Runs Rough Under Load

[97Yukon4dr]

Hello, I own a 1997 GMC Yukon 5.7L v8 4x4 with the 4L60e trans.
I've been stumped over an issue for a while now and I'm trying to find a real place to start. If I'm going up a hill and continue at or exceed 1500rpm going 70-90kmph the truck will run extremely rough, (feels like a terrible misfire) the more I try to accelerate the worse it will become. I have to coast it up the hill while slowly bumping the gas because it loses power. It only sometimes throws check engine codes but never stores them so I cant scan them while driving.
A shop that's familiar with mine said it was a drivetrain issue but wouldn't diagnose it, it can sometimes have trouble shifting into fourth which causes the issue and it can last sometimes after it shifts. It doesn't cause any weird noises.

If anyone has experienced this or thinks they can help let me know before I throw hours of diagnostics into it. I've dumped a ton of time and money into this truck and this is the last real problem its got left

 

[Schurkey]

1. What is the fuel pressure UNDER LOAD? How old is the fuel filter?
2. Distributor cap is a very common problem; I've heard that #3 is the worst.
3. Test ignition coil with a spark-tester calibrated for HEI ignitions. Tool costs about $15.
4. Connect a scan tool, look at pending codes and history codes in addition to current codes.
5. While the scan tool is connected, look at cylinder-by-cylinder misfire counts.
6. Scan tool still connected, look at the data stream under load. Long- and short-term fuel trims, O2 sensor data, MAP, EGR, TPS, Knock sensor, Coolant temp, etc.

 

[97Yukon4dr]

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1. What is the fuel pressure UNDER LOAD? How old is the fuel filter?
2. Distributor cap is a very common problem; I've heard that #3 is the worst.
3. Test ignition coil with a spark-tester calibrated for HEI ignitions. Tool costs about $15.
4. Connect a scan tool, look at pending codes and history codes in addition to current codes.
5. While the scan tool is connected, look at cylinder-by-cylinder misfire counts.
6. Scan tool still connected, look at the data stream under load. Long- and short-term fuel trims, O2 sensor data, MAP, EGR, TPS, Knock sensor, Coolant temp, etc.

1. The fuel pressure at idle is 52 at idle and 60 when revved up. and the fuel filter is brand new it did not affect the issue
2/3.It was a new distributor cap, rotor, plugs, and wires as of October 2020.
4.I have already connected my shops Autel scan tool and it doesn't have any stored codes (current, pending, or history) aside from my abs light
5/6.I haven't done that yet I will try when the scan tool is available thank you for the thought


It seems the problem is a little worse in 4wd

 

[stutaeng]

It's weird that it throws a code but it doesn't store it?

Even when I've disconnected a sensor by accident and the CEL goes on, the code is stored if I hook up my basic handheld scanner.

 

[97Yukon4dr]

I know right. The one day it was so bad the light was flashing, by the time I got to scanning it (without turning off the truck) no codes were there.

 

[DonYukon]

Any shop that tells you "its probably this " but won't bother diagnosing it is a shop Id stay 100 miles from

 

[stutaeng]

I know right. The one day it was so bad the light was flashing, by the time I got to scanning it (without turning off the truck) no codes were there.

The only time I recall my SES light flashing is when I had a misfire due to a damaged plug wire on my 4.3 Vortec. IIRC, the flashing SES light means you are risk of damaging your catalytic converter.

 

[Schurkey]

IIRC, the flashing SES light means you are risk of damaging your catalytic converter.

Yup.

 

[Revan73]

My bet is fuel regulator and/or MAF.

Fuel reg is hard to test unless you have an O2 code. MAF is easy to test just disconnect and if the problem goes away or gets much better buy a new one.

 

[Pinger]

What is SES? The same as CEL?