LS Swap - stumbling at low RPM

[ScottEwine]

Good Evening All,

I swapped out an LS6 into my '75, the details of the swap are below. I have been driving it and working on the tune, but have not been able to get it to run as well as I had hoped. It starts and idles fine and it runs as I would hope while warming up, but then once it warms up it 'stumbles' in certain conditions, mainly at low RPM when I am off idle, so when I start from a dead stop and when I am on the highway at 55 in 4th and give it just a bit of gas, I can hear/feel it misfiring. It does not record any misfires. The intake is behind the radiator so the inlet air temp can get pretty high, not sure if that is throwing off some of the fuel calculations or if there is perhaps a loss of compression somewhere. Let me know if there is a good set of parameters I could log and post that would help me get to the root cause.

Thanks, Scott

-----------------------------------------

2002 Camaro Z28 LS1(6) / 4L60E

~115k original miles, swapped into a '75 Corvette - the leakdown tests on all cylinders were good

Engine
CamMotion LS1 Stage 2 Camshaft (214/220-115+3)
Duration at .050 - 214/220
Lift with 1.7 - .553/.553
Lobe separation 115
Intake center line 112
New valve springs
Original LS6 intake
Original injectors
New knock sensors
New oil pump/pan/pickup
New timing set
Basic intake using original MAF - located partially behind the radiator so the intake air is warmer than I would prefer for now.
New IAT sensor
97-99 Corvette manifolds into 2 1/2 dual rear exhaust
Radiator - 98-02 F-body with dual cooling fans

Upgraded to LS6 Lifter Valley Plate - I believe this may allow more air to be pulled from the valve covers into the intake

Transmission
No modifications, pulled the pan to make sure it looked ok and replaced the filter and fluid.
Original torque converter

[horsepowerguru427]

Have you tuned VE and MAF yet? Post a tune and data log if you want some help. We cant guess at whats going on in the computer. To post files go to advanced mode on the reply box and click on the paper clip up top. That will allow you to add files so we can view. Thats shouldnt be a hard cam to tune as far as things that needed adjusted.

Your intake being right in front of the radiator can definitely cause high IATs which will definitely affect fueling and spark. Cant give you very specific answers without a tune file though. Adding the OG stock tune and the tune you have altered will help as well, since we'll be able to use the compare feature that way.

Kudos on listing all the mods though.

[ScottEwine]

Have you tuned VE and MAF yet? Post a tune and data log if you want some help. We cant guess at whats going on in the computer. To post files go to advanced mode on the reply box and click on the paper clip up top. That will allow you to add files so we can view. Thats shouldnt be a hard cam to tune as far as things that needed adjusted.

Your intake being right in front of the radiator can definitely cause high IATs which will definitely affect fueling and spark. Cant give you very specific answers without a tune file though. Adding the OG stock tune and the tune you have altered will help as well, since we'll be able to use the compare feature that way.

Kudos on listing all the mods though.

Thanks for offering to help. I did tune the VE and MAF, it was my first attempt so not sure I got it all correct, but it definitely runs better than it did before I started. I will post the original tune and my current tune and some log data highlighting where it stumbles. Do you have a good list of parameters I should be logging? I know with this era ECU if you try to log to many the data can get flaky (or so I am told).

Scott

[edcmat-l1]

Where's the tune and data log?

businessman-looking-into-crystal-ball.jpg

[ScottEwine]

Where's the tune and data log?

businessman-looking-into-crystal-ball.jpg

Heading out to get some data now!

[1dirtybird]

Pay attention to the KR and add the Burst Knock to the recording table. Could be seeing false burst knock in the low RPM's during tip-in.

[2xLS1]

Check the fuel pressure at the rail while driving. Long hose on tester with the gauge where you can see it on windshield. #1 problem I see with swaps is the fuel delivery system.

[ScottEwine]

Here is the as found tune, the current tune, and the most recent log file.

75 as found tune.hpt75 2023-211-19.hpt2023-11-19-04.hpl

The issue really arises when it is at ~1500 rpm and you give it some throttle, roughly 20-40%, you can feel/hear the car stumble and hear occasional backfires. This is common in overdrive at 55 when you give it some gas, or when starting from a stop. If I am doing 60 in 3rd and give it a bit of gas it responds nicely.

Scott

[ScottEwine]

Here is the as found tune, the current tune, and the most recent log file.

75 as found tune.hpt75 2023-211-19.hpt2023-11-19-04.hpl

The issue really arises when it is at ~1500 rpm and you give it some throttle, roughly 20-40%, you can feel/hear the car stumble and hear occasional backfires. This is common in overdrive at 55 when you give it some gas, or when starting from a stop. If I am doing 60 in 3rd and give it a bit of gas it responds nicely.

Scott

Here is what it looks like under the hood.

IMG_9238_sm.jpg

[ScottEwine]

If you look at the 11:33 mark you can see where I am doing about 55 and give it some gas several times, that is where it stumbles and misfires.

[1dirtybird]

Ok there are a few things.

Turn off the LTFT especially when tuning, next unplug the MAF and do a VE tune. When that is done then go and retune your MAF scaling.

