P38 Diesel Idle

Hi all,

I've got a strange issue with the idle on the P38. When starting from cold it idles ok but when I've driven about half a mile or so and come to a stand still the idle momentarily goes to the correct RPM (750 from what I've read) but then goes down to about 650 and hunts very badly - I also can't correct it by using the throttle as something seems to override it.

It also doesn't like light throttle at low revs(when in gear), the engine just 'chatters' (can't think of any other way to describe it ) until I increase the revs slightly and it's smooths out again.

So far I've changed the in tank pump (which completely gave up), the ECU coolant temperature sensor, changed the fuel filter, checked for air in the clear pipe between the filter and pump (no air since the in tank pump replacement), given it a service etc, also put a new seal on the crankcase breather to air intake as the air intake end wasn't sealing properly but still no difference.

Apart from the idle / low rev issue it drives really well and always starts straight away hot or cold.

I've scoured through most of the threads concerning a rough idle but haven't found one with the same symptoms as mine so I'm hoping someone can shed some light on this

It's a 1996 2.5DT Manual with 144000 miles.

Cheers

Mike

When the throttle potentiometer is below 9% the idle switch is active. Idle is then controlled subject to load by the ECU adjusting the quantity servo to give more or less fuel to maintain 750 RPM subject to speed signals from the CPS. Diag would tell you the problem. As long as the bloke using it knows what he is doing of course.

Smoother with aircon off?

Ok cheers, there's a decent LR specialist near me so I'll get it booked in - what sort of price should I expect to pay for a diagnostic check?

It's a base spec model so no air con unfortunately

mcmurray said:

Ok cheers, there's a decent LR specialist near me so I'll get it booked in - what sort of price should I expect to pay for a diagnostic check?

It's a base spec model so no air con unfortunately

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Don't know it depends on how far down the road he sees you coming from. Shouldn't take more than about 15 minutes but depends on his kit.

A local specialist diagnosed mine and reset the idle to position 4. Now it idles spot on with no vibration. (mine had an odd vibration coming through the chassis which was cured by replacing the autobox mounts and correcting the idle)

Rob Edge said:

A local specialist diagnosed mine and reset the idle to position 4. Now it idles spot on with no vibration. (mine had an odd vibration coming through the chassis which was cured by replacing the autobox mounts and correcting the idle)

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What was it idling at before reset and what does it idle at now. Position 4 is unknown to me what diag did he use?

wammers said:

What was it idling at before reset and what does it idle at now. Position 4 is unknown to me what diag did he use?

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Land Rover T4 used and it gives if I am not mistaken, values from -1 to +4

Rob Edge said:

Land Rover T4 used and it gives if I am not mistaken, values from -1 to +4

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Ok never used that. What was the idle speed before they changed setting and what was it after reset? Have you had the pump apart?

Got it on the diagnostic machine today - no codes or errors apart from a cruise control one (not surprising considering it's non existent) so I'm a bit stumped again - only charged £40 though Is there anything else I can check?

mcmurray said:

Got it on the diagnostic machine today - no codes or errors apart from a cruise control one (not surprising considering it's non existent) so I'm a bit stumped again - only charged £40 though Is there anything else I can check?

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Yeah, I've got the manual DT and I always get that one too. I think it is because the ECU is split across 2 chips and the information that stores whether cruise is fitted or not is stored on the other one and a default value passed through.

Did you check the little pipe from the fuel filter? If that is cracked or the MAP (is that the right one? Get mixed up between MAP and MAF) sensor is knackered could that cause it?

Grrrrrr said:

Did you check the little pipe from the fuel filter? If that is cracked or the MAP (is that the right one? Get mixed up between MAP and MAF) sensor is knackered could that cause it?

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It's the MAP, Manifold Absolute Pressure sensor.

wammers said:

Ok never used that. What was the idle speed before they changed setting and what was it after reset? Have you had the pump apart?

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Original idle speed was 675. Now just over 750. Only repair to the pump was replacing to copper washer on one of the outlets on the pump.

Rob Edge said:

Original idle speed was 675. Now just over 750. Only repair to the pump was replacing to copper washer on one of the outlets on the pump.

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Ok so fuel increased to raise idle speed.

wammers said:

Ok so fuel increased to raise idle speed.

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No. The pump was leaking at the nose of the injector. We only replaced like with like on one cylinder. This is on the injection side

Rob Edge said:

No. The pump was leaking at the nose of the injector. We only replaced like with like on one cylinder. This is on the injection side

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Yes, changing the setting raised fuel by moving the position of the control collar to increase idle speed.

wammers said:

Yes, changing the setting raised fuel by moving the position of the control collar to increase idle speed.

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Now I get you.

I'll have a look at the fuel pipe / MAP sensor over the weekend - the LR place recommended using a fuel system cleaner just in case the pump or injectors are clogged up. Not used any cleaners like this in the past - any particular brand I should be using / is it worth doing?

mcmurray said:

I'll have a look at the fuel pipe / MAP sensor over the weekend - the LR place recommended using a fuel system cleaner just in case the pump or injectors are clogged up. Not used any cleaners like this in the past - any particular brand I should be using / is it worth doing?

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Strange as it may seem some injector cleaners do seem to give improvement. They are mostly all the same, put in with fairly low fuel quarter tank or so for max effect. Don't think it will cure your problem though.