Is this a weak spark

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edwardbahaw
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Is this a weak spark

Post by edwardbahaw »

I have a NAS spec 3.9 V8 fitted on a 110 defender and the problem is that it starts poorly. It takes about 5 attempts before it starts. It also runs quite rich as determined by the strong scent of the exhaust fumes.Today I have replaced the plugs with a set of clean used ones and took a video of the sparks from one of the plugs which I removed. See the you tube link below.

https://www.youtube.com/watch?v=x7MkNxV ... fIksbrG4eQ

These sparks seem very weak to me. In case you are wondering the plug is earthed via a jumper lead.

Also, swapping the plugs has not rectified the starting problem nor the excessive fumes from the exhaust.

Previously in an attempt to solve this problem I have already changed the following using new parts. ( I do not use the brand which begins with a B and ends with a T due to very bad experiences with their quality)
Ignition coil
Rotor
Distributor cap
Ignition module/amplifier
HT Leads


Any thoughts or suggestions are highly appreciated.



Spongo
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Post by Spongo »

Well it doesnt look too bad to me, I couldnt see the plug properly but it does look a bit darker than expected, the poor starting could be a fuel issue I guess, have you checked the injectors are they leaking or staying open at all.

Also might be worthn checking that the engine coolant sensor for the injection is faulty

Thius is assuming its injection and not carbs :)
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edwardbahaw
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Post by edwardbahaw »

Spongy

Many thanks for the reply. I forgot to include in the list that I have new set of injectors installed as well (OEM).

I should add that I must step on the accelerator in order for it to start otherwise I would be cranking over and over and over and over and it would not fire.

Is this a tell tail sign of low fuel pressure?

Ed

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Post by edwardbahaw »

Update. After alot of online research I just advanced the ignition timing a bit and it starts spot on now BUT when it is driven there is significant pinging.

Is this a stretch timing chain?

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Post by Spongo »

If it is you will hear it, a kind of swooshing sound when you rev it.

Or could be the dizzy that needs attention
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ChrisJC
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Post by ChrisJC »

edwardbahaw wrote:Update. After alot of online research I just advanced the ignition timing a bit and it starts spot on now BUT when it is driven there is significant pinging.

Is this a stretch timing chain?
Check that the vacuum and centrifugal advances both work in your distributor. And that the vacuum pipe is correctly connected and air-tight!

Chris.
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Post by DaveEFI »

edwardbahaw wrote:Update. After alot of online research I just advanced the ignition timing a bit and it starts spot on now BUT when it is driven there is significant pinging.

Is this a stretch timing chain?
I'd be inclined to see what the timing actually is by using a timing light - and check the centrifugal advance is working correctly. You may first have to check that the TDC mark is correct, though. It's quite easy to print up a degree strip on your computer, print it out and glue to the damper - to give an accurate reading beyond the engraved ones. Or you could possibly buy one ready made.
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edwardbahaw
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Post by edwardbahaw »

Hi all

Just an update to everyone on my findings and plan going forward.

If I advance the ignition timing by turning the distributor anti-clockwise by about 10 degrees it starts perfectly but if I test drive there is significant pinging.

To solve the pinging I gradually returned the distributor to its original position in 2 degree movements. Each time I fired it up and test drove and each time it became even harder to start. The pinging only goes away when the distributor has been returned to its original position at which point to get it started is extremely difficult. It cranks fine but it does not fire. I have to step on the accelerator and then it struggles to fire up until about 5 attempts with each attempt being about 2 to 3 seconds of cranking. With the ignition advanced its starts on the first attempt without having to step on the accelerator.

So my plan forward in the absence of a timing light and timing marks on the crank pulley I am going to remove all spark plugs except no. 1. Then with the distributor cap removal I am going to rotate the engine by hand using the 24 mm nut on the crank pulley. My aim is to see if i get full compression just at the point when the distributor rotor aligns with the no1 spark plug (5 o clock position) If it fails this test then my conclusion is the engine timing (valve, piston TDC and ignition) is off and I would have to strip the front and correct it.

