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Testing injectors?

SCW

SCW

NAXJA Forum User
Location
SLC (yuck) UT
I'm running warm when I've got my foot in it (uphill or down), getting great fuel economy and have a terrible miss/hiccup that I can't nail down- I think it's running lean. The miss went away for about a week after I hit it with BG-44K, but it just keeps getting worse now.

Is there any way to test injectors? This is in my '92 with 135K miles. I assume testing the fuel pressure would be indicative, what should the pressure be?

Thanks-
 
The only test I know of that you can do without expensive diagnostic equiptment is an ohms test...I don't know what the resistance is of the top of my head though....

The fuel pressure should be 30psi with the fuel pressure regulator vacuum line hooked up and then 39psi with it unhooked.
 
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How would fuel pressure tell you if the injector is firing? I have always used a stethescope of long piece of hose to listen to each injector and make sure it is clicking, however if it is just clogged it may still click.
 
A bad sensor would be more likely than bad injectors.
O2, MAP, MAT, CTS, sensors all set the mixture.

O2 or MAP sensor are the most likely.
 
langer1 said:
A bad sensor would be more likely than bad injectors.
O2, MAP, MAT, CTS, sensors all set the mixture.

O2 or MAP sensor are the most likely.
Click to expand...


Temp sensor also, Ive see them read -38 F when the engin is at operating temp. This tricks all the other sensors and makes the engin run like crap.
 
i never really looked at my jeeps injectors but have tested one on my chevy pick up... you can check if they are working electricly but getting a "noid" light kit like this http://images.google.ca/imgres?imgurl=http://www.thepartsbin.com/cartools/images/otc-3054c.jpg&imgrefurl=http://www.thepartsbin.com/cartools/diagnostic_tools-noid_light_and_iac_test_kit_with_harness_extension-d.html&h=286&w=300&sz=31&hl=en&start=1&tbnid=S9j77wY923lucM:&tbnh=111&tbnw=116&prev=/images%3Fq%3Dnoid%2Blight%26gbv%3D2%26svnum%3D10%26hl%3Den you also might be able to see if you have a poor firing cyl by cehcking the colour of the spark plugs....
 
the noid light only tells you if the injector is getting juice to it,not if the injector is really functioning....you can accomplish the same thing by hooking a 12v test light up to the injector wire and running the motor.
 
I've seen well equipped tractor trailer places with test fixtures, they clean them and then check them for pattern. No idea of the price as I have only eyeballed one shop around 95 or so, up until then all my stuff was carbed till dec of 97 when I got my 98XJ.
 
Thanks gentlemen. I added a secondary heater core last week to try to keep the heat down a little, spliced it inline with the existing core and put it in the engine bay between the oil filter and air conditioner compressor. For the first 300 miles or so my temps dropped about 30 degrees, right down to about 190*. Now the temp is starting to work it's way back up after about 500 miles, so I am leaning towards a sensor problem and the computer 'learning' to do what it needed to make the temps increase again.

So- back to square 1. I don't want to blindly replace parts and sensors, how should I test them and which would be first on the chopping block? O2 sensor maybe, but I've never seen one give BETTER fuel economy when it goes, could happen though. What is involved in testing a MAP sensor?

Thanks again-
Shane
 
langer1 said:
Good place to start, but it won't tell you if its stuck open or closed.
Click to expand...


Yup. For that you'll need an injector pulsing tool and a fuel pressure gage to determine the pressure drop of each injector when pulsed.

Alternatively, you could pressurize the fuel rail and check and make sure the system holds steady, correct pressure with the engine off.

Also - run the diagnostic test on the fuel pressure regulator while you're at it.

If you need the correct specs for the tests, let me know.

-Ted
 
SCW said:
Thanks gentlemen. I added a secondary heater core last week to try to keep the heat down a little, spliced it inline with the existing core and put it in the engine bay between the oil filter and air conditioner compressor. For the first 300 miles or so my temps dropped about 30 degrees, right down to about 190*. Now the temp is starting to work it's way back up after about 500 miles, so I am leaning towards a sensor problem and the computer 'learning' to do what it needed to make the temps increase again.

So- back to square 1. I don't want to blindly replace parts and sensors, how should I test them and which would be first on the chopping block? O2 sensor maybe, but I've never seen one give BETTER fuel economy when it goes, could happen though. What is involved in testing a MAP sensor?

Thanks again-
Shane
Click to expand...

In theory a biased sensor could trick it into running lean, but the O2 sensor should sense it running lean, and correct the problem.

You can back probe the O2 sensor with an analog volt meter and see if it is rapidly crossing the 0.45 volt threshold or if it is stuck above or below the 0.45 volt set point. One of the three wires is a 12 volt heater wire, the other two smaller wires are the O2 output signal and the ground. Testing it first will give you some clues as to where to look next.

The CTS (coolant temperature sensor) and MAP (Manifold absolute pressure) and IAT (Intake air temp) sensors are simple variable resistors. Their test values are posted in various threads here, three letter acronyms and words do not work in the search tool unless you add * at the end like CTS*, or spell out the word Coolant temperature sensor in the search tool. You could also just test them and post the values and ambient temperature of the sensor here. The MAP resistance varies with vacuum. It is 0.5 to 1.5 volts at idle on pin B to ground, pin C should be about 5 volts input. The '87 CTS and IAT are both 3400 ohms at 70 F (with sensor harness disconnected!), 450 ohms at 160 F. I think the 92 is the same for IAT, but the CTS is a different part in a different location on the 92 so it may have changed.

Also a restricted exhaust, damaged muffler, loose muffler baffle plates, or broken catalyst inside the cat converter can cause it to run hot and make it appear to be running lean.

Have you done the rapid key on/off trick to flash the computer for OBD-1 error codes?
 
If an electrical fault stops an injector from firing, it's almost certain to set a check engine light and a code.

If an injector goes out, it will usually idle all right, and accelerate fairly well, but miss when running normally. You can do a basic test for a dead injector by pulling them one by one at idle. The XJ's engine management system will compensate the idle speed for a bad one, so when you pull a good one, the engine will bog a little, then recover. If you pull a bad one, nothing will happen.

This doesn't tell us anything about spray pattern or even whether the injector is sticking open or closed. For that you really need some other testing equipment.
 
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