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#1
02-22-2005, 10:13 PM
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I've had this pontiac 400 in my firebird for a few years now, bought the block freshly built, haven't driven it much (still messing with other stuff) and it runs well. but when I listen to it idling, it has that "diesel sound". I used to worry about knock, ping, etc, but timing doesn't seem to change it, and like I said it runs good. It is around 10.5:1CR, but I always wondered why it sounded like that. any Ideas?
heres the specs: pontiac 400 (.030 over) forged pistons 230/240 112* LCA craneRAIV double roller chain cam set neutral edelbrock 750 cfm performer e-performer RPM hedman hedders delco hei w/ msd coil+msd 6a box run around 102 octane at least (mix 93 and 110 race 1:1, same on straight of either)
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hmmmm.... which car do I work on today? -answer: all of them, they all need something |
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#2
02-22-2005, 10:13 PM
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I've had this pontiac 400 in my firebird for a few years now, bought the block freshly built, haven't driven it much (still messing with other stuff) and it runs well. but when I listen to it idling, it has that "diesel sound". I used to worry about knock, ping, etc, but timing doesn't seem to change it, and like I said it runs good. It is around 10.5:1CR, but I always wondered why it sounded like that. any Ideas?
heres the specs: pontiac 400 (.030 over) forged pistons 230/240 112* LCA craneRAIV double roller chain cam set neutral edelbrock 750 cfm performer e-performer RPM hedman hedders delco hei w/ msd coil+msd 6a box run around 102 octane at least (mix 93 and 110 race 1:1, same on straight of either)
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hmmmm.... which car do I work on today? -answer: all of them, they all need something |
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#3
02-22-2005, 10:29 PM
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the term dieseling when talking about a gas engine means run off when you turn it off.
the engine still "spits" and "spats" and tries to run for a bit more. since your engine is running and making a dieseling sound it makes we wonder what it sounds like. maybe go out there was a digital camera that records video and sound and take a sample. maybe someone else here can suggest something.
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
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#4
02-22-2005, 10:31 PM
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Try firing it up after unbolting the convertor and sliding it back. If the sounds gone... Got a convertor that's been like that, off and on, for 10+ years. POS P/U truck, but the noise IS diesel. It's so uncanny that I've actually had two people assume that it had one.
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#5
02-22-2005, 10:32 PM
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does it diminsh as the eng temp increases?
i'm thinking piston to cyl wall clearance here. forged psitons will chatter a bit when cold. mike
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so many pontiacs, so little time.................. moderator is a glorified word for an unappreciated prick.................. "Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." --Albert Einstein "There is no such thing as a good tax." "We contend that for a nation to tax itself into prosperity is like a man standing in a bucket and trying to lift himself up by the handle." - Winston Churchill |
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#6
02-22-2005, 10:39 PM
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did you use the fuel pump with the yellow nozzle??? just kidding!! do you know what the piston to wall clearance is?? was the block machined before you got the pistons?? were they fit to the block? has it always made this noise??
kal |
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#7
02-23-2005, 07:41 AM
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Crappy hydraulic lifters....Install lifter bore restrictors and run a solid. Main reason I don't run hydraulics anymore. Probably goes away under load right?
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1971 Base Firebird..505ci IA 2A Round port E-heads by Butler 348 cfm.. Comp street SR 266/[email protected] Victor intake/Quick Fuel Q950. 1 7/8" Dougs headers..3" mandrel bent exhaust with Hooker Max Flow mufflers, T-350 well built with 10" 4000 stall nitrous Continental converter..3.73 posi . Caltracs and 10" slicks. Belt driven water pump, alternator, Pwr steering and brakes,Flex fan. 11:1compression, straight 93 octane pump fuel. 10.35 @ 129.88 with 1.45 60ft N/A at 3700lbs race weight. |
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#8
02-23-2005, 10:26 AM
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things I have seen in the past.
