Are you getting spark at number 4 spark plug?

 

That's what I asked yesterday;

"Does the plug get a spark on number four? And please share with us the basics of what readings you got on what".

OP, and anyone else reading this; it's important to answer as many of the questions asked of you as possible; they're asked for a reason.

 

Just went outside to check,have good spark.I have a code P0204 injector number four.The truck also didnt start today,it just popped through the throttle body.I did just buy the truck and drove it home.Tomarrow I will check the output from the crank sensor.Could it be a crank sensor?

 

If you get good spark on number 4 (and it is important to test it from the number four plug wire.....) but, no injector pulse, the crank sensor is doing its job, and you have a driver problem for that injector. New PCM time.

 

Highly doubtful, likely the injector itself.

 

"Could it be a crank sensor?"

Also from yesterday:

"I have my doubts about a PCM being at fault when just one particular cylnder is involved; sounds more like a sensor problem.

I don't want to sound like a broken record, but I just fixed a problem where, maybe coincidentally, #4 wasn't firing, and it turned out to be a bad crank sensor. The procedure for finding, removing, and (crudely) testing it can be found in these three threads:

01 5.9 no signals to coil Help
Interesting spark problem
Really rough at cold idle

Dodge ignitions seem to me to have a reputation for being well-designed and very reliable, and as with all electronic ignitions, really pretty simple. Crank sensor, cam sensor, temp, O2, manifold pressure, throttle position sensors, coil, distributor, computer/PCM. Lots of relay, fuses, links, and a few other goodies, but still pretty simple.

I'm far from the most experienced guy here, but it seems to me that PCM's are probably replaced far more often than they are actually bad. (Code 53 is "An internal PCM failure or PCM communications failure has been detected." Have you ever gotten that?)

Just do you best to rule out the basics first before you plunk down the big $$$ for a new computer; especially the crank sensor".

Those threads contain a lot of information on the crank sensor. I think it should be first on your list of things to check out. Rule it in or out, then move on to the next thing if you have to.

And have you ever gotten a code 53?

***

"If you get good spark on number 4 (and it is important to test it from the number four plug wire.....) but, no injector pulse, the crank sensor is doing its job .... "

Very good chance you're right Brian, but as I learned the hard way, the crank sensor doesn't either A) do its job or B) not do its job. Just like mine wasn't reading certain cylinders (cutouts on the flywheel) well enough to allow the coil to build up a sufficient spark, maybe it's not giving the PCM a strong enough signal to know to fire the injector.

I know you said the spark at #4 was good, but maybe it's not as good as you think it is. Does it give a good zap across about a 3/8" gap? Is it as strong as the other cylinders?

I'm really doubting it's the crank sensor, but I'd still take it out and ohms test it before you spring for a computer.

On the other hand, if a PCM is faulty and won't fire a certain injector, will it usually set a code, like #53?

 

I thought the camshaft and distributor did the spark,and the crank sensor sent a signal to the PCM to ground the injector.The injector OHMS ok and when I ground it,it fires.

 

"I thought the camshaft and distributor did the spark,and the crank sensor sent a signal to the PCM to ground the injector.The injector OHMS ok and when I ground it,it fires".

The cam sensor and crank sensor work together to tell the PCM when to ground/unground/fire the coil. The crank sensor tells the PCM when the next cylinder is coming up, and the cam sensor tells the PCM whether a cylinder is at (around) TDC of its compression stroke, in which case it gets a spark, or TDC of its exhaust stroke, in which case it doesn't.

***

"The PCM is not grounding the injector. If I supply a ground to the injector it fires. Is it possible the crank sensor is faulty and for some reason it wont fire number four injector".

With apologies to Hahns, that doesn't sound like a bad injector.

Neither does "The truck also didnt start today, it just popped through the throttle body".

The crank sensor has to be tested and ruled in or out.

 

Thank you everone for the help.I will test the crank sensor tomarrow after work.I just bought the truck.I was planning to do a tune-up with the intake gaskets anyways.Might as well test or replace the crank sensor while Im there.

 

"I will test the crank sensor tomorrow after work".

Good deal. If it's OK, then it just might be the computer/PCM.

I definitely don't know as much about how the PCM controls the injectors as I'd like to. I think I'm correct that it has no idea which cylinder is firing, spark-wise; to it, it's just "the next one", and the distributor directs the spark to the right one; but how does the PCM keep the injector firing order straight, and know which one to control? Brian, maybe you could clear that up for me. Thanks.