Could valve float happen at 5200 on a stock LS1?
Thread Starter
On The Tree
Joined: Dec 2008
Posts: 187
Likes: 0
From: San Anselmo, CA
Could valve float happen at 5200 on a stock LS1? In the "external engine" forum, I've got a long thread going on a big power dip issue that I'm seeing on a very stock LS1 motor that's been swapped in to a Miata race car chassis.
The problem has all the characteristics of valve float:
- It's at a very specific RPM (power dip is from 5200-5800, then power starts to come back)
- It happens on a part-throttle dyno pull at the same RPM (so this seems RPM, not load, related)
- After a dyno pull up in to this range, the car stumbles and coughs and misfires for a few seconds. (I read that the lifters can get pumped up on valve float and cause this).
The problem I'm having is that this is happening at such relatively LOW rpms.
I've chased this across an old tired LS1 I bought off craigslist and now a new GM crate LS1 - I'm seeing the exact same behavior.
As I said, this motor is in a miata chassis, so all the wiring is non-stock, but the motor itself is. Other than shorty headers, a pretty open exhaust and a large K&N cone filter intake, there are no other mods. The internals are 100% stock.
Can I rule valve-float out?
Is there any definitive test for valve float?
Cheers,
Dean
The problem has all the characteristics of valve float:
- It's at a very specific RPM (power dip is from 5200-5800, then power starts to come back)
- It happens on a part-throttle dyno pull at the same RPM (so this seems RPM, not load, related)
- After a dyno pull up in to this range, the car stumbles and coughs and misfires for a few seconds. (I read that the lifters can get pumped up on valve float and cause this).
The problem I'm having is that this is happening at such relatively LOW rpms.
I've chased this across an old tired LS1 I bought off craigslist and now a new GM crate LS1 - I'm seeing the exact same behavior.
As I said, this motor is in a miata chassis, so all the wiring is non-stock, but the motor itself is. Other than shorty headers, a pretty open exhaust and a large K&N cone filter intake, there are no other mods. The internals are 100% stock.
Can I rule valve-float out?
Is there any definitive test for valve float?
Cheers,
Dean
The fact that you are seeing it with two different motors (in what i assume is the same car) tells me its not the motors. What components did you carry over from your original engine to the current. Those would be what i would be looking at.
yea if the power goes away and then comes back that is most likely not a valve float issue. The stock cam does peak right around 5200 though but will still hold power. Have you checked your coils?
Thread Starter
On The Tree
Joined: Dec 2008
Posts: 187
Likes: 0
From: San Anselmo, CA
Ok, a bunch of questions and answers:
Most of the sensors, intake, exhaust, harmonic balancer, TB, injectors, coils, etc. all moved over from the other motor.
Here are some of the things we've tried so far:
1 - tried a second MAF
2 - removed the MAF and are now using an SD tune
3 - tried a completely separate PCM with same tune
4 - have tried 3 completely different tunes
5 - removed muffler and did several dyno pulls
6 - unplugged knock sensors
7 - watched timing at the crank - timing didn't change during the breakup (5200-5800)
8 - did a part-throttle dyno pull - engine still broke up at same RPMs
9 - watched and logged voltage. unwavering 13.8 volts
10 - watched voltage at one coil
11 - new plugs and wires
12 - gapped TR55's down to 0.035"
13 - watched fuel pressure at the rail - unwavering 58psi
14 - changed to 36lb injectors and retuned
15 - cleaned up all ground - pulled apart connectors and reground metal that they are connecting to
16 - tried a different set of coil packs - THIS IS INTERESTING - the car was spitting and sputtering and misfiring through the entire RPM range. The power dip was still there among all the other sputtering, but WHY WOULD THAT HAPPEN? These coil packs are known good, because they were on another LS1 that dyno'd perfectly earlier in the week.
17 - 100 octane gas
After each of the changes above we did dyno pulls. Every single pull could be laid on top of every other pull and the dip is almost identical (within a couple HP) over the entire trace.
Most of the sensors, intake, exhaust, harmonic balancer, TB, injectors, coils, etc. all moved over from the other motor.
