OT-ish: Engine knocking or pinking

Wotcher all, OT-ish because it's not a landy engine, but I'm sure it applies just as well.

I'm getting what I *think* is knock or pinking in the pinz. At low revs under load if I put my foot down, I get a sound like a few bits of lead shot falling onto a solid chunk of metal, not a "ping" more of a rattle really. If I slacken off the throttle it goes away, or if I change down a gear to increase the revs it also goes away. It really only happens with low revs, WOT and when I'm going up a hill.

Does this sound like knocking/pinking to you, and if so I've read that older low-compression engines are often designed to withstand it as part of their normal daily life under the above conditions (up hill, WOT, low revs) and that on such engines I should only worry if it's happening all the time, e.g. on the flat. Any users of older petrol landies have any thoughts on this?

According to a chunk of text from a magazine apparently written by an engine designer, pre-ignition is something to worry about but you tend not to get a warning other than a blown piston, but with detonation, it happens after the main spark has fired and ignition has already begun so the piston is already on the way down. It's detonation that you can hear as pinking. Thing is I don't know what pinking sounds like but the situation this noise is happening under (WOT, low revs, engine under load) makes me think it is.

Cheers for any thoughts.

Reply to
Ian Rawlings
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sounds about right Ian once you have heard pinking its quite distinctive lifting off the throttle should make it go away. A small amount of pinking is acceptable when you first put the boot down almost inevitable with some engines but is should go away very quickly as the revs start to build Overly lean mixture ( air bleeding in or carb setting) or too much advance can cause it and the plugs then run hot as a result and you can also get excessive heat in the cylinder not something to leave for a while before sorting as stuff tends to get burned .Check the timing is ok then the plugs if they are on the white side then you are running lean- you might want to post the question on the Pinz user group cos I'm sure I recall somebody posting about Pinz valve problems on the 4x4 site.

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Derek

They burned down the gambling house It died with an awful sound

Reply to
Derek

Yep it does that, doesn't pink for long, and not at all in the lower gears as the revs build so quickly. Every time I've checked something on this truck it's turned out to have been set up right by the previous owners (swiss army) and recently sorted, and I'm fairly sure the pinking's not really gotten worse, it's only now that I'm noticing it.

Plugs look like I think they should, insulator is very slightly browned with some white bits and not wet, I've checked gaps and valve clearances, all were spot on.

I've posted on the pinz forum on yahoo groups but despite some of the chaps on there knowing a lot, they can be slow to respond and also sometimes they don't follow through once they start feeding the information so you can suddenly get left high and dry. I think it's down to there being so few people around who know these machines, so the groups are a bit sparse. I'll be taking it to a local old school mechanic tomorrow too, and phoning my pinz expert in scotland so hopefully I've got the bases covered! I know there's lots of engine nuts in here though so thought I'd prod you all. Typically someone on here has given me some useful info within minutes of me posting, cheers for that ;-)

Reply to
Ian Rawlings

"Ian Rawlings" wrote........

Are you sure you have read that last paragraph properly and/or quoted it correctly? Now why do you have to richen the mixture or retard the ignition timing if you have got pre-ignition (detonation)? And why does it fire before it is supposed to do. And how do you get a detonation when the mixture is already burning if, as you say, 'it happens after the main spark has fired and ignition has already begun', think about it? And yes, that's what pinking sounds like. I think I'm losing my marbles. :-)

Martin

Reply to
Oily

"Ian Rawlings" wrote

Sounds like pinking, sounds like the 2.5 petrol engine in my 90. If the settings are all correct and you are using the right grade of fuel then it's something getting hot in the engine (acting like a glow plug) and causing the engine to act like a diesel. In the case of my engine, when I renewed the Valve Stem Seals I noticed a slight bit of very old damage to a piston but as it hadn't caused any problems, and after a discussion with a knowledgeable friend I let it be. Just ground in the valves etc and reassembled. Now it pinks, probably because that cylinder now has valves that seal properly. I'll eventually have to take it all apart again and replace the pistons.

Reply to
Bob Hobden

I didn't say that, you are losing your marbles ;-) I said I've got detonation, I threw something in about pre-ignition because that's what I'm *not* getting, they're two very different things.

According to the article, pre-ignition isn't detonation, pre-ignition is when the fuel mix fires prematurely due to some other heat source, e.g. hot plug element or burning ember, and almost always happens when the cylinder is near the bottom of the fuel intake stroke as that's when the fuel mix is happiest to ignite, it's harder to ignite under pressure. The end result is that the piston ends up trying to compress an expanding ball of hot exhaust gasses and burning fuel. This then causes the engine to die very quickly. You can't hear pre-ignition because it happens when the cylinder pressure is very low so the sudden rise in pressure doesn't cause any shock in the engine, the damage is caused when the piston rises to meet the expanding explosion so the first you know about it is when the engine has done it once or twice and then dies. So this isn't a cause of "knocking".

Detonation is very different, it's when the fuel ignites but the pressure jump caused by this fuel burning causes the fuel elsewhere in the cylinder to also ignite, so you end up with two flame fronts. This causes a rapid rise in pressure at a point when the cylinder pressure is already high and rising, so the shock can be heard outside of the engine as "knock", however as the piston is aready on the way down it doesn't do as much damage as pre-ignition, and many older engines can put up with it without harm as long as it's not constant. It's basically caused by the pressure suddenly rising faster than it was designed to due to the controlled burn rate of the fuel being ruined by a second ignition point due to the fuel spontaneously igniting under pressure, e.g. due to low octane, too much air or whatever other problem the engine has.

