Undercar - Service Repair

Search Autoparts/Motorage/Technicians/Undercar-service-repair/

All choked up

Just like you, an engine can't run very far if it can't breathe.
Tuesday, May 1, 2012 - 07:37
Print Article
Just like you, an engine can't run very far if it can't breathe.

There are a couple of ways you can test the volumetric efficiency of your problem child. You’ll need a scan tool capable of reading and recording Global OBDII, and an area near your shop where you can perform a few wide open throttle (WOT) runs from a rolling start safely (and legally). The first, and easiest, method uses a standard Parameter Identifier (PID) called LOAD or CALC LOAD. Look for it in the datastream and select it as one of the PIDs you want to display and record. In addition, add these PIDs to the list:  RPM (engine speed), STFT (Short Term Fuel Trim-both banks if applicable), and LTFT (Long Term Fuel Trim-both banks if applicable).

The LOAD PID is a calculated value that is indicative of volumetric efficiency. Be sure to check those fuel trims to make sure the MAF is telling the truth.Here’s a tip for you. Most scan tools allow you to choose what PIDs you want to display. By selecting only those you need or want to observe, you speed up the refresh rate of the data.

Now you’re ready for your test run. Connect your scan tool and start recording. From a rolling start, drop the hammer and accelerate at WOT until you feel the shift. You can do this in first or second gear, whatever your area will allow safely. Of course, allow the engine to reach normal operating temperature before you floor it!

Now check your recording. I like to graph the data. It makes that peak RPM easier to see. Locate that peak just before the shift and then look at the corresponding value for LOAD. If it is lower than it should be, check fuel trims. If fuel trims are in normal ranges, you’ve probably got a restriction. If, however, fuel trims are correcting for a lean condition (adding fuel, positive numbers), take a hard look at the Mass Airflow sensor (MAF). It is likely under-reporting airflow to the Engine Control Module (ECM).

No, this is not a low cylinder. This is typical for a running cylinder at idle. Now snap the throttle to wide open and record the peak for comparison to that cranking compression reading you took earlier.Be sure to make a few passes to insure the readings you’re getting are correct.

The LOAD PID is a calculated value, not a measured one. That’s why a bad MAF can skew the whole deal and why it is a valid test of the MAF sensor itself. If you want to be more high tech, there are several VE calculators online that allow you to input the values yourself. If you use one, you’ll need to add a few more PIDs to your list: Intake Air Temperature (IAT) and airflow (in grams per second). You’ll also have to enter the engine displacement precisely for the calculator to return an accurate number.

Article Categorization
Article Details
blog comments powered by Disqus