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Hello folks,

My diesel auto has developed a strange problem. Once it gets up to about 70mph (or about 50mph up reasonably steep hills) it changes up a gear and despite having my foot to the floor the power actually backs off as do the engine revs resulting in a slight deceleration.
There are no warning lights showing, everything else appears normal.
Does anyone have any ideas?

Regards,

Si.

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I'd change the fuel filter and run a flow test on the intank pump whilst the old filter is off.

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Thanks for responding. It is due a service but the engine performs and pulls as normal up until the point the symptom kicks in.
The ECU appears to be actively reducing the power (electronic throttle on these diesel engines) as if responding to a signal to limit the power. I would have suspected it was going into limp mode but it is not quite as restricted as that and the engine light does not come on.

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Sounds like a fuel flow issue to me. In tank pump or fuel filter.

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It could be the intank pump. Verify by doing a flow test as suggested. Book says min 180 mls in 10 secs.
Stop and open the filler cap when it looses power and cuts back to check if there is a vacuum (don't do it when moving Ha Ha).
You have got an earlier model without a MAF and EGR, so I am not as familiar with it.
There will an inlet manifold pressure sensor instead. Clean the sensor and check the tubing is OK.
The turbo puts out a lot of gunge into the intercooler and through to the inlet manifold. It may need a clean out.
When it goes into limp mode, it locks up into 3rd gear and has hardly any power. Just enough to get home.
I don't think it has gone into limp mode.

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One thing you all forgot ------ small vacuum hose under inlet manifold to fuel filter housing ---- the little brass nipple gets blocked causing the engine to lose power under load

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Hello all, realised I never posted the outcome of this. Whilst preparing to carry out the fuel pump flow check I thought I'd just make sure the small diameter pipe to the MAP sensor was good. Turned out the end had perished and become loose on the map sensor and leaking. I cut the last 10mm off the end, refitted it (firm fit) and all is well again.

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It's the same size tubing (3.2mm) as the leakoffs. Worth getting Gates brand and changing the lot for piece of mind.