If the chap who wrote the article really does design engines (and I've got no reason to think he doesn't) then chances are he knows what he's on about, it's an interesting piece of text, hold on I'll dig up the URL;

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Have a look, one of the most interesting articles on the subject I've read, and sounds convincing. Reprinted from some magazine in the states I think. Mind you the magazine is called "contact", I think it's quite different from the "contact" magazines you get on the top shelves in newsagents in the UK ;-)

Reply to
Ian Rawlings

That sounds like a confusion of two issues, pre-ignition (which happens due to extra heat sources) and detonation (which happens due to fuel igniting itself). Detonation can apparently lead to pre-ignition, but pre-ignition just leads to a fooked engine. The pressure rise that causes detonation is supposedly due to the ignition of the fuel by the spark, so you get a double-detonation and too rapid a burn rate, that's why retarding the ignition helps to stop it because the spark fires at a point when the pressure in the cylinder is reduced.

There's a list of common causes of detonation which you can get by googling for stuff like "engine pinking", if you're sure it's pinking (which was the bit that I wasn't so sure about, never having heard it before), it shouldn't be *too* hard to stop it from happening without needing to take too drastic measures. Also have a peek at the article link I posted elsewhere in this thread and see if your piston damage matches the description in there (mostly at the skirt I think), ISTR sommat interesting about that but am too tired to go and re-read it to remind me!

Right, time for bed methinks!

Reply to
Ian Rawlings

Oops sorry I meant "it's when the fuel ignites due to the spark plug, but the pressure jump etc etc"..

Reply to
Ian Rawlings

That just depends on how you interpret the English language, any explosion howsoever caused is a "detonation", but what we are talking here is how a diesel works, compression ignition, and the expanding gases from the initial firing is upping the compression to the point that the rest of the fuel ignites as the diesel because ideally you need a controlled burn. And this is down to the fuel being mismatched to the design of the engine or compression ratio used.

Pre ignition can be caused by lots of things as you say, some fuels leave more deposits on the pistons than others leading to de-coking more often, plugs running too hot or sharp edges in the combustion chamber staying hot etc.

Well, it was late :-)

Martin

Reply to
Oily

Well yes, but in context it meant something specific, just like "ignition" can mean just lighting a match, and "fuel" can mean logs ;-)

Indeed, although apparently it's also just down to unsophisticated engine design, it seems older engines will do it from time to time, and it's not something to worry about unless it's happening a lot. It happens in modern cars even on the right fuel but the knock sensor and ECU just sort it out. I've spoken to a few pinz mechanic types now and pinking in the situations I've been getting it is fairly normal and nothing to fret about. Now I know that of course, it's stopped doing it ;-) I've even cranked the ignition nearly back to where it originally was and it didn't pink on a run I did this morning, I suspect I've been deliberately provoking it to see if it would happen and now I'm driving normally it's just not happening.

Still, it's lovely to have the old beast back on the road after I broke it through some unrelated fiddling!

Reply to
Ian Rawlings

On or around Wed, 6 Jun 2007 21:08:22 +0100, Ian Rawlings enlightened us thusly:

sounds exactly like pinking.

one of 2 things:

Ignition timing (static or dynamic adjustments) is out of kilter and it's running too much advance

Mixture weak due to lack of fuel.

The latter would probably show up in other ways like lack of top end power. weak mixture due to air leaks doesn't normally give symptoms under the specified operating conditions.

Check basic timing adjustment and function of vaccuum advance/retard system, that'd be my bet.

Has it been set correctly for 95 octane fuel (unleaded)?

Reply to
Austin Shackles

Someone on the pinz forum suggested that too, cleaning up the distributor is on the cards, although to be honest it looks pretty clean already. I'm not sure when the weights start to have an effect but it only pinks at low revs when they're not supposed to be doing much, so perhaps not the dynamic advance. I've twisted the distributor until I get noticable power loss, it helps for sure but since speaking to other pinzers I've twisted it back and modify my driving to reduce the pinking as it's apparently fairly normal. There is still some diesel in the tank from my duel fuel experiment, that's on hold for now as I dilute the diesel out until I get a handle on this pinking. It was happening before I added diesel.

A possible one, carbs could do with being rebuilt at some point, not something I have any experience having only worked on an EFI engine before, might be time for me to start ;-) Mind you I never have to use the choke or engine start setting which add extra fuel, so I have always thought it's probably running rich.

It's a pinz, it doesn't have any ;-) It can however reach the stated maximum speed (60MPH) and doesn't feel like it's lost any power relative to what you'd expect from a 2.5 tonne truck with a weedy 33 year old engine and lots of drivetrain drag.

Will do, never done it before so time to learn! There are charts and graphs in the manual to map out the advance/retard so should be easy to find out if it's on spec.

As far as I'm aware, it's got "unleaded" plastered all over it in stickers, so I'm told anyhow, it's all in swedish/german, which is greek to me!

Reply to
Ian Rawlings

Well after setting the ignition up properly twice (checking for TDC) and still getting the pinking I went back to the way my Dad used to do it, by ear, set the tickover at high and turn the distributor until the engine sounds nice and even etc. Worked a treat, no pinking and feels better to drive too. One day I'll check how far off the book setting it is, certainly more retarded than it should be.

Reply to
Bob Hobden

On or around Tue, 12 Jun 2007 23:34:53 +0100, "Bob Hobden" enlightened us thusly:

I've had a few engines which don't run nicely when on the book setting.

although if the book setting is too retarded, that won't give you pinking.

Reply to
Austin Shackles

"Austin Shackles" wrote ... after "Bob Hobden" enlightened us thusly:

Sorry Austin, I meant it's now more retarded than the "Book" setting (judging by the position of the distributor).

Reply to
Bob Hobden

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