Sigma VVT misfiring

[njhendo]

I was out last night for about 30 minutes and once the engine was properly warmed up I started accelerating harder. Suddenly at about 5-6,000 revs the engine started half cutting out / misfiring/ losing power and the tachometer was all over the place- up to 9,000 and then down to 2,000 even though the engine was probably only running at about 3-3,500. I slowed down to 30 and it recovered but then did the same when I accelerated hard to about 60 again.

Any ideas what the problem might be. Last time out was at Castle Combe with the L7trackday where I drove it to the rev limiter and it didn't miss a beat all day?

Might it be a lamda, TPS or MAP sensor problem? 

thanks,

Neil

 

[njhendo]

Sorry, forgot to say it is a 135hp sigma

[revilla]

Long shot but sounds remarkably similar to a K Series issue I had to diagnose for somebody. Ran fine up to about 6000rpm then suddenly started cutting in and out with everything going haywire. Turned out to be as simple as a wire that had been badly crimped and was loose in the crimped terminal. In this case it was where the ECU supply had been extended down to the starter main terminal when fitting a master switch. The wire was loose in the ring terminal but the joint was sealed with heat shrink which obscured the problem. If was remarkable how it ran fine to the same RPM each time then started cutting. I guess the wire resonated at a certain frequency.

[njhendo]

Thanks. Perhaps I'll start by making sure the ECU is firmly in place.

Its a 2019 car and never had any engine issues before. 

[CharlieC]

This sounds very similar to an issue I have had with my car recently (310R - 152 bhp Sigma). It was resolved - after much trouble-shooting by both PGM and Northampton Motorsport by reconnecting a 12V power supply from the battery to the ECU (apparently these are not always installed int eh factory). I suggest a call to MBE Systems (who manufacture the ECU), or take it to Troy at Northampton, who fixed mine.

For reference, my misfire happened consistently at 5,500rpm at high throttle settings (low throttle input didn't create the same issue, leading us to think initially it was a fuel issue).

PM me is you want more of a discussion.

Charlie

[Eccles]

Sounds similar to a problem I had on my Sigma, I had changed the flywheel to a lightweight one and on testing it it had what I can only call a very hard rev limit around 5-6K. Turned out the crank sensor was slightly too close to the flywheel and by spacing off .5mm fixed it. Must admit I didn't notice the tacho going mad - too busy wondering what the hell was going on probably.

Probably not the same reason but may be worth checking the crank sensor and wiring as I think these pulses are what the ECU will use to work out the RPM.

Just a thought.

 

Ivan

[njhendo]

Thank you for your replies. Fortunately on Sunday I took it out and it ran like a dream. I'm hoping it was some dodgy fuel or air in the fuel line as a result of running it a bit low at Castle Combe. Fingers crossed...

[DJ.]

Just to rule one thing out, it is unlikely to be the lambda sensor. When mine failed after hitting a badger! The car was fine accelerating hard but was terrible at a constant speed/ throttle when it goes into closed loop mode.

 

I hope it has fixed itself

 

Duncan

[Scotland North AR]

Also check the alternator/starter motor/battery connections, plus the cleanliness of the earthing bolt by the battery. This is the one with about four wires on it.

The nut fell off my starter motor connection on a run out and gave all the same symptoms. I now know a little tiny bit about the electrics now as a result.

Mine's a Sigma 150 with F175 upgrade.

[K7 VCT]

Try the earth terminal  nect to the ecuans battery tray