Rough running and weak spark from coil

[TWB86]

Evening chaps,

Today i've finally got around to investigating a rough running problem i've been having which seems to be getting worse, initially it was just a bit lumpy but has now taken to intermittently cutting out at idle and stuttering badly during driving.

The car is a '98 4.0 litre

So far i have...

Plugged in Rover Gauge and checked fault codes. There were none.

Removed plugs and inspected, they were fouled up as is often the case, i have ordered a set of 6's as the plugs are fairly old now and i had intended to change to the hotter type anyway, particularly as i do a lot of short distance driving.

Removed HT leads (Magnecor KV-85) and inspected, visually they look fine and when tested with a multimeter don't provide any more resistance than would be expected for their length. I suppose they have been on the car since probably 2012 ish but haven't done a huge amount of miles, they were fitted by the previous owner.

Opened distributor cap and inspected, again it looks fine.

Removed stepped motor and cleaned, tested movement with Rover Gauge, the motorised arm pinged off with the spring but otherwise moved cleanly.

Then i took the HT lead from the coil to the distributor off and held it against the rocker cover whilst turning the engine over. There was a spark, but not a very big one. This isn't something i've done before so I'm not sure exactly what to expect but i would have thought that the spark coming off the business end of the coil should be pretty strong, is that the case?

So my questions are...

Should i be suspecting any kind of problem with the coil or condenser and is there any further testing i can do?
How can i identify a bad sensor reading which could be causing the ECU to over fuel?
Can i just ditch the plug extenders and protect the HT leads from the manifold or would i need to change the type of plug leads I'm using due to losing the resistance from the extenders?
And any other advice on what i should look into?

Thanks in advance for your help

[ClassiChimi]

I had an intermittent connection at my coil where the spade connectors go on, causing sudden loss of power and odd running at times, worth just checking them. Just make sure there not a sloppy fit !

[QBee]

How long/how many miles since the problem started?

[TWB86]

I would say it's probably been "a bit lumpy" for a few months but it's only driven a couple of times a week, the cutting out and heavy stutter first appeared a couple of days ago when I last used it.

[WOO5IE]

Hi TWB the temp sensor that the ECU uses is at the top of the engine near the dizzy. They are cheap to replace but the brass bodied ones are the type to get. I got mine last year off EBay for around a tenner from a reputable supplier that was mentioned on a thread on here.
I also changed my plugs to 6 'Es and have the same leads as you,and I installed the resistive type that has an R in the code NPR6 xx I believe

I also did away with the plug extenders as my problem was mainly when the car was hot and it ran intermittently ran like a pig.

You will need some plug temp protectors if you away with the extenders . Do a search on here as it has been discussed many times recently.

If the spark is weak then it could also be the ignition amp but try one thing at a time

Good Luck

[TWB86]

Thanks WOO5IE,

The plugs i've ordered are BP6ES, so presumably non-resistant. Is that an issue?

I went out just now to test the primary and secondary resistance on the coil to see if i could get any closes to identifying it as being at fault. I couldn't find what the resistances should be specifically looking online but i measured the following...

Primary Resistance: 0.29 measured at range 200 Ohm
Secondary Resistance: 7.16 measured at range 20K Ohm

Does anybody know if that is correct before i replace it?

Im thinking that is a logical next step as the spark is present so therefor the ignition amp is switching the current to HT but the spark is weak so potentially the coil is on the way out.

[TV8]

Hi Tom,

I am in south London if you want to try a parts before having a spend-up. I have most of the parts you need?

[TWB86]

Hi TV8,

Thanks for the offer, i'll PM you.

[blitzracing]

The coil will give about 35kv at its tip- so if you have just a single HT lead with a gap to ground it should easily jump 10 to 15mm with a bright blue spark. The coil resistances look OK- but a simple resistance test wont show up any internal shorts at higher voltages, so its only fractionaly better than guesswork. The coil output will also be low if the ignition amp is failing. Personally I hate Magnecore leads with a passion and Id throw them away as a matter of course. RoverGauge is purely an ECU tester, and wont pick up ignition faults unless they are so bad it skews the fuelling, and you will see very high plus lambda trims as the ECU mistakenly puts in more fuel. If you want definitive HT readings, I have some Gunson HT testers that will give a bar code reading up to 30kv that I normally sell as part of a RoverGauge kit for an extra £20. Drop me a mail if you want one.

