Need new pcm what to do?

[tuff]

The last couple of nights we been trying to fugure out my problem and it looks like its going to be a bad pcm.What do i do as far as tuning a diffrent pcm with my software when i get it.Ill be getting a used one so the vin is not going to be right.Whats the best route to go.Thank mike

[GMPX]

What makes you think you have a bad PCM?.
They are pretty robust things, we have a saying around here "It's never the PCM's fault".

Cheers,
Ross

[tuff]

Well to make a long story short we were getting code p0121 tps.The car just started running bad and the 6 and 8 cylinder seem not to be in timing.Eather firing to soon or late this is what i think.Things we checked are map was replaced maf crank senser swaped coil packs and wires pulled intake check for cracks and vaccum leaks.Made sure all grounds are good on back of block and on motor. fuel pressure good and did a cylinder leak down test.tps senser sweeps voltage nice and smooth.I just don't know what else it is.It just feels like the motor is just out of timing on some cylinders.We have spark to i forgot to mention.This is why i lead to a pcm problem.Any info would be great mike

[Dirk Diggler]

Its not the PCM's fault hahahahahahaha


P0121 can be taken car of in table B3907 just put a 0 in the first 3 rows and that will get rid of it. From the sounds of it it sounds like a bad tune or in need of tuning. Did you just put a cam in the engine

[tuff]

No the car has ran great all season.I run at the track most of the time and when we got home and took it off the trailer it just started running like crap.It has no throtte responce.It feels like a turd.

[DrX]

I think its a good sign that you are getting DTCs. If the PCM was bad it might not recognize a malfunctioning circuit.

I had a somewhat related experience the other night:
I went for a logging drive. Did a u-turn on the highway and everything shut down. "reduced Engine Power"> no start. No flashlight and GM has done away with under hood lights! At least I had EFILive. Had a bunch of TPS and related DTCs. P0121 is "not reported" in my cal but it would have been lit up if it was. I had P0120(TPS sensor 1 Circuit), P1516, P2135. Once I turned off the enablers for these DTCs I was able to get it started and crawled home in "reduced engine power" mode(no throttle response).

I had been to the carwash earlier so I pulled the TPS connector . Seemed dry inside. Blew everything out with compressed air and it works fine now. Must have been some water on the back end of the connector shorting the circuit. Think I will seal that up.

So I would check the circuits for possible shorts, the integrity of the connections at the connectors, ensure that the connectors are clean and dry(front and back), replace the sensor etc. prior to the PCM replacement. You may have already done all this.

[tuff]

I checked all the connectors all are clean and dry.If it was a tps sensor why would it act like its loading up and have no throttle responce but when you watch the values live they all check out good.BTW my car has egts and when the car is running i log cyl temps and num 6 is showing weak.Ive ruled out coil packs wires plugs cylinder leak down.I don't know what else it could be.BTW the valve train was also checked.Num 6 cyl is about 300degrees cooler than the others.And no missfires. Im hoping someone comes up with a fix and im wrong about a bad pcm.

[DrX]

What year/vehicle/engine is this? Any other DTCs set? Did you work through the GM diagnostic for DTC P0121?

[tuff]

Its a 99 firebird with a stock cube motor and heads and cam.Ive had this setup for years.As far as doing a full checkup on the p 0121 i just watched the values in efi live.It acts like its working fine so i don't know how this would effect the way it runs.And how would it effect just a couple cylinders.P0121 is the only code.

[DrX]

Anything unusual in the freeze frame(history=Shift+F9) data stored when the DTC was set?

PM sent

[Black02SS]

Are your oxygen sensors switching properly? I had the same problem before and it turned out that they were shot but didn't throw a code. Replaced them and the car came alive. Happened right after I got back from the track.

[tuff]

Thanks i found a writeup simmlar.Do you think its not the pcm but more the tps sensor.Im just looking at the% values in the scanner maybe the ground on the tps is not working is that possible.Would you still be able to read % on the scanner even with a bad ground wire.

[tuff]

Are your oxygen sensors switching properly? I had the same problem before and it turned out that they were shot but didn't throw a code. Replaced them and the car came alive. Happened right after I got back from the track.
Ive ran open loop all season.I run leaded fuels.It felt funny coming down the return road after my last pass for the day but didnt think nothing of it.When i unloaded the car is when i knew somthing was wrong.I,ve had this p0121 code pop up before and the car never skiped a beat.

[DrX]

Thanks i found a writeup simmlar.Do you think its not the pcm but more the tps sensor.Im just looking at the% values in the scanner maybe the ground on the tps is not working is that possible.Would you still be able to read % on the scanner even with a bad ground wire.

The way I understand it, the TPS has to be functioning for this DTC to be set. But the predicted throttle angle(based on RPM and "other" inputs) does not match the % throttle that the PCM is seeing. If you are running OL, then I guess "other" would be MAP and MAF((if it is not disabled). The sensors could be OK, but if something else is causing an RPM drop, this DTC will set. i.e. there is a mismatch between RPM and TP. It could be related the other issue you mentioned. Does the "cylinder balance test" in EFILive correlate with your egt differences?

[tuff]

I tryed the cyl balance test but the car was running so poorly i had a hard time to keep it running.When the test was complete some cyl didn't even show up on the chart.

