misfire problem

[munky]

Have a problem with what I'm assuming is a misfire, over 3,200 revs only while under load (so I can't replicate it in the driveway) there is a frequent, momentary loss of power and the rev counter dips & then bounces back. It does this when accelerating and when cruising. I'm guessing it's an ignition problem of some sort, have changed all the HT leads but the problem persists. No other electrical problems, so shouldn't be battery or alternator. What's next to check - distributor or coil, and what am I looking for? Had a look in the dark for any evidence of tracking, couldn't see any, although as I say the problem only seems to occur under load. What about timing - is this controlled by the ECU?

cheers

Edited by munky on Sunday 11th November 16:41

[Paul (93 Chim)]

You could try a couple of things, if you have a multi-meter pop off the mini plugs on the injectors and measure the resistence across the two terminals, should be Approx 16.9 ohms, if any are u/s they will go open circuit.

Another check is to find out which pot isn't firing, a thin piece of plastic on the exhaust outlet (to melt it or not) or temp meter should show this, then you can check for a spark on that cylinder. Or buy some of those flashing spark detectors from Halfords.

It's difficult to tell if the engine is missing on one cylinder at tick over. I found mine the hard way by checking a lot of components on the ignition system and finding it was a duff injector.

[blitzracing]

You have the answer as the rev counters dipping. This means there is a fault on the primary / switched side of the coil, such as coil 12 volt supply, coil, coil wiring or the ignition amplifier. HT faults tend to make the revcounter "bounce" eratically. The ECU has nothing to do with the ignition firing, its purely fuel control.

Mark

[munky]

blitzracing said:

You have the answer as the rev counters dipping. This means there is a fault on the primary / switched side of the coil, such as coil 12 volt supply, coil, coil wiring or the ignition amplifier. HT faults tend to make the revcounter "bounce" eratically. The ECU has nothing to do with the ignition firing, its purely fuel control.

Mark

thanks - how do I check which of those it is? Maybe it isn't possible since it only occurs when driving, not when stationary. Paul - I doubt it's a single cylinder not firing as the loss of power is so violent when accelerating.

Just been out in it again and need to revise my description slightly. The problem occurs all across the rev range while accelerating hard (or trying to), but only over 3200 revs when cruising at constant speed.

[T5AMX]

I had this problem on my 4 ltr Chimp.
One of the cats had broken loose in the exaust. It was one of the front ones located by the Lambarda sensors in the engine bay so I had them both cut out. So it could be that.....
hope it helps

Sam

[Paul (93 Chim)]

I've had a few problems with the coil over the years, ACT sell a bosch type, if it's not been changed over the past few years it may be a good place to start, also I've had a rotor arm fail, they are cheap enough but again you need to buy a Lucas one, my old one was a pattern part.
Hope this helps.

Paul

[blitzracing]

It can get a bit more complex. The voltage required to fire the spark goes up as you put more fuel and air in, if the coils failing it may only show up then. Having said that I dont know why the rev counter should "drop" if the coil output is low, its only running on the switched 12 volts on the primary. Some simple diagnostics would be to run a cable back to the ignition side of the coil to a lamp in the cockpit, and see if it flickers when the car cuts out. Least then you would know the 12v is good. Apart from that you really need to test swap the coil / ignition amp as next point. Its also possible there is some sort of intermittent connection to the ignition amplifier / distributor trigger thats coming and going dependent on the distributior vaccume (assuming this is connected??), as the timing plate moves. If it is connected, unplug it from the distributor and block it, and see what happens under load.

[Bensonv8]

I've had similar symptoms before

You should check the HT leads. Make sure the connections are clean and a tight fit.
(HT lead to the plug cap and the plug cap to spark plug).
i had a poor connection which left sooty scorch marks in the cap on the ht-lead.
on higher revs the vibrations lift/shake the connections apart so you get a spark in the cap/ht-lead and a weaker one in the engine !!!???
well that diagnosis is a bit of a stab in the dark,
but a new set of ht leads sorted me out

[munky]

Bensonv8 said:

I've had similar symptoms before

You should check the HT leads. Make sure the connections are clean and a tight fit.
(HT lead to the plug cap and the plug cap to spark plug).
i had a poor connection which left sooty scorch marks in the cap on the ht-lead.
on higher revs the vibrations lift/shake the connections apart so you get a spark in the cap/ht-lead and a weaker one in the engine !!!???
well that diagnosis is a bit of a stab in the dark,
but a new set of ht leads sorted me out

cheers - already replaced the HT leads though, so next I've borrowed a spark plug spanner to check whether the dealer tightened them properly after the last service, when they should have been replaced. After that I'll move onto the other suggestions on here - thanks everyone

[munky]

result - it now runs perfectly again!

checked all spark plugs were tight (they were), checked firing order was correct (it was) so then moved onto the HT leads, a couple (including the coil lead) were crimped in a bit too much, thus possibly not making a great connection and the coil lead in particular had blackened a bit indicating perhaps the connection wasn't great.. so cleaned those up & adjusted, then took out the coil which was rather dirty and cleaned it up especially the LT contacts, cleaned the connectors on the wires that connect to it (which were also a bit loose) and put it back in tightening it all up properly, and hey presto!

[munky]

It started playing up again - in the end turned out it was the coil being loose - the mounting is the earth. Thread had somehow stripped at some point. Fixed by castle TVR who helicoiled it or something. Pick it up very soon so will find out how it runs!