[TWB86]

Thanks Blitz,

Im aware that some people don't rate the Magnecor leads, personally i wouldn't have fitted them but as i know the problem is upstream of them at the moment they can stay for now. Im hoping TV8 will be able to help me out with a bit of diagnosis by substitution with known good parts but if not then i may well be in touch for a HT tester.

The car is currently not starting and I'm pretty sure the engine is a little on the flooded side, am i right in thinking the procedure is to remove the fuel pump fuse and turn the engine over to clear?

[blitzracing]

Best to drop the plugs out and let everything dry out, otherwise disconnect the amp to prevent ignition and fuel, then crank the engine over holding the throttle wide open to try and flush plenty of air through the engine to try and dry it out for say 20 seconds. The V8 can be a pain to start if the plugs are wet or sooted (This can be a sign of a bad misfire as the ECU richens the mixture if it detects a misfire).

[TWB86]

OK then, my new plugs have arrived today anyway so ill take the old ones out in preparation.

Worth asking again at this point, the plugs i ordered are BP6ES, any reason i shouldn't fit them if I'm planning to get rid of the plug extenders? i.e. should they be resistant plugs?

Cheers

[blitzracing]

You need some resistance in the HT circuits. This is normally in the OEM HT leads, but low resistance Magnecores can cause the rev counter to glitch due to less controlled spark discharge so you may need to add extra resistance somewhere. That could be a standard resistive king lead from the coil to the distributor, resistive plug extenders, or resistive plugs.However the worse the lack of resistance does is glitch the rev counter, or possibly cause any electronics on the car to mis-behave, not that's there a lot in a TVR. The 14CUX ECU itself is pretty tough, but a lot of after market systems need resistive plugs so they dont get glitched.

[TWB86]

I see, in which case I'll keep the plugs and if I have any glitching I'll know what to do.

[TV8]

Hi TWB - no pm received.

Edited by TV8 on Thursday 11th February 16:27

[TWB86]

oh, ok. Hopefully you have something now Graham.

Chers

[Paulprior]

Just out of curiosity why do some people really dislike the magnecore leads? I have them on mine and have some occasional rough running and cutting out issues?

[blitzracing]

To be fair to Magnecore they dont say anything untrue on their web site about what the plug leads can and can not do- they make no power increase claims what so ever, and state quite simply what peak voltages the leads can take- be it black, blue or red. The colours of the leads are indicators of the peak EHT the lead can take, and to some extent the spiral cores resistance to an HT spike. Problem is People like RPI will sell you £100 set of blue or red leads that are way in excess insulation wise of anything an RV8 system will ever throw at it, so all you end up with is bulky and less flexible leads, and absolutely no power gain. For stock engines black Magnecore leads are just fine- but I bet you've never seen a set of them! There are also plenty of story's about the leads breaking down as well- Ive certainly seen it first hand, that you don't expect from a premium product thats around 5 x more expensive than OEM leads that work just fine. The cores resistance also drops going from black to blue to red, and very low core resistance leads to nasty glitches on the coil primary as the coil fires that can get into the cars main 12v supply. Sure if you are running a very highly turbo'ed race engine you might need red "race leads" to keep the spark intensity- but old Rover V8 at 55 BHP per litre- no way.

Edited by blitzracing on Thursday 11th February 20:40

[QBee]

Assume you are trickle charging if you drive it occasionally.
The thought just crossed my mind that you might be running on battery alone, so have you checked the 100 amp fuse? The one under the car.

[TWB86]

No I'm not trickle charging it, it gets used fairly regularly but generally over short distances. I'll have a look at the fuse just to be sure,

Thanks for the suggestion.