Rattle under Boost
Discussion
Need some advice on a strange sound that my car is making. It’s now been in 2 different garages for investigation, but nothing has been found.
It sounds to me like a rattle as if something is loose, or a bolt has been dropped onto the block and got trapped somewhere but it only happens on full boost. An air leak was suggested, but to me it's too loud and too 'mechanical' for that. The car is a sequential twin turbo, so when the primary turbo gets to full boost, the noise will start, and then boost drops, sound disappears. When it gets up to full boost again on both turbos, the sound comes back. Apologies for the poor video, I’ll try to get a better one.
The engine was fully rebuilt a few months ago (crank, bearings, piston rings etc due to big end failure) and was fine for about a month, then the noise started. The turbos were obviously inspected when it was rebuilt, and they were both in perfect condition (car has done 110,000kms – 68k miles).
Performance doesn’t seem to be affected, but it’s not a nice noise none the less. Anybody have any ideas on how to track the source down or places to look? Thanks
https://www.youtube.com/watch?v=UuY_zXy57IY
It sounds to me like a rattle as if something is loose, or a bolt has been dropped onto the block and got trapped somewhere but it only happens on full boost. An air leak was suggested, but to me it's too loud and too 'mechanical' for that. The car is a sequential twin turbo, so when the primary turbo gets to full boost, the noise will start, and then boost drops, sound disappears. When it gets up to full boost again on both turbos, the sound comes back. Apologies for the poor video, I’ll try to get a better one.
The engine was fully rebuilt a few months ago (crank, bearings, piston rings etc due to big end failure) and was fine for about a month, then the noise started. The turbos were obviously inspected when it was rebuilt, and they were both in perfect condition (car has done 110,000kms – 68k miles).
Performance doesn’t seem to be affected, but it’s not a nice noise none the less. Anybody have any ideas on how to track the source down or places to look? Thanks
https://www.youtube.com/watch?v=UuY_zXy57IY
Max_Torque said:
Seriously? Two garages don't know what Detonation sounds like?? Find some better people to work on your car!!
To be fair, would you really expect a normal garage to know ? Plenty of those pretending to be tuners dont even know !Seek a proper tuner/specialist and have them look at it before you blow another engine up.
Although I can barely hear anything in the video lol
stevieturbo said:
Although I can barely hear anything in the video lol
Seriously? Turn yer deaf aid up old man 
Anyhoo, the sound quality isn't good enough to definitively say it's detonation but obviously that's by far the most likely cause given the symptoms. If it's been making that much noise for a while now it's unlikely that there's no damage. I'd expect the pistons, chambers and head gaskets to be well on their way to turning into Swiss cheese.
Easy way to tell if it's det is to back a load of ignition advance out of the map and see if the noise goes away but DO NOT keep driving it while it's doing this. Better have a read of this thread. Sounds like the same problem. Not properly mapped for the boost level.
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
Detonation often happens under load. What happens if you lower the boost? You may have reached your boost threshold because of your compression ratio especially if it's not been lowered or as Dave says, your ignition advance. If removing boost cures it then it's clearly heavy pinking or detonation. Doesn't mean you've shagged your engine unless you've carried on driving it when it's detonating. I'd check the compression ratio across all cylinders. Also have a look at the spark plug tips for aluminium flecks. Fingers crossed that all's well.
Well thats not good news. Looked up a few videos of detonation, and it does sound similar.
What would be the cause of this (especially after an overhaul), and are there any remedies?
I can't change the map, as the car is standard. All the advice I've read online is for very old cars with very primitive systems, or highly tuned stuff. I'll pull the plugs at the weekend and have a look.
Thanks for your thoughts
What would be the cause of this (especially after an overhaul), and are there any remedies?
I can't change the map, as the car is standard. All the advice I've read online is for very old cars with very primitive systems, or highly tuned stuff. I'll pull the plugs at the weekend and have a look.
