Any clues on an enging stutter on a LUX?

Bow Runner · Jan 10, 2016

My LUX is experiencing an occasional stutter when running on low revs. It only happens when the engine is warm.

I have replaced the following but it still happens:

In-tank fuel filter (Porsche OEM)
Fuel line fuel filter (Porsche OEM)
Fuel pump (Bosch)
Coil (Bosch)
Distributor Cap (Bosch)
Rotor Arm (Bosch)
HT Leads (BERU)
Spark Plugs (Not sure)

I have also had the following done but again no solution:

Injectors ASNU cleaned
Throttle Body Cleaned
Checked the AFM and adjusted the arm to realign the track

It starts fine, runs really well and pulls hard. I have no problems over its performance but this blipping is annoying.

Any ideas would be really welcome.

other clues are that a phishing sound can be heard under the bonnet when it does it. You can also sort of hear a click under the dashboard.

Paul 290T · Jan 10, 2016

When you say low revs, do you mean when idling or driving with slight throttle?

Monkeythree · Jan 10, 2016

Air leak? Go over all the hoses and joints on the intake side and check for splits or loose connectors.
Failing that, try removing and cleaning the idle control valve.

bmnelsc · Jan 10, 2016

I agree with M3. I would think the clicking you hear might be the DME relay which controls both fuel and the idle control valve although the "phishing sound" sounds like a vacuum leak. So you may have a bad connector/split in the line (which I would expect to hear all the time) or your control valve itself may be cracked/leaking while its trying to engage (which you may only hear only while the valve is operating or I suppose if there is a bad connector/line downstream of the valve you might only hear it while the valve is operating?)

Bow Runner · Jan 11, 2016

Bow Runner · Jan 11, 2016

Sorry I thought it would include the original post. It does this both when idling and when driving with slight throttle.

I will check the intake hoses and joints for splits or loose connections.

Would a failed intake manifold gasket cause this?

Monkeythree · Jan 11, 2016

Bow Runner said:
Would a failed intake manifold gasket cause this?

Yes. Any air entering the engine downstream of the AFM is air that the ECU doesn't know about.

Bow Runner · Jan 12, 2016

I have a vacuum pressure tester so will plug that in and review the results. I've been looking at what it can tell you about an engine. Looks to be a really useful tool to know about on these pre-computer cars.

Bow Runner · Jan 15, 2016

I've run a vacuum test on the engine. The engine was warm after a run and was exhibiting the stutter.

At idle the gauge was reading a steady 19 InsHg. When I blip the throttle it drops and returns ok. Each time the stutter happens (I'd say it happens mostly once every minute when the engines hot and sometimes closer together) the reading drops to around 16 InsHG momentarily then straight back up to 19 InsHG.

I read on one site this could denote a sticking valve. Any suggestions?

Bow Runner · Feb 3, 2016

I've now been through the process of fitting a replacement brand new Bosch Reference sensor and still no resolution to the problem. Fitted it in the reference sensor location and no change. Replaced the speed sensor with the old ref sensor and still no change. So its not the reference or speed sensors.

Its definitely not a constant issue so I can't see it being a vacuum leak.

I'm running out of ideas. It must be something to do with a warmed up engine as it never happens cold.

The only thing I can think of is something adrift in the ECU.

Would a failing temperature sensor cause this?

golfprorich · Feb 4, 2016

Bow Runner said:
I've now been through the process of fitting a replacement brand new Bosch Reference sensor and still no resolution to the problem. Fitted it in the reference sensor location and no change. Replaced the speed sensor with the old ref sensor and still no change. So its not the reference or speed sensors.

Its definitely not a constant issue so I can't see it being a vacuum leak.

I'm running out of ideas. It must be something to do with a warmed up engine as it never happens cold.

The only thing I can think of is something adrift in the ECU.

Would a failing temperature sensor cause this?

The temp sensor is easy to check with a multimeter [

]

Bow Runner · Feb 4, 2016

It's so intermittent I'm not sure it would show on a brief multimeter test. I think describing it as a momentary downward blip in the revs is a better description than a stutter.