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Discussion Starter #1 (Edited)
:confused: Hey guys, Ive looked on Advance Auto parts site for the o2 sensors on my 1991 3.9 Auto Dakota and they are only giving me the "Upstream" one. From what Ive researched there are two o2 sensors no?? ..one being before the Cat and one after the Cat..correct? Do these o2 sensors have the same part number? If not can someone provide me a part number and/or place to purchase the one after the Cat?

I"m narrowing down my trouble shooting on the 3.9 by replacing parts! lol hopefully this process will fix what I believe to be a sensor problem.

*Symptoms: Under load the truck has really noticeable and loud valve chatter. I have put premium 93 octane in it and ran it for 30 miles so far and under acceleration it still pings/chatters excessively(thinking it may have been bad gas or cheap gas). Timing is perfectly set at 10* as recommended by manual. Im hoping this isnt a sign of a leaking head gasket or something of that nature. I have not adjusted the valves. Im starting with the o2 sensors and then the EGR valve as Ive read those can cause problems with gas/air mixture that may cause the chatter/pinging.

In the last month I have replaced the following when the truck died unexpectedly on me:
* all new distributor which includes the pickup sensor
* coil
* wires
* all plugs
* cap
* rotor
* fuel filter
* ASD relay
** replaced the water pump as well as it was leaking threw the weep hole pretty bad.

Thanks ahead of time!!
 

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Discussion Starter #2
Just found out the ole LA blocks have a none adjustable valvetrain so that illuminates one possibility of the valves needing adjustment.
 

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Discussion Starter #3
Bump ^^ Still experiencing excessive valve chatter. Have not replaced the o2 sensors yet but I have deleted the EGR and disabled the smog pump. I made a delete plate for the EGR and I plugged the vacuum line associated with the EGR. Any input would be helpful.
 

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Discussion Starter #5
A mechanical oil pressure gauge was installed by previous owner and when the truck is moving while accelerating the oil pressure is 52/54 psi or so. Cant remember exactly..just know that thats about normal. I also run a full bottle of Ristone Zinc treatment in with my high mileage oil every oil change ((which is only about once every 6months as I dont drive the truck but maybe 400miles a year)). The truck currently has 122130 miles. Occasional ill start the truck and get a stuck lifter but it calms down after a few minutes. Ive messed with the timing as well trying to sort this valve chatter out. Retarding it helped minimally but it did make it less of a noticeable sound while driving. That truck would run and drive and pull with the timing at 3* before DTC! If I set it exactly at 10* before DTC(which is factory recommended setting) it falls on its face. ..runs like crap at 10*. I found a happy medium at about 7/8* before DTC. The truck starts great, runs great and pulls great at 7/8*. Pulled multiple bump-stop smashing loads of dirt off this summer with it and it did great. ...just chatters.. :/
 

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That's odd about your timing. I have a truck that's the same way though. It's supposed to be set at 10, but runs so much better at around 8.

Just found out the ole LA blocks have a none adjustable valvetrain so that illuminates one possibility of the valves needing adjustment.
And did you mean to say it illiminates one possibility? If not maybe someone could take a look at 'em for you? Or do it yourself if you know how. I never liked messin around with em.

Well for what its worth, if it was me, I'd run some sort of cleaner/desludger through it, dump the oil, and put new oil in. The lifters could be gettin sludged up. I had an old truck with a straight 6 that was chattering pretty good. Eventually it ended up throwing a rod through my oil pan. The new engine I put in was a little chatty. I started running marvel mystery oil in its oil/gas and it's quited down quite a bit. I always run MMO in all my vehicles. It seems like it helps, but who know.
 
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