Thanks for your thoughts
There's a variety of reasons why it might be detonating after a rebuild. Were the heads or block skimmed, wrong pistons fitted perhaps, anything else that might have raised the compression ratio? Cam timing wrong. Ignition timing components not refitted properly hence more ignition advance than stock. Maybe a knock sensor isn't working if it has one as standard.
I would do a cranking compression test as a first step and see if it's generating the right numbers for a stock engine and that all cylinders are the same. Also get a strobe light on it and see if the ignition timing is correct.
I suppose it's just conceivable it's a load induced mechanical rattle but it's hard to tell from that recording. To my ears it doesn't sound quite right for det but I think the recording has lost some of the higher frequencies I'd expect to hear. It also sounds like it might only be coming from one cylinder or one component. The sound pulses are too discreet for an all cylinder noise. It actually sounds a bit like a hydraulic tappet playing up but that's not normally associated with boost level.
You need an experienced engine guy to have a listen. Maybe a local rolling road operator. Forget garage mechanics. They don't know their arse from their elbow.
Where in Scotland are you?
I would do a cranking compression test as a first step and see if it's generating the right numbers for a stock engine and that all cylinders are the same. Also get a strobe light on it and see if the ignition timing is correct.
I suppose it's just conceivable it's a load induced mechanical rattle but it's hard to tell from that recording. To my ears it doesn't sound quite right for det but I think the recording has lost some of the higher frequencies I'd expect to hear. It also sounds like it might only be coming from one cylinder or one component. The sound pulses are too discreet for an all cylinder noise. It actually sounds a bit like a hydraulic tappet playing up but that's not normally associated with boost level.
You need an experienced engine guy to have a listen. Maybe a local rolling road operator. Forget garage mechanics. They don't know their arse from their elbow.
Where in Scotland are you?
Here's a good recording of what det really sounds like. Much higher pitched than your noise, but as I said it might just be the frequencies coming through on your equipment.
http://www.youtube.com/watch?v=6xTWHJADFe8
http://www.youtube.com/watch?v=6xTWHJADFe8
Yes, it is the Legacy. Knock sensor is a possibility, I asked about this when the engine went the first time - but we decided that this was most likely due to the previous owner running it low on oil as all but one of the bearing surfaces were as new (I had only owned the car 2 weeks before that happened).
Pumaracing, the sound from the pickup is different, but definitely similar. There is a big exhaust on the car, so that may be helping to mask certain sounds. I'm in SW Scotland, Ayrshire but willing to travel for someone who knows what they're doing. I had the engine built by someone in Edinburgh because he came highly recommended. I pointed the noise out to him once the car was all back together and had been run for a week (took it up for a check over), but he didn't say anything about detonation, which I am surprised at as he seemed to know what he was doing.
The heads were lightly skimmed and acid dipped, used mls head gaskets instead of composite, so that I had the option of turning the boost up slightly in the future. The same pistons were used, as they were found to be ok (the bores were wet when the engine came apart - so he thought that the ringlands had gone, but amazingly the pistons were fine). Compression tests & strobe lights are out of my comfort zone experience wise, and anyway I agree that someone experienced should look at it. Going to be so angry if it has turned to swiss cheese after what I've spent on it.
ETA: just checked the car for error codes - all clear. I suppose this could still mean that the sensor is playing up though?
Pumaracing, the sound from the pickup is different, but definitely similar. There is a big exhaust on the car, so that may be helping to mask certain sounds. I'm in SW Scotland, Ayrshire but willing to travel for someone who knows what they're doing. I had the engine built by someone in Edinburgh because he came highly recommended. I pointed the noise out to him once the car was all back together and had been run for a week (took it up for a check over), but he didn't say anything about detonation, which I am surprised at as he seemed to know what he was doing.
The heads were lightly skimmed and acid dipped, used mls head gaskets instead of composite, so that I had the option of turning the boost up slightly in the future. The same pistons were used, as they were found to be ok (the bores were wet when the engine came apart - so he thought that the ringlands had gone, but amazingly the pistons were fine). Compression tests & strobe lights are out of my comfort zone experience wise, and anyway I agree that someone experienced should look at it. Going to be so angry if it has turned to swiss cheese after what I've spent on it.
