Flashing engine light / no power under WOT or heavy load

[east302]

I would be wondering about catalytic converter back pressure. If you have an infra red temp gun, start the engine and let get warm and then give us front of and rear of temp readings at the cat(s).

The stumbling under load does sound like an exhaust restriction. You can get a back pressure gauge for around $40 on Amazon to check for damaged catalytic converters. Pull the upstream oxygen sensor. You’ll need a gauge with an M18 adapter which most come with. Also, some don’t come with a swivel fitting making installation a pain. A 90-elbow could be used to make it easier.

There could be two issues, though— The misfire causing the damaged converter(s) which then causes the sluggish performance. Does your scanner show misfire counts by cylinder?

 

[04SS99Denali]

I asked about the trans cuz a wonky MAF can affect trans feeling. Most common is delayed shifting and/or sudden, more harsh shifts.

That power loss could just be common heat soak. The intake tube and IAT sensor gets warmed up as it sits in a hot engine bay. The PCM reads this as ambient air being really hot so timing is pulled. A few seconds of running cools the IAT so the timing is restored. This is exaggerated if you have headers, especially uncoated.

this is definitely not heat soak. The engine will not rev past 3000 in 2nd gear and it will lose power temporarily for a few seconds after even under minimal throttle.

The stumbling under load does sound like an exhaust restriction. You can get a back pressure gauge for around $40 on Amazon to check for damaged catalytic converters. Pull the upstream oxygen sensor. You’ll need a gauge with an M18 adapter which most come with. Also, some don’t come with a swivel fitting making installation a pain. A 90-elbow could be used to make it easier.

There could be two issues, though— The misfire causing the damaged converter(s) which then causes the sluggish performance. Does your scanner show misfire counts by cylinder?

At idle and with “cruising around” under 50% throttle everything runs perfect. Idles smooth accelerates smooth. Only when it goes WOT it falls on it’s face in 1st and won’t even go past 3000 in 2nd. It will start to shutter and lose all power. If a cat was clogged this bad wouldn’t it throw a code?

 

[iamdub]

This is definitely not heat soak. The engine will not rev past 3000 in 2nd gear under load.

Lol

I didn't know this symptom was in relation to your main problem. No, a heat-soaked IAT sensor will just result in retarded timing and the resulting loss of power for a little bit, but it would still rev out and drive like normal otherwise.

 

[iamdub]

Just went back and re-read your symptoms. In Park or Neutral, can you rev it as normal? What if you slowly accelerate with the gear selector in 1, up to around 3,000RPM, then sharply floor it?

Low fuel supply and clogged cat present very similar symptoms. I'm inclined to rule out the fuel supply (volume and/or pressure) since you're not getting any lean codes. Kinda suspecting a clogged cat. But, what throws that off is you said none of your issues happened before the intake manifold gasket replacement. Doing those gaskets isn't going to suddenly destroy the cat. Unless this started a little while after that job? Or was it right after?

 

[04SS99Denali]

Just went back and re-read your symptoms. In Park or Neutral, can you rev it as normal? What if you slowly accelerate with the gear selector in 1, up to around 3,000RPM, then sharply floor it?

Low fuel supply and clogged cat present very similar symptoms. I'm inclined to rule out the fuel supply (volume and/or pressure) since you're not getting any lean codes. Kinda suspecting a clogged cat. But, what throws that off is you said none of your issues happened before the intake manifold gasket replacement. Doing those gaskets isn't going to suddenly destroy the cat. Unless this started a little while after that job? Or was it right after?

It will rev to rev limiter in neutral
Park without issue. And yes this started right after which is why I was suspecting timing. But timing checked out already. I also changed plugs / wires, water pump.

 

[OR VietVet]

As I said in the beginning, I still think you need to eliminate the cats as the source of the problems. Your problem sounds like cats or fuel pressure and I lean toward the cats.

 

[alpinecrick]

No I do not think its been changed at least in the last 50k.

Good grief. No wonder your fuel pump died, and not doing the new one any good either. Change the fuel filter every 20k or so--regardless of the recommended service interval.

These trucks can be picky about electric/electronic components--AC Delco or Delphi only. And particularly picky about cap and rotor. Other brands have improved in recent years apparently, but still........

Really sounds like collapsed muffler or Cat(s). I have seen similar symptoms and that was the culprit.

 

[04SS99Denali]

Ok here is the big update everyone was waiting for. I finally had some time to work on my pile. Put a new MAF and MAP sensor. Then ran some logs on the cam position sensor. It was -9 degrees so I zeroed it back at zero. Took it for a test drive and it it’s great. Truck runs so much better. After some driving around engine light comes on! Po 420 bank 1 Cat lollll!! Just can’t win!