Grif 500 high rev misfire advice please

[pum]

Hi. Whilst I'm here I thought I'd take the liberty of asking another question of the gathered illuminaries

For a long time I've had this annoying misfire at high revs on my 1994 Grif 500. It was totally standard, and the misfire is a kind of stuttering one that comes in at about 5250 revs. Felt pretty much like a rev limiter cutting in.

I've had an MSD after market ignition and coil fitted, partly to try to cure the misfire and partly just cos I wanted the improved engine running. The engine has also been complete rebuilt/bottom end replaced. Fuel supply has been looked at, and pump replaced. I'm suspecting fuel injection ECU, and considering replacing that with an after-market unit, but want to get some advice before commiting to dishing out moocho dosh.

Can anyone offer advice on this such as:

- other stuff to check (and how)
- mechanics/places that are particularly good at identifying and fixing this sort of problem.
- recommended after-market replacement ECUs.
- low cost other stuff to do at the same time as ECU upgrade (eg: induction stuff).

many thanx in advance, Pum.

[steve-v8s]

Misfire on mine turned out to be one of the “suppressor” caps that fit between the plug and the plug lead. Got another one from the local Vauxhall dealer which fixed the problem, then did the proper job and replaced the rest along with new leads. As a test you can run with the cap out of line but the lead then tends to rest on the manifold.

[pum]

Ah, forgot to mention I had new 8mm leads fitted with ignition stuff (can't remember the brand).

[steve-v8s]

OK, not that then.

[JonRB]

I was going to say that I had exactly the same symptoms on my 1997 Chimaera 500 and that they went away when the HT leads were replaced as part of a routine service.

But if you've changed the leads then it presumably isn't that.

[tgriff]

The misfire at high revs is without a doubt your coil amplifier , look at the side of the HT coil you will see a little silver box maybe black , well its breaking down under load , change it , Its not expensive and will solve all your problems, sexual mechanical, monetry, in fact its almost the meaning of life.
You can shower me with praise when you find you can now hit the rev limiter with the blip of your loud pedal.
happy motoring
Steve

[pum]

Steve, thanx - I shall check it out. You don't think this is a fuel ECU sounding thing then?

thanx, Pum.

[jamiet]

Not neccessarily the amp. I had exactly the same problem. It felt just like the limiter cutting in - very frightening when it happens while overtaking If I took my foot off the pedal and then tried again, it would sometimes be ok.

I tried leads, shrouds, ign amp, throttle pot, etc etc, etc. Turned out it was the curved air duct after the airflow metre. The air registering at the air flow metre was correct and it was sending the correct signal to the ECU, but then the liner was collapsing inside the next air duct that carries the air to the plenum. So the air supply was almost completely choked off. It wasnt permanently collapsed, only when the vacume got high enough.

Its not all that unusual apparently. Might be worth a check before forking out for amp module.

P.s, if it is the amp module, get it for £40 from a lucas dealer rather then £90 from a main dealer like I did.

Also might be worth checking out the thread I started to see other suggestions: www.pistonheads.co.uk/gassing/topic.asp?f=9&h=&t=95656

>> Edited by jamiet on Tuesday 3rd May 12:27