Low RPM+WOT rumble
Thread Starter
Heel & Toe
Joined: Aug 2010
Posts: 15
Likes: 0
From: Munich Bayern
Low RPM+WOT rumble
My corvette ('95 manual) has a strange Problem:
At low rpm and WOT, till about 1600 rpm the engine is knocking and rumbling. The problem gets worse the lower the rpm and is and at 1000rpm it even beginns at about 80% throttle.
Except from this, the car runs fine, it just needs a bit more fuel than before. High octan fuel decreases the problem.
Optispark, O2 sensors, fuel injectors, spark plugs and cables are new. MAF has been cleaned. The IAC valve seems to be lazy, but I don't think this will be the problem.
Before this started, I've alread had and still have a mysterious #24 VSS error.
Any ideas?
Max
At low rpm and WOT, till about 1600 rpm the engine is knocking and rumbling. The problem gets worse the lower the rpm and is and at 1000rpm it even beginns at about 80% throttle.
Except from this, the car runs fine, it just needs a bit more fuel than before. High octan fuel decreases the problem.
Optispark, O2 sensors, fuel injectors, spark plugs and cables are new. MAF has been cleaned. The IAC valve seems to be lazy, but I don't think this will be the problem.
Before this started, I've alread had and still have a mysterious #24 VSS error.
Any ideas?
Max
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
You need to change your driving habits. I assume your car is a manual. Unless you're in first gear, wot or 80% throttle at 1600 rpm or lower in any other gear will cause knocking and pinging in most cars.
Thread Starter
Heel & Toe
Joined: Aug 2010
Posts: 15
Likes: 0
From: Munich Bayern
I don't know if my knocking sensors are working correctly but the engine shouldn't shake so bad even if they don't. High rpm are also no problem.
Before I had this problem the car accelerated well at this load conditions, but now it feels like trying to accelerate in 6 gear with 500RPM, not like 1500...
At 70% throttle now the car is faster than at 100%...
Before I had this problem the car accelerated well at this load conditions, but now it feels like trying to accelerate in 6 gear with 500RPM, not like 1500...
At 70% throttle now the car is faster than at 100%...
Drifting
Joined: Sep 2014
Posts: 1,579
Likes: 59
From: Trieste Italy
My corvette ('95 manual) has a strange Problem:
At low rpm and WOT, till about 1600 rpm the engine is knocking and rumbling. The problem gets worse the lower the rpm and is and at 1000rpm it even beginns at about 80% throttle.
Except from this, the car runs fine, it just needs a bit more fuel than before. High octan fuel decreases the problem.
Optispark, O2 sensors, fuel injectors, spark plugs and cables are new. MAF has been cleaned. The IAC valve seems to be lazy, but I don't think this will be the problem.
Before this started, I've alread had and still have a mysterious #24 VSS error.
Any ideas?
Max
At low rpm and WOT, till about 1600 rpm the engine is knocking and rumbling. The problem gets worse the lower the rpm and is and at 1000rpm it even beginns at about 80% throttle.
Except from this, the car runs fine, it just needs a bit more fuel than before. High octan fuel decreases the problem.
Optispark, O2 sensors, fuel injectors, spark plugs and cables are new. MAF has been cleaned. The IAC valve seems to be lazy, but I don't think this will be the problem.
Before this started, I've alread had and still have a mysterious #24 VSS error.
Any ideas?
Max
Though you mean your engine knocking and rumbling or is it just poor on low RPM?
Last edited by Christi@n; Apr 6, 2016 at 05:50 PM.
Thread Starter
Heel & Toe
Joined: Aug 2010
Posts: 15
Likes: 0
From: Munich Bayern
Just low rpm or idle is ok, but together with a high gear and full throttle, the engine starts to shake and rumble.
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Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
I think the OP needs to post a vid of the symptom so that we can understand it better. Otherwise, we're just WAG'ing. Could be EGR, could be vacuum leak, could be injectors, plug wires, etc. etc. We need to understand the actual sound/symptom better.
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
No way. Not in any electronically controlled engine I've seen. That is the point of having electronic controls, and feed back like KS. Knocking at 80% throttle at 1600 RPM is unacceptable. Here is my LT1 at 100% throttle starting at 1000 RPM. No alarming or unpleasant noises/sounds or sensations at all....just a smooth pull.
I think the OP needs to post a vid of the symptom so that we can understand it better. Otherwise, we're just WAG'ing. Could be EGR, could be vacuum leak, could be injectors, plug wires, etc. etc. We need to understand the actual sound/symptom better.
I think the OP needs to post a vid of the symptom so that we can understand it better. Otherwise, we're just WAG'ing. Could be EGR, could be vacuum leak, could be injectors, plug wires, etc. etc. We need to understand the actual sound/symptom better.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Most modern 5.7-6L V8's Make ~300 lb-ft @ 1000 RPM. Nothing special about mine...other than I started my pull down low, so you can see that, where most people do not start a pull that low.
GM does their pulls from "down low", too....
Last edited by Tom400CFI; Apr 9, 2016 at 02:44 PM.
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
Considering this thread is going no where why don't we start somewhere.
You state it "rumbles" and such at low RPM AND WOT?!?!
