Hey ls1tech,

I've put about 300 miles on my LS swapped 1968 Camaro. I'm trying to isolate an issue I've been having with light throttle surging. It only happens at light throttle and will completely go away around 3500 rpms, and higher. I have no problems at idle, but will start to surge around 1300 while driving. Background on the motor: I built the top end of my engine, SBE LS1, with LS2 cam (and front cam conversion), LS6 valley cover, stock 799 heads, LS6 intake and 85MM MAF with a CAI.

The car runs great at WOT and above 3500 or when I'm giving the car moderate-heavy throttle. The motor almost seems as if it is gulping for air. I can't really tell if it's too much or too little, but that's the feeling it gives. I've confirmed that I have 58PSI through the whole rpm range, I checked for vacuum leaks with no luck of finding one, and I just did a TPS and IAC reset last night. Still no dice.

When I first put the car together I didn't realize that I had the old style LS6 valley cover without the integrated PCV valve, so I did have an issue with the engine pulling a large amount of crankcase vacuum. I've since added an inline PCV valve and that solved that issue. I also tried last night what the car would do if the crankcase was vented to atmosphere, and that didn't change the surging either.

I confirmed that the IAC is functioning with my HP tuners program and it moved while I added the counts, although it didn't appear to completely close? I also tried completely unplugging the IAC last night to see if that would change things and it didn't. It also didn't give me any codes which I thought was a little strange.

I'm currently in the middle of doing a LTFT adjustment for the VE Tables, so the MAF is set to fail at 1Hz so it shouldn't have any affect on the tune at this point. I've added roughly 10% to some cells but it had zero change to the surging problem good or bad. At this point I'm leaning towards either a very small vacuum leak or a bad IAC.

Anyone have any thoughts or worked through a similar situation?

Thanks,
Ryan

 

I tried the throttle body from my truck that has a known functional IAC and the results were the same surging issue.

 

one thing that might help is to find out on the ve map where it does this and see if there's any big differences in the values between cells either for fuel or for timing... if it is jumping between values that are too different it might cause what you're experiencing. maybe smooth the cells in the area and see if it helps...especially for timing, find out where on the map that is happening and then change that cell and the ones surrounding to a single value so timing doesn't fluctuate and see if the problem goes away.

 

I know its obvious but check your plugs and wires too. I had a surging problem that was only at cruise and I went through all the timing and fuel cells and couldn't solve it. Started checking basics and found a burnt plug wire and fouled plug. Replaced them and the surging was completely gone.

 

Thank you both for the input. I'll check both of these things tonight and report back. Hopefully it's a simple spark plug wire or something.

 

good point, start with the basics, plugs and wires and then the tune

 

I double checked all my plugs and wires and they were all in good working order. I did find one plug wire that didn't seem to clip down to the plug as hard as I'd like, so I replaced it. Didn't change anything though. I'm hoping to check the tune tonight so see if there are any sharp dips or anything causing the hunting rpm issue.

 

the term surging to me means at a constant throttle input the engine/car speeds up and slows down in a slight manner. As if you pinched the fuel line on and off. I have also had people describe surging at lower rpm and soft light throttle and it be a misfire due to a cracked spark plug insulator or a boot thats carbon tracked to ground. These things only show up at cruse and light throttle because of the very
lean mixtures demand much more KV to bridge the plug gap. Hope you get it corrected !!

 

I just went through a similar situation with an LS1 Miata with the Z06 air intake. At low rpm in the higher gears- 5th and 6th below 1500- I had what felt like a lean surge that I could not tune out at all. It was really aggravating because I like to cruise in that range often. Long, long story short. The LS6 intake had a direct shot to incoming air in the nose of the car and all the turbulence was freaking out the maf sensor. I use the LS7 style cartridge style maf. I used a piece of thick plastic and made a stand off for the front of the filter. I spaced it out about an inch so the air had to go around it and then into the filter. It 100% completely cured the drivability issue. I can go down the road at 1100 rpm in 6th at 45 mph smooth as butter and it just pulls away with any throttle input. It helps the car only weighs 2500 lbs. Hope this helps.

 

Use HPT scanner while driving to manually adjust a few things.
While cruising and during the surge, try these one at a time to narrow the problem to airflow, timing, or fuel:
1. Go into Special VCM Controls and lock the IAC in a set position to see if that helps. Don't let the IAC move. Put it at both low and high extremes to see if that effects the surge.
2. Next, go to spark control where it says Spark Control - Absolute. Set it to about your average cruise advance. Then knock out 5 degrees from the base timing, and then try adding 5 degrees to the base timing.
3. Last, go to the AFR control. Try low and high extremes. Usually a very rich AFR will stop the surge.

That should help you find your main focal point.

 

Thanks for that! I'm tuning my LS3 engined Miata with the same intake, and fighting the same issues. I'd seen the picture of the Flyin' Miata air intake shield, and forgotten all about it.

Here's a link to the picture I'd seen ... Flyin' Miata Intake Baffle

I owe you a beer. :-)

 

Thank you for all of the help, I really appreciate all of the responses. I seem to have fixed the issue by flashing another know to be good stock LS1 tune. This is the tune that I had on my 76 T/A with the LS1/6 speed. I drove it around and didn't have any of the issues I've had before. What is also pretty interesting is that the STFT's weren't even off by that much? The only modification that I did to the otherwise stock tune was added some data I had for an 85MM MAF. I still need to put a few hundred miles on it to confirm that the surge issue has been resolved, but it would typically come on immediately so I'm pretty sure it's resolved.

What I want to do is compare the tune that I build using the 01 Z06 tables to the one that is on the car now to see what tables are different and hopefully isolate the issue. Once I determine the cause I'll update this thread for someone in the future. I'm guessing when I was copying the tables, something was conflicting that was causing the rpm's to "hunt".

 

Grubinski,
Let me know how that works out. I do all the conversions for V8 roadsters. I might just start incorporating that from the get go. Which chassis did you start with? Seems to be biggest problem on the NB.