460 no power and random idle
 

Hi guys, I'm at my wits end here on this old truck. I have an 88 F350 4x4 with a 460 and zf5. I have replaced a lot of parts including the entire fuel system short of the injectors themselves. My biggest issue is the truck seems to be gutless when above 3rd gear when warm. When cold, it seems to have more power. The other issue is when driving and shifting into neutral, rpm stays at around 1100-1200 and sometimes drops down to 700-800. On startup, idle is correct at 800 give or take rpm. It has a new ECT, map, iac, o2 sensor and more that I can't think of at the moment. As for codes, nothing important. Just an 81, and I believe 33/31 or so. I can check soon here but these codes are just about the emissions, nothing important. The random idle I feel like is pointing towards something. I also forgot to mention that it does not high idle for 30 seconds or so like most trucks on the forum do. I feel like once I tackle the idle issue, it might help me find more about the no power problem, whether it's a vacuum leak or something.

Any help is greatly appreciated!

Sounds like maybe the TPS going silly?
Or the EGR is stuck?

I thought that could be the problem too but I have testing the TPS with a meter, and checked the egr for correct function as well. Last night I checked for codes and only got the 81. Then I put the original ECM back in the truck. I got no codes at all then after driving around for a bit. I don't seem to has the hunting idle problem with this ECM, but the no power is magnified, at anything more than 1/4 throttle, it will backfire and bog down 500 rpm from wherever I was at. I would almost say it's fuel related but does not happen as bad with the computer so I could only guess ignition. Not a cut and dry problem :-X16

I'd keep that ECM in there, and pull the Spout connector and set the timing on it. See what she does then.
I had to think you have 2 bad ECU's? Sounds like the timing isn't advancing with the original ECM. Or maybe out got a problem with the spout connectors/ECM timing advance.

That makes sense. I'll check that and let you know thanks!

Timing appears to be working correctly unfortunately. What strikes me as odd is that both computers give the same problem, but the original computer is so much worse. With the junkyard ecm, I only get a loss of power while the original, I get very loud backfiring as well.

That is odd. Someone smarter than me is going to have to diagnose this one. I feel like it's going to be something simple that we're both overlooking.

One other issue that I just thought of also is my truck has never high idle warmed up for the 30 seconds or so that normally happens. I have a new ECT installed and like stated before no codes. I wonder if the ecm is seeing a different temperature and messing with the timing that way?

It's not far fetched to believe you got a bad ECT out of the box, and that's been the problem all along. There's a way to resistance check the ECT, I've seen it before here on the forums.

That's one thing yet I have to check. Since it could just be reading cold/hot all the time the computer wouldn't throw a code then right? Or is it smart enough to detect no change?

It won't throw a code, it will just change the parameters to match whatever random temperature the ECT is throwing at it.

Well, both the ACT and ECT are reading correctly. I'm at a loss. Even with the engine running roughly and no power, it should still go through it's warmup cycle and it does not. There is a new IAC I put on a while back so that's functional. At a certain point if I can't get this sucker to work right I'm going to snag the entire engine bay wiring harness from a junkyard...

I can't think of what else it would be. Maybe the ignition coil is crap, and you're getting a weak spark? Or maybe a whole cylinder not firing because of something in the distributor?
Have you put a pressure gauge on the fuel rail, and stepped on the pedal to make sure you're getting an appropriate pressure increase?

Fuel pressure specs out, spark verified good a long time ago, as far as one cylinder not firing I don't know. I've pulled the injector connectors to kill the cylinder and all seem to be contributing. This fall I'm headed for college and won't really need the truck, but since I'll be 2.5 hrs from home, and up north, I'd like it to run right and be reliable. This is sort of my last ditch effort before I take it to the local speed shop where one owner is a major ford guy specializing in these years.

Yeah, I feel ya. Maybe someone will jump in here and save the day with the answer. Unless you have 2 bad computers, I'm not sure where else I would look.
Clogged Cat maybe?