Engine running rough below 3k RPM. Fine above it. ??? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chevy72blu · 11-17-2023, 09:30 PM

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.

**Getter-Done** · 11-18-2023, 11:43 AM

I suggest Sea-Foam to start with.

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

chevy72blu · 11-18-2023, 01:17 PM

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?

**Getter-Done** · 11-18-2023, 01:40 PM

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.

**Accelo** · 11-18-2023, 04:06 PM

Spark plug wires will do exactly what you described.

chevy72blu · 11-18-2023, 04:08 PM

Quote:

Originally Posted by Accelo

Spark plug wires will do exactly what you described.

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.

chevy72blu · 11-18-2023, 05:22 PM

Yep! Passenger side, cylinder 8. Cracked boot.

Of course it was the last plug and wire I changed

Runs great now. Thank you!

**Getter-Done** · 11-18-2023, 08:52 PM

Quote:

Originally Posted by Accelo

Spark plug wires will do exactly what you described.

Great Diagnosis

Quote:

Originally Posted by chevy72blu

Yep! Passenger side, cylinder 8. Cracked boot.

Of course it was the last plug and wire I changed

Runs great now. Thank you!

Glad you got it running great.

LS short box · 11-18-2023, 09:35 PM

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

**Accelo** · 11-19-2023, 06:42 PM

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.

Rich84 · 11-20-2023, 02:27 PM

LOL..Same here.

cj847 · 11-23-2023, 11:17 AM

Try something like this.

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

**Accelo** · 11-23-2023, 11:42 AM

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.

11-17-2023, 09:30 PM	#1
chevy72blu Registered User Join Date: Apr 2004 Location: Las Vegas, NV Posts: 2,188	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. __________________ RIP EastSideLowLife '72 C20 SWB convert. 5 lug, LS1/4l60e Last edited by chevy72blu; 11-17-2023 at 10:28 PM.

11-18-2023, 11:43 AM	#2
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 8,404	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 __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

11-18-2023, 01:17 PM	#3
chevy72blu Registered User Join Date: Apr 2004 Location: Las Vegas, NV Posts: 2,188	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? __________________ RIP EastSideLowLife '72 C20 SWB convert. 5 lug, LS1/4l60e

11-18-2023, 01:40 PM	#4
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 8,404	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. __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

11-18-2023, 04:06 PM	#5
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,295	Re: Engine running rough below 3k RPM. Fine above it. ??? Spark plug wires will do exactly what you described.

11-18-2023, 05:22 PM	#7
chevy72blu Registered User Join Date: Apr 2004 Location: Las Vegas, NV Posts: 2,188	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 Runs great now. Thank you! __________________ RIP EastSideLowLife '72 C20 SWB convert. 5 lug, LS1/4l60e

11-18-2023, 09:35 PM	#9
LS short box Registered User Join Date: Jun 2015 Location: Carlos MN Posts: 2,131	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.

11-19-2023, 06:42 PM	#10
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,295	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.

11-20-2023, 02:27 PM	#11
Rich84 Registered User Join Date: Mar 2012 Location: Woodbury, Ct. Posts: 1,692	Re: Engine running rough below 3k RPM. Fine above it. ??? LOL..Same here.

11-23-2023, 11:17 AM	#12
cj847 Registered User Join Date: Sep 2016 Location: Ozark, MO Posts: 573	Re: Engine running rough below 3k RPM. Fine above it. ??? Try something like this. https://www.summitracing.com/parts/s...+5ba29de7c3faf

11-23-2023, 11:42 AM	#13
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,295	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.



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