Here are some of the things we've tried so far:
1 - tried a second MAF
2 - removed the MAF and are now using an SD tune
3 - tried a completely separate PCM with same tune
4 - have tried 3 completely different tunes
5 - removed muffler and did several dyno pulls
6 - unplugged knock sensors
7 - watched timing at the crank - timing didn't change during the breakup (5200-5800)
8 - did a part-throttle dyno pull - engine still broke up at same RPMs
9 - watched and logged voltage. unwavering 13.8 volts
10 - watched voltage at one coil
11 - new plugs and wires
12 - gapped TR55's down to 0.035"
13 - watched fuel pressure at the rail - unwavering 58psi
14 - changed to 36lb injectors and retuned
15 - cleaned up all ground - pulled apart connectors and reground metal that they are connecting to
16 - tried a different set of coil packs - THIS IS INTERESTING - the car was spitting and sputtering and misfiring through the entire RPM range. The power dip was still there among all the other sputtering, but WHY WOULD THAT HAPPEN? These coil packs are known good, because they were on another LS1 that dyno'd perfectly earlier in the week.
17 - 100 octane gas
After each of the changes above we did dyno pulls. Every single pull could be laid on top of every other pull and the dip is almost identical (within a couple HP) over the entire trace.
what year pcm are you using?
What base tune? Fbody? Truck? Vette?
What coils were used as the replacement? Same origional square stock style or a truck style? Are the origional coils the origional 98-02 square coils?
Can you post up a HP tuners log scan of it doing the issue?
What base tune? Fbody? Truck? Vette?
What coils were used as the replacement? Same origional square stock style or a truck style? Are the origional coils the origional 98-02 square coils?
Can you post up a HP tuners log scan of it doing the issue?
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You have done your homework so far. I have a couple of thoughts. I highly doubt it's valve float because the problem goes away at higher RPM's.
Does your scanner have a misfire counter by cylinder? Can you check the cam and crank signals as you move acroos 5200 RPM?
Does your scanner have a misfire counter by cylinder? Can you check the cam and crank signals as you move acroos 5200 RPM?
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It might be worth pulling one spring and testing it. Any loss of valve control can be very hard on the valve springs and they can lose stiffness. Almost sounds like an electrical issue of some type since the coil swap netted different albeit worse results. Any way to monitor the coil trigger circuit?
Thread Starter
On The Tree
Joined: Dec 2008
Posts: 187
Likes: 0
From: San Anselmo, CA
Ok, a couple different questions so far:
1 - Scanner isn't picking up ANY misfire codes at all. We're using HPTuners, currently.
2 - We can't check the cam and crank angle with any sort of precision, but we're lining up and expert with an oscilliscope to come to the dyno with us on our next round of testing.
3 - Sensors - we're monitoring everything we can with HPTuners. O2 sensors are new and are giving absolutely reasonable readings throughout the pulls. Have replaced MAP sensor and crank angle sensor. IAT, Throttle position, etc. all are giving normal readings, but I haven't changed out any other sensors. Is there one in particular I should chase?
4 - TR55 gap. We originally had them gapped at 0.05" and tried 0.035" just to try something different. No change in behavior.
5 - No spray! This is a bone stock motor. It's only making like 285HP at 5200 before it takes a crap.
Regarding the valve float, several different guys have surfaced in the last week who are all having a similar problem after installing underdrive pulleys/harmonic balancers. The theory is that an undamped or underdamped vibration in the crank can interact with the valve train, resulting in a resonant frequency that can cause early valve float. A couple of these guys got breakup around 5000rpm after installing cheapo u/d pulleys, went back to the stock pulleys and were fine.
Thoughts on that?
1 - Scanner isn't picking up ANY misfire codes at all. We're using HPTuners, currently.
2 - We can't check the cam and crank angle with any sort of precision, but we're lining up and expert with an oscilliscope to come to the dyno with us on our next round of testing.
3 - Sensors - we're monitoring everything we can with HPTuners. O2 sensors are new and are giving absolutely reasonable readings throughout the pulls. Have replaced MAP sensor and crank angle sensor. IAT, Throttle position, etc. all are giving normal readings, but I haven't changed out any other sensors. Is there one in particular I should chase?
4 - TR55 gap. We originally had them gapped at 0.05" and tried 0.035" just to try something different. No change in behavior.
5 - No spray! This is a bone stock motor. It's only making like 285HP at 5200 before it takes a crap.
Regarding the valve float, several different guys have surfaced in the last week who are all having a similar problem after installing underdrive pulleys/harmonic balancers. The theory is that an undamped or underdamped vibration in the crank can interact with the valve train, resulting in a resonant frequency that can cause early valve float. A couple of these guys got breakup around 5000rpm after installing cheapo u/d pulleys, went back to the stock pulleys and were fine.
Thoughts on that?
