Engine running rough below 3k RPM. Fine above it. ???
 

I have a stock '98 LS1 in my truck which has been there for years. It sat for a while. t

There is a stumble and hesitation at low RPM that seems to go away around 2500-3k RPM. First thought was bad gas, but I pumped out what I could and I've ran a few gallons of fresh 91 octane. Maybe helped a little? Cleaned MAF and TB. No change.

It also takes several cranks to fire, whereas it used to fire on the first crank.

I would think any issue fuel related would be worse at high RPM.

Ignition issues? I was thinking maybe coils? No codes. O2 sensors?

The truck is down probably 100HP, but otherwise runs and drives just fine.

Thank you for your thoughts!

Edit: Pulled one plug, it looked fine.

OBD2 scanner shows good output from the O2 sensors (rear deleted).

Sitting in it at idle for a long period, it is definitely idling rougher than it ever did before. Stable, but bumpy, like a cam with a lot of duration (it is stock). Took it on another drive and while it was hesitating I floored it - bogged and lost all power until it recovered from that (engine did not die).

I feel like this should be fairly obvious but it's been a while since this thing was my hobby.

Re: Engine running rough below 3k RPM. Fine above it. ???
 

I suggest Sea-Foam to start with.

Link: https://www.amazon.com/Sea-Foam-SF-1...99526675&psc=1

Re: Engine running rough below 3k RPM. Fine above it. ???
 

I've always been skeptical of that stuff but I suppose it wont hurt. I read I can add it to a full tank and it also acts as a fuel stabilizer, so why not?

If this was a carbed SBC I would have already placed the dizzy cap and the plug wires. But on this thing, I should be getting a code if it is misfiring, no?

Re: Engine running rough below 3k RPM. Fine above it. ???
 

This product has help several of my fuel injected engines run better.

Some of those had sat for a while like you mentioned yours has been.

Some of the Other products I have tried have not worked as well.

Maybe some of the LS gurus can help you with diagnosis of coils and fuel system stuff.

:chevy::chevy::chevy:

Re: Engine running rough below 3k RPM. Fine above it. ???
 

Spark plug wires will do exactly what you described.

Re: Engine running rough below 3k RPM. Fine above it. ???
 

I was thinking that also, but wouldnt it throw a misfire code in that case? I have no codes in the system.

Otherwise yeah, that was my first instinct - arcing plug wire.

Re: Engine running rough below 3k RPM. Fine above it. ???
 

Yep! Passenger side, cylinder 8. Cracked boot.

Of course it was the last plug and wire I changed :lol:

Runs great now. Thank you!

Re: Engine running rough below 3k RPM. Fine above it. ???
 

Great Diagnosis :metal:

Glad you got it running great. :metal:

Re: Engine running rough below 3k RPM. Fine above it. ???
 

Just FYI in regards to LS plug boots. On my LS swaps I always use the stock metal boot protectors.

Re: Engine running rough below 3k RPM. Fine above it. ???
 

I change the wires on LS motors every time I change the plugs. I have never been able get them off in one piece anyway.

Re: Engine running rough below 3k RPM. Fine above it. ???
 

LOL..Same here.

Re: Engine running rough below 3k RPM. Fine above it. ???
 

Try something like this.



https://www.summitracing.com/parts/s...+5ba29de7c3faf

Re: Engine running rough below 3k RPM. Fine above it. ???
 

I have the plug wire puller also!
LS Wires last over 100K and have typically been on there for years. I gave up and just buy wires with the plugs. The factory heat shields don't make it any easier.
Maybe one could get them off, in one piece, if they have been replaced and the silicon gel used on them. The tool is a knuckle saver and I suggest it as a safety item.
This is what works for me.