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Thread: RPM dropping off badly
01-27-2012, 05:11 PM #1
RPM dropping off badly
Hey all im new to the forum. just picked up a 94 1500 ext cab 5.7 tbi 4x4 4l60 tranny. heres the prob. i went to get the codes ran at oreillys it has 4 or 5 codes. one was a map sensor problem. i just went ahead and replaced it. it also had 2 codes about the knock sensor and a code for the ESC. what happens is it starts fine, idles wonderfully cold or hot. but if i touch the gas ever so slightly it lugs n glugs, and starts making that sound that like a crackling spark retard. if i actually use the gas a lil more from a idle, the idle will drop off when i let out say backing up or parking it wont start crackling it will have big lopes trying to correct itself from the idle dropping so low. it only did the crackling in park. it runs down the road fine shifts nice thanks to B and m shift plus. just coming to a stop idle will drop off and come back. its becoming annoying and needs to be dealt with. it also does the idle drop/and correct a lot under 1200 rpm in gear when stopping at a light and so on. if im in OD and start slowing it does not drop it idles back down fine. it also has a hesitation spot on takeoff sometimes. can anyone point me in a good direction on what to start looking into. im no efi genius. im a carb guy so bear with me. things done are. new map/2 new vacumm hoses. one was cracked one i broke. pulled egr to clean and check for proper diaphram operation/leaks, cleaned TB. can someone explain why a esc would go haywire and cause the knock sensor to throw a code aswell. i do know they are both working in conjunction. what should i do.
01-27-2012, 09:49 PM #2
ESC?? that is a new part to me? but the knock sensors do go bad after a time or else a broken wire or bad connector.. they are all common. As to the idle issue i would look into an Idle air control valve and see what it is doing since that will cause those symptoms, and take a close look at the wiring harness. one thing with EFI esp OBD1 is they have a habit of blaming other parts for the codes, and if the codes havent been cleared i would clear them then run the truck for a while, and see what the new codes are.. that way you have an actual known problem since a code will store on the computer until reset which can be 100K or more miles for a simple blip.;Mike
1997 Chevy S-10 Blazer 4X4 with 330K miles and counting (Hunting rig).
2009 Saturn Aura XE (wifes car)
2011 F-150 Crew 4X4
"Hold it to the floor till you see God.... Then Brake!!!!"
01-28-2012, 03:13 AM #3
I did a lil research, and a little youtubing. i learned that the IAC gets nasty and causes all sorts of problems. i will disconnect battery tomorrow morning for a while and reset the computer. i will likely pull it out tomorrow. i heard they get gum/varnished. i heard cleaning can do wonders. im glad u mentioned the iac. i was wondering about it and didnt wanna make a move until i get advice. what can a bad knock sensor do anyways. what symptoms would it cause. is there a way to check it with a test light. i dont have a voltometer but ive got a light. i will keep posting on my progress. could a bad tps sensor aswell cause low rpm malfunction. doesnt really do it when cold just warmed up. BTW already love the site.
01-28-2012, 04:09 AM #4
ESC= Electronic Spark Control, or the computer/brain. They can be damaged in many ways like not grounding the vehicle when working on it. Many moe precautionary steps need to be taken when working in or around systems connected to the ESC/Ecm.2010 Chevy Silverado Z71 ext cab.4x4
72 Monte Carlo 350-415 hp- sold (June 2011) to pay medical bills
79 Impala 350 4bbl 375 hp- sold (June 2011)to pay medical bills
"If He asks you to go a mile go two"
01-29-2012, 04:15 PM #5
okay heres what i got. cleaned IAC. nothing changed. disconnected batt. terminal to reset computer. reconnected this morning drove to parts store. now down to two codes a code 42 and 43. electronic spark timing error and electronic spark control failure. so i bought a new esc. BUT... where is it. its not next to the tbi like most. does anyone have a picture/diagram. or some knowledge for a noob. it runs slightly better with the new map sensor and vac leak fixed. the other codes must have been old codes that had been dealt with but not reset. i have looked everwhere on the little metal bracket for it. i heard one person at the shop today that he had one on a van located under the brake pedal. where could it be and thanks for all your help.
01-29-2012, 10:08 PM #6
Well news to me. shoulda got the hint from different forum posts. the ESC is built into the computer. so my question now is why did i get a module for a 94, IF ITS BUILT IN. what do i have an external or an internal. its also a 4l60 tranny has the tbi with the black bracket on the pass. side. i could not find a module connector. what should i do now. replace a 200 dollar computer and hope it works. or is there something triggering a failure. being its a computer and its an 18 year old computer. im sure she could be on its way out. any idears.
01-30-2012, 04:46 PM #7
Alright the ESC (has alot of meanings and sometimes people mix up the acronyms sorry about that). You were probably given an engine or ignition control module.. Sadly parts jockeys dont always have a clue what they are looking at nor getting for you.
The Engine control computer is only $57 (plus $100 core charge but you get it back) it is behind the glove box, and actually quite easy to change. All of the main ignition components other than the coil are under the dist cap and under the rotor, and are a pita to get at to change.
01-30-2012, 07:59 PM #8
yah i know whatya mean i got the correct module. but it was for a 93 and under. 94 they switched to integrating it to inside the computer. i returned it today ordered a new computer. and yes i got all the numbers from my old computer. i went ahead and got my new knock sensor and my temp sensor that goes on the side of the block. idiots who had it before me put an aftermarket guage in that hole. i bought a new sensor for the manifold and the block and wala it registers perfectly on the dash. the knock sensor was fairly easy just messy i took a bath in antifreeze. also had a lil antifreeze eyewash lol. if the computer goes in and nothing changes ill go with a spare ignition module i have. its an aftermarket mallory one. but i dont want to install till then cause i still have the reciept haha. where did you find a 57 dollar computer. also is there a chip i need to switch over. i believe the prom.
01-31-2012, 07:57 AM #9
The $57 one was on Advance auto parts. and the prom you shouldnt need to switch over as far as i know since the new computer should already come flashed, but i would read up a bit on that from the manufacture of the new computer.
01-31-2012, 09:16 PM #10
Well got the computer home switched out the chip and put it back in. i also unplugged battery before procedure. started it up and ran the exact same. so out it came took it back and got an oem AC DELCO OEM ignition module. reason i did not use the aftermarket one is because everything else is stock so a HP module isnt going to really do much more than a highquality oem part. got it out. noticed the chip in it was a sorenson made in taiwan. the cap had all the conductors in it completely rusted. i was shocked that this thing even ran. i didnt get a new cap. so i broke out the sand paper and clr. long story short i made it look like new though it will be replaced here in a day. got it all back together started faster, ran totally different. it idle smoother was more responsive. i didnt drive it i just let it idle because gas tank is almost empty. my check engine light is still on. but if a light is on and it drives good. its fine with me. ill know more if its fixed once i drive. if that doesnt do it ill try coil,cap, rotor, wires, plugs.
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