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
With oscilloscope, get these waveforms (separately):
1. CKP and CMP, with #1 secondary (for syncing) (if o-scope has enough channels),
2. secondary kV on all cylinders (may require 2 or 4 passes depending on channels),
3. fuel pump voltage and current (using amp clamp probe),
4. current thru INJ1 and INJ2 fues (this is injector and coil current), and injector voltages.
1. CKP and CMP, with #1 secondary (for syncing) (if o-scope has enough channels),
2. secondary kV on all cylinders (may require 2 or 4 passes depending on channels),
3. fuel pump voltage and current (using amp clamp probe),
4. current thru INJ1 and INJ2 fues (this is injector and coil current), and injector voltages.
Ok, a couple different questions so far:
1 - Scanner isn't picking up ANY misfire codes at all. We're using HPTuners, currently.
2 - We can't check the cam and crank angle with any sort of precision, but we're lining up and expert with an oscilliscope to come to the dyno with us on our next round of testing.
3 - Sensors - we're monitoring everything we can with HPTuners. O2 sensors are new and are giving absolutely reasonable readings throughout the pulls. Have replaced MAP sensor and crank angle sensor. IAT, Throttle position, etc. all are giving normal readings, but I haven't changed out any other sensors. Is there one in particular I should chase?
4 - TR55 gap. We originally had them gapped at 0.05" and tried 0.035" just to try something different. No change in behavior.
5 - No spray! This is a bone stock motor. It's only making like 285HP at 5200 before it takes a crap.
Regarding the valve float, several different guys have surfaced in the last week who are all having a similar problem after installing underdrive pulleys/harmonic balancers. The theory is that an undamped or underdamped vibration in the crank can interact with the valve train, resulting in a resonant frequency that can cause early valve float. A couple of these guys got breakup around 5000rpm after installing cheapo u/d pulleys, went back to the stock pulleys and were fine.
Thoughts on that?
1 - Scanner isn't picking up ANY misfire codes at all. We're using HPTuners, currently.
2 - We can't check the cam and crank angle with any sort of precision, but we're lining up and expert with an oscilliscope to come to the dyno with us on our next round of testing.
3 - Sensors - we're monitoring everything we can with HPTuners. O2 sensors are new and are giving absolutely reasonable readings throughout the pulls. Have replaced MAP sensor and crank angle sensor. IAT, Throttle position, etc. all are giving normal readings, but I haven't changed out any other sensors. Is there one in particular I should chase?
4 - TR55 gap. We originally had them gapped at 0.05" and tried 0.035" just to try something different. No change in behavior.
5 - No spray! This is a bone stock motor. It's only making like 285HP at 5200 before it takes a crap.
Regarding the valve float, several different guys have surfaced in the last week who are all having a similar problem after installing underdrive pulleys/harmonic balancers. The theory is that an undamped or underdamped vibration in the crank can interact with the valve train, resulting in a resonant frequency that can cause early valve float. A couple of these guys got breakup around 5000rpm after installing cheapo u/d pulleys, went back to the stock pulleys and were fine.
Thoughts on that?
I dont think valve float would ever cause a noticable stumbling or backfiring problem. Your dampener theory is interesting. Maybe its causing a resonance that is interfering with the crank sensor or reluctor wheel?? When valve float is experienced its usually just a loss of power.
My bet is on an electrical issue or grounding issue.
My bet is on an electrical issue or grounding issue.
Joined: Aug 2007
Posts: 24,241
Likes: 89
From: Turnin' Wrenches Infractions: 005
All this crap about valve float and you haven't even pulled a valve spring? Swap em out to PAC 1218s. Stock valve springs aren't that great to begin with.
Thread Starter
On The Tree
Joined: Dec 2008
Posts: 187
Likes: 0
From: San Anselmo, CA
The valve float theory is our latest theory since we were last at the dyno just 3 days ago. I'm resisting the urge (as dictated by the thickness of my wallet) to just start replacing parts. I'm with you - I *should* replace them anyway, but I'm trying to come up with low cost experiments that will help us figure out the specific problem. I already have spend a few hundred bucks on bigger injectors since we were SURE that was the problem... 
Point taken, though!
Putting on another harmonic balancer will only cost me $7 (new bolt) since Orion is going to loan me one. I'm going to start there, as I suspect it isn't the SPRINGS causing the issue, but rather another vibration.
Cheers,
Dean
Point taken, though!
Putting on another harmonic balancer will only cost me $7 (new bolt) since Orion is going to loan me one. I'm going to start there, as I suspect it isn't the SPRINGS causing the issue, but rather another vibration.
Cheers,
Dean