ETA: just checked the car for error codes - all clear. I suppose this could still mean that the sensor is playing up though?
Edited by Viperz888 on Monday 24th November 21:23
Viperz888 said:
I'm in SW Scotland, Ayrshire...
Ach so yer a bloody heathen lowlander are ye? I'll no help any bloody heathens wi their poxy engines when there's plenty of god's own highlanders needing my advice so they are.Ok kidding
Tak yersel along to Russ Patton at Dastek, Dalgety Bay or HQPerformance or fitever they call themselves the noo. Tell him I sent ye. He'll hae a listen and ken fit to dae.
A compression test is dead easy and frankly obligatory after any rebuild. Ye'd be a fool to yersel ah tell ye if ye dinnae dae one and mak sure all four pots are pulling their weight. There's nae point trying to fix a problem if the engine's got a basic mechanical fault.
If the engine needs another rebuild then post back on here first. I don't do much these days but I can at least advise.
Pumaracing said:
Ach so yer a bloody heathen lowlander are ye? I'll no help any bloody heathens wi their poxy engines when there's plenty of god's own highlanders needing my advice so they are.
It's worse - I'm a pasty at heart! I'll give them a phone tomorrow to see when I could head over; thank you. Who should I say sent me?Until then, just stay off the boost, right?
I would think that a faulty knock sensor is more than a possibility, I'd be fairly sure it is the problem.
Why have we decided the engine will be buggered now just because it's been pinking?
Physically check the knock sensor first, it should be on the top of the block in the centre under the inlet manifold.
It's shaped like a banjo housing with a bolt through the centre, make sure the bolt is tight and check the connection.
It has been known for some engine "tuners" to reposition the sensor to a less sensitive position in order make it less likely to pick up knock and therefore increase the boost and performance.
Why have we decided the engine will be buggered now just because it's been pinking?
Physically check the knock sensor first, it should be on the top of the block in the centre under the inlet manifold.
It's shaped like a banjo housing with a bolt through the centre, make sure the bolt is tight and check the connection.
It has been known for some engine "tuners" to reposition the sensor to a less sensitive position in order make it less likely to pick up knock and therefore increase the boost and performance.
MLS gaskets are normally much thinner than composite ones (although not always) but combined with a head skim your engine may no longer be quite what you think it is, or what it originally was. For example on the Peugeot TU5J4 and JP4 engines the J4 composite gasket is 1.4mm and the JP4 MLS is 0.6mm to 0.7mm. That alone bumps the CR up by a fair chunk if you swap them.
I would check exactly what parts you've had fitted at the rebuild. You might be able to get a rough idea of the current gasket thickness just by examining the cylinder head area.
As to other comments on the knock sensor, if the engine is truly standard, in good order, on stock boost and decent fuel it shouldn't be knocking anyway for the sensor to play any part in things. I note the noise starts at only 9 psi boost which isn't exactly earth shattering territory for turbo engines. I mentioned it for the sake of completeness but it may have been masking a build problem when it was working and then let that problem show up if it failed.
I would check exactly what parts you've had fitted at the rebuild. You might be able to get a rough idea of the current gasket thickness just by examining the cylinder head area.
As to other comments on the knock sensor, if the engine is truly standard, in good order, on stock boost and decent fuel it shouldn't be knocking anyway for the sensor to play any part in things. I note the noise starts at only 9 psi boost which isn't exactly earth shattering territory for turbo engines. I mentioned it for the sake of completeness but it may have been masking a build problem when it was working and then let that problem show up if it failed.
Boosted LS1 said:
^The OP can hear the engine knocking or making mechanical noise. The knock sensor would only pull spark if faulty but it can't invent an audible noise of it's own.
I'm not trying to say the knock sensor will cause the detonation, but that its job is to stop it happening.Edited by oakdale on Tuesday 25th November 22:42
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