Perplexing final speed restriction on Disco Td5, ECU causing it?

I've been helping out a fellow Td5 owner with a performance problem in his Disco. It is a MY01 manual Disco with almost 120000 kms that has been thoroughly maintained. It pulls extremely well across the entire range (reaching 125 km/h @ 4200 rpm in 4th, which is definitely not limp mode) however once in 5th gear the car will NOT speed up over 125-130 km/h, throttle floored and everything else in good order.

Have advised this chap over email for the last few months --without luck-- so he finally drove over to my place & had a chance to plug in Nanocom:

- NO faults logged in engine ECU
- MAF values are perfectly healthy (60 gr/hr @ idle, 630 gr/hr @ full load)
- AAT/IAT/MAP/FT/ECT live readings are all normal
- Throttle pot supply and track values seem correct (pot 3 values are all over the place but vehicle has a 2-way throttle pot)
- Engine runs very smooth to the ear, idle cylinder balancing is near perfect
- EGR has been blanked off
- No oil in loom or ECU
- Fuel pump pressure as well as regulated fuel pressure were checked ok

He bought it this way from the previous owner so there's no reference of how & when this problem started.

Below is a caption from Nanocom showing fueling input data at 5th gear and throttle floored. Note road speed at 124 km/h, it simply didn't go above that. Manifold pressure of 214 kPa & air flow at 630 gr/hr should allow for plenty of fuel to be injected. Temperatures are all fine. Also no SLABS faults or anything that I reckon could make the ECU restrict fueling.



Here's the ECU settings, injectors were physically checked and codes correspond to those written in the ECU. Also both config tune ID and fuel tune ID do seem to correspond to a (late EU2) manual Disco.



Any ideas?

Looking at your figures I would say the 630gm reading on the MAF was too high. Usually they struggle to get high enough. In your case it appears to be too high. Around 400 is usually the upper mark to be wanting to reach.

Has the engine been tuned in any way? Does it feel like the engine's losing power or that it just doesn't want to go any higher?

Two things are crossing my mind: MAP sensor and turbocharger wastegate modulator.........but i'm a bit dumb in engine management system

Engine has not been tuned per se (remap or chip), but EGR blanked off, turbo wastegate actuator rod adjusted a tad, & critical sensors are cleaned on a regular basis.

It does not loose power, just flattens out in the same way it'd do when reaching 'normal' top speed.

Have to disagree with you (Ratty) over the MAF readings. In my book a 400 g/hr reading at full load indicates a 'tired' MAF sensor, 6 out of 10 perhaps. Took my Nanocom for a ride this morning, read a maximum of 646 g/hr at full load (outside temp -12 °C, atmospheric pressure on the high end). Also looked at recorded fueling data from a few 'godfathered' Td5s (still not charging so can't call them clients), about half of them have MAF readings above 600 g/hr. Could this be due to the diagnostic system used perchance?

The vehicle in question had a funny behaving MAF, low end values were good at 50-55 g/hr (usually anything below 45 g/hr is a bad sign), top end just over 500 g/hr (not the best but still not a performance restriction factor), however it lagged response in between, as if the values in between 'dropped' rather than staying in a straight line. Threw in a new MAF sensor which improved the mid range response dramatically as well as increased the top end readings to the shown values.

On the other hand (Sierrafery) turbo reaches full boost at 2100-2200 rpm and MAP values look right all around, which rules out the wastegate modulator doesn't it?

With so exhaustive explantion even i understood now...as i said i'm quite meagre regarding the engine and it's management...... based on pure logic then it seems that when it reaches that point the engine doesnt get enough of something...fuel/oxigen/inputs fromECU/etc......If u are certain that all the filters involved are new, the fuel pump is working Ok.....it's above my knowledge.......what i've seen reading engine management from RAVE, trying to learn and understand more there is an issue with the fuel's temperature... the inputs from Engine Coolant Temp. and Fuel Temp. sensor are used by ECU to to allow optimum fuel delivery and injection controll........maybe these are making the tricks if the fuel temp is not the optimal on that specific speed when it wont rise further......i dunno more...sorry

Hi, I am new to this forum and have been searching the net as I have a very similar problem with my Disco 2 TD5 ( 2003 ) only bought a few days ago.. Did you ever find the fix for this issue please?

mikebaxter said:

Hi, I am new to this forum and have been searching the net as I have a very similar problem with my Disco 2 TD5 ( 2003 ) only bought a few days ago.. Did you ever find the fix for this issue please?

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Have you even read through the thread

You don't win the prize for reviving the oldest thread or for introducing yourself either

Is there any oil at the red plug of your ECU?

Hi, I have not been on any of these forums before as I am new to quirks of Landrovers so please excuse my lack of introduction.

I have to say I don't even know what the red plug on the ECU is. Could you explain please?

mikebaxter said:

Hi, I have not been on any of these forums before as I am new to quirks of Landrovers so please excuse my lack of introduction.

I have to say I don't even know what the red plug on the ECU is. Could you explain please?

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Ha you'll be fine and pick it up as ya go along

Firstly you say you have only had the vehicle a few days??

So why not take it back??

Your manual will show you where your ECU is in the engine bay.

Once you have found that the red plug will be obvious

Not got a manual?? Download free RAVE manual.

If you do a search for "RAVE manual" there will be a link somewhere on one of the threads the search throws up

Thanks for your advice. I will take a look at that. Can't take it back unfortunately but as ever thing else is fine, hopefully it will be a good one once I have got this issue sorted out.

If I have oil at the plug is it just a case of cleaning it up?

mikebaxter said:

Thanks for your advice. I will take a look at that. Can't take it back unfortunately but as ever thing else is fine, hopefully it will be a good one once I have got this issue sorted out.

If I have oil at the plug is it just a case of cleaning it up?

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No

You may need to replace the injector loom.

T'is not a tricky job nor particularly expensive (Newun costs about £30) just a bit messy. You'll then need to flush the oil out of the loom to the ECU brake cleaner is probably best.

But we're getting ahead of ourselves have a look at the plug first it might not be that.