914 doesn't go above 4500RPMs! Help needed!

Hello again Joaquim,

I don't know what position the servo should be in at full throttle when the engine is stationary, I can't find a reference for that in the manuals. Mine shows 96% at 115% throttle position, photo attached. Another thread from 4 years ago shows an engine with similar figures to mine. Verifying 914 TCU Sensor Data Input

To commence trouble shooting this I would ensure a few things are correct per the Rotax manuals.

Firstly I would consult MML 12-20-00 page 65 and carry out the 4 steps in that table.

Secondly I would check the adjustment of the servo cable as per MMH 78-20-00 steps 1-12, commencing on page 22. There is also a video on this site that may assist. Wastegate Adjustment - 914

Thirdly, although you didn't mention your MAP at the time of the aborted take-off, I would double check the condition and installation of the entire induction system including ducting and the air filter housing. There is a possibility the airbox temperature reached or exceeded 88 degrees which would force the TCU to reduce boost if the engine was sucking hot under cowl air. The IM 72-60-00 page 4 refers. Unless you have an airbox temperature sensor and indicator installed, you will not know this about to happen!

The MMH has a reasonable description of the TCU system commencing 76-10-00 page 2 and continues on page 10 with guidance on extracting and inspecting data logs using the BUDS software. 

I don't wish to offend you with a stupid question so please excuse me but was the propeller in the fine pitch take-off position with green light on?

Good luck and I hope this is of some help to you.

Cheers,

Des.

Hi Des,

Thanks for your inputs! Don't worry, at this point there are no stupid questions. I need to think that anything is possible.

I am sure the propeller was in the takeoff position, because it's part of the check list, and I attempted to takeoff a few times, so the chances that I missed that step all the times are very slim.

The servo cable and wastegate mechanism were checked and lubricated. I am almost sure everything is OK here.

I don't have a MAP gauge. For that reason I have just bought an Aviasport Turbo monitor, that connects to the TCU serial port (the same used for TLR46 software), and reads all TCU parameters.  https://www.aviasport.com/UPDATE_2022/Instrumentacion/Turbo/IM_3917/IM_3917.aspx Unfortunately I don't have it yet. 

The temperature being too high is not very probable, because it was almost freezing outside temperature. Though, now that you mention it, I remember to observe the CHT just before takeoff was quite high, almost 110C. In this low temperatures I warm up the engine with almost closed cowling flaps. And for taxing I needed to use abnormally high RPMs because the taxi way is grass, uphill and it was a bit soft due to all the rain in the previous days. I have an intercooler, which reduces considerably the air temperature that comes out of the Turbo to the airbox.

I haven't flushed the oil circuit (MML 12-20-00 page 65 and carry out the 4 steps in that table). The annual service is approaching, so it's something that I will need to do sooner or later, but I don't understand the reasoning. Can you explain why you think this can help me to find the issue and potential solution?

At this moment, I have 3 potential causes:

1- Something is wrong with TCU/sensors, which makes the servo not go above 80% in throttle at 115%. If this is the case I am totally lost because I don't know how to proceed. I am not aware that I can do anything in BUDS or TLR46 to solve this.

2- Air leak in the intake system. This continues to be the most probable cause. I have thoroughly checked all hose clamps of the big hoses (air filter, turbo to intercooler, intercooler to airbox and airbox to carbs). I have also checked the small hoses coming out of the airbox, but due to accessibility, I am not very sure that I have done a good inspection job here.  

3- The airbox temperature was too high. The probability is not high, but the possibility is there.

Game plan:

1- Connect BUDS and warm up the engine with cowling flaps totally open. Check what's going on at 115% (Airbox temperature, servo position, Airbox pressure (and target). 

2- Check all the hoses for air leaks. This is not an easy job for a normal human being in my Stemme. The accessibility is awful and you would need very slim arms with several elbows to do a proper job! The inspections that I have done so far were all with the engine not running. Is there any inconvenient in having the engine running and put soap water in the hoses to check for leaks? 

For some unknown reason the text, at least in my computer, is partially hidden on the right side, so I attached a screen shot where you can read the text as I have written it.

Aviasport Turbo Monitor

Something to note on the 914.  If the incoming air to the carbs is over 90C the TCU will back off the boost to save the engine from detonation.  If you have closed off all the airflow to the intercooler, assuming you have one, that may be an issue.  

It is normal at engine off that the TCU puts the wastegate fully closed, you have no inputs until you run.  Setting the throttle position is a 2 step function, see your Rotax heavy manual for that process with BUDS.  

Cheers

Rotax Wizard wrote:

Something to note on the 914.  If the incoming air to the carbs is over 90C the TCU will back off the boost to save the engine from detonation.  If you have closed off all the airflow to the intercooler, assuming you have one, that may be an issue.  

It is normal at engine off that the TCU puts the wastegate fully closed, you have no inputs until you run.  Setting the throttle position is a 2 step function, see your Rotax heavy manual for that process with BUDS.  

Cheers

I was expecting the servo to be at 100% on both throttle ends, 0% and 115%. What is puzzling me is that at 115%, the servo is only at 80%. In the manuals there isn't a reference value to compare. Though, here in the forum there is already 2 cases where the servo is almost at 100% with 115% throttle.

I have intercooler, so if the CHT was at 110C, I would expect that the incoming air in the carbs would be around 70 to 80 maximum. I think I read somewhere that it lowers the air temperature by 40C.