92 lumina 3.4 , What generates RPM for ecm

I have been having a problem with the 3.4 running right .... Then recently it tripped a code .. egr failure ... so i replaced it ... but it still wasnt running right.... when it was cold it would run up to the rev limiter... 3000 rpm ... and cut out .... till it warmed up.. then the idle would drop and run ok .... The gas mileage has always been poor at best ...
After replacing the Egr it continued to have a problem holding idle... So i looked at the data stream .... and i noticed that my engine rpm reading was haywire .... 25 then 50 then 75 ... then when the idle would drop it would jump up to 225 ... in any event i take this as a bad sensor ... in the data stream i can see the crank single and the rpm match the tach... and i know where that sensor is ... also gm normally used the crank as a starting sensor , and not a accurate rpm... .. i have worked on most of this engine and i havnt seen any cam sensors ...
So the question is what generates the signal for the ecm??? .... and where is it located..... After having been on this problem for a couple of days now ... i havnt gotten any for sure answers ... but im thinking theres another crank sensor ... like gm used on those modules that poked through the engine block , on the side closest to the firewall... have to drop the carriage , engine and trans to get in that area ... so i want to be sure before i go there...
Thanks


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If you don't get a reply here check out W-Body.com or MYMonte.com that's where a lot of the 3.4 DOHC guys hang out. Or got to ChevyTalk.com and post in the '90-'93 Forum, there's a mod there by the name of Pat Gizz that is pretty sharp on 3.4 Lumies.

thxs

My cousin had a 3.4l Z34. The engine would idle horribly and while driving, low rpm performance was extremely rough!! Oddly enough, as the engine rev'd up while driving - it didn't seem as bad.

He had changed the EGR valve also.....and he had changed ONE of the 3 coil packs. Even after changing the EGR valve, the computer scanner STILL called for a bad EGR valve. He finally got disgusted with the car and sold it cheap to a mechanic. The mechanic ended up finding out that one of the 2 remaining unchanged coil packs was bad.....that was what caused all the trouble.

Engine runs fine now!!

I would suggest starting there.....this isn't the first one of these engines that I've heard of having a similar problem.

Being it's a 92, I would think the timing belt has been changed, but I would check that for poor performance, the egr should come on in three stages when warm, solenoid 1,2&3 progressively, if they're coming on, check for a vac leak there in case the pintle's aren't seating when off at idle.Also be sure to clear the egr code!