[DrX]

I tryed the cyl balance test but the car was running so poorly i had a hard time to keep it running.When the test was complete some cyl didn't even show up on the chart.


Did you try setting P0121 to"not reported" with the EFILive Tuning Tool (as a diagnostic test not a fix)?

[Dirk Diggler]

P0121 can be taken car of in table B3907 just put a 0 in the first 3 rows and that will get rid of it.


This will tke care of the P0121 code.....

[DrX]

P0121 can be taken car of in table B3907 just put a 0 in the first 3 rows and that will get rid of it.


This will tke care of the P0121 code.....

What is B3907 anyway? It's not present in my tun/cals.

[GMPX]

I think C3907 was what he meant.

Cheers,
Ross

[tuff]

I don't think trying to clear the code or cover it up is going to work.Like i said my car runs like crap and it will not rev up.Its like limp mode but wayyyy worse.

[DrX]

I don't think trying to clear the code or cover it up is going to work.Like i said my car runs like crap and it will not rev up.Its like limp mode but wayyyy worse.

I didn't suggest permanently disabling the DTC enabler. Just wanted to know if that gets you out of "Reduced Engine Power" mode and to assess the functioning of the TPS. Of course if you have other issues it still will not run perfectly but it should respond to the throttle. I couldn't get over 50-60 km/hr in REP mode. All stepping on the throttle did was keep it from stalling. Good thing there are no hills around here or I would have been going backward!

[tuff]

I don't think trying to clear the code or cover it up is going to work.Like i said my car runs like crap and it will not rev up.Its like limp mode but wayyyy worse.

I didn't suggest permanently disabling the DTC enabler. Just wanted to know if that gets you out of "Reduced Engine Power" mode and to assess the functioning of the TPS. Of course if you have other issues it still will not run perfectly but it should respond to the throttle. I couldn't get over 50-60 km/hr in REP mode. All stepping on the throttle did was keep it from stalling. Good thing there are no hills around here or I would have been going backward!
DrX thanks for your help and everyone else.Ill give it a shot tomarrow and see what happens.Ill let you guys know. Thanks mike

[tuff]

I tryed the cyl balance test but the car was running so poorly i had a hard time to keep it running.When the test was complete some cyl didn't even show up on the chart.


Did you try setting P0121 to"not reported" with the EFILive Tuning Tool (as a diagnostic test not a fix)?
Well i tryed that as well as seting the first three lines to zero.I am having no luck :cry: I even put my stock injecters back in to see if there is a diffrence.How many grounds are on the car that are important?Do you guys still think its not the pcm.The car even pops a little when its cold i can't even take my foot off the gas without it dieing out.

[Black02SS]

Maybe I missed it, but you are in open loop, have you checked your Fuel pressure and AFR with a wideband by chance?

[tuff]

Maybe I missed it, but you are in open loop, have you checked your Fuel pressure and AFR with a wideband by chance?
Fuel pressure is at 58-60 psi i didn't hook up my wideband.The car runs so poorly you couldn't even get it out of the driveway.

[DrX]

Did any new codes set after you cleared the DTCs and disabled them with EFILive? Or was it still running bad just with no DTCs set?

When you re-enable DTC P0121 and it resets, retrieve the associated "stored history data"(GM's "Freeze Frame") with EFILive's Scan Tool. Is there anything that looks out of whack in there?

Were all the injectors firing when you changed them?

I think you said you already checked for vac leaks, spark, compression, etc.

Did you work through that GM diagnostic to determine if the TPS, MAP or PCM could be at fault? (I'll check if there are any other related diagnostics)

[tuff]

It all looks good im going to try a tps senor and see if there is not a problem that im not seeing with it.If that is the problem i would have bet a ton of money that it was not the problem by the data ive seen.btw no more codes.

[kp]

make sure your MAP sensor is working..

[tuff]

make sure your MAP sensor is working..
That was one of the first things i changed.Map is working fine.I think im going to get a pcm and try that i think were all out of ideas.

[tuff]

Well the pcm didn't work.This car must have a ground or a short i don't were else to look.I might just put a carb on it and call it good.

[GMPX]

What makes you think you have a bad PCM?.
They are pretty robust things, we have a saying around here "It's never the PCM's fault".

Cheers,
Ross

As I was saying........ :cry:

Sorry to hear that did not fix it, ground problems etc can cause REALLY weird stuff to happen. Don't put a carb on, the logging rates are really slow with them.

Cheers,
Ross

[tuff]

Im not putting a carb on it i was just being sarcastic.I think thers a ground issue somewere. When i start the car whens its cold and try giving it gas it will pop threw the exhaust and almost sounds like it wants to pop threw the intake to.Most people would say to tight of valves but thats not the case.I need to find all key ground wires and trace them one by one.

[Trippin]

tuff,

Any chance you broke a valve spring or bent a valve?

[joecar]

.... my car has egts and when the car is running i log cyl temps and num 6 is showing weak.... Num 6 cyl is about 300degrees cooler than the others....

You get good compression/leakdown on #6...?

You get good spark on #6...?
You get fuel on #6...?
Is injector on #6 good (flows properly)...?

Any chance the crankshaft reluctor wheel got chipped...?