Mechanics

98 GRAND

1998 Dodge Caravan • 6 cylinder FWD Automatic • 104 miles

98 grand caravan 3.3 ltr flex fuel. NEW cmp ckp sensors (just in case) Runs perfect on initial start up. Always between 28 and 36 mins later engine dies. No difference if you are driving or in park at idle. My scan tool pulls a intermittent loss of cmp /ckp dtc. However looking back over the sensor history graphs in my scanner I see no such loss of signal. Motor will not restart until the next day. ALL wiring from both senors have been visually inspected and ohm meter tested from pcm to sensors passes all jiggle tests and water too. I AM STUMPED for the first time in all my days. YES fuel pressure and compression have been tested before during and after failure. Motor has EVERYTHING it needs to run I.E. Fuel injectors fire, coil fires at correct time(9 degs btc on start up), Also I checked ALL components for proper grounding ( trans-axle too!).I thought perhaps I was loosing ground but Unless there is something I have missed but god I have been from start to finish 5 or 6 times now. HELP. Thanks in advance! Matt
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Truckitt2u
June 28, 2011.




For your recording, are you actually watching the crank sensor signal, or are you looking at "CRK: Present" under "sensor data"? If you're watching the signal, watch if the amplitude drops off or if the low part of the signal stops going down to 0 volts. If the amplitude drops enough, it will stop triggering the Engine Computer which will set that code. You might still see that as a good signal. That's how they often fail when they become heat-sensitive. A less-known cause for the same symptom is an incorrect air gap. That must be set with a thick paper spacer stuck to the end, or, aftermarket sensors often have a thin plastic rib molded to the end. If you remove and reinstall one of those, you're supposed to cut the remaining rib off and use a paper spacer. After replacing a bunch of transmissions, I got so "good" at setting the sensor's air gap by feel that I quit using the spacers. I got lucky on the first dozen, but that last one developed intermittent stalling two weeks later. Someone else replaced the crank sensor but it most likely just wasn't spaced properly.

If you're watching sensor data in your recording, you may not catch the dropout because the engine will stop too quickly, then both sensor signals will stop. Once it doesn't start, watch for the presence of the cam and crank signals. If both are present during cranking, check if the ASD relay is being commanded on. If it is not, I hate to suggest it, but it would suggest there's a problem inside the computer. Chrysler historically had extremely little trouble with their Engine Computers, but that reliability ended with the redesigned '96 model. If the ASD relay is being turned on, that proves BOTH sensors are working on the older engines, but somewhere between the mid '90s and early '00s, the engine will still run if the cam sensor fails. The crank sensor determines spark firing times. The cam signal synchronizes injector timing, but on newer engines the injectors will fire on the crank sensor pulses as a backup strategy. When the ASD relay is listed as "on", and the engine doesn't start, that's when you have to determine if you're missing spark or fuel pressure.

When it comes to missing fuel pressure, Chrysler pumps rarely quit while they're running. They will fail to start up when the brushes in the motor are worn. Once the engine stalls, if you don't hear the hum of the pump for one second after turning on the ignition switch, it's more likely you will find something intermittent in the circuit running to the pump. Corroded / overheated contacts in the fuel pump relay are a possibility, but also look for signs of melting in the fuse box around those relay terminals. I've also run into a plug that wasn't fully seated at the fuel pump, and those terminals built up resistance and overheated. That caused stalling too but you never knew when it was going to happen. The diagnosis was done by using the scanner to command the fuel pump relay on, then voltage readings were taken, and 12 volts was found all the way to the connector where the plug was found to be melted.

If spark is missing, look for an intermittent connection in the wire going to the coil pack. There's no electronics in the coil pack, so when they fail, they are generally not intermittent. If the ASD relay is being commanded on during cranking, you should find 12 volts on the dark green / orange wire to the coil pack connector. You shouldn't have to check the other three wires since it's not likely all three coils would fail at once. You'd have a severe misfire if one failed but the engine wouldn't stall.

Caradiodoc
Jun 28, 2011.


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