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

"Scott C" > wrote in message
...
> 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
>
>

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.

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.

"Scott C" > wrote in message
...
>A friend of mine has a 1986 Ford Explorer. It's runs very rough from 700
> (idle) to about 1500 RPM.
>
>
> 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

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?

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.

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?

================================================== ===========================================

"Jim Warman" > wrote in message
news:mb9Tj.251$Yp.126@edtnps92...
> HT, you have a very good grasp on the subject of pre-ignition... A gold
> star for you 8^)
>
>
> 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...
> 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...>

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.


"Happy Traveler" > wrote in message
. ..
> 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?
>
> ================================================== ===========================================
>
> "Jim Warman" > wrote in message
> news:mb9Tj.251$Yp.126@edtnps92...
>> HT, you have a very good grasp on the subject of pre-ignition... A gold
>> star for you 8^)
>>
>>
>> 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...
>> 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...>
>
>

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



"Jim Warman" > wrote in message
news:0XsTj.469$Yp.100@edtnps92...
> 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.
>
>
> "Happy Traveler" > wrote in message
> . ..
> > 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?
> >
> >
================================================== ==========================
=================
> >
> > "Jim Warman" > wrote in message
> > news:mb9Tj.251$Yp.126@edtnps92...
> >> HT, you have a very good grasp on the subject of pre-ignition... A gold
> >> star for you 8^)
> >>
> >>
> >> 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...
> >> 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...>
> >
> >
>
>