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L28 DEATH rattle!


pnoland

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OK I have a modified l28 it has N42 head F54 block with flat top pistons and a .480 lift .280 duration cam. I get a rattle from the engine if I'm cruising at 80mph in 5th and I punch it, kinda sounds like someone shaking a pcv valve it will also do it stopped in natural if I give the throttle a quick hit as well as in other gears under the same conditions with different speeds. I have removed all adv. from timing I'm running 97 octaine fuel and I have installed new NGK B7ES plugs any help will be much appreciated thank you.

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hi!

 

I had a mustang with an engine (that I rebuilt) that would make a rattling/ticking sound under certain circumstances.  couldn't find it, and mechanics couldn't... one told me it sounded like it could be a bracket that I forgot to secure that vibrated only at a certain RPM/vibration combo.  drove it like that for 2-3 years, never gave me issues. . 

 

not saying this is the case for you, but i'd check if everything is screwed in tight just in case.hope it's nothing. 

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I'm running 15* BTDC timing and as far as anything being loose it's not everything is tight iv gonna through and checked that already. Everyone I talk to says it's detonation.

How fast does it advance from idle?  Put the light on it and take a couple of simple measurements.  Maybe you have a broken advance spring.

 

Also, for troubleshooting purposes drop the timing to 5 degrees.  Don't hold on to the number you want and try to make it work,  Figure out what the real problem is first.  15 is already higher than spec.  You have more advance than spec,, and a higher CR than spec. on an engine that likes to detonate. 

 

And, when did it start doing this?  Are you familiar with the engine or is it new to you?  Could be you're spinning your wheels on a bad tank of gas.  Just because the pump says 97 doesn't mean it's 97.  When you say you tried race gas, do you mean you drained the tank and refilled with "race gas" or just put a few gallons in?  And, as I understand things, in certain parts of the country "race gas" means methanol.  And they mix it with Mountain Dew and drink it, then go to the hospital.  What's the octane rating of your race gas?

Edited by NewZed
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How fast does it advance from idle? Put the light on it and take a couple of simple measurements. Maybe you have a broken advance spring.

 

Also, for troubleshooting purposes drop the timing to 5 degrees. Don't hold on to the number you want and try to make it work, Figure out what the real problem is first. 15 is already higher than spec. You have more advance than spec,, and a higher CR than spec. on an engine that likes to detonate.

 

And, when did it start doing this? Are you familiar with the engine or is it new to you? Could be you're spinning your wheels on a bad tank of gas. Just because the pump says 97 doesn't mean it's 97. When you say you tried race gas, do you mean you drained the tank and refilled with "race gas" or just put a few gallons in? And, as I understand things, in certain parts of the country "race gas" means methanol. And they mix it with Mountain Dew and drink it, then go to the hospital. What's the octane rating of your race gas?

As of right now adv. at idle is 15* at 5000 rpm it it 32* I am not running a vacuum adv as my vacuum diaphragm is torn as far as the race gas I went to the local track and filled my tank it is 110 octaine I am aware that I am running high CR but that's why I bought the motor I wanted a high compression N/A motor and it started just recently it seems I currently have the adv set at 10 every day I bump it back a few degrees and drive it to work and back to see if there is a change.

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Why inch your way down from what is damaging your engine?  Crank it way down to where it's gone and inch your way back up.  If the head has been skimmed, you could have pretty high CR.  

 

But, the fact that it "started just recently" would indicate that the setup can work without detonation.  More detail than "just recently" might offer clues.  How long did it run well, what changes have you made recently, etc.

 

Ideally, you would just dump as much information as you can in to a thread like this, in the first post. People will get bored and go away if it takes hours to just to get to the simple stuff.  It took 10 hours just to find out what your timing was set to, and now 8 hours later, it's not even there, it's at 10.  Good luck.

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I apologize for my vague replies and lack of info as far as the adv goes I can set it at 15* retarded and it will still make the rattle and just recently is in the last two months maybe three it ran great when I put it in back in December I haven't done anything to the engine since January and I didn't notice the rattle then. I'm going to pull the valve cover and make sure nothing has come loose.

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Check for slack in the timing chain. I have seen one engine that had a weird rattle similar to what you described. Turned out to be a busted keyway in the crank allowing the timing gear to slap back and forth.

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Check for slack in the timing chain. I have seen one engine that had a weird rattle similar to what you described. Turned out to be a busted keyway in the crank allowing the timing gear to slap back and forth.

Wouldn't the timing gear just spin freely if that happened.

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Kind of went thru a similar situation with a buddy and his fresh rebuilt L24. After the engine was warm it would rattle like detonation at around 2500-3000 rpm. We did every thing timing including a different dizzy. Tore the motor down for fear of damage. A different engine shop looked at and determined the bores were over honed and left too much piston clearance. The original engine shop tried to save original bore size and bones the sh!t out of it. I think it was like 6-8 thou over spec. Couldn't find any other reason for the sound. Engine is NOT back together yet. The bores looked like crap for what little miles it had on it.

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If you think its detonation get some copper tubing and a cheep stethoscope from harbor freight pound one end of the copper tube flat and drill a hole in it bolt it on the motor close to the head were the heater hose runs is good run the tube into the car and hook up the stethoscope hose to the tube if you here detonation you know its timing if you don't a least you wont be guessing there are electronic versions of this that I own but I still use this for tuning 

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