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Unread 06-24-2013, 12:53 AM   #1
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1998 WJ 
Join Date: Jun 2013
Location: Niznhny Novrorod, Russia
Posts: 1
Wj P1391 after engine overhaul

Hi there,

I faced a problem that MIL lights on my Grand Cherokee WJ 4.7. Self diagnostic shows P1391 - Intermittent Loss Of CMP or CKP. At first I replaced CMP sensor (Mopar), but without any success. After that I used DRBIII tool and observed following: engine runs on idle speed, revs are good according to tachometer (about 650 rpm), but once per about 20 seconds tool shows engine revs 5000-7000 rpm, and CMP/CKP sync fails, then returns to normal state. The same issue is for at least two sensors - Intake Air Temperature and Engine Coolant Temperature (they changes explosively for a moment, e.g. IAT sensor shows -15 instead of +70).

I used oscilloscope and checked for a signal from both CMP and CKP sensors. Here are oscillograms:

Are signals correct? I suppose that CMP's peak down to ground is not quite good. Is it?

It seems timing chain is installed correctly, as engine works good, without loss of power or any additional sounds. My idea is that something is wrong with wiring, but I checked a lot of grounds and connectors and everything is good.

Guys, have you any ideas?

P.S. Sorry, my english is not very well

Last edited by Serje; 06-27-2013 at 01:40 AM..
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