Thread: 3.9 EFI Flat spot / slight misfire

Originally Posted by jimbob292

Recently carried out 1,000 k's ago my 10,000k service all standard OEM parts etc, champion plugs, oil filter, air filter etc.

Starts right on the button from cold / hot, idle's perfect with the A/C on & off, no signs of arcing HT leads, these are OEM ones, must be getting on for three years old now.

In the last 100k's i seem to have flat spot, up slip roads (slight inclines) but if i plant the right foot down it kicks down and fly's!

Could it be bad fuel, I'm going to buy another fuel filter and just double check, to make sure there's no crud floating about, fuel tank was replaced 15months ago under a recall.

I suspect a misfire would be more noticeable / louder the feeling is like you've just cut the injectors out for a split sec, then back on again, like you've driven into a head wind, then just disappears again.

Any ideas or pointers?

Cheers

Jim

As the injectors are driven from the ignition pulse, a misfire by the ignition amplifier will instantly drop out the injectors as well. I would suggest first checking the security of the distributor pickup wires as they move with vacuum advance. Also the air gap in the pickup. Try disconnecting the vac advance for the next test drive.
I have replaced ignition amps based on this kind of symptom with great success. There are Chev 4 pin HEI modules that can also be used, based on what an AULRO member did in southern Africa.
BTW neither OEM leads or Champion plugs rate highly with me. Bosch or NGK seem to be better.

Sorry to hijack, I have a similar fault with my 3.9, and thought instead of having two very similar thread i would just throw my issue in the bundle and follow this thread.

My symptons have been around for ever but I am lazy and ignored it to date

I have noticed just today thou while under medium acceleration the car gained power instantly at the same throttle point... almost as if something just started working, that wasn't working before.

My misfires have always been light load throttle points (coasting or holding a certain RPM range), the plugs have been changed and I did have some earthing issues which I believe is sorted.

I did however for the first time yesterday turn on the AC by accident and the car misfires badly and is not drive able with AC on, it seems OK at first (maybe a bit rougher at idle) but driving between the 2000 & 3000 rpm band it is all sorts of messed up.

I had similar issues with this car when I owned it previously and since then the ignition module has been updated and relocated, and a number of distributors have been used.

NGK plugs have been fitted, and I just ordered a set of Magnecor leads to replace the old.

Hopefully we can both get to the bottom of this!

Originally Posted by bee utey

As the injectors are driven from the ignition pulse, a misfire by the ignition amplifier will instantly drop out the injectors as well. I would suggest first checking the security of the distributor pickup wires as they move with vacuum advance. Also the air gap in the pickup. Try disconnecting the vac advance for the next test drive.
I have replaced ignition amps based on this kind of symptom with great success. There are Chev 4 pin HEI modules that can also be used, based on what an AULRO member did in southern Africa.
BTW neither OEM leads or Champion plugs rate highly with me. Bosch or NGK seem to be better.

Thanks for the reply, took the dizzy cap off, and the rotor arm must be glued on to the shaft, hell, I'm worried if i pull to hard i will damage the dizzy.

Had a look all around the coil and amp to see if i had any suspect wiring, going to get a new one on order and give it a go.

If the dizzy was at fault wouldn't this given me all round bad performance even when starting / idle & running?

But, going for a run with the vac advance disconnected would confirm the above am i correct in thinking this?

Originally Posted by Grimace

Sorry to hijack, I have a similar fault with my 3.9, and thought instead of having two very similar thread i would just throw my issue in the bundle and follow this thread.

My symptons have been around for ever but I am lazy and ignored it to date

I have noticed just today thou while under medium acceleration the car gained power instantly at the same throttle point... almost as if something just started working, that wasn't working before.

My misfires have always been light load throttle points (coasting or holding a certain RPM range), the plugs have been changed and I did have some earthing issues which I believe is sorted.

I did however for the first time yesterday turn on the AC by accident and the car misfires badly and is not drive able with AC on, it seems OK at first (maybe a bit rougher at idle) but driving between the 2000 & 3000 rpm band it is all sorts of messed up.

I had similar issues with this car when I owned it previously and since then the ignition module has been updated and relocated, and a number of distributors have been used.

NGK plugs have been fitted, and I just ordered a set of Magnecor leads to replace the old.

Hopefully we can both get to the bottom of this!

If you feel this at the same throttle point it might be the throttle potentiometer, not responding correctly, its my understanding of the 14 CUX, when the A/C is on it adjusts the idle valve accordingly with an input from the VSS (gearbox transducer) below a certain speed to active the idle valve & the throttle position.

check out this link on the testing of the throttle pot

http://www.g33.co.uk/images/PDFS/LR_...ionSystems.pdf
http://www.g33.co.uk/images/PDFS/14c..._injection.pdf

LR electronic dissies are funny beggars, high voltage systems are a bit like suspensions, the correct damping makes the ride smoother/maximises spark energy delivery. As for the rotor button, some tips here:

Lucas V8 dissy nylon clip repair method.

Anyway disconnecting the vac advance is about whether the pick-up movement on vacuum is causing problems. If there is no difference I would say possibly you should get modern plug leads, ie spiral core ones. They have better internal damping than plain resistive leads and lower loss.

Originally Posted by jimbob292

If you feel this at the same throttle point it might be the throttle potentiometer, not responding correctly, its my understanding of the 14 CUX, when the A/C is on it adjusts the idle valve accordingly with an input from the VSS (gearbox transducer) below a certain speed to active the idle valve & the throttle position.

check out this link on the testing of the throttle pot

http://www.g33.co.uk/images/PDFS/LR_...ionSystems.pdf
http://www.g33.co.uk/images/PDFS/14c..._injection.pdf

I have checked and adjusted my TPS a number of time so supect it is ok...

LOL just looked under the car... VSS is bent up and facing the chassis

Often these misfires can be traced to the fitting of a Bosch alternator interfering with the ign signal.
If you have fitted a Bosch alternator, you must either use coax grounded at both ends from dizzy to coil, or reroute the wire well away from the alternator.
This is particularly true of electrical load sensitive misses, the bigger the load the more the miss.
Regards Philip A

Originally Posted by Grimace

I have checked and adjusted my TPS a number of time so supect it is ok...

LOL just looked under the car... VSS is bent up and facing the chassis

That's a bit of a result, sounds like your on the home straight now.