misfiring V8

[ferlin]

My car has developed an annoying misfire and I cannot pin it down.
I have cleaned the plugs which were dirty as if running rich, changed the ignition coil, ignition leads, distributor cap and rotor arm.
When it has been misfiring I have run it without the cold start injector being connected which made no difference. I have changed the coolant temperature sender too.
I have recently noticed that if its ticking over and I load the electrics up by turning on the lights it stalls. Its not ticking over too slow as nothing has been adjusted.
Maybe the ignition is collapsing somehow when the voltage drops?
Should I be looking at changing the ignition amplifier next under the coil?

[Wedg1e]

Bad earth, ignition amplifier, pickup coil in the distributor, corrosion of the bullet connector in the loom near the airflow meter (the white/black wire that triggers the injection system), dying fuel pump relay (or even the fuel pump)... if the car's cutting out when you turn the lights on I'd go with bad earthing first.

[honestjohntoo]

ferlin said:

I have cleaned the plugs which were dirty as if running rich

Assuming its a flapper system?

Your only significant clue so far is that the system appears to be running rich.

The scattergun replacement of coolant temperature sender, ignition coil, ignition leads, distributor cap, rotor arm, running without the cold start injector yielded nothing because most of those, including the potential replacement of the ignition amplifier, have little to do with a rich running system.

You could try looking for fault with things that are likely to cause an over-rich mixture.

• CTS connections (to the ECU) including,
• a bad earth connection to the engine block behind and below the LH rocker cover,
• failing FPR,
• noisy throttle pot triggering the ECU enrichment circuits,
• blocked air filter,
• blocked crankcase ventilation system,

* EAV not fully opening due to bad connection to the internal bimetal coil Error - does not affect AFR - sorry!

• AFM CO adjustment gallery blocked
• AFM connections
• Weeping CSI or main injector(s) tested by measuring for fairly rapid fuel pressure drop after switching ignition off.

FYI about 80% of flapper system faults are due to poor connections and air leaks into the plenum system.

As air leaks generally tend to cause uncontrollable weak mixture, you should concentrate on the connections possibilities, cleaning connectors, wriggling cables etc.

A health check may be a spiffing wheeze too!

Testing (as opposed to just replacing) components is generally better because, if done methodically, it checks out the connections too!

Good luck!

Edited by honestjohntoo on Tuesday 15th October 11:49

[ferlin]

Thanks for the advise guys. I think I will investigate the bad earthing first as the ignition does appear to collapse when I load up the electrics.
Its so difficult to tell whether its ignition or fuel John and changing the ignition leads, cap, rotor arm etc was something that hasn't been done for 28 years so not a bad thing!

[wedggie]

I feel your pain ferlin, I'm in the middle of a similar search. a new ignition amp has made a world of difference to mine. I tried the change everything approach at first and ended up with a variety of new parts replacing the old, perfectly functioning items One thing to look at, i had a corroded connector on the ignition amp, which when warm broke down... might be worth a look.

HonestJohn, I've been left with a strange niggle - when travelling at about 2,000rpm, if i give her some oomph (technical term) it stammers and stutters until about 2,500rpm when it runs like a dream until 4500rpm when its judders and shakes again.. I'd say it was using a huge amount of fuel too....

any ideas appreciated.

[honestjohntoo]

ferlin said:

changing the ignition leads, cap, rotor arm etc was something that hasn't been done for 28 years so not a bad thing!

Is it possible there is some routine system maintenance that has been overlooked? You'll need that health check - methinks? You could be amazed when you get inside the plenum and follow up on internal cleaning, settings, connectors, air leaks and such-like.

[honestjohntoo]

wedggie said:

I'd say it was using a huge amount of fuel too....

Our engines don't mind running rich, indeed they can shove out good performance too, but not always across the whole range - that bit in the middle where you are cruising as opposed to accelerating with loads of torque or getting near to full load, can be noticeably affected.

Tough on the wallet, and of course tough on bore lubrication - oil being washed away by fuel.

You also need to follow up on the mentioned causes of rich running and do the health check - I would say!

[adam quantrill]

Hi Ramon, nice to hear from you again. As this question comes up all too frequently, maybe it's time we put together a flowchart that's optimised for time or money (cost of substitute parts that it) which we can leave up on the forum and/or can form part of your publications?

Then people can work through the symptoms and make measurements or substitute parts methodically, rather than the scattergun approach (perhaps call it the "machine gun" approach?)

[ferlin]

update and thanks for all sugestions... This might help somebody else as I havent read about this fault before.
The problem that I had with the miss fire was caused by a poor connection to the power resistor which sits on the wing next to the air flow meter. The white multipin connector and connections have been cleaned up and the connector is now held in tight with a tie wrap. Also there was a broken earth lead that was attached to the fuel pump as well which probably didnt help although that didnt cause the miss fire.
It runs like an absolute dream now as I also changed the leaking HT leads and distributor cap and rotor arm trying to find the problem!
Pity the weather has changed for the worsed and I have just got to decide on whether to keep the wedge now.

[mrzigazaga]

Mine is held in with cable ties also...


It was when i got it but there was only one on it and it was fairly loose...I was getting an intermittent flat spot so i cleaned the plug connector and then replaced it with two ties...Seems to of done the job!..I also cleaned out the N/S rocker cover breather as it supposed to have a pin hole inside the small metal tube..The foam was also rotten so i have replaced that..It has made it feel more responsive..Effectively its the crankcase breather..Just be careful if you remove it..You will need to give it some WD for a day or so and then use a spanner underneath it and gently tap it off...Cheers...Ziga