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This car uses a Bosch ME17.8.3 ECU. Whilst the boost control is strictly closed loop especially on OEM software, like most boost controllers it needs the initial wastegate duty to be reasonably in the ball park to reach target boost but not overboost and close the throttle, then the PID control takes over.
 

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On some cars you can get away with running catless downpipes and no tune as the pressure differential is not so great that the ecu can no longer control target boost. On the Mclaren clearly the stock downpipes are a huge restriction.
 

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looks that way, its back pressure related. thinking about it, I can see there being a spike and cause an issue until everything pressurizes but I guess at that point its too late.
 

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This is a easy one. Injector failure. You mentioned metal shavings. But you did not mentioned a complete fuel system clean and injector inspection. In my opinion smaller shavings got lodged in the injectors, one in particular and misfire killed the rod.
Whoever did the work would be have to be held responsible, but you have mods so, you might be able, but if you have the funds, it's a nuisance not really worth the trouble, but do push for it.
I would just get another engine block, take your fuel system out and clean it thru, or just forge this block and go full beast with it.
 

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Nonsense they wouldnt have made it past the filter
Cannot do anything but laugh at the idiocy of those words. Google is a wonderful tool.
This happens across all brands and models, in dfi engines it is more common when the hpfp fails, but mclaren is not and metal dust and grime is not a new thing to be found inside the injectors. Hell it's metal could very well had cut the filter, or just clogged it.
Quite simple to cross out tho, check injectors.
 

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honestly, its really unlikely. even metal dust wouldnt make it through. have you ever seen inside of a fuel filter? Its so unlikely.
 

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Discussion Starter · #68 ·
I’m sorry I haven’t updated everyone. I’ve been dealing with this issue and trying to get it resolved. Again I’m writing this in hopes of resolving the problem and avoiding this problem for the next poor enthusiast that may run across it.

Heres the update:
Cylinder detonated and threw a rod. No oil issues. No rental abuse causing bearing damage none of that. Engine was as healthy as an ape right up to moment of failure.

Removed the engine, Sourced known good engine from salvage car. Installed the engine along with the fuel rail, injectors and all from the donor engine. New plugs. Swapped over the Ryft exhaust and catless pipes as well as the inlets. Installed yet another fuel pump out of precaution. Drive the car for 2 hours easy then do a pull only to find the same 3rd gear cutout still existent. Needless to say I had a lot of different emotions at the same time.

I think the theory makes sense that due to the catless pipes it might overboost and fuel cut but I’m not fully convinced.

Car is now at a reputable tuners shop who says he will for sure find the problem and said he has seen one before same model year that leaned out and threw a rod exactly like mine. Said that besides leaning out or fueling issue they are hard to break.

When I get an update I will share it for everyone as I have often read up on various problems with cars I’ve owned and appreciated seeing the outcome.
 

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I’m sorry I haven’t updated everyone. I’ve been dealing with this issue and trying to get it resolved. Again I’m writing this in hopes of resolving the problem and avoiding this problem for the next poor enthusiast that may run across it.

Heres the update:
Cylinder detonated and threw a rod. No oil issues. No rental abuse causing bearing damage none of that. Engine was as healthy as an ape right up to moment of failure.

Removed the engine, Sourced known good engine from salvage car. Installed the engine along with the fuel rail, injectors and all from the donor engine. New plugs. Swapped over the Ryft exhaust and catless pipes as well as the inlets. Installed yet another fuel pump out of precaution. Drive the car for 2 hours easy then do a pull only to find the same 3rd gear cutout still existent. Needless to say I had a lot of different emotions at the same time.

I think the theory makes sense that due to the catless pipes it might overboost and fuel cut but I’m not fully convinced.

Car is now at a reputable tuners shop who says he will for sure find the problem and said he has seen one before same model year that leaned out and threw a rod exactly like mine. Said that besides leaning out or fueling issue they are hard to break.

When I get an update I will share it for everyone as I have often read up on various problems with cars I’ve owned and appreciated seeing the outcome.
This is interesting. At what RPM is this cut out may I ask and how does it feel? Do you get any flashing check engine light after this happens?

does it happen every single time you open the throttle fully or is it intermittent?
 