Piston Slap Wrist pins, going bad |
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#9
02-23-2005, 11:33 AM
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my 2 cents,
If your car is in park, and idling, do you hear this dieseling sound? If yes try stepping on the gas to rev up the engine, not alot just maybe to 1500 to 2000 rpm's, do you still hear the dieseling? If the dieseling goes away as you rev up the motor, I would have to say you may have sun a bearing, sometimes when you rev it the sound may get louder before it gets quieter, but the sound will always come back when the car comes down to idle again. My guess is a spun bearing. Mark
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1965 GTO: 467 (Built By Dan Willever), 400 Block, Eagle 4.25 Stroke, Eagle 6.8 Rods, BRC Pistons, Custom Grind Hydraulic Roller Cam, Edelbrock 72cc Heads Ported, HS Rockers, Doug's Headers, Edelbrock Victor 4150, QTF 850 Carb, AutoGear M22 4 Speed, SPEC Clutch and Pressure Plate, 12 Bolt 3:73 Posi. 
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#10
02-23-2005, 11:55 AM
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SD455 engines sound like that when cold.
They had large piston-to-wall tolerance. Pontiac addressed this problem in service bulletins issued to line-techs.... SD455, "audible piston slap" with forged pistons, as normal.
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Home of WFO Hyperformance Shaker induction. |
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#11
02-23-2005, 10:50 PM
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well that gives me a few things to look at. I'm swapping trannies and convertor in a week or two, that will check that. I didn't build this engine myself, I bought it newly built at a machine shop, don't know the piston/wall clearance and the shop got blown away. as far as I remember, it has always made the noise. I don't think it goes away when up to temp, will have to take a ride and double check it. the spun bearing theory worries me a bit, since I haven't enjoyed the car yet or raced it. I will have to try to narrow down the possibilities. I don't notice the sound when drivin or higher rpms, but never heard it get louder. I'll run it some this weekend to check the symptoms. I'm still open to any more suggestions, like always, you guys are a big help.
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hmmmm.... which car do I work on today? -answer: all of them, they all need something |
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#12
02-24-2005, 07:10 PM
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I don't think it's a spun rod bearing because that is a definate knock. If you suspect that it is you can isolate it by removing plug wires from the cap one at a time. The reason a rod knocks is there is extra clearance in the bearing and when the cylinder fires it slams the rod down on the crank pin. If you remove the cylinder firing you take away the force that slams the rod into the crank pin and the knock goes away. What I would suspect in your engine is excess piston to wall clearance, if this is the case it should go away at least get less as the engine warms up and the piston expands from the heat. Forged pistons take more clearance than cast pistons and are usually noisier.
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Go Fast... Return... Go Fast Again... Who cares about left turns??? |
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#13
02-25-2005, 03:35 PM
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Do you have a oil pressure gage?I bought a "rebuilt" 389.The oil pressure was 60 when it
was started and when warmed up the pressure went down to 5!At least 2 spun rod bearing and ALL the bearings were very scratched/scored.The engine was rebuilt by a pro...... 
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#14
02-25-2005, 04:53 PM
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This might be a silly question but does it have Rhoads lifters on the camshaft? This could make it sound like that if you were expecting hydraulic. Also a standard crank and .010 bearings would do it also.
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#15
02-25-2005, 06:51 PM
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Check all around on motor for the word "Cummins".
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#16
02-28-2005, 08:00 PM
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Ive got a 455 in a 79 T/A,3900lbs.Ross pistons, SD Rods, H/O intake,750 holley,Crane Hyd. Cam, 3:42 gears, H.D.4+3 nash, it runs low 12s.on slippery street radials. The guy who built it for me set the piston to wall clearence a little loose (he builds blown alky tractor pulling engines which he would also build with extra clearences). The engine is a little noisy when cold after it warms up the forged pistons expand and the noise goes away.This engine was built in 1989 runs great. Does the noise go away after it warms up? Later Mark
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