[Mk2] [Turbo] Engine Cutting Out Completely on Boost

[jrleech]

A friends Rev 1 Turbo has a problem.

It drives OK 'off boost', but when he put his foot down, the car accellerates, the engine light comes on, and then the engine dies. Once the revs drop to ~2000rpm, the engine comes back to life (self bump starts), and it will boost again.

I did think maybe he's confused and it's not cut out, but gone into limp home, but limp home requires the ignition to be cycled to reset it, this didn't.

Also odd, is there's no boost at all when the car is cold, only when it gets warm.

He thinks he's getting ECU Code 34, which is Boost related.

He's just had a de-cat fitted, not sure who's de-cat it is, it was 'off a guy'.

My guess is that the new downpipe is interfering with the wastegate valve and preventing it from fully opening. Not sure about the cold performance though??

Any thoughts?

Thanks,
Jon

[Peter Gidden]

You have PM.

[JMR_AW11]

it sounds like overboost fuel cut to me.

This will give error code 34.

For fuel cut to occur the ECU has to do the following:

It looks at the computed engine load (based on AFM and RPM readings) and then it uses this to look at a map in the ECU of load vs rpm.

basically this is a variable load threshold map depending on rpm.

If the load is higher than the map value at the current rpm then the ECU decides to see if it needs to run the "do I need fuel cut?" routine.

This is a similar map based routine but this time it's MAP sensor vs rpm and if the MAP sensor reading exceeds the map value then the ECU starts a timer.

If the MAP sensor reading stays above the threshold for the duration of the timer then it activates fuel cut and the engine will cut out.

It then stores code 34 and lights up the engine light.

It also sets a flag in ECU memory and this affects how the car will drive for the rest of the journey.

Basically the ECU gets scared for the engine and will cause fuel cut to occur again at a much lower engine load. This secondary fuel cut routine doesn't look at the MAP sensor, just engine load based on AFM and rpm.

So for the rest of the journey the ECU will cut fuel every time you try and boost.

If you switch off and on again you will get one run at high boost before it goes into the secondary (lower) fuel cut threshold.

The rev 1 has lower values in the map tables for fuel cut so it is more prone to this problem if anyone mods the system.

[JMR_AW11]

Also note that if anyone Unichips the rev1 or rev2 then they run the risk of killing this fuel cut protection entirely because if they alter the AFM sensor readings to lean off the fuelling (i.e. tell the ECU the load is lower than it really is), the ECU may never get past the first load check and so the fuel cut routine may not even get accessed

However, it might still get accessed at certain rpm levels (because of the way the system is mapped) but it could be totally inactive at other rpms leaving the engine prone to damage from overboosting at certain parts of the rev range.

[JMR_AW11]

The other thing is that the ECU will limit the boost during warmup using the TVSV valve so I don't know if this explains the symptoms you are getting when the engine is cold.

Also it switches in a warmup correction map for the ignition which operates below 2500rpm and this looks like it spoils performance during warmup as it retards the ignition quite a bit in places (coolant vs load vs correction)

[jrleech]

Thanks, but I don't think it's fuel cut, as it kills the engine then recovers... If it was the wastegate fouling, then the VSV wouldn't have any effect, and therefore I'd expect it to still overboost when cold, where as he seems to have no power at all.

Might be something else, maybe a faulty AFM or pressure sensor.

Realised my mate's only 20 miles from S-BITS, so he's going to take it there for Pete to have a look over.

Cheers,
Jon

[JMR_AW11]

Thanks, but I don't think it's fuel cut, as it kills the engine then recovers... If it was the wastegate fouling, then the VSV wouldn't have any effect, and therefore I'd expect it to still overboost when cold, where as he seems to have no power at all.

Might be something else, maybe a faulty AFM or pressure sensor.

Realised my mate's only 20 miles from S-BITS, so he's going to take it there for Pete to have a look over.

Cheers,
Jon

TBH I've never driven a MR2 turbo and felt fuel cut but looking at the code the engine cuts and will recover once the load is reduced. It sounds like you've felt fuel cut already though so maybe it cuts/recovers in a different way to normal with this fault?

Fuel cut does give code 34 though which is why fuel cut is the likely suspect unless it had a genuine fuel cut ages ago already stored in the memory anyway? Try clearing the code 34 and seeing if it comes back?

It looks like a faulty pressor sensor will give code 35 (low volts from sensor) and the AFM gives code 32 (high volts from sensor)
there's also code 31 but I haven't worked that one out yet...

maybe there's a fault that makes it prone to fuel cut eg the MAP sensor signal is stuck at 5V or there's something upsetting the MAP sensor and triggers code 34 fuel cut when it shouldn't?

I can't explain the cold symptoms of no power though. Maybe there's two things wrong with it?

[jrleech]

Indeed... it's an odd one. I hit fuel cut on my first Rev1 Turbo after it had the boost upped, just caught it if you nailed it in 4th up a motorway 'hill'. That killed off loads of power, but like you say, I had to the ignition off and back on again to get it to run correctly again. His just recovers which is odd! Could be the ECU is going into a different limp home mode...., one which clears itself.

Anyway, I'll post once it's sorted and report back what the problem was.

Thanks,
Jon

[Loser]

Holy thread revival!

I've just had the exact same symptoms as this, i've not checked the code (i really did JUST have the problem), it came on boost like normal but reaching around 5 psi it stopped pulling and the CEL came on, then when revs dropped (possibly when boost dropped though) it recovered and drove perfectly normally, until i tried to go past 5 PSI



Did you ever find out the problem??

[jrleech]

Yeah, I meant to post back, sorry!

Turned out it was just fuel cut, caused by the increase in boost due to the de-cat, an FCD was fitted and it's now fine.

The description of the problem that my mate gave me was wrong, the engine wasn't cutting out, it was just going into limp home mode.

Not sure about your problem... once in limp home mode, you generally don't get any boost until you turn the ignition off and back on... You say 5psi, is this on an aftermarket boost gauge? Have you done anything to your car recently?

Cheers,
Jon

[Peter Gidden]

I agree with Jon..

What modifications?

Boost gauge?

Error codes?

[Loser]

It was code 34, literally did it going up the road home so i've not tried turning the ignition off or anything, just drove home like normal.

It's got an MBC, FCD and an omni boost gauge, i suppose the FCD could be fooling the ECU into thinking 5psi is zero boost and then it's fuel cutting?


I was almost hoping it was something more serious than fuel cut which would explain my lumpy running when cold (running out of stuff to replace )

But it's not.. pants.

[hucky]

mine was doing similar but only in 5th gear when i boost.... now it just wont start

only mods its got that could poss be related would be induction kit and boost guage. dont think the turbo timer would effect if or the water temp guage.