It could well be the turbo vanes sticking but you can check them yourself, or get your mechanic to by coupling a vacuum source to the actuator and watching the actuator arm move as you progressively apply vacuum. If the arm moves smoothly all is well.
The ecu would be the last thing I would suspect in this case. If it was faulty you would probably have either full or no boost continuously.

 

Getting someone to start the car and turn off a couple of times while watching the actuator will also give you a good idea as it should move smoothly into position when started, and back when switched off. Map sensor is the turbo pressure sensor, so if that was replaced its not that. There is a vacuum controller (trace the small rubber pipe from the turbo actuator back to it) with 3 small pipes going in and out of it, this gives trouble too, as do the small pipes, which develop cracks and leak vacuum. So if arm is not moving fully and smoothly to its open/closed positions, start here by seeing if you have sufficient vacuum coming through the pipe which moves the arm, and work back. Does that make sense ? If the arm is moving correctly, come back to us.

 

Many thanks to you both. I'll check the actuator and pipes this weekend. If they are sticking, does the 'clean' the mechanic have in mind (pouring a liquid cleaner into the turbo, leaving it a few hours then taking the car for a spin to clear it out) sound a good idea. Should I be concerned about all the dirt being blown through the engine?

 

When i first got my 147 JTDm it regularly popped up the fault light if asked for acceleration above about 2500 revs in 4th or higher gears, but it could be cleared by simply switching off and on again....annoying but not too major an issue. After a few weeks of regular use and a few good runs it started to improve and now it can go several weeks without tripping the fault - I put it down to it having some reasonably hard use and having "blown out the cobwebs"......I really should get round to checking all the pipes for leaks, though, as I do get some turbo wailing and some clouds of black smoke on hard acceleration - more noticeable at night when the car behind's lights dim!

 

The engine light does go off after a few days, and limp mode either only lasts a few minutes or can be eradicated by switching off and on again. The guy I bought it off said it was virtually unused during the week as he had a company van, and as I also have a company van, my wife drives it to work (a round trip of 4 miles). I must admit that I've kept it to between 1500 and probably under 2500 revs using the six gears apart from the odd spurt in the lower gears (when there has been the odd cloud of black smoke). So I'll take your advice and give it a bit of a blast. Want to take it on a 80 to 100 mile trip shortly but was concerned about the engine light/limp mode and causing lasting damage, but have had no peculiar noises from engine so can I assume a longish trip might do it some good?

 

Some recommend the liquid clean like your mechanic advises, others prefer to remove the turbo and clean by hand, which is 100% guaranteed to clean it. Former is much cheaper than the latter. Don't worry about the dirt with the liquid method, it doesn't go near the intake side, its purely the exhaust side your dealing with so it all goes out the exhaust. You could also try some injector cleaner into the fuel, millers is good, and as recommended above, a good hard drive, lots of full throttle from 1500 RPM to the red line.

 

Is it worth £50 in your opinion to clean the turbo with the liquid clean?

 

Give it the bit of a blast first, then give it a go, but off is best. Assuming of course that the actuator arm is not moving, if it is, waste of time.

 

1) remove the EGR and the blanking plate
2) clean the EGR and refit it without the blanking plate
3) clean the MAP sensor

Basically, no point in having the EGR blanked if it aint running right in the first place!

Then see how you go and take things from there.

What is the fault code? p0235 or P0238?

 

Ask your mechanic to test the turbo for sticking vanes, using a vacuum pump (Mityvac or cheap Chinese clone off eBay, either are fine) plugged into the actuator. The turbo actuator should move in a smooth, linear fashion from fully rearward at 0ins.Hg to fully forward at ~17insHg. If it does, the turbo isn't overboosting due to sticky vanes.

If it doesn't do that, the most common cause is the slim vacuum hose from the vacuum solenoid to turbo actuator has a small crack or hole. That causes overboosting. A meter of new 3mm ID silcone hose will cost you about £5 on eBay.

If the vanes are sticky, oven cleaner or similar will remove carbon if that is the cause. But it will do nothing to deal with rust (which is the problem I had, thanks to previous owner only running the engine briefly over a year while he tried to diagnose what turned out to be swirl flap failure. I presume it just never got properly warm, and then water condensed inside the turbo). Removing and dismantling the turbo then cleaning up the rust with emery paper is the only way to address that.