SURGING ON LIGHT THROTTLE

Kev · Post by **Kev** » Sun Jan 21, 2007 3:40 pm

Any ideas what might cause this. On light throttle and on the overrun when the engine revs get below 1500rpm, I have to dip the clutch as the car starts to lurch back and forth with increasing ferocity until I fear for the transmission. It's OK when being driven, but bad in heavy traffic when I need light throttle; it's a pig to drive in rush hour.

Kev

Ian Anderson · Post by **Ian Anderson** » Sun Jan 21, 2007 3:46 pm

Snap!

I have the same problem andbeen fighting with it for a while - better now with a treplacement ECU from Fleabay and telling it not to look for the lambdas

But at the same time the power seems a lot lower than the original kit!

Ian

Alley Kat · Post by **Alley Kat** » Sun Jan 21, 2007 5:39 pm

Air leak on the induction side or timing? Is it carb'd or efi?

GreenV8S · Post by **GreenV8S** » Sun Jan 21, 2007 5:57 pm

That's very common in RV8 powered TVRs. If possible, change down to keep the revs out of the problem area - but once you've dropped down to first there's not much else you can do. Some TVRs are more prone to it than others, and small amounts of backlash in the transmission or any wear in the diff or engine mounts can make it much worse. I don't believe that all these cars have air leaks, and even having them mapped and set up by Mark Adams doesn't always solve the problem (although it often seems to help) so I don't think it's simply caused by miss-fuelling although this is another thing that could make it worse. One thing that seems to make a big difference is disconnecting the vacuum advance. This will mean you get worse fuel economy and the engine runs hotter at idle, so you may not want to use this as the long term solution (but you might, if those things don't matter to you). The theory is that without the vacuum advance you need more throttle to produce a given power level, and this means higher gas speed and less manifold depression which reduces EGR induced distribution problems. Anyway, whatever the cause it does seem to help.

Kev · Post by **Kev** » Sun Jan 21, 2007 6:26 pm

I'm MS efi.
I've tried altering timing and fuelling, but todate without sucess.
If it's significant I'm using a fairly warm cam (piper285) on a 4L, so there will be some valve overlap, hence lower induction vacuum.

I'll seal the plenum to the base and see if this helps as there will probably be some air leaking there.

Does anyone know what in theory is happening during the surging? If I could understand why then I would have something to go on, as it is I don't know the cause never mind how to cure it.

cheers
Kev

GreenV8S · Post by **GreenV8S** » Sun Jan 21, 2007 6:53 pm

Kev wrote:I've tried altering timing and fuelling, but todate without sucess.

How is your ignition timing set up? What did you try? Did you try dialling all the vac advance out at low rpm?

daxtojeiro · Post by **daxtojeiro** » Sun Jan 21, 2007 7:44 pm

Kev, post a log of it at the point of the problem, this is tuneable unless you have a very very mad cam, normally its due to too much advance or too weak, what advance and AFR is it running at that point?
Phil

Kev · Post by **Kev** » Sun Jan 21, 2007 8:51 pm

Phil go to http://uk.briefcase.yahoo.com/bc/kevin. ... =bc&.view=

I've never tried this before so if you can't see it let me know and I'll email the files.
On the datalog go to 1130s or thereabouts you'll see the effects at 1300rpm ish.

I've posted the msq file so you can upload this onto mslvv as well.
This datalog was used to tune so the MSQ file has been changed but only in the fuel ve table.
To me the O2 seems to be varying a fair bit, but I've no idea why. I'm using a LM1 WBO2.

If anyone is interested to see what's going on you can download mslvv from the web; import the xls datalog and see what engine parameters were logged and what the engine was doing at varying load/rpm points during this afternoons runout.

Kev

Ian Anderson · Post by **Ian Anderson** » Sun Jan 21, 2007 8:59 pm

Kev

I also have a "Fast Road" cam called a JE102 in the 3.9.
I'm still on 14CUX

I was told to get a can of quck start (ether based stuff) and spray it all around the intake area - if it is sucking the revs would rise - mine never moved off idle.

Mine was runnig really hot - blackened the stainless exhausts but now seems to be running cooler (with the new ECU)

I have also got a serious thirst at about 9mpg (running gently as still only 260miles on the engine)

So something is still not right but may just need further bedding in on my kit.

Ian

daxtojeiro · Post by **daxtojeiro** » Sun Jan 21, 2007 9:12 pm

Hi there,
looking at it the problem area is where you leap from 11/12 deg advance at 1000RPM to 27deg advance by 1500RPM, I would try retarding it down to 18deg at 1500 around the 30-50KPa area, again at 2000 reduce that to around 26deg and 2500 bring it down to 32deg and try it again,
Phil

Kev · Post by **Kev** » Sun Jan 21, 2007 9:22 pm

Ian
to me the main problem with your setup is you can't actually see what's going on unless you put the car on a rolling road with an exhaust gas analyser. You've got a problem and like me you don't know what's causing it, but I can datalog the engine management system and can at least get a pointer to where to try.
9mpg suggests a significant problem somewhere. I would suggest getting a wide band lambda sensor and datalog the O2, that may give you a starting point. Don't forget that the standard ecu was designed to drive an engine with a soft cam and all the inlet gubbins and restrictive exhaust system. Your engine is so far off standard that imo the management system isn't coping. Is you dissy standard, because again I doubt whether the timing will be correct either.
The info you've posted here gives conflicting symptoms, engine running hot suggests too weak or too retarded, but black exhausts suggests too rich. It could be of course that it's too weak in parts of the rev/power range and too rich in others. Problem is I don't know and I suspect no-one will unless you get it on a rolling road.

Of course I would suggest

that you get a MS and junk the standard stuff, but I'm biased I like it. Download mslvv and view the datalog I posted, you'll see what I mean.

Hope this helps
Kev

Kev

Kev · Post by **Kev** » Sun Jan 21, 2007 9:23 pm

daxtojeiro wrote:Hi there,
looking at it the problem area is where you leap from 11/12 deg advance at 1000RPM to 27deg advance by 1500RPM, I would try retarding it down to 18deg at 1500 around the 30-50KPa area, again at 2000 reduce that to around 26deg and 2500 bring it down to 32deg and try it again,
Phil

cheers Phil, I'll try that when it stops raining

daxtojeiro · Post by **daxtojeiro** » Sun Jan 21, 2007 9:26 pm

Just as an example, here is my advance curve:

Kev · Post by **Kev** » Sun Jan 28, 2007 2:14 pm

I've been out today with the new ignition map and it's worked

Thanks Phil

Ian
on my setup the problem turned out to be ignition timing, it may be worth checking your dizzy, maybe disconnecting the vacuum to see if it has any beneficial effect.

Kev

Ian Anderson · Post by **Ian Anderson** » Sun Jan 28, 2007 4:24 pm

Kev

thanks for that - that's one thing I have not done - give it a try next time it's out

Ian