Does this make sense or is there a better way?

Ed

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Post by edwardbahaw »

Update:

I already removed all the plugs which makes the engine easy to turn by hand and I discovered that I can move the crank about 4 degrees back and forth and without the distributor rotor moving.

This can be a worn distributor drive gear or a stretched timing chain.

Thoughts anyone before I strip out more parts?

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Post by kiwicar »

Hi
4 degrees slop on set up with a chain like he rover without any proper tensioner is pretty good for an engine that has done more than 2K miles.
I would get an engine stop, an old plug with the innards removed and a bolt jammed in the end, and accurately mark TDC on the engine damper (front pulley).
If this has just started happening on a previously OK engine then something has changed. I would bet the innards of the mechanical advance have jammed up, or one or the return springs has fallen off whatever it is have an investigate and fix it. What is happening with the vac advance, is that returning to no advance when there is no Vacuum?
Once you have sorted the distributor and TDC mark then look at setting modifying the the advance curve so it will start and run properly.
Best regards
Mike
poppet valves rule!

edwardbahaw
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Post by edwardbahaw »

Update:

I can now verify that the timing chain is stretched since I have removed the left hand rocker cover and I am able to turn the crank shaft pulley back and forth approximately 8 degrees before the push rods begin to move.

In addition with a probe fitted down in no. 1 cylinder as I turn the crankshaft pulley toward TDC on the compression stroke I can see that there is upward piston movement before the intake valve is fully closed which means less air and less compression leading to difficulty starting and rich running which is the symptom and the cause is a stretched timing chain.

So solution is to change timing chain and sprockets.

Am I right?

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Post by kiwicar »

Hi
on a normal 4 stroke engine the inlet valve should hit .05" lift point around 25 to 70 degrees after bottom dead center (crank). My cam hits .05" lift point at 58 degrees after bottom dead center and is a medium harsh race cam. A miller cycle would be later still but you most likely never have to worry about one of those unless you drive a Prius or a F1 car.
I wouldn't bother changing the chain and sprockets without changing the cam and tappets as well and certainly not for only 8 degrees of slop. 8 degrees of slop is still not a lot for a timing chain set up like this with more than 10K miles on it. How far has the engine gone since it's last rebuild?
What I do not understand is why you are looking at cam slop when the issue is the distributor not retarding the ignition correctly at idle? has the advance stop dropped off the mechanical advance? How old is the engine? How old is the distributor? how long has this been a problem?
Best regards
Mike
poppet valves rule!

edwardbahaw
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Post by edwardbahaw »

Kiwi

I've been thinking about you said and decided to do more checks.

The crank pulley actually moves 4 degrees before the rotor begins to move which is less that what I initially thought.

So I'm beginning to think that the timing chain stretch/slop may not cause the difficulty starting after all.

I can confirm that all the sensors are okay except for the O2 sensors which are unplugged. I can't see how this can cause difficulty starting though.

I will be receiving a new set of spark plugs and an inline fuel pressure gauge to rule these two out.

The engine has 100,000 miles and it does not consume any oil.

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Post by DaveEFI »

The amount of backlash you see by turning the engine backwards is a bit of a red herring, as the engine only turns in one direction. Friction in the cam etc will tend to take up the 'slack' and keep the relationship fairly constant.
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Ian Anderson
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Post by Ian Anderson »

Is it EFI?

If so is it Hotwire?

If so reset the ECU
Disconnect battery for 30 secs

Reconnect and the fault codes will have disappeared

Retry and perhaps it will start as it may have been in limp home mode, where it goes sloppy rich but runs normally other than that

Ian
Owner of an "On the Road" GT40 Replica by DAX powered by 3.9Hotwre Efi, worked over by DJ Motors. EFi Working but still does some kangaroo at low revs (Damn the speed limits) In to paint shop 18/03/08.

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