W203 C270Cdi still limiting revs and turbo after coming to a stop.

[JoeHorner]

As posted previously, the car goes into a sort of "semi" limp mode when you come to a stop after the first 10 mins or so of driving. Drives fine up to that point but then no turbo and limited to 2000rpm.

The only code being thrown is MAP sensor reading implausible (read with icarsoft MB v.1)

The code gets thrown sometimes even if you don't start the engine : read & clear codes, turn ignition off and on again, and it comes back.

That suggested to me that it has to be the sensor itself, wiring, or an ECU issue because, with the engine not started, nothing else could be affecting it.

So, changed the sensor today as a relatively cheap and nice quick job but still the same.

Have cleaned all connections thoroughly at the sensor and ECU, and live data (through OBD) seems to be reasonable but I realise that has a low refresh rate so spikes might be missed.

Getting it onto Star diagnostics isn't feasible around here at the moment (the joys of North West Wales!) so, anyone have any ideas or see any flaw in my logic above re causes?

 

[supernoodle]

A plausability error could be checking against an absolute threshold, a model or another sensor, eg barometric. What other pressures does icarsoft offer? I expect it doesn't offer anything useful.

Freeze frame data would should you a snapshot of several key bits of data when the fault actually set, hopefully including the MAP value. Can be useful in such cases. Unfortunately icarsoft only offers freeze frame when in OBD/EOBD mode, not MB mode. Xentry would show the full freeze frame data.

 

[JoeHorner]

Yeah, beginning to think I might need to invest in an xentry setup seeing as the car's officially a keeper now (just failed mot on one front spring, no advisories).

Don't think icarsoft shows the baro data but suspect that may be where the problem lies - seems to be integral with the ECU if it is. And no idea what's involved in an ECU swap.

Been trying to avoid spending on xentry but, on the other hand, it would give me an excuse to trade in my old E34 525TD for an E class

 

[JoeHorner]

So, finally bit the bullet and got an xentry setup. Still waiting for the mux to arrive from China (hopefully a good one!) but, playing with the simulation, it seems that it's likely to lead to a fault with the baro sensor in the ECU.

Still, she flew through her MOT with just one front spring needed a couple of weeks ago so, if an ECU is needed, probably worth the cost

 

[oigle]

Have you examined the possibility of pressurised air escaping? I had similar with my ml270 due to air escape. A smoke test is the best diagnosis if you cant locate a split or similar.

 

[ajlsl600]

If u can't get ur boost press u hav a screwed turbo(unlikely) . Problem with turbo control or a pipe or filling allowing boost to atmosphere.(most likely)

 

[JoeHorner]

Have you examined the possibility of pressurised air escaping? I had similar with my ml270 due to air escape. A smoke test is the best diagnosis if you cant locate a split or similar.

Yep. Been through all that over the last year & it's all good.

It also sets a "MAP reading implausible" code as soon as you switch the ignition on (IE: before you start the engine) despite the reading being accurate according to a reliable barometer, all wiring checked out, and a new MAP sensor just in case (I don't like "diagnosing by swaps" but it was a cheap enough check)

With the engine not started, the only thing it has to compare that reading with is the barometric sensor in the ECU. Annoyingly, no aftermarket scanners seem to be able to report the Baro reading so it's xentry or nothing.

The new Mux is now held at our local delivery office with a customs charge to pay. Will see if they'll let me collect later today, If not it should be here on Monday.

 

[JoeHorner]

Already been through all

If u can't get ur boost press u hav a screwed turbo(unlikely) . Problem with turbo control or a pipe or filling allowing boost to atmosphere.(most likely)

Already been through all of those and they're fine. Running through Xentry's guided tests (using readings from my icarsoft while I wait for the mux) is pointing squarely at the baro sensor but not ready to invest in a new ECU until a

 

[JoeHorner]

And, we have a solution!

Took a drive down to the sorting office to collect the mux and, sure enough, the ECU is reading WAY too low on atmospheric pressure.

Also finally got to apply a correction to the trip meter that was claiming we were averaging about 65mpg instead of the 45 or so that the fuel bill suggests

 

[ajlsl600]

So ur sorted?

 

[JoeHorner]

So ur sorted?

Well, I know what the problem is. Just need to get a replacement ECU sorted out, but having spent out on the xentry setup that can wait until next month

 

[supernoodle]

Good stuff. It'd be great if icarsoft supported a decent range of DIDs so you wouldn't need the Xentry just to read a simple pressure.

ECUs with on board baro have a vent with a goretex filter. Don't know if that gets blocked it would give your symptoms?

