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if you can figure this out ill be amazed..2000 z28 idle help

Question 
6K views 24 replies 7 participants last post by  97z28convt 
#1 ·
ok, if someone has seen this or might know what I got going on ide greatly appreciate the help.


Car: 2000 Camaro z28 convt, got the car undriving and recently replaced the trans.


Car has multiple misfires across all cylinders and idle surge.. Replaced maf,iac, and TPS and have diagnosed it down to a TPS wire issue or computer.


Hooked it up to a good scanner, on the engine side the tps voltage jumps all over the place from .5v to 4.5v. however when we look at the trans side , the tranny is seeing a constant .57 volts , that goes up with throttle increase.


im thinking its gotta be a computer , cuz if it was wiring I don't know how the trans would be seeing the proper voltage,


thoughts, help?
 
#2 · (Edited)
If car is having multiple misfires have you checked your plugs, wires, coils etc? Have you misted wires with water and ran at night to look for spark leakage?

Might want to put an oscilloscope on your crank position and cam position sensors to see if they are working correctly. One or both could be having drop out problems or crazy wave forms. Neither of which would set a code, but would cause crazy running problems.

If you have a known good spare pcm, go ahead and substitute it. But I don't think it's going to be your problem. Ls1 type pcm's have an excellent track record for reliability.

Go ahead and check TPS wiring to confirm or deny your suspicions.
 
#3 ·
yeah thats where we started, but the misfires go away when not at idle. they also go away when i unplug the MAF. checked the plugs and wires, and swapped out a few wires and plugs and it made no effect.. also swapped out the MAf and tps and previously the TB itself all made no effect..

we definately narrowed it down to something with the signal /reference or circuit on the TPS

using the scan tool you can see the engine side TPS voltage going crazy regardless of throttle position. using the scan tool the throttle position is reading correct but the voltage just jumps everywhere, not a linear voltage increase like it should.. hooked it up to my good running 98z28 and the voltage is steady and with a linear voltage increase as the throttle opens as it should be.

i found a pcm pinout and ill be checking continuety on the tps wires tonight and between the wires as well, just to confrim no broken/rubbing wires... im assuming the wires are fine because the TPS voltage that the trans is seeing is correct... .6volts with a linear increase..

Its so crazy, because the trans is getting the correct voltage reading from the TPS , but the engine is not.

im thinking its either the wires or the PCM itself, could it be anything else? am i missing something else that could mess with the TPS?
 
#5 ·
we definately narrowed it down to something with the signal /reference or circuit on the TPS
rottle opens as it should be.

i found a pcm pinout and ill be checking continuety on the tps wires tonight and between the wires as well, just to confrim no broken/rubbing wires... im assuming the wires are fine because the TPS voltage that the trans is seeing is correct... .6volts with a linear increase..

Its so crazy, because the trans is getting the correct voltage reading from the TPS , but the engine is not.

im thinking its either the wires or the PCM itself, could it be anything


It sounds like the wires. Either a bad connection/broken joint or frayed insulation.

TPS usually has 3 wires Vref, gnd and signal out.
So if trans is seeing that i would check cable/route between tps, pcm and trans.
Have you checked supply & ground at TPS and the signal out with a voltmeter ?
Trace wires to PCM & trans. Could be a bad connector.

HTH
Mitch
 
#4 ·
would the crank or cam sensor malfunction cause the voltage problem with the TPS?

i was thinking of doing the pcm swap.. will i have problems with the VATS system?
 
#7 ·
We were ising a really nice scan tool where you can look at the engine readings and the trans readings... I dont think the wiring goes to the trans , but we thiugh the computer sent that voltage reading to the trans for kickdown purposes .

Update, I checked the actual voltage of the signal wire from the tps coming into the ecu.. It's correct.. But the scan tool showed the voltage jumping all over the place, so the thought is that the ecu is not interperating that reading correctly...

Ordered a new ecu.. We'll see if that fixes it.
 
#9 ·
thx man, yeah hopefully thats the ticket, if not i might have to take it to the dealer, as ill offically be stumped.

if that does fix it, itll be the first pcm ive seen go bad on these cars.. and ive owned about 8 of um, but theres a first for everything.
 
#10 · (Edited)
Update: The idle/random cylinder mis remains!!!!


Just got the car back from the dealership with a #4 rate GM tech in the country... his advice was intake decarboning and fuel injector service.. but he wouldnt guarantee it will fix the problem... so basically he has no idea whats wrong with it.... his thinking is that the fuel might be landing on carbon on the intake valves and not properly getting into the cylinders.... my thinking... is "on every cylinder" not likely... when i asked him if he borescoped it.. he just kinda was like.. uh i did not... so im not happy about a bunch of $ spent there.. he spent most of my $ redoing what i had already done.. so annoying..


To date: ECU replaced, Intake leaks fixed, PLUGS/Wires/MAF/Throttle Body/EGR all replaced or tested no change.. ecu has been reprogrammed to factory settings, cam/crank sensors reset, fuel pressure is fine...


man this is killing me...


does anyone know if lt1 stock fuel injectors are the same as ls1? im thinking about swapping out a set i have just to rule them out...
 
