Car just started misfiring out of nowhere and PO121 code

I just did my cradle swap last weekend and the car ran perfect for about 2 days since then.
Then I started it up to drive home from work yesterday and it was missing right from the start-up. I limped it home and it was driving fine other than that. Just missing one or two cylinders it felt like. Since then, the misfire has continued unchanged.

I scanned the car today and the new code that comes up is PO121 - throttle position sensor range. I also have a PO300 but that's always there because of the cam. I don't get any specific cyl misfire codes even though it really feel like it's that way.

Just for sake of checking, I've tried a spare set of coils and wires. I checked the connections and ground for the ICM and played with the wiring around the TP - No leads there. Oddly, it doesn't seem like anything changes if I unplug the cyl 1 fuel injector or spark plug wire but I don't want to base my assumptions of anything off that. I did check for spark there and voltage at the injector connector and those look fine. The spark plugs look great.

I guess I'll go through the diagnostic procedure in my FSM for that trouble code but I don't feel like this is a problem that a TP signal can do.

Any ideas would be greatly appreciated
this is such a slap in my face after all the work I did last weekend

Life seems to be like that. I had a coil pack go bad a day after installing my Gen V. I thought for sure it was an injector clip that got damaged from the install. Just happened to take a crap all on its own.

Are you still running a stock TB or do you have a N*?

I've been running a northstar & LQ4

I went through the PO121 diagnostic flow chart from my FSM and concluded that my PCM is bad. I'm just a tiny bit skeptical of that verdict... but it's possible I guess.

I ran the car and using the DHP scanner, looked at the individual cylinder misfire counts and they all misfire a little bit but cyl 1 is off the chart compared to the others. The others will get to like 3 or 4 before they reset and cyl 1 will hit like 140 before it resets.

I have work tomorrow so I need to drive the car there and back (about 10 min drive) and then I'm supposed to drive 30mi on the highway to see my parents for Easter so this sucks. I don't know what the f*** to do with this thing right now.

I'm trying to double check that misfire and now I'm not reading anything for misfires on the DHP or aeroforce - stays at zero WTH?

If cyl 1 seems to be the main one showing misfires try swapping coils around and see if the misfire changes cyl.
Any chance something got bumped around the balancer when you did the sub frame? A loose crank or cam sensor will throw random misfires without there actually being a misfire.

Ummmm, something in the programming go wrong?? Do you know how to check the TPS with an Ohm meter? If not, I can explain it. It's real easy....

I gotta run but I tried a spare set of coils and ICM - same stuff.
My aeroforce keeps showing crazy misfires on cyl 2 today.

Scott, this problem just started up out of nowhere - I didn't do any programming. Just drove the car to work - fine and then started it up to go home and bam! misfire that hasn't stopped yet. As far as misfires go, it's very consistent, I'd say. It doesn't really matter if the car is idling or I'm on the gas, it's severity is the same. Idling, the car just sounds like it has a enormous cam in it and it's been pretty much exactly the same since it started. The only thing that I'd say changes is sometimes the car won't want to shift out of 1st gear until i pretty much let off the gas. I think that's just a side affect.

As for the TPS, I didn't actually ohm it. Just observed the % that the scanner sees and it looks ok.

Unfortunately, I need to drive this thing on the highway to my parents house now for a family obligation and of course I'll try to monitor things and report but something tells me, nothing very notable is going to happen. Quite frankly, the car is drivable, I just don't like it, especially without knowing what's going on.

I found the misfire problem.




Cyl#1 exhaust valve

Just another victim of the Comp Cams 105lb spring problem apparently.
I don't have any extra springs laying around except the 130lb springs that'll be going on with the new heads hopefully this summer so I used one. The timing chain isn't going to be happy about that but I don't think one hard spring is going to kill it.

I cleared the DTC's so we'll see if that PO121 stays off now.

Excellent instincts Codith, I'm glad you found what looks like the problem, hope it solves the misfire.

OWIIEEEEEEEE

Wow dude! u are a Sherlock Holmes!

Car has been running fine now and that code hasn't come back. That code was terribly deceiving. As for the valvesprings, they are known for having failure problems since CC moved manufacturing to Mexico but something I didn't know is that they can also loose strength after a a brief amount of use, but not break. Kemp was telling me he tested some of those springs he took off a car after 5 or 10k miles and found they were not holding even close to the lbs they were supposed to. All I can say is I'm glad I'm changing all these out for the 130's this summer when the heads go on...