When is it appropriate to look for carbon deposits

Joe from Jersey City Diagnostician Posted   Latest  
Discussion
Driveability

Hello everyone. I have a question for all of you driveability techs. First, let me start by saying I am a huge fan of the ats scan tool and especially the training dvd that is included with the kit. Needless to say the topic of this dvd is on using your time efficient and yielding solid diagnostic decision you can use facts to back up. Here is my question...

With any vehicle that sources gasoline for fuel (especially GDI fueled engines) how do you prove carbon build up is present. ??? 

I know several engines may require the service more often than necessary. So some use mileage as reason to take a peak. Has anybody come up with a dso test to point them in the direction of carbon build up prior to inspection? You know so when you use your videoscooe your not wasting your time?

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Jeffrey from Yuma

 

Manager
 

Joe,

I haven’t, but I haven‘t had many GDI engines in for service yet. Maybe the dry climate here is less conducive to carbon buildup. 

I think I’d rather know what customer concerns, fault codes, and fuel trim/VE data would indicate a need for an intrusive look at the intake valve faces. 

A DSO seems like the long way IMHO.

What usually leads you there?

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Ray from North York

 

Diagnostician
 

There was a post in Autonerdz with a 4 cylinder GDI engine with misfire codes. 

A pressure transducer was connected to the intake manifold while cranking and the waveform showed some cylinders were pulling less vacuum then the other cylinders during the intake strokes.

A photo showed the carbon buildup on the intake valves.

My experience with misfires in GDI engines is that the injectors drip and the scope secondary spark time shows the rich, fouled, dripping injector. 

Ray

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Jim from Frederick

 

Curriculum Developer
 

Without mentioning specific vehicle in this reply, an often overlooked resource is scan tool data. There can be PIDs related to "throttle trim" or modifiers like "loss flow control" available. These can be an arrow toward changes in air flow expected under specific operating conditions. The culprit could be carbon. 

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Rudy from Montebello

 

Technician
 

I like that line of thought Jim. Can you elaborate a little more?

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Jim from Frederick

 

Curriculum Developer
 

Hi Rudy,

Sorry I missed the detail of a "reply to me" in the email alerts. Still getting used to this beta.

To elaborate a little more I will provide on example from one manufacturer. This is the type of data that is starting to show up in vehicles that we may not yet be looking for. The example I will use is a late model Toyota. Toyota has a series of PIDs related to ISC (idle speed control) flow. In data we can see ISC Flow, ISC Learning Value, ISC Feedback Value, Electrical Load FB Value, AIr Conditioner FB Value, Engine Stall FB Flow and Deposit Loss Flow. When looking at the repair manual under Data and Active Tests we can get an idea of what these PIDs indicate. 

ISC Flow is defined as the sum of ISC Learning + ISC Feedback Flow + the modifiers. ISC learning is the learned idle rate as most experienced techs would read it. ISC Feedback can be thought of as short term "trim" or adjustments to maintain the target idle without outside changes. Then we get to the modifiers. Electrical Load FB is the air flow rate attributed to electrical load. There can be a value listed for all loads off and one listed for specific additional load like headlights and defrost grid active. The AC FB Flow is the additional air for AC operation and a nominal flow rate is given. All of these can provide insight and direction. 

Next we get to Engine Stall Feedback. If the engine is NOT tending to stall the number should be ZERO. If the engine RPM is dropping and the computer adjusts air flow and timing to prevent it this number will trend higher. Deposit Loss Flow is an adjustment made outside the other FB rates and modifiers. Normal is ZERO so anything up from there may be the result of Carbon buildup. 

Adding these PIDs to others, like Low Rev For ENG Start and Low Revolution Control can provide insight into how the system is operating and WHY it is making adjustments. 

Other manufacturers have similar PIDs that offer direction. Ford for example uses "trim values" for modification of idle strategy. For me, being able to provide documentation in the data and effected changes post service is a win in communication with a client.

I hope that helps explain where my thinking is. 

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Ray from North York

 

Diagnostician
 

I haven't seen it myself, but In the scan tool Generic data, the MAF and the Calculated Load percentage would read lower with carbon on the intake valves.

Ray

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Albin from Leavenworth

 

Diagnostician
 

That should be reflected in the engine load, which would be easy to find on a FRTD (flat rate test drive).

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