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I’m sorry I haven’t updated everyone. I’ve been dealing with this issue and trying to get it resolved. Again I’m writing this in hopes of resolving the problem and avoiding this problem for the next poor enthusiast that may run across it.

Heres the update:
Cylinder detonated and threw a rod. No oil issues. No rental abuse causing bearing damage none of that. Engine was as healthy as an ape right up to moment of failure.

Removed the engine, Sourced known good engine from salvage car. Installed the engine along with the fuel rail, injectors and all from the donor engine. New plugs. Swapped over the Ryft exhaust and catless pipes as well as the inlets. Installed yet another fuel pump out of precaution. Drive the car for 2 hours easy then do a pull only to find the same 3rd gear cutout still existent. Needless to say I had a lot of different emotions at the same time.

I think the theory makes sense that due to the catless pipes it might overboost and fuel cut but I’m not fully convinced.

Car is now at a reputable tuners shop who says he will for sure find the problem and said he has seen one before same model year that leaned out and threw a rod exactly like mine. Said that besides leaning out or fueling issue they are hard to break.

When I get an update I will share it for everyone as I have often read up on various problems with cars I’ve owned and appreciated seeing the outcome.
I have attached a datalog of a stock Mclaren 720S on catless downpipes and no tune closing the throttle after overboosting by 0.5bar. This was fixed by tuning the car. Fit stock downpipes or tune the car!

When you launched the car before it failed did it close the throttle as it does without launch?

0.5 bar of boost is a lot of boost when the car only runs 1.1/1.2 from the factory iirc

Have the car data logged, it will be clear why you are getting throttle cut. It is not safe to drive the car like this!
 

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I have attached a datalog of a stock Mclaren 720S on catless downpipes and no tune closing the throttle after overboosting by 0.5bar. This was fixed by tuning the car. Fit stock downpipes or tune the car!

When you launched the car before it failed did it close the throttle as it does without launch?

0.5 bar of boost is a lot of boost when the car only runs 1.1/1.2 from the factory iirc

Have the car data logged, it will be clear why you are getting throttle cut. It is not safe to drive the car like this!
[/QUOT
Listen to him, I have been speaking with him over the days and he knows what he is talking about and I trust him and he will be tuning my car.
 

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I have attached a datalog of a stock Mclaren 720S on catless downpipes and no tune closing the throttle after overboosting by 0.5bar. This was fixed by tuning the car. Fit stock downpipes or tune the car!

When you launched the car before it failed did it close the throttle as it does without launch?

0.5 bar of boost is a lot of boost when the car only runs 1.1/1.2 from the factory iirc

Have the car data logged, it will be clear why you are getting throttle cut. It is not safe to drive the car like this!
Please can you explain more
1. Afaik the car runs 20psi stock = 1.38bar, and not 1.1/1.2bar, where from did you get that boost level of 1.1-1.2bar?
2. Assuming the car does high 1.4bar - low 1.5bar (only 0.15bar or so above stock) at maybe the same amount of ignition timing as a stage1 car, why is that dangerous?
 

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That's brutal , sorry to hear. Its 100% the catless downpipes tho and I know this cause I installed mine when the car was new and got overboost on my first drive before I got the tune done .

Go back to stock or tune it . Switching back to stock downpipes to at least test if that's the issue takes like 2hrs . I have swapped mine myself a few times .
 

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1bar boost on stock downpipes isn't the same amount of air flowing through the engine as 1bar boost with catless downpipes.

So you can imagine that 1.5bar boost on catless downpipes is even more air than 1.5bar boost on stock downpipes would be.

The standard ECU is calculating the fuel and timing it needs for the airflow it expects as if the car had standard downpipes.

By removing that restriction and not reprogramming, it's having to adjust well outside the range it was expecting to see. At some point, it may no longer be able to adjust quickly enough or give up completely and throw errors or limp modes.

Do what AirdrieVentador suggests - go back to stock or get the ECU recalibrated for your modified engine..
 

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1bar boost on stock downpipes isn't the same amount of air flowing through the engine as 1bar boost with catless downpipes.

So you can imagine that 1.5bar boost on catless downpipes is even more air than 1.5bar boost on stock downpipes would be.

