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[Kevin Wood]

I took a datalog from a working DTi this weekend but haven't had a change to look at it in detail yet. What was immediately obvious was that the MAF reading was much higher when pressing-on but about the same at idle. Boost pressure was very slightly higher but there was a lot more power being delivered.

Theories?

The MAF is lazy and not reporting true airflow to the ECU, so fuel quantity is low, hence power output is low.

The actual airflow is low for some reason despite boost pressure being there. Guessing boost pressure is measured at the manifold, though, so pretty much eliminates a leak or blockage upstream of that.

[Marks DTM Calib]

So pressure is there but not flow.....

Well the turbo pressure is controlled and the variable vanes will be modulated to achieve this.

So the question is why isnt the engine consuming the air, does the MAF reading v revs match (it should as its a diesel engine) ?

[Kevin Wood]

As said, haven't looked closely yet but MAF was roughly the same at idle. P1ULK's was topping out at about 600 at mid revs and MAP at just under 2 BAR whereas the working example was showing a MAF reading well over 1000 and a shade over 2 BAR.

[Marks DTM Calib]

So, that to me says that the MAF is ok...I wonder if the MAP sensor is over reading and the turbo is being knocked back.

I suspect the mechanics may well be working as expected given that the lack of apparant physical boost from the turbo is contradicted by a manifold pressure reading which is correct.

It would be interesting to see a pressure gauge put on the manifold to allow the ECU MAP reading to be compared with a real manifold pressure reading.

If the MAP is correct then the next thing to consider is what could seriously reduce the air consumption of the engine.....or is there a shed load of air getting into the inlet between the MAF and turbo (cant be post turbo or boost pressure would be lost). This would result in the MAF reading backing fueling off.

[Marks DTM Calib]

Was thinking EGR stuck open but, that would affect the boost pressure I suspect (or maybe not!)

[Kevin Wood]

The EGR and turbo vane actuators were checked manually as described in TIS procedure.

MAF readings at idle with EGR open and closed were about what TIS indicates, as were the boost pressure readings under the conditions described in TIS (IIRC, hold the engine at 3000 RPM and check indicated boost pressure builds when you apply vacuum to the actuator). Electrical vacuum valves were working fine too.

The intake hoses on both sides of the intercooler swelled and the turbo could be heard to spool up, and there was a change in engine note when operating the vane actuator. Intake hoses couldn't be squashed by hand at this point, so boost pressure was there to some extent although it would indeed have been nice to have measured that with a gauge as well.

I think the fact that the readings follow what TIS indicates gives me some confidence that the MAP and MAF inputs are OK and that the EGR is working, under those test conditions, at any rate. The rubber hose between the MAF and the turbo inlet was checked and found to be sound.

EGR might be popping open under boost, of course. Maybe that's causing the low MAF reading, but not bleeding off enough boost to make the pressure look obviously low?

[Kevin Wood]

Then again, I guess the EGR opening uncommanded is going to do one of two things: bleed boost into the exhaust and increase the MAF reading, or introduce exhaust gas into the intake, reduce MAF, reduce O2 into the engine and boost will probably be about the same? Depends on relative pressure between exhaust system and intake.

Given that it's well down on power, boost is normal-ish, MAF is down and it's diluting the oil maybe the second of the above is most plausible?

[Marks DTM Calib]

When EGR sticks open you generaly get the turbo spool down as the exhaust gas speed drops (gases passing into inlet) and the MAF value then drops (so fueling follows it)

[Kevin Wood]

Ahh, OK. No sign of that whatsoever. Boost was there reliably and from lowish RPM - according to the MAP sensor, at any rate.

[Kevin Wood]

OK, some graphs of the engine parameters under full load, I've picked a point passing through around 2500 RPM on full throttle.

Good engine : https://dl.dropboxusercontent.com/u/26795734/Omega/DTIGraphGood.bmp

Bad Engine: https://dl.dropboxusercontent.com/u/26795734/Omega/DTIGraphBad.bmp

So, actually, there is considerably more boost on the good engine (well above the "boost pressure command" value!), which I hadn't initially spotted on the live data. MAF and fuel delivery are up in proportion with this increase in boost - so maybe it's simply down to a lack of boost.

[Kevin Wood]

... and I suppose I should have plotted the vane actuator signal on that graph, to see if it's the ECU holding back the boost pressure or something else.

[P6UL K]

Ok, now this is all going right over my head!

If I knew where the map sensor was I'd remove it and give it a clean

[P6UL K]

Weather wasn't too bad here today so i stripped down the intake and de-coked it all and with the help from a parts washer it all looked new again in no time at all, tbh it wasn't all that bad & wouldn't have been causing any problems... But its another thing ticked off the 'possible issue list'

[Kevin Wood]

Hmm. Had another look at the data logs and the ECU certainly isn't holding back the boost via the actuator.

I think it's not making the boost for some reason. Coming back to thinking it's the EGR leaking or a problem with the turbo itself, assuming the exhaust hasn't blocked.

[P6UL K]

Hmm. Had another look at the data logs and the ECU certainly isn't holding back the boost via the actuator.

I think it's not making the boost for some reason. Coming back to thinking it's the EGR leaking or a problem with the turbo itself, assuming the exhaust hasn't blocked.

Only thing I haven't replaced as yet is the EGR valve, but the exhaust is a complete new system as it was needed for the MOT and the DPF/Cat has been cut open and the honeycomb removed as the initial thought was that had collapsed.