misfire help

[bobfather]

I've been suffering misfiring for the last few days, 4ltr serp. Very lumpy when first started from cold especially when damp. Once warm the misfire is only present when at low revs and low power, i.e. when in slow traffic. There is evidence that it may be more prevalent when travelling around a righthand bend. There's no hint of it when the car is at speed.

I'm suspecting HT lead breakdown or some other ignition related fault.

Any help would be greatly appreciated

Edited by bobfather on Monday 3rd August 17:40

[jojackson4]

AFM

[bobfather]

Really? The symptoms appear to be ignition related

[Danblez]

Always start with the basics when was the last time you replaced dizzy cap, rotor arm, ht leads and plugs?

Mine had exactly the same symptoms when I bought it and replacing the above completely transformed it.

I am not sure if it was the leads or the plugs that made the most difference as I changed both at the same time but I suspect it was the leads.

Another suspect could be the plug extenders. Easy enough to test by running without them for a short time.

[igiveup]

I would check every HT lead is seated properly, both at Spark plug extender and Dizzy.

The pressure builds under the rubber boots and every so often its more than enough to push them off, I had number 2 cylinder HT lead pop off, thought I had broken something as it was really unbalanced.

Also the King HT Lead from Coil to Dizzy had arched, not noticeable unless you remove them and check.

Also remove dizzy cap and arm and give the surfaces a light sanding with a fine grit paper.

[blitzracing]

Its worth checking the HT leads in the dark- if they are shorting out you might well see a spark. A misfire may not be the HT side of things, but an issue with the coil as well. Ive recently bought a Job lot of Gunson HT testers for this very purpose on the RV8, that allow you to easily measure the HT voltage in each of the plug leads, so you can check for spikes if the lead is open circuit, or low voltages if the lead is shorting out. They are now out of production unfortunately but have a simply LED display showing the peak voltage on each line, they are excellent units. Ive not got around to doing TVR fault finding notes for them yet- but if you are interested in one drop me a line- They are £35 inc postage.

[bobfather]

Plugs, dizzy cap, roter and leads were replaced last service 2k miles ago. I've popped the old leads back on, just waiting for dry roads to test it now

[QBee]

One useful test is to find out which cylinder is misfiring.

Either:

Buy a digital thermometer off Ebay, about £10. Make sure you get one that goes to over 450 degrees C, as manifold temps can be as high as 400, so the 300+ ones are no use.

link to advert
Then fire her up, wait for the misfire, and take the temperature of each manifold pipe in turn about two inches from the gasket. The misfiring one will be about 100 degrees cooler.

OR:

perform the same test, but using the tip of a cable tie. It will melt on the good ones, not on the misfiring one.

My best guess is a loose plug extender.

[ChimpOnGas]

Lets be 100% frank here from the outset... it's impossible to diagnose an engine fault over an internet forum so you need to accept all the answers you'll get will at best be well intentioned guesses.

We all do it

My advise as a starting point is do some logical & systematic diagnosis, this way you're not just taking a scatter gun or wild stab in the dark approach to solving the problem.

The most valuable diagnostic tool you can have is experience (and a multimeter), if you're not sure how to go about diagnosing such faults I recommend finding a good professional that does.

It'll be cheaper in the long run!

But as everyone else is guessing away I may as well join in

My advice (other than what I've already said above) is to ditch those dreadful extenders, they are nothing but trouble.

I did just that, and have proved if you protect your HT lead ends properly you really dont need those troublesome extenders.

Here's what I did using the excellent MSD Super Conductor leads, MSD Pro-Boot Guard and DEI Titanium Protect-A-Boots over the whole lot:


Top quality MSD Super Conductor lead




MSD Pro-Boot Guard




Not show here but the MSD Pro-Boot Guard actually goes right to the tip of the grey HT lead end where it meets the spark plug




DEI Titanium Protect-A-Boot



The completed set




It's not a cheap exercise, but neither it would seem is buying 8 brand new extenders that may or may not last 10 minutes themselves.

Good value is often where you pay a bit more up front to do the job right in the first place, because you inevitably end up only doing the job once!

[bobfather]

Thanks ChimpOnGas, I was running without extenders when this fault started. The leads were protected with heat resistant socks and the cables pulled to maximise distance from the headers. There was no sign of heat damage to the cables but I was conscious of the fact that some of the leads were stretched due to having no extenders. This caused the leads to have firm contact on metal parts of the engine and the potencial for the cables to separate from their caps. This is why I decided to eliminate lead faults first by refitting my old leads complete with extenders.

After fitting the engine ran okay but I still need to drive it to see if the misfire is still present

[bobfather]

Thanks ChimpOnGas, I was running without extenders when this fault started. The leads were protected with heat resistant socks and the cables pulled to maximise distance from the headers. There was no sign of heat damage to the cables but I was conscious of the fact that some of the leads were stretched due to having no extenders. This caused the leads to have firm contact on metal parts of the engine and the potencial for the cables to separate from their caps. This is why I decided to eliminate lead faults first by refitting my old leads complete with extenders.

After fitting the engine ran okay but I still need to drive it to see if the misfire is still present