Your VE Table is very important especially on quick throttle transitions. Its also strange that you are getting 0 kr not even blips did you de sensitize them (I haven't Looked At your tun just the data log}

[ScottEwine]

Check the fuel pressure at the rail while driving. Long hose on tester with the gauge where you can see it on windshield. #1 problem I see with swaps is the fuel delivery system.

I used the setup from Tanks Inc. which seems pretty solid. Using the stock C5 fuel filter/regulator. Where in particular is the issue with the fuel system that you see?

https://www.tanksinc.com/index.cfm/p...rod/prd443.htm

[ScottEwine]

Ok there are a few things.

Turn off the LTFT especially when tuning, next unplug the MAF and do a VE tune. When that is done then go and retune your MAF scaling.

Your VE Table is very important especially on quick throttle transitions. Its also strange that you are getting 0 kr not even blips did you de sensitize them (I haven't Looked At your tun just the data log}

I already did the VE tune and I think I got it pretty close.

[ScottEwine]

Ok there are a few things.

Turn off the LTFT especially when tuning, next unplug the MAF and do a VE tune. When that is done then go and retune your MAF scaling.

Your VE Table is very important especially on quick throttle transitions. Its also strange that you are getting 0 kr not even blips did you de sensitize them (I haven't Looked At your tun just the data log}

Also when I first got the engine in and started tuning it, I got some errors indicating the knock sensors were bad, so I replaced both of them.

[ScottEwine]

I think I am onto something! It seems I have a bad passenger O2 sensor? It says O2 B2S1 Ready: No. When I got back I unplugged the passenger side O2 sensor and it made no difference so it seems I will replace the o2 sensor and see if that helps.

2023-11-19_16-00-22.jpg

[edcmat-l1]

This is the screen shot at 11:33 real time, 6:00 minutes into the log.

First thing is, you're logging way too many pids. It slows the scanner way down on the GEN3s and changes the way the line graph looks. I'd get rid of 75-80% of your pids. You don't need any trans pids right now. You don't need every available timing advance pid.

The screen shot you posted with the O2 "not ready" looks in the little bit of line graph included, that O2 sensor is flatlined at 445ish mv which is a default value. The long drive data log, both O2s are working. So it may just be taking that sensor longer to come on line. Either way, that sensor is not the cause of your misfire.

The screen shot I posted of your example of the stumble looks like a classic misfire. Most likely cause is a cracked plug or a plug or wires not fully clipped on either the spark plug or the coil. That's the cause of 99% of all LS misfires.

[ScottEwine]

This is the screen shot at 11:33 real time, 6:00 minutes into the log.

First thing is, you're logging way too many pids. It slows the scanner way down on the GEN3s and changes the way the line graph looks. I'd get rid of 75-80% of your pids. You don't need any trans pids right now. You don't need every available timing advance pid.

The screen shot you posted with the O2 "not ready" looks in the little bit of line graph included, that O2 sensor is flatlined at 445ish mv which is a default value. The long drive data log, both O2s are working. So it may just be taking that sensor longer to come on line. Either way, that sensor is not the cause of your misfire.

The screen shot I posted of your example of the stumble looks like a classic misfire. Most likely cause is a cracked plug or a plug or wires not fully clipped on either the spark plug or the coil. That's the cause of 99% of all LS misfires.

Thanks for the feedback. I replaced the o2 sensor this morning and got the same results, so I agree that is not the issue and that it is reading correctly. I will check the plugs and wires and see if I find anything. Do you have a good list of pids I should scan to help troubleshoot issues like this?

Scott

[Alvin]

I used the setup from Tanks Inc. which seems pretty solid. Using the stock C5 fuel filter/regulator. Where in particular is the issue with the fuel system that you see?

https://www.tanksinc.com/index.cfm/p...rod/prd443.htm

They make a solid product but I've had two with problems. Both where the pump was mounted too close to the floor blocking the bypass causing the pump to literally melt down. Both where hard to diagnose because they would have fuel pressure until the pumps warmed up.

I'd do a fuel pressure gauge ASAP

[ScottEwine]

This is the screen shot at 11:33 real time, 6:00 minutes into the log.

First thing is, you're logging way too many pids. It slows the scanner way down on the GEN3s and changes the way the line graph looks. I'd get rid of 75-80% of your pids. You don't need any trans pids right now. You don't need every available timing advance pid.

The screen shot you posted with the O2 "not ready" looks in the little bit of line graph included, that O2 sensor is flatlined at 445ish mv which is a default value. The long drive data log, both O2s are working. So it may just be taking that sensor longer to come on line. Either way, that sensor is not the cause of your misfire.

The screen shot I posted of your example of the stumble looks like a classic misfire. Most likely cause is a cracked plug or a plug or wires not fully clipped on either the spark plug or the coil. That's the cause of 99% of all LS misfires.

WE HAVE A WINNER! It was a couple bad plug wires on the passenger side. I have replaced them all with a set of straight boot wires that seem to fit better with the early C5 corvette stainless manifolds. Thanks for the advice!

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IMG_9294_sm.jpg