LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

messed up idle ???

Old 08-13-2008, 09:56 PM
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messed up idle ???

ok here is the rundown.. i bought my 97 SS M6 back from my buddy. it runs decent and has been newly rebuilt. it is now a 355 with a cc306 camshaft, 1.6 rockers and supporting mods i.e. exhaust, throttle body and whatnot, and also has a madz28 tune..

here is the deal.. it drives fine but has an extremely hard time holding an idle. this is what all has been replaced since the build to try and remedy the problem..

new MAF
TPS, IAC, MAP and o2 sensors, as well as new plugs and wires (again)

i have also changed out the original harness for another obd2 harness that wasnt hacked up, and took out the adjustable fuel pressure regulator, and put in a new delco regulator. i even changed the intake manifold gaskets as well as throttle body gasket to see if maybe it was a vacuum leak. it doesnt run hot or anything, so i dont know what the problem is.. anybody have any other advice so that maybe i can get this car to where i can drive it daily without any issues???
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Old 08-13-2008, 10:04 PM
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In what way does the idle differ from the RPM its programmed for? Erratic, surging, steady but significantly above or below the programmed value, etc.?

Have you ever scanned it to see how the PCM attempts to control the idle using the IAC "counts"?
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Old 08-13-2008, 10:07 PM
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in my opinion it seems like it isnt catching itself quick enough, and it is falling to about 400 rpms before trying to 'adjust' itself in which case it dies. if i feather the throttle is will evntually work itself out to where it idles fine, with no assistance, until the throttle is pushed again. i dont know how to read a scanner, but had a local mechanic who does business with the parts house i work for look at it and he said that everything looked like it was functioning properly. he basically feathered the throttle as well to get it to stabilize while reading it....
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Old 08-13-2008, 10:09 PM
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oh yeah.. he also basically said to him it seemed as if the idle had been programmed to low, and it wasnt able to stabilize before dyin ...

i asked my buddy who i got the car back from what the idle was set at and he said 950 rpms.. that sound right ?
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Old 08-13-2008, 10:29 PM
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950 sounds right. if the guy had a scan tool, he should have been able to see the "desired idle" speed. it should say 950 then. you need to tell us what the IAC counts are, when its too low. I know ion does a great job with tunes, as he did mine too for my 383. but poss. he didnt get something right? what about the valve adj.? if its too tight, the cyl.s wont quite seal pefectly and you end up with low cyl. press.. just a thought. something that wont cost you anything and can try. either readj. them with the eng. running, or just do a compresion test to see what the cranking compresion is.

its also possible the iac valve wiring or connector is messed up. but that should generate a code if the idle speed is not matching the "desired idle speed".

what spark plugs did you use? if you got too shallow of a spark plug it'll do the same thing. seen that happen before. idled horribly, but would rev, and you COULD get it idle, but it didnt really want to.

good strong spark coming from the coil?
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Old 08-13-2008, 10:36 PM
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the coil and icm are new as well.. and the spark seems to be good.. i will have to ask him to put it on the scanner again.. the valves are fine i checked them as well... i dont know what it could be i have been trying to process of elimination but nothing is helping. i know ion did my old trans am tune with this same cam and it was dead on... i wouldnt imagine this one being any different as far as the tune is concerned.

also i am using autolite 106 spark plugs. and they are stock unported heads
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Old 08-13-2008, 11:02 PM
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when did the problem start happening? right after doin some work? or just out of the blue?
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Old 08-14-2008, 03:12 AM
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to my knowledge it started happening right out of the blue.. it actually started this on my buddy, i didnt think it would be too hard to fix, but it has me stumped thats for sure ...

i know when hefirst finished the rebuild it ran good, and idled fine..
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Old 08-14-2008, 04:59 AM
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I had the same problem when I messed a little bit to much with my TB blades set screw, I had to open the blades back up to get more air at idle. The IAC counts was also very high. But im sure you already tried it
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Old 08-14-2008, 10:21 AM
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ok here are the codes i just pulled out of it at work. i cant get it on an actual scanner today, but thought i would post these..

p0122-TPS sensor low input
p0200- injector circuit malfunction
p0336- crankshaft position sensor circuit malfunction

and a code 1642- radiator fan relay circuit malfunction.. i do know that the relay is bypass on this vehicle and the colling fans are ran via an external temp controller
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