M42 running lumpy/stuttering

[MouseRat]

Bit of a problem with me M42B18.
Basically most of the time it runs fine, but sometimes (maybe 50% of the time) when I accelerate from low RPM the car stutters, sounds kind of like it's misfiring, makes no power, and then suddenly it's all good again and makes full power.

This usually happens low down in the rev range, but sometimes higher in the rev range it'll stutter for a second and then be back fine again. Exhaust also smells pretty strong when idling (running rich?)

So far I've cleaned the MAF crank and cam position plugs, checked the ICV/vacuum lines.
The Fuel Pressure Reg is good because the vacuum line going to it has no fuel in it and if I pull it off the idle rises (so this must mean the diaphram inside is good?)

What's next? Car doesn't have an O2 sensor so can't be that and surley it won't run at all if it was crank/cam pos sensor

Any help appreciated

[Brianmoooore]

If the injectors are more than 70,000 miles old since they were rebuilt or were new, then I'd start with those. The engine is set to run fairly lean on idle, and if the injectors are flowing less than they should, the extra air available as the throttle opens can make the fuel too weak to ignite.
Exhaust smells of petrol for both weak and rich mixtures - weak mixtures because the fuel simply hasn't ignited, and rich mixtures because there is left over fuel coming out the rear.

[rix313]

I had something which sound similar, it was higher up the Rev range around 4.5krpm. It would cough and fart but eventually would push through. I was at an event and only had a throttle sensor available so swapped that on a whim. The problem went away.

[Brianmoooore]

I had something which sound similar, it was higher up the Rev range around 4.5krpm. It would cough and fart but eventually would push through. I was at an event and only had a throttle sensor available so swapped that on a whim. The problem went away.

It may well turn out to be something like this, but you'll notice I wrote "start with the injectors". There' a world of difference between what I assume is a road car, and one like yours, used for competition. I assume that your engine is meticulously tuned and maintained, and you are most unlikely to be running around giving away 20 BHP to the opposition, simply because of worn injectors, or because of any other neglected 'service' item.

[MouseRat]

If the injectors are more than 70,000 miles old since they were rebuilt or were new, then I'd start with those. The engine is set to run fairly lean on idle, and if the injectors are flowing less than they should, the extra air available as the throttle opens can make the fuel too weak to ignite.
Exhaust smells of petrol for both weak and rich mixtures - weak mixtures because the fuel simply hasn't ignited, and rich mixtures because there is left over fuel coming out the rear.

Engine has around 55,000 on it. There's a company called injectortune that cleans out injectors and cleans them up, so could do that.
Could injectors cause it to run rich too?

[Brianmoooore]

Injectors can gum up, making the cylinder they serve run lean, or they can leak continuously, making the mixture rich.
No lambda sensor means no feedback loop to correct the mixture, but even with one, the correction applied will only be to the average strength of the mixture of all four cylinders.

[martauto]

I had a very similar situation with an m44 b19 in my 318ti and it turned out to be the coiil pack and leads

Mart.

[MouseRat]

Thanks for the advice guys, I'm gonna take the injectors out and get them cleaned up, I'll also test the throttle position sensor somehow too.