308 rotational rattle at cold start | FerrariChat

308 rotational rattle at cold start

Discussion in 'Technical Q&A' started by ashie, Apr 5, 2020.

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  1. ashie

    ashie Rookie

    Aug 26, 2015
    14
    Brisbane
    Full Name:
    Greg Ashe
    Hi all,

    Hope there is someone out there who might be able to assist with this strange problem.

    I have a 1985 308GTS QV. When I start the engine from cold, there is a very pronounced and sharp rattle. As I increase the engine revs, the rattle gets faster, so it's directly linked to engine rotation. It doesn't necessarily get louder.

    After 4-5 minutes of running, it goes away and does not return again until the engine is cold again, suggesting that it is related to either engine heat or lubrication or both.

    The noise started a couple of years ago and has gradually got worse - maybe over about 3,000 miles? In this time, I have replaced the water pump, all three drive belts and the timing belts. This did not change the situation.

    The rattle appears to be coming from the vicinity of the aircon compressor but that doesn't make sense as the noise happens regardless of whether or not the aircon is turned on. If it was the aircon clutch or the associated bearings, the noise would be there all the time.

    I would have thought that same logic applies to the alternator.

    This all makes me wonder if it's something a little more sinister such as valve train.

    Has anyone ever come across this before? Any ideas would be greatly appreciated!!

    Cheers
    Greg
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,038
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #2 Steve Magnusson, Apr 5, 2020
    Last edited: Apr 5, 2020
    Many reports of this, and most likely a "slapping" noise of the AC Compressor V-belt as it rides up the flank of the idler pulley groove and then "snaps" back down. The 308AC belt path design is terrible with the low wrap angle on the idler pulley. This noise sounds surprisingly very metallic, and only occurs with a cold belt (so goes away when the belt heats up and softens). Easiest first thing to rule in or out by just spraying some V-belt dressing on the belt.

    The other option is an exhaust leak that closes up when things heat up (and your report that it takes many minutes to go away makes this an option, too, as the belt noise usually only last 1~2 minutes or so).
     
    ashie likes this.
  3. ashie

    ashie Rookie

    Aug 26, 2015
    14
    Brisbane
    Full Name:
    Greg Ashe
    Thank you Steve, I'll try that tomorrow morning.

    I hope your suggestion works but that then leaves me wondering why a new belt didn't change the situation? Due to lack of Ferrari experience, is it possible I have worsened the situation when replacing the belt by incorrect tensioning or something like that?
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,038
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    A new belt usually is a little "softer", so less prone to this cold noise, but it's just a terrible design. About the only thing you can do is shim the AC Compressor to be closer to the crankshaft to add as much belt wrap on the idler pulley as possible, but even the maximum possible is still about 3X lower than is should be in a good design. As I mentioned, your report of it taking 4~5 minutes to go away raises the probability of it being an exhaust leak, but it's easy (and tool-less) to try the belt dressing to quickly rule the belt in or out first.
     
    ashie likes this.
  5. BillyD

    BillyD Formula 3
    Silver Subscribed

    Feb 28, 2004
    1,774
    Pacific Northwest
    Full Name:
    Bill
    I heard a timing belt tensioner bearing sound like what you’re describing years ago. At least that’s what the owner said it was.
     
  6. ashie

    ashie Rookie

    Aug 26, 2015
    14
    Brisbane
    Full Name:
    Greg Ashe
    Thanks, Billy. Sound was there before I changed the timing belts and was still there after - I used the "expensive" tensioner bearings - so it can't be that. Can't logically be bearings at all seeing it goes away after a few minutes.

    Going to get the wheel arch liner out next week and get right in there for a proper listen but my money is still on the aircon belt, as ridiculous as that sounds.
     
    flash32 likes this.

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