Runs decent every where else. Higher octane fuel decreases the problem. This is correct right?
Sounds like it could be pinging. This can be caused a LOT of things.
Check fuel pressure at idle, WOT (snap test), etc...
Check your Opti-Spark for proper signals-Timing Issues
KS system operation
Overheating
Etc....
I posted a video series on this here.
https://www.corvetteforum.com/forums...eo-series.html
You state it "rumbles" and such at low RPM AND WOT?!?!
Runs decent every where else. Higher octane fuel decreases the problem. This is correct right?
Sounds like it could be pinging. This can be caused a LOT of things.
Check fuel pressure at idle, WOT (snap test), etc...
Check your Opti-Spark for proper signals-Timing Issues
KS system operation
Overheating
Etc....
I posted a video series on this here.
https://www.corvetteforum.com/forums...eo-series.html
Last edited by 93Rubie; Apr 9, 2016 at 10:14 PM.
Melting Slicks
Joined: Aug 2008
Posts: 2,082
Likes: 50
From: Palmdale CA
How are your temps behaving? Have you had a coolant recovery tank push fluid out? Have you had/have any cooling issues?
The reason I ask this is because I had a similar problem with my 95 making torque in the Low RPM range, it would Hesitate and stumble. This all starting happening after an overheat. Turned out I had leaking head gaskets.
Now I can pull pretty hard in 6th gear at 1200RPM and all of the Stumble went away.
Hopefully you have a simple problem like Fuel pressure etc.
The reason I ask this is because I had a similar problem with my 95 making torque in the Low RPM range, it would Hesitate and stumble. This all starting happening after an overheat. Turned out I had leaking head gaskets.
Now I can pull pretty hard in 6th gear at 1200RPM and all of the Stumble went away.
Hopefully you have a simple problem like Fuel pressure etc.
Thread Starter
Heel & Toe
Joined: Aug 2010
Posts: 15
Likes: 0
From: Munich Bayern
Thanks for your replys.
I've made a video yesterday where you can see this strange behaviour.
Teperature, oil pressure, etc are all ok.
At 00:12 you can see what happens when I floor it, with the shifter starting to shake and a strange engine sound.
Btw, unplugging the MAF doesn't change anything, it even seems to be get worse then.
Max
I've made a video yesterday where you can see this strange behaviour.
Teperature, oil pressure, etc are all ok.
At 00:12 you can see what happens when I floor it, with the shifter starting to shake and a strange engine sound.
Btw, unplugging the MAF doesn't change anything, it even seems to be get worse then.
Max
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
Thanks for your replys.
I've made a video yesterday where you can see this strange behaviour.
Teperature, oil pressure, etc are all ok.
https://www.youtube.com/watch?v=nDIIW7C2peg
At 00:12 you can see what happens when I floor it, with the shifter starting to shake and a strange engine sound.
Btw, unplugging the MAF doesn't change anything, it even seems to be get worse then.
Max
I've made a video yesterday where you can see this strange behaviour.
Teperature, oil pressure, etc are all ok.
https://www.youtube.com/watch?v=nDIIW7C2peg
At 00:12 you can see what happens when I floor it, with the shifter starting to shake and a strange engine sound.
Btw, unplugging the MAF doesn't change anything, it even seems to be get worse then.
Max
That can be caused by a TON of things. Time to start testing.
Thread Starter
Heel & Toe
Joined: Aug 2010
Posts: 15
Likes: 0
From: Munich Bayern
...I've already thought about this, but I can't explain why less throttle or higher rpms should cause a better ignition?
Last edited by n8sh4de; Apr 11, 2016 at 07:01 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Open the throttle to WOT,cylinder pressure goes way up...spark doesn't want to jump that gap...it looks of other, easier gaps to jump. I'd look (at night or in a dark garage) for spark leak from wires, boots, etc. Another place for spark to short to ground is a fouled plug (unlikely on a modern engine but possible) and inside the distributor.
Your misfire looks rhythmic to me, meaning it's one cylinder that has an issue. If you agree that it's rhythmic, that eliminates the coil, ICM, coil wire, and you should focus on plug wires, boots, plugs, and dist cap.
.
Last edited by Tom400CFI; Apr 11, 2016 at 07:35 PM.
Safety Car
Joined: Sep 2001
Posts: 4,160
Likes: 554
From: Glenbeulah Wi
I feel that it's most likely an ignition issue. The higher the cylinder pressure, the harder it is for spark to jump a gap across a plug. So, when you've got your throttle more closed, you have lower cylinder pressure and the spark easily jumps the gap, ignites the mixture.
Open the throttle to WOT,cylinder pressure goes way up...spark doesn't want to jump that gap...it looks of other, easier gaps to jump. I'd look (at night or in a dark garage) for spark leak from wires, boots, etc. Another place for spark to short to ground is a fouled plug (unlikely on a modern engine but possible) and inside the distributor.
Your misfire looks rhythmic to me, meaning it's one cylinder that has an issue. If you agree that it's rhythmic, that eliminates the coil, ICM, coil wire, and you should focus on plug wires, boots, plugs, and dist cap.
.
Last edited by bjankuski; Apr 12, 2016 at 10:12 AM.