5.3LS won’t rev?

[iamtherealJayy]

So I ls swapped my 78 k10 about a month ago and I’ve been waiting on exhaust. I got my exhaust at done today and now the truck won’t rev up? I can hold it to the floor and it will just bounce and return to idle repeatedly. If I crank it I can rev it twice then it returns to no rev. The MIL isn’t on currently. My tachometer isn’t accurate but it shows it stopping at 1000 rpms.

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This is my first ls engine, it took me four days to get the 350 out and the ls in and started. I’ve been waiting three weeks to get exhaust done and now the truck is left useless currently.

 

[Camar068]

After my swap, I had the front o2 sensors backwards and it was sluggish unless I laid into it. In my case, I guess it doesn't look at o2 sensors as much when heavy on the throttle. Ran like a scalded dog until I let off.

To identify which was which (drivers vs passenger) it was I wanna say a blue wire and a blue and white wire. Anyway, it turned out to be a white stripe that ID'd one or the other side. Wish they had made them different colors lol.

Another thing to check is all your grounds at the PCM. Disconnect the battery, and OHM from the battery ground cable to each ground pin on both PCM connectors. You may need to use a length of speaker wire to extend your test lead from the battery to your meter.

 

[iamtherealJayy]

The truck ran fine before getting exhaust done other than occasional idle surge. I just had them connect the existing exhaust on the truck to the manifolds (2” pipe) and apparently that’s too small? I’m looking into going to 2.5” pipe since it’s got dual exhaust the whole way back

 

[iamtherealJayy]

The MIL came on with code po354, could that have any correlation with the issue? I’ve ordered a complete 2.5” exhaust kit and plan on redoing the exhaust. I just don’t have any idea why it’s all of a sudden an issue.

 

[rusted nuts]

2" should be plenty.

 

[iamtherealJayy]

I ordered 2.5” pipe and I’m going to redo it just incase since the manifolds have 2.5” pipe. I just don’t understand what all of a sudden caused this issue. It will rev fine to 1500ish then just cut off. I can hold it to the floor and it repeatedly just bounces up and returns to idke

 

[gmbellew]

I ordered 2.5” pipe and I’m going to redo it just incase since the manifolds have 2.5” pipe. I just don’t understand what all of a sudden caused this issue. It will rev fine to 1500ish then just cut off. I can hold it to the floor and it repeatedly just bounces up and returns to idke

no real idea, but if you are drive by wire it could be the pedal position sensor

 

[Ricko1966]

Po354 is a circuit code a wire, a coil,or pcm problem look for something maybe got laid against the exhaust and melted to ground. Look for anywhere wires could have grounded or got disconnected

 

[iamtherealJayy]

@gmbellew its cable, with full range of travel
@Ricko1966 po354 would be the second one on passenger side right? That cool is loose in the bracket if that matters? All the others don’t move but that one jiggles. I observed my o2 sensor wiring after the exhaust and it all appears fine along with the wire going up to the MAF. I wonder if it is something with the trans stuff since I’m running a manual. Dude warned me in the beginning he doesn’t do many manuals. I’ve read about “abuse mode” and it seems pretty similar to my issue. Either way I’m still going to a 2.5” exhaust I’ve got everything but a muffler because I can’t decide on one

 

[CalSgt]

IDK man, you’re set on doing another exhaust but I would doubt that is the issue.

Po354 is a circuit code a wire, a coil,or pcm problem look for something maybe got laid against the exhaust and melted to ground. Look for anywhere wires could have grounded or got disconnected

^^^^ Rick is leading you in the right direction, I’ll add to look for any pinched wires too. It’s possible something got burned or broken when the exhaust went on but also possible the short just showed up finally. It’s not like there’s a ton of hours on it since the swap. I wouldn’t limit my search to spots near the exhaust.

Did you make your own harness or buy one? Either way trace the whole thing back to the PCM looking for signs of damage

 

[iamtherealJayy]

A guy in Murray did the harness, the wiring around the o2 sensors looks fine, the grounds seem fine, I also hopped on that Facebook group and a couple people mentioned abuse mode, I’m cutting the exhaust today since I’ve got all the pipes now and I’ll see how it acts being just manifolds and downpipes again

 

[GTX63]

Step 1 Look for the obvious. You have a fault code.
What has changed about your truck? You had exhaust put on and now there is an issue. 1/2" diameter pipe difference is not causing the problem.
PO354 is electrical. Try to clear the code. If it resets then you have your lead.
I have been down your road many times and usually, when I skip over step 1 and head for 3, 4 and 5, I end up back at step 1.

 

[iamtherealJayy]

The code does come back but just observing around the #4 coil pack everything seems fine. The coil jiggles in the bracket but that’s the only thing separating it from the other seven. I followed the wires from the o2 sensors up to the maf and it all seems fine. The only code the truck is showing is the ignition coil d circuit(po354) someone else on that fb group mentioned to ground pin 32 on the blue connector(trans safety switch?) I did so and it did not help. Someone else mentioned I could unplug the tps and see if it would drive but I don’t know what that means the issue would be?

 

[RecklessWOT]

I ordered 2.5” pipe and I’m going to redo it just incase since the manifolds have 2.5” pipe. I just don’t understand what all of a sudden caused this issue. It will rev fine to 1500ish then just cut off. I can hold it to the floor and it repeatedly just bounces up and returns to idke

is that just in park/neutral, or does it act that way when you try to drive it? I have an '08 GMC with a 6.0 and if I just sit there in the driveway and try to rev it up it does not let me, a lot of new cars are that way. It is like a separate rev limiter to protect the engine since there is no real reason one would need to sit there in park with their foot to the floor. Too much computers and sh!t, thinking they know more than me, please just let me manipulate this machine the way I see fit. Hah that's why I love old technology, all computers do is piss me off.

Edit: Just saw the part where you said you were using a manual transmission. Maybe the computer is confused because it can't sense what gear you're in and it's trying to protect you from yourself. Man new sh!t is dumb.

 

[iamtherealJayy]

The truck is a 4 speed manual with all the auto harness stuff cut off. I grounded pin 32 on the blue harness as per request someone in the fb group with no benefit, I’ve also read to ground pin 34(both pins are PRND related) I might try that but I don’t want that to cause it to think it always in park or neutral. I will also put the VSS in I completely forgot about it. It’s the last thing I have to wire into the harness until it’s complete. Maybe an input there will solve some of my issue?
Edit: it does it in gear also @RecklessWOT I took off driving around the yard after grounding pin 32, It went smooth for a minute then I kissed the steering wheel when it cut out, I shoved it down in low and idled the truck back to its parking spot.