pre-ignition - need explaination

A friend of mine has a 1986 Ford Explorer. It's runs very rough from 700 (idle) to about 1500 RPM. He's found several things wrong and fixed them,
but the problem is still present. He took it to a mechanic who removed the plugs and found the element (for lack of a better word - where you gap the plugs), burned totally away - it was not there any more. The mechanic explained this as a pre-ignition problem, it's getting way to hot - and he was not surprised the pistons did not have holes in them. The compression is good, but the mechanic seems to think the valves are being pushed into the valve seats very hard. To me it seems like the valves are closed to long when the engine fires - but I don't know.
Can someone explain pre-ignition in more detail than it fires early. What is the cause of pre-ignition? A timing belt problem? Seems the timing is off,but checks OK.
Thanks
Scott
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Well, I can't explain pre-ignition but rough idle is often caused by a dirty IAC valve (located on the intake manifold on a 1996 4.0 V6). If you really have a 1986 Explorer I'm not sure what you have--a pickup, perhaps?
It sounds like this vehicle has been neglected so if it was me I'd replace the air cleaner, serpentine belt, spark plugs, plug wires (Motorcraft or at least Bosch--don't get the cheap ones), fuel filter, check all the vacuum hoses, check the vacuum cluster on the intake manifold for loose or contaminated fittings (look for automatic transmission fluid on the fittings), check the coolant level and determine if there is a head or intake manifold gasket leak, and clean the throttle position, idle air control (IAC), and mass air flow (MAF) sensors with some brake or carb cleaner and then see what you have. Also check the battery cables and make sure they are solid.
If you have clouds of white smoke at startup it might be burning coolant. If it's burning transmission fluid it'll probably run rough at low speeds but your transmisison would probably be slipping due to low fluid level. The good news is that if your compression is even it's probably not a head leak.
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1986 Explorer? Explorer was introduced around 1991. Perhaps a Bronco?
Our very knowledgeable Jim Warman hopefully will provide a thorough explanation. Just in case he is too busy at the moment, here is my humble understanding:
Pre-ignition means that the mixture ignites too early in the cycle. It could be that the plugs fire too early, or that the mixture ignites before the plugs even fire. That can be caused by glowing carbon deposits in the cylinder head or by a defective (or disabled) EGR (exhaust gas recirculation) system. Lack of EGR flow will raise the combustion temperature, which could lead to pre-ignition. However, EGR only kicks-in at cruise conditions, and you are talking about low RPM. So probably not the case. Any half-decent mechanic should be able to determine which it is.
If this is indeed a mid-80's vehicle, it will have adjustable 'base timing', which is done simply by turning the distributor body a few degrees here or there. It's very easy to check with a simple timing light (your mechanic will have to disable the advance mechanism by disconnecting a wire or a vacuum tube, depending whether this is a computer-managed system or not). However, something is telling me that the problem is not ignition timing, because due to the advance mechanism, incorrect timing will ordinarily manifest itself as pinging under heavy load.
In a vehicle that old, a jumped timing chain (there is no timing belt in a 1980's Ford SUV, regardless of model) could create some real havoc. Not sure it that would lead to pre-ignition, and the good compression (whatever that means) seems to tell that this is not the issue. But I am going above my head here -- Jim will hopefully enlighten us.

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Guess that yet another cause of pre-ignition would be lean mixture. Unmetered air (better known as vacuum leak) could be the culprit. if it's gross, you will hear hissing somewhere, perhaps around the intake manifold. If this is any kind of fuel-injected engine (as a 1986 should be), there will be trouble codes stored in the engine computer; perhaps not very specific, but worth taking a look, and it takes only a test light to read them. Does this vehicle have a CEL (check engine light)? If so, is it on? Again, what kind of mechanic was it that couldn't diagnose the source of pre-ignition?
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HT, you have a very good grasp on the subject of pre-ignition... A gold star for you 8^)
Anything that will cause the combustion event to occur early can give pre-ignition... this can be as simple as improperly adjusted base timing, inadequate fuel quality, any kind of "hot spot" in the combustion chamber or even having the compression ratio being influenced by deposits....
Unmetered air might be a vacuum leak or, if it is upstream of the throttle plates, it wouldn't be a vacuum leak and there would be subtle differences in the affect. In 1986, however, I don't recall Ford having much to do with anything other than speed/density based systems.
For the OP.... it is normal for spark plug electrodes to suffer from erosion... Pre-ignition will tend to leave deposits on the ground electrode if it is severe or has been left unattended for any length of time... There will be an accompanying , unmistakeable noise under acceleration. At this point, I am left to doubt that pre-ignition or engine "ping" is part of the complaint. If your mechanic equates worn electrodes with pre-ignition, I would get worried (Elmer Fudd told us to "Be afwaid... be vewy, vewy afwaid").
If this vehicle is, indeed, a 1986.... IIRC, Ford used a "negative feedback" type EGR valve... There is a small port inside the valve that can carbon over and this would have the effect of having the car "nose over" on light acceleration... In effect, the EGR valve opens too far, too early. Going to WOT (wide open throttle) seems to "cure" the problem - this is because at low RPM WOT, manifold vacuum drops to very close to atmospheric pressure. Without sufficient vacuum, the EGR valve closes and the problem seems to go away.
If the car is an Explorer... one would need to know the year and, possibly, the engine "flavour" to say anything meaningful.
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Hi Jim,
Thanks for the compliments!
A couple comments: 1) I still have one of those mid-80 Fords (to be donated to a not-so-well-off friend, as soon as I can replace the worn control arm bushings). The EGR is not a negative feedback type, but uses a position sensor - a potentiometer mounted on the top. 2) In my humble experience of repairing my own vehicles for the last 30+ years, I have never seen a ground electrode that's totally burned - not even close. So what else would cause such thing?
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The "negative feedback" term is with the way the EGR valve operates internally... this http://tinyurl.com/5ehap9 probably explains it better than I can (with pictures added for clarity, no less 8^) ). The feedback I am referring to isn't feedback on valve position to the PCM.
The OP didn't say ground electrode but did mention the "element"... I may be wrong, but I am assuming that this is the centre electrode he is referring to... At the same time, it is common for some erosion to occur on the ground electrode, especially on plugs at or beyond their expected service life... And we must also consider what motors are fitted with a wasted spark system... The "ground" electrode will suffer more erosion than the centre electrode on the negative side plugs. But here is where careful inspection is required... Is the electrode eroded or is it melted away?
Erosion can occur from oxidation, corrosion, fuel quality (perhaps even fuel additives) or even the metallurgy of the plug itself. A spark plug with a burned away electrode could be because of pre-ignition, detonation or faulty EGR operation. or any other circumstances that keep the spark plug hotter than intended.
Remember, the spark event typically lasts for 1.5 milliseconds (most COP systems are now "multi-striking" at idle)... during this time, the spark plug is at it's hottest and, depending on the system, one electrode or the other is being bombarded by a stream of electrons... and this can create the condition...
HTH.

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Great info guys, thanks.
I will find out a bit more about the Explorer, thought it was a mid 80's, but I'll check.
sc

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