Saturday, October 31, 2015

Formula Vs. Smog Check: Testing Fuel Pressure and the MAP sensor

On this page are a couple of videos showing you the results of my Fuel Pressure Leak-down test and how to test a MAP sensor on an LT1 V8.  These are two important tests when you've got an emissions problem especially if the car is running rich.  

The fuel rail will bleed off some pressure over time but not all of it in a few minutes!  Anything more than a couple of PSI drop after a few hours means there's a leak in the system somewhere.  

It's important to mention that if at all possible, you really want to isolate the individual components of the fuel system when troubleshooting.  If you're testing the fuel rail and you don't know the condition of the fuel pressure regulator, for example, then ideally you'd want to block off any return lines to the tank so you don't confuse a bad fuel pressure regulator with say a stuck check ball in the fuel pump. 

If that's not possible then you're only going to get a general idea that there's a problem.  In which case you'll have to take into account other factors.  In my case I know the Fuel Pressure regulator is good ( brand new ) so I'm fairly confident that any pressure loss seen at the rail is going to be isolated to a component of that rail.  Note that all of this testing is done with the engine off.  With the engine running other factors like the Fuel Pump, Injector pulse time and engine vacuum come into play which can mask a basic fuel pressure problem.  

Incidentally, you won't see it in the video but the way to isolate the fuel system to check for what part is causing you the headaches involves either pinching off the return lines or installing a shutoff valve.  I don't have the valve and I'm not about to start pinching anything made of plastic that's been baking under the hood of a car for 20 years.   So I didn't do that test but a rich mixture is more likely to be a fuel injector than a check ball in a fuel pump at the back of the car.  I was more interested in seeing if there was a drop in pressure in general.  

Once I can afford a new set of Injector O-rings I'll be pulling the injectors out, priming the rail and seeing which one(s) are leaking.

As for the MAP sensor testing, well, it's always the first thing blamed right behind the O2 sensor when an engine is running to rich or lean.  That makes sense since those sensors as well as the MAF (Mass Air Flow) provide critical data to the engine's PCM to adjust the mixture rate.  There are others like the Coolant Temperature sensor, Throttle Position Sensor and others but basic emissions troubleshooting will have us starting at the MAP sensor since it's usually the easiest one to access and check.  

The MAP sensor's job is basically to read engine vacuum and send a signal back to the PCM.  With an LT1 the PCM takes that vacuum value and compares it with the Air flow data from the MAF to see if the two values complement each other.  That allows the PCM to make more minute adjustments to the fuel mixture for performance and emissions.  It's also a sanity check for each sensor since the failure of one can be compensated for by the other.  Either one can provide enough data to allow the engine to continue to run but not as efficiently.

Some cars only have a MAP sensor but LT1's are a bit of a wild animal when it comes to tailpipe emissions. Getting the engine into emissions compliance takes a lot more data points than your average grocery-getter especially with high compression that lends itself to problems with NOx and CO issues.

So if you ever wondered why there's so many wires under your hood, now you know....

For me this is all kind of a reference exercise but for  you it might be instructional.  Enjoy!

Fuel Pressure testing...

Checking the Manifold Absolute Pressure Sensor (MAP)

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