I'm totally new to this but this may be my last resort. My 1998 Ram 2500 5.9 Magnum suddenly wouldn't start. After scanning codes, etc., I replaced the crankshaft position sensor, the camshaft position sensor, and the ignition coil. The camshaft position sensor was the last item changed and it was the one that fixed the no start condition. (The coil was rusty enough to have the laminations being pried apart by the rust, so I replaced it on general principles). However now my tach needle is jumping wildly, my scan tool says RPM's vary from a few hundred to 4400 or so randomly, and the engine is running so rich it quickly loads up and stalls. In the short time it will stay running it has not set another code, there is no fuel trim indicated, and what I can see of the parameters in my scan tool otherwise look normal. I've got a new battery and alternator (from a separate issue with dealing with the draw from my snowplow pump triggering a low voltage warning) so my system voltage is up to normal. I've disconnected the battery for half a minute or so to allow the computer to reset, all to no avail. Does it seem logical that the new cam position sensor is causing the jumpy tach, and at the same time overfueling the engine when the tach is saying "high rpms-more fuel" when the rpms are actually low? I hate to blame the cam position sensor, but it seems that it might be factory defective. Would the crank position sensor (new) also be able to produce these symptoms? Does this sound like anything anyone has had experience with? My scan tool won't read individual components on demand, so if there is any help you can offer, I'm all ears! Thanks.

 

Crank sensor will give you jumpy tach. Put the old one back in, see how it runs.

 

Thanks--when the weather breaks, I'll try it! Does the crank position sensor provide the RPM signal that controls the fuel injector pulse width, also? If it does, that would explain my over-rich condition as well.

 

Pretty much. Swap it out, reset the PCM, and see what happens.

 

Good advice: The crank sensor was the culprit this time, but the original problem was the cam position sensor. Thanks for the help!

 

I've got another question. I've installed a new IAC because of an erratic idle problem. The idle speed sometimes dropped below curb idle when I suddenly lifted off the throttle, and then camesback up to normal idle speed. At other times, idle RPM "hunts" a few hundred RPM up and down. With a standard transmission, idle speed quirks are very noticeable. I thought perhaps the IAC, being original, was sluggish. With the new one installed, fast idle lasts longer than I think it should on start-up, and curb idle is somewhat higher than before (950 rpm +/-). How long does it take the PCM to learn a new IAC? What can I do to help it relearn beyond clearing the current trouble codes from my no-start adventure when my scan tool will do so? Should I pull the PCM fuse for a few minutes? Thanks for the help!

 

What brand IAC? I had problems with the aftermarket IAC I got at the local parts store....... put in an old mopar unit I had laying around, and my problems went away.

Did you reset the PCM after swapping the IAC?

 

I think I didn't power down my PCM long enough before. I disconnected the battery for about half an hour just now, and when I started the truck, it seemed to be back to normal with the new IAC installed. I'll drive it for a while, clear my codes and see what happens. Thank you so much for your help!

 

When I change parts of that nature, I will generally leave the PCM disconnected for a while, hook everything back up, turn the key to "on" (not start) count to ten, and THEN start the truck. (without touching anything else.) That lets the PCM learn the 'zero' values of various sensors. (maybe it's just a placebo, but, it seems to shorten the time it takes for the PCM to figger out how to properly idle the engine as well....)

 

If I have subsequent problems, I'll definitely try that procedure. Establishing baseline values that way seems logical to me. Thanks.