Hi All,

Big fan, I've learned a lot here. Lemme start by saying this issue is with an engine swapped car (LQ4 / 4L80E in a 68 El Camino)

I've been having this issue with my car for about the last year now... It is not running consistently, and feels like its getting worse. It'll start right up, even on a cold day. Most of the time, it idles ok after startup... as i start driving around (not much engine load) / warming it up (130ish degree engine temps), it feels like it drops down to only running 6 maybe even 4 cylinders-- huge loss of power, running rough. If i stop, put it in neutral and rev it up to 2k rpm and hold it, after 5 to 10 seconds, rpms will drop 200-500 rpm and sound like its missing. While doing this, once the revs drop as stated, they'll kinda jump around
When i'm driving and experiencing this issue, more throttle = more noise, but no more go. I can go to 3/4 throttle and just get a bunch of noise, but a HUGE loss in power. If i go to WOT, it seems to remember what its doing and run fine, for the most part. Hard to tell at this point if WOT is 7 or 8 cylinders. When it goes in to this <8 cylinder mode, it sounds as though it is running out of gas-- lots of intake noise. Every once in a while, it'll backfire when i go to WOT after being in this 'mode.'

It has gotten more and more inconsistent, and i'm annoyed. Throughout this past year, it has thrown only 1 code: 'Injector Loop Open.' Since then, I've taken out my harness and rebuilt it. I just got the harness in and still have the same issue. The harness was modified by a friend of mine a couple years ago, and entirely repinned by me over the last few months.

History / parts:
68 El Camino,
2004 LQ4: Edelbrock ProFlow intake manifold w/ 90mm tb, factory sensors, mustang cobra injectors (39#), Texas Speed 228R cam, Hooker long tubes, Dart Pro 1 225cc heads, walbro fuel pump.

2006 4L80E, 3k rpm converter, 9" in the back with 3.55 gears.

Car made 393rwhp, 352 ft-lbs. Ran great for about a year.

Does anybody have any insight on what this could be? I'd love it if this was as simple as a sensor going bad, but i need to figure this issue out already! Any help is greatly appreciated.

 

PCM? Only reason I am even suggesting that is I have had the same problem on one of my friend’s trucks I fixed a while back. It would start and run fine for about 5-10 minutes and then once it warmed up it started bucking and just not running right. After searching it turned out to be a bad computer. Now I am not saying this is your problem but I wouldn't count it out. Hopefully someone that has had more experience with the exact problem you have can chime in and give some better advice. Good luck and keep us posted.

 

Thanks for the ideas, i appreciate it. One of my bosses had his computer go out on his truck too (2004 Silverado SS), so i've heard of this happening before too..

I still need to check a few of the basics (compression, vacuum, and plugs) but having done that a few months ago with the old harness, everything looked fine except for 1 plug-- showing lean. I moved the injector, discovered that the injector was fine and figured it was the soldered injector pigtails. Having repinned everything in the engine bay, i figured this ought to have fixed my issue. I don't have a check engine light anymore, but it is as inconsistent as ever.

 

Sounds to me like its fuel related, whats your fuel pressure? How bout o2 sensors after it goes into closed loop? Gonna need to look at some live data.

 

As far as the most common issues to the least common issues I would say first check for an intake leak , next check the operation of the egr , unplug the connector and power it to the correct pins ,,,,,,,start engine and put power to it until it is fully open ,,,,,the engine will die unless the passage is clogged and if that is the issue will resemble what's happening when You drive around . Next check the evap ,,,,,,,this 1 is a little tougher to test . Next a quick check of the pcv and if none of this seems to be it then I would have the pcm checked .