#12 ·
TPS is brand new ( no change). no i havent done a compression test, Its been awhile since i had my buddies tech 2 on it.. but if i remember correctly, the miss goes away when you get on the throttle.. i do know for a fact the miss went away when we unplugged the maf.. but the GM tech thought that was only because it wasnt registering because it was in default mode.

I was potentially thinking maybe the torque converter with my new tranny was bad, might explain the idle jump.. is it idles fine when just sitting, but when you come to a stop is when it starts bouncing and can die..

but that wouldnt explain the random multiple cyl misfire...

and the two gotta be related...

last night i seafoamed the intake and that seemed to get a ton of carbon out of the intake... but problem didnt change.

now im thinking about changing an injector or two.. to see if that helps at all.. ive got some 30# mustang ones i was gonna see if they fit.. or lt1 injectors laying around although i think those are only 24#.. and im reading i need 29#., dont really wanna cannibalize one of my other ls1 cars right now, but i may.. ( dont know if the 24# or 30# will be acceptable to just throw in?)

my buddy with the tech 2 was doing a bunch of reading and hes starting to think maybe 02 issue.. although they are reading as they should on the scanner.

but so are the injectors... so its just a matter of throwing stuff at it at this point and hoping....

definately getting sick of throwing parts at this car, and having no change...
 
#13 ·
Ls1's run a higher pressure stock but I don't think the lt1's will work. You'd want a tune for bigger injectors, you'll just introduce more problems. If you have a known working set on another car, I would borrow from that first including other suspect parts. Free is for me.
 
#15 ·
our setups aren't exactly the same or anything but I found myself tortured by the exact systems only to finally hook up a smoke machine and make sure I didn't in fact have a vacuum leak at idle.. turned out after several minutes smoke would start leaking somewhere around the rear of my FAST 92 intake. Since parts weren't that expensive I resealed the entire top end (shortly before I purchased the car the P.O. did the heads, cam, intake, etc, so I basically redid everything he touched buttoned it back up and problem free ever since.
 
#17 · (Edited)
sorry for the delay...

Im done with this car.. i cannot figure it out.. ive officially given up...

to summarize... code p0300..random cyl misfire
new ecu.. both reprogramed and replace...
new TPS, New MAF,
EGR Tested
intake smoked.. leaks found and fixed, but made no change,
Fuel pressue is good and trim levels appear to be good on scan tool, so replaceing the fuel injectors seems like a waste of time.. ide be assuming every one is misfiring
new front o2 sensors, rears removed for testing pursposes..
intake de carboned..
no compression test done.. but that seems like a waste as literally every cylinder would have to have bad pressure, since misfires are affecting every cylinder.
plugs/ wires are fine.. swapped around for tesing purposes...
took care to dealer they suggested intake decarbon and injector cleaning... morons!

aside from a potential crank sensor?, which according to scan tool seems to be working. or something else.. i dont know... and as you can see from the above.. im sick of throwing good money after bad.... im just done and think im gonna sell it....
 
#19 · (Edited)
yep.

yeah its a stumper.... i have and have had many many camaros.. prob like 15.. never encountered a problem i wasnt able to fix/solve, etc... until now...

I even called a second dealership to see if they have encountered anything like this and see if they want to take a crack at it...they said no...
 
#21 · (Edited)
http://www.ls1.com/forums/f6/p0300-code-random-misfire-31077/

Some good checks in that thread, read through all of it and past some of the dumb posts haha.

I think you are assuming all cylinders are misfiring which isn't the case. A tech2 can report what cylinders are misfiring and narrow your diagnosis to the affected cylinders. I would personally pull the plugs, look them over carefully, and do a compression test on every cylinder. Its the basic stuff you need to know and may point you in the right direction for repair.



GearChatter.com :: View topic - DTC P0300 Engine Misfire Detected
 
#22 ·
thanks guys....

the ls1 thread is really helpfull... i think i might end up making the car a winter project ...

I have pulled like 5 plugs and they are all fine, also swapped um , and some wires.. that being said, per the tech 2 the misfires are on all cylinders, but some more than others.. and you can hear it in the engine..

the weirdest thing to me is acutally how high the idle at cold startup.. its like 2K then slowly comes down..

things i havent done that im willing to do...
1. loosen the exhaust to create leaks, incase my cats are plugged, the car did smell like sulfer when i first started running it this year.
2..swap fuel injectors, ill steal um from a fine running 98 i have.
3. change the crank sensor.. just incase.. and maybe plugs...

if none of that does it itll 100% be time to cut and run.
 
#25 · (Edited)
ok... so.... problem is solved...

here goes .. car had a very mild cam in it..

when the previous owner removed all his goodies he took everything except the cam.. a tune from pcm for less and boom problem is solved , car runs good.. no misfires... idles good.. etc... talk about going down every road possible.

All is well in the world.... boy that took far to long to figure out.
 
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