Craig P38D said:

Hello everyone, I am having a sort of similar issue with my 2.5 DSE auto recently. The top speed seems almost limited to 70 MPH and the speed drops steadily to about 57 MPH on long steep hills. Before the problem started, I was able to accelerate uphill without an issue. So far I have changed the oil and filter, air and fuel filter. I've changed the MAP sensor and checked the MAP pipe is clean, with no cracks and is securely fitted. I found during investigation that the turbo hoses were a bit soft and replaced those. The acceleration is now really good and the engine revs smoothly up to 3000 RPM ish when pushed, before changing up. However the problem persists on the motorway and I'm still stuck at about 72 MPH at about 2500 RPM. I don't normally drive faster than 65 MPH, but like to have a little in reserve which is now lacking.

At some point a tuning box was installed, which used to flash when the engine was running, but no longer does so. I'm wondering if this might be the cause, although it seems odd that the performance is there at lower speeds.

I read elswhere that changing the fuel pump relay might help and I plan to do that tomorrow.

Has anyone had a similar experience? Any advice greatly appreciated.

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Datatek said:

A failed power box may well limit performance. Also, you don't say what year your car is, but if it has a MAF sensor and EGR, failure of the MAF will affect performance. You can check this by running with the MAF unplugged.

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Craig P38D said:

Hello and many thanks for your reply. My car is a P plate 1996 with 235K on the clock. I suspect that the engine may not be the original.
I am wondering if gear box might be a factor, as the car goes really well in first to third gears, but then runs out of steam when it shifts to 4th. Box was serviced 4 years and 20 K ago and has been topped up once. Perhaps it needs topped up, I will need to check. I am rubbish with electrics so was a bit scared to tackle the tuning box myself. There aren't any warning lights on the dash.

I will take your advice and that of Grrrrrr and report on results. Thanks again for your help.

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Craig P38D said:

Hi everyone, progress has been made and the problem is solved. I topped up the gear box with 500 ml of gear box oil while engine was cold and moved the gear stick from park to 1st and then up through the gears before and after topping up. Took the car for a run down the dual carriageway, but the problem persisted, with the speed topping out at 72 MPH. Not really fancying checking the fuel pump as a next step,I pulled over and had a look at the tuning box. There isn't a name on it, so not sure of the manufacturer. As previously mentioned the box used to flash when the engine was running, but had stopped flashing recently. While I was checking for a switch on the box , incase it had been turned off, the wires must have moved and the light on the box started flashing briefly. Further wiggling, application of insulating tape to hold the wire in place and bingo the box looked like it was working. A test drive then confirmed that all is now well. In combination with new hoses, sensors, filters etc, the landspeed record for a P38 Diesel could be in reach. The improvement is so pronounced, that I will have to tread lightly on the throttle, as only half the peddle pressure is needed for steady acceleration.

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RangeRoller dt said:

Is the tuning box blue and black?

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RangeRoller dt said:

PSI maybe?
View attachment 329502View attachment 329503

I’m just curious not seen one for quite a while. There are other types, either way best thing you can do for your engine is take it out and put it back to standard, especially when diagnosing. healthy engine you won’t need it.

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