The standard ECU is calculating the fuel and timing it needs for the airflow it expects as if the car had standard downpipes.

By removing that restriction and not reprogramming, it's having to adjust well outside the range it was expecting to see. At some point, it may no longer be able to adjust quickly enough or give up completely and throw errors or limp modes.

Do what AirdrieVentador suggests - go back to stock or get the ECU recalibrated for your modified engine..
agreed go back to stock and see the outcome
 

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Discussion Starter · #77 · (Edited)
For those who ask, when it happens it feels like throttle shutting briefly. Like car hits a brick wall then comes back to life right after. I have driven a friends tuned car by M engineering and it was a lot of fun so I will try the route with the dyno tune first. I’ve driven stock cars too and the exhaust note from my pipes is the best I’ve heard. I didn’t want to go stock because I still believed I had an injector or fuel delivery issue as the car performed great for 4000 miles with these pipes before ever a problem, but I realize it may not have been safe in hindsight. I am at peace knowing it’s in good hands and should emerge better and faster than ever. @AirdrieVentador you make a good point and I have followed you on Instagram for a while! Sick car! As I said when this issue first developed the cutout was multiple times during a pull in 3rd or 4th and after the fuel pump controller the issue changed dramatically to a single abrupt cut. So it very well may be shutting the throttle as speculated.

So until I hear from the shop I will speculate that I did run into the same fueling issue as @taw6189 and when I swapped my fuel setup, specifically the controller I then changed the issue from fueling to a single abrupt cutout to protect the engine from the better airflow into and out of the turbo’s due to my mods.
 

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For those who ask, when it happens it feels like throttle shutting briefly. Like car hits a brick wall then comes back to life right after. I have driven a friends tuned car by M engineering and it was a lot of fun so I will try the route with the dyno tune first. I’ve driven stock cars too and the exhaust note from my pipes is the best I’ve heard. I didn’t want to go stock because I still believed I had an injector or fuel delivery issue as the car performed great for 4000 miles with these pipes before ever a problem, but I realize it may not have been safe in hindsight. I am at peace knowing it’s in good hands and should emerge better and faster than ever. @AirdrieVentador you make a good point and I have followed you on Instagram for a while! Sick car! As I said when this issue first developed the cutout was multiple times during a pull in 3rd or 4th and after the fuel pump controller the issue changed dramatically to a single abrupt cut. So it very well may be shutting the throttle as speculated.

So until I hear from the shop I will speculate that I did run into the same fueling issue as @taw6189 and when I swapped my fuel setup, specifically the controller I then changed the issue from fueling to a single abrupt cutout to protect the engine from the better airflow into and out of the turbo’s due to my mods.
Much appreciated!

And yea when I had the cutout before my tune with juat catless downpipes . It's exactly like u say , I would hammer it and it would start going and then BAM close the throttle and feel like I hit a brick wall . The only difference is mine threw an overbooat code and went into limp mode after thw second time it happened .

I knew I was getting a tune I just figured I could drive it back from my shop to my house before I pulled the ecu to send to Mengineering. But nope , it didn't like thw catless downpipes . I also had 3.5inch not 3inch like most ppl tho.

That was 2.5years ago and since I started tuning with Mengineering it hasn't had a hiccup again ever.
 

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less restriction means lower boost as the volumetric efficiency increases, agreed. But if the standard car runs almost 1.4bar and now running 1.5bar, that should be something the ECU can't compensate pretty easily, as the car has enough fuel to run 1.8bar on catless exhaust with pretty aggressive timing too. So besides having codes thrown, why is that a safety problem? I still dont get it... the car doeesnt run 1bar stock to compare to 1.5bar...
 

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I guess it's reaching its limits of what the ECU feels it can safely compensate - and there's a delay between the time it notices the overboost/lean condition and reacting to increase fuelling, or adjust wastegate duty and ignition timing. During that time, the engine could encounter some knock events which could leave some permanent damage on the hardware (pistons, bearings, crankshaft journals, cylinder head, head gasket)).

You'd need to do some longer term logging to find out what exactly the ECU is doing - but the safest way as an owner is to at the very least have a tuner look at the car after any major modifications are made and if they can make adjustments to bring the ECU into an area that isn't having to make significant adjustments then they should do that.
 
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