rev limit / misfire 5000rpm

[angel88]

Hi all

Having owned my stunning tuscan for 3 months now (daily driving ) im am still in love ( is this normal)

Anyway i visit this forum daily just to check on whats what. I did read on here somebody else developed a misfire at 5k rpm and fixed it with a new fuel filter, is this the best thing to try first.

The car idles - runs like a charm but as soon as u hit 5000rpm the shift lights flash green and orange and it hits a rev limiter / misfire. I have my lights set 6- 6.5 -7.

If anyone could shed some light that would be great.

I have plugged the laptop in and reset and efi fault...but it keeps returning, could this be a clue.

Many thanks John

[s6boy]

Can you drive through the mis-fire/ cut out, ie accelerate up to the point and then ease off and then squeeze back on? If so it could be the coil or leads beginning to break down, quite common and the symptoms you describe are similar though don't remember the change up lights going on mine.

Also in this weather make sure it is right up to temperature as the ecu will cause the 'misfire' if it feels the temp is too low.

Let us know how you get on.

[angel88]

Thanks for the speedy reply.

She was up to 69degrees so she was wamed up nicley.

You can drive through it but it flashes the lights and jurks a lot and sounds like a rev limiter. Had a couple of times it pulls straight through and scares the life outa me hehehe.

[Arapo]

Had a similar incident in June. I was driving on the motorway and at 3000rpm I felt something similar to a rev limiter kicking in, then lifted for a sec, accelerated again and it was fine. Took the car to the garage, they diagnosed a misfire, changed the leads, and been working fine since..

[Robertjp]

Have a read of my posts on throttle bodies, this sounds fairly typical of worn throttle bodies.

Its worth checking the ignition first though.

Rob

[angel88]

ok thanks i will do.

also have a lambda fault on my laptop which ive heard can also lead to this. lambda 2 error

so i guess i will try that first. i think they are £80.

fingers crossed lol

[BGB autosport]

If you look on the MYTUSCAN web site there is a list of issues that can cause the high end miss. I have worked my way through it and ended up doing every thing apart from the throttle bodies.

They are currently off the car and have been machined, ported and reassembled with roller bearings fitted and are ready for refitting once i have done a few other tweeks.

[Curdster]

I had the same miss fire probs but while car was in at RG last week I had the HT leads and plugs changed to see if that cured the problem before starting on the throttle bodies, and it has. Worth doing to eliminate.

[Robertjp]

Worth bearing in mind you can check your ignition without spending much money!

Whilst the engine is running you can check the coil pack, HT leads, injector and spark plugs by unplugging and plugging in, golden rule is that if the engine note DOESNT change you have an issue with one of the above.

If you have a problem, swap leads and coil pack ports and injcetor leads and by process of elimination you will find the problem.

(second golden rule....use insulated pliers for the HT leads )

Its pretty easy to hear and saves replacing everything!!

Edited by Robertjp on Wednesday 17th March 09:13

[Tuscanuwe]

"Its pretty easy to hear"

But not at 5000 rpm!!
Easy is it only when engine has lumpy idle, but you cannot
check under load!
And Speed 6s do like to have misfires also under load!

Uwe

[Robertjp]

The point is if you have an ignition problem you can diagnose at idle, you dont have to run @5000rpm!!

If it is an HT lead or something actually breaking down (but not yet defunkt), what are the chances of it doing the same thing everytime at 5krpm? low i would say.

My money is on the TB's though to be perfectly honest.

[angel88]

well at the weekend i ran it up to temp and gave it another boot full. if you wait 2 or 3 seconds of the mini revlimter at 5.5rpm it does eventually pull through like a train right the way up to 7rpm.

very odd

[Robertjp]

angel88 said:

well at the weekend i ran it up to temp and gave it another boot full. if you wait 2 or 3 seconds of the mini revlimter at 5.5rpm it does eventually pull through like a train right the way up to 7rpm.

very odd

it isnt that odd. my words taken a thread i started earlier this year, i have solved my problem by the way, my car now pulls to 7k+ comfortably with no hint of a misfire...


"Have talked to several different engine builders and it seems that ANY PLAY AT ALL in the speed six throttle bodies will cause you an issue of some kind, whether this be going out of tune, hesitation at light throttle or proper misfire at the top end of the revs. Typically they are worn at 25-30k miles. My throttle spindles have about 0.0065" or about 0.17mm of play at worst - it seems it doesn't take much at all to cause an issue! They have done 32k miles.

With regards going out of tune - well, this is the easiest to diagnose i guess, small movements in idle position can have a big proportionate affect on airflow, and the throttle pot internals can also start to wear with vibration which affects their accuracy.

With regards to hesitation at very light throttles, butterfly valves still fairly horizontal in the body will vibrate up and down due to airflow 'pulsing' i.e. no real vacuum. This vibration has a double whammy effect, it will change the airflow to a small degree, but overriding this it will rapidly change back and forth the apparent position in the throttle pot, causing fuelling to change and a rich / lean mixture in a very mixed up way!

With regards to the misfire at high revs, this is again due to vibration. at around 6k the airflow is extremely high and causes high frequency vibration (there must be a harmonic around this combination of airflow and engine speed i guess) on the now vertical throttle butterfly, which in turn vibrates the hell out of the throttle pot and causes the misfire, in some bad cases it can momentarily stall the ignition system due to an overload of data from the throttle pots.

The absolute engineering solution seems to have been undertaken by Dreadnought TVR, I had a good chat with them and their understanding of this problem is extremely good. Roller bearing fixes are done by Str8 Six and Racing Green, but talking to Dreadnought a roller bearing fix isn't ideal either due to issues around low stiction with bearings and a further vibration caused by airflow, but, i guess these must work adequately or people would not have bought them and been happy. Im not advocating one or the other but simply to say that the bearing fix is cheaper than a special, bespoke bush arrangement that Dreadnought use. I havent been able to find an engineering company that will do it any cheaper that the guys that currently do it and are practiced hands.

Im going to go with a roller bearing fix simply because i don't have the funds for the Dreadnought solution at around £900. It will solve my going out of tune issue and high end misfire if nothing else.

Rob"

edited to add the link to the thread...
http://www.pistonheads.co.uk/gassing/topic.asp?h=0...

Edited by Robertjp on Wednesday 17th March 12:25

[robsco]

s6boy said:

Also in this weather make sure it is right up to temperature as the ecu will cause the 'misfire' if it feels the temp is too low.

Is this correct as it may give me a clue towards my high rev misfire? My water temp is low at circa 65 degrees, I have tried several coolant temp sensors so presumably I need a thermostat replacement. Does the car adjust fuelling based on water temp?

[s6boy]

Hmm, I think I remember the situation. Outside temperature was very cold and had I'd only been driving for about 10 minutes on the motorway, water was about mid 70's but oil low 20's. Foot down at about 4k revs (I really don't remember exactly though) and it felt like the engine misfired until I backed off. All OK later when oil was above 50 and no other instances since so I put it down to software/ECU looking after the engine, might be wrong but it makes sense.
Since then I don't pay much attention to the water temperature only the oil.