I'll be pulling the motor again. Oh yippy !!!! ( . . . not)
I'm just about to build my garage, and I refuse to do another motor under the patio, so my XE will have to go into temporary hibernation while it's new home is being built.
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