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


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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?

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The one thing that you have not mentioned is fuel pressure at the regulator - it should be a steady 58psi all the time the pump is running.I've seen these pumps run down to less than 20psi when failng - with symptoms of poor performance.At the sort of speeds you mention the engine needs to make full power to make progress above it.Also remember that the turbo can make full boost at as little as 2100rpm under load,but if the fuel pressure is down there will be a limit to the injected amount.

In a similar situation TD4 Freelanders wont show overboost in the first 4 gears when the boost modulators fail,its not till you get to 70mph in fifth that there is enough load on the engine to make it work hard enough.

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Does the nanocom read absolute inlet pressure or above ambient?

if its absolute your boost pressure is about normal but if its above ambient its over booosting

sorry but not familiar with nanaocom readings

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It's absolute - when the turbo isn't spinning, it's around the same as ambient.

I'd second the fuel regulator being a possible culprit. I changed mine recently (had to do the god-awful job three times...long story) but the car's performance picked up noticeably with the new reg in.

Prior to swapping it out my Nanocom stats were all fine (quite similar actually) and they looked the same after swapping it out.

How did you check the pressure on the regulator and fuel pump? As Ally V8 mentioned, they produce a fair amount of pressure as standard.

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Fuel pressure was measured with a home made device: a quick connect fuel line extension (male x female) with an inline "T" where an electrical pressure sender unit is hooked up. This way fuel pressure can be monitored inside the cabin (0-10 bar fuel sweep electrical gauge) whilst driving (up & downstream of the FPR since it can be connected in either line). Upstream was 5-6 bar & downstream was steady 4 bar (~58 psi), throughout the gears.

Was a bit surprised when first tested Td5 'raw' fuel pump pressure, as it can be rather noisy as well as high (recorded up to 7 bar in a new pump). Went through 5 different Td5s & 7 fuel pumps before convincing myself it's just the way they are.

As said, the car drives like a charm up to that 'top' speed; also in 4th at 4200 rpm & 120 kph fuel demand should be much higher that in 5th at 3000 rpm, which indicates fuel delivery is not a problem.

Presently believe it is some sort of logical speed limit switch in the ECU (as required by some specific markets). Planning on trying a different ECU next time this fellow comes around.

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Ok,you are happy with fuel pressure.All the live ecu readings are in the parish so you need to look elsewhere,my suggestion would be to change the injector lower washers - very possible that one or more of them are just and just starting to fail.This allows combustion gases to be forced into the fuel gallery and the overall injected quantity will be lessened.

My other thought is to try a set of "known good eui's". I once had to check out a 110 Defender which was down on power,my test set of eui's restored its performance - and then the customer wouldnt pay for replacements as he thought his employees would not be able to thrash it so much if it was flat.....

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Owner did head gasket himself not long ago replacing plastic dowels for steel ones, bank testing EUIs and fitting new washers & o-rings on them. One of the neatest sounding Td5s (in my limited experience though) and powerful all along, so I'd dare say nothing wrong with the EUIs.

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  • 9 months later...

I was interested in this thread as I encountered a similar problem.

Apparently, correct market settings and fuel map must be programmed into the ecu.

My customer bought an ecu from ebay ; when installed, it did the same thing, ran quite well but only upto 120kph. Upon close inspection of ecu values I noticed the market setting was wrong. Upon further research, i read on one of the LR forums that aside from market settings, a corresponding correct fuel map must be programmed. We headed to the local dealership and had the correct market setting and fuel map programed - that solved the problem.

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