Strange Misfire Problem??

[450Nick]

Hi guys,

I've got a strange intermittent problem and I'm a bit lost on what it could be..

The car was run on the dyno yesterday and the guy running it said he had no problems with any kind of misfiring. When I took the car out for a run around the block afterwards though, I put my foot right down and reved the car right out to 6k and when I backed off, the car sounded like it was only running on 7 cylinders - though was still pretty smooth. I cruised for a few seconds and the misfire seemed to clear. Put my foot down again and it was fine, and then again and it misfired.

As far as I can think, this must just be a loose connection somewhere, as this only occured when the car was on the road, and only seems to occur when full throttle is used. So what could cause a temporary misfire like this then? And what would be the best way to go about tracing it?

For info, the car is a 450 with an Emerald ECU and SC Power supercharger. Anyone got any ideas before I start pulling apart the wiring loom??

[450Nick]

No ideas??

[Jammy Dodger]

have a bump on me !

[steve-V8s]

I was battling a intermittent Missfire last year and think, or rather hope it was one of the plug leads. I discounted the leads because I changed them for a different set and still had the problem. However, I think I was misled because whilst misfiring it fouled a plug so swapping the leads didn’t remove the symptom although they were the cause. Changing the plugs however did until the lead failed again and fouled up another plug. Just to add to the confusion a wide band sensor can incorrectly show lean rather than rich if the charge is completely un-burnt which got me blaming an injector. An infra red heat sensor is helpful to tell which pot is missing if you can measure the manifold temp whist the fault is apparent.

[mrdarcy]

Hi, I had a similar problem on a TVR serviced, well maintained, new cam, injectors, 400. I went for the old trick of engine runiing at night with bonnet open on drive. Result, one lead arcing to coolant jubilee clip, one arcing to dip stick one arcing to alternator mount. I replaced the leads and re-routed so that no lead to metal contact, now running consistent dream. I believe the leads were chaffing and arcing intermittently when driven, only really noticeable under load, ie welly. Hope simple solution, usually are.

[Simon says]

Hi Nick, are you set up for EGT monitoring on your car? at least you can detect what pot is giving you jip one thing I will say though is cars usually run on RR's do take some punishment (+ no natural airflow/ventilation) so your trouble may be nothing more than a duff spark plug think I would be inclined just to fit a set any-ways and then start from there

Edited by Simon says on Wednesday 18th April 15:22

[450Nick]

Ok thanks guys, I'll look into getting hold of a new set of plugs and leads when I service it at the end of the month and hopefully get it back on the road for the first time in 18 months!

[daxtojeiro]

450Nick said:

Ok thanks guys, I'll look into getting hold of a new set of plugs and leads when I service it at the end of the month and hopefully get it back on the road for the first time in 18 months!

Dont forget to use resitive plugs! NGK with an R in them.

Phil

[hiltonig]

I had a similar problem it was being caused by fuelling and a air leak on the plenum

[CHGRIFF]

Check your HT leads are not lying close to signal harness to ECU. If contaminating the signal to ECU this can cause all sorts of irregularities resulting with mis-fire. Seem to remember Hamish had similar issue.

[TVR_owner]

450Nick said:

Hi guys,

I've got a strange intermittent problem and I'm a bit lost on what it could be..

The car was run on the dyno yesterday and the guy running it said he had no problems with any kind of misfiring. When I took the car out for a run around the block afterwards though, I put my foot right down and reved the car right out to 6k and when I backed off, the car sounded like it was only running on 7 cylinders - though was still pretty smooth. I cruised for a few seconds and the misfire seemed to clear. Put my foot down again and it was fine, and then again and it misfired.

As far as I can think, this must just be a loose connection somewhere, as this only occured when the car was on the road, and only seems to occur when full throttle is used. So what could cause a temporary misfire like this then? And what would be the best way to go about tracing it?

For info, the car is a 450 with an Emerald ECU and SC Power supercharger. Anyone got any ideas before I start pulling apart the wiring loom??

Are you pulling water into the engine on high vacuum Nick?

[450Nick]

TVR_owner said:

Are you pulling water into the engine on high vacuum Nick?

Sorry John I'm not following? The misfire occurs intermittently under full throttle on the road, at which point the induction system is running at around 8psi boost.

hiltonig said:

I had a similar problem it was being caused by fuelling and a air leak on the plenum

How did this cause a misfire? And I assume you mean the main plenum, not the SC one?

daxtojeiro said:

Dont forget to use resitive plugs! NGK with an R in them.

Phil

Yup, had that problem before too! I run BR8ECS so will be replacing the same.

CHGRIFF said:

Check your HT leads are not lying close to signal harness to ECU. If contaminating the signal to ECU this can cause all sorts of irregularities resulting with mis-fire. Seem to remember Hamish had similar issue.

This may be a good shout.. The leads have been on there rather a long time so I will replace them and check the running against the loom to make sure nothing funny is happening. This will be a job for next weekend I think so I'll report back with findings... Thanks for all the suggestions

[TVR_owner]

450Nick said:

This may be a good shout.. The leads have been on there rather a long time so I will replace them and check the running against the loom to make sure nothing funny is happening. This will be a job for next weekend I think so I'll report back with findings... Thanks for all the suggestions

Sorry Nick, thought it was when you went back on the has after lifting off. Was thinking valley gasket leak .

[EGB]

450Nick said:

Ok thanks guys, I'll look into getting hold of a new set of plugs and leads when I service it at the end of the month and hopefully get it back on the road for the first time in 18 months!

Any news Nick. Hope you have it solved. Good luck.