 

[JoeHorner]

Good stuff. It'd be great if icarsoft supported a decent range of DIDs so you wouldn't need the Xentry just to read a simple pressure.

ECUs with on board baro have a vent with a goretex filter. Don't know if that gets blocked it would give your symptoms?waa

That's a good point.

I did pick up a dead ECU a couple of weeks back when I started suspecting this was the issue just to see what the chance of replacing the sensor would be.

It's only 3 solder connections but basically zero chance because the sensor fitted has no data at all out there so no way of tracking down a replacement.

Two options seem to be to get a clone done (about £250) or get a reset one and code it to the car (about the same). First means sending the existing one away but has warranty if anything goes wrong. Second let's me keep the car going while it's sent but if I screw up the coding there's no comeback.

Either way, the car only cost about £700 and has already done 4 or 5 decent trips without issues - including Anglesey to London & back in a day - so worth investing a bit on it

 

[supernoodle]

I'm sure you'd be able to swap the sensor from the dead ECU to yours if it's the same vintage. I expect all EDC16s for MB would use the same part.

With 3 pins I suspect it is a simple analog sensor, power(prob 3.3v), signal and ground. Newer modules use sensors on a SPI or I2c bus, but they would use 4 or more pins.

I would have thought an ECU repair place would be able to fix/swap it, but if the cost is close to the £250 you have for the other options it might not be worth it.

 

[JoeHorner]

The one I got to look at was rotted out from a drowned car - just wanted it for component IDs.

It is a 3 lead sensor but not easy to desolder because it's soldered UNDER the sensor & not sure how it'd take to a hot air gun and reluctant to damage the existing ECU because it does at least allow the car to run.

Would probably be cheaper to learn to wipe / reprogram the eeproms with the contents of mine but that's a whole new learning curve!

Have emailed a few repair places but none seem to be answering

 

[JoeHorner]

And...... it's cured (I think)

Brief recap:

Since we've had it, it's been going into a semi-limp mode (max 2000rpm, no kickdown, and holds the highest gear it can) as soon as you come to a stop after about 10 mins driving.

The only code showing with an icarsoft scanner was "Map sensor plausibility".

Went through all the regular suggestions for that - wiring, intercooler pipes, the sensor itself - but no faults.

So eventually got access to an xentry / das system which showed the barometric sensor in the ECU reading a permanent 650hPA or so - about what you'd expect 10000 feet up a mountain. The sensor is built in to the ECU so looked like headed for a fairly expensive replacement.

Could get a replacement coded to the car for about £250, or a virginised one to code myself for the same price. Or I could get one off eBay for £40 and work out how to renew it myself. I love a challenge.....

A lot of reading round later and it all came down to the contents of a 1kb eeprom in the ECU which holds the data locking the ECU to the car. So, taking a used ECU and either resetting it or copying the eeprom contents from the old one to the replacement SHOULD work, and has for lots of others.

The simplest, but highest risk, way is to desolder the chip and swap it to the new ECU. But, if you mess it up you have no working unit. The chip communicates by SPI and SPI eeprom programmer's are easy to get, but most won't write this particular chip because of some oddities in its design.

But the good old Arduino has SPI available and you have complete freedom with the software on those to write something that will deal with the chip's oddities. There's also Windows software out there which will "renew" the file in the eeprom so the car sees it as a brand new unit. Alternatively, some forums have members who're happy to reset a file if you supply the original.

So, spent a few happy evenings playing with an Arduino Uno I had lying around and an old (rusted) ECU that the ROM chip was ok on until I could reliably read the contents, change them, write them back and verify the write was good.

eBay ECU turned up today so connected it to the car and confirmed that (a) it would crank and fire, then stop, as you'd expect with wrong immobiliser info stored and (b) the baro sensor reading was more like it should be at sea level.

Got the lid off, clipped the wires from the Arduino onto the chip, and hit "read". Seemed good, so gave the resulting file to the ECU clearing program and wrote the results back onto the ECU.

Connected the car up to xentry and hit "initial setup" for the engine control. It read the old one (as expected) then didn't complain when I connected the replacement. After a minute or two and several "are you sure?" type warnings,cand a request for the chassis number, it confirmed that the coding had been successful.

Started up, coughed and spluttered for a minute or two while the ECU forgot it's old engine adaptations and learnt this one, and off I went on a 20 min test.

Final result: no dash lights, no codes, and turbo all the way

 

[supernoodle]

Well done, so much more satisfying fixing it yourself.

 

[JoeHorner]

Well done, so much more satisfying fixing it yourself.

Absolutely.

Not to mention cheaper (if you don't count the time involved) and a great learning curve in terms of getting to grips with far more modern cars than I'm used to