Hard bucking under part throttle?
Thread Starter
Heel & Toe
Joined: Nov 2015
Posts: 24
Likes: 0
I've also recently noticed a knocking sound coming from underneath the passenger side, right behind the tire that seems to increase and decrease with idle and it's becoming slightly louder within the last two weeks. I should have gotten under this thing today. 
Race Director
Joined: Mar 2015
Posts: 15,926
Likes: 579
Race Director
Joined: Mar 2015
Posts: 15,926
Likes: 579
this one has me stumped. do you have something else to drive?
Last edited by antfarmer2; Nov 29, 2015 at 07:43 PM.
Le Mans Master
Joined: May 2005
Posts: 7,362
Likes: 2,281
From: Central Maryland
Before you start throwing a lot of cash and parts at this problem, I'd strongly recommend you get it hooked up to a good scan tool that can give you performance data on such items as ECT, TPS, MAF, MAP, O2S, IAT, EGR, LTFT, STFT, and so much more.
You can burn through a lot of cash replacing parts that are not misbehaving. If you don't have the diagnostic skills and the necessary equipment to diagnose the problem yourself, you'll be ahead of the game in the long run if you take it to a talented diagnostician.
Nobody here can do anything more than guess at what's causing this fault based solely on the engine's behavior, in the absence of useful data. Fuel pressure is a good data point, but there is so much more data that the ECM makes available that you aren't seeing yet. You're flying nearly blind, and that's no way to isolate and repair a driveability fault.
Live well,
SJW
You can burn through a lot of cash replacing parts that are not misbehaving. If you don't have the diagnostic skills and the necessary equipment to diagnose the problem yourself, you'll be ahead of the game in the long run if you take it to a talented diagnostician.
Nobody here can do anything more than guess at what's causing this fault based solely on the engine's behavior, in the absence of useful data. Fuel pressure is a good data point, but there is so much more data that the ECM makes available that you aren't seeing yet. You're flying nearly blind, and that's no way to isolate and repair a driveability fault.
Live well,
SJW
Max G’s
Joined: Nov 2008
Posts: 2,751
Likes: 79
From: Monroe OH
NCM Sinkhole Donor
I had a similar symptoms with my LT4. It would idle rough and buck at part to 3/4 throttle. I found the issue when we put the car on the dyno. The issue ended up being the connector at the coolant temp sensor in front of the block just under the air intake piping. Try to wiggle yours and see if it clears up. It is not too difficult to splice a new one in. You can find them at the Corvette specialty stores. Mine has been fine ever since. 
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Those are the exact symptoms of a bad opti. That’s exactly how my car reacted (and others) to the first and second opti I installed. Runs good cold but not hot. Bucks under load and smoothes out with higher RPM is a lower gear. RPM in idle would wander as did the tach.
Sorry you have 10K on the opti but that’s my call.
Sorry you have 10K on the opti but that’s my call.
Last edited by pcolt94; Nov 29, 2015 at 11:05 PM.
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Race Director
Joined: Mar 2015
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Those are the exact symptoms of a bad opti. That’s exactly how my car reacted (and others) to the first and second opti I installed. Runs good cold but not hot. Bucks under load and smoothes out with higher RPM is a lower gear. RPM in idle would wander as did the tach.
Sorry you have 10K on the opti but that’s my call.
Sorry you have 10K on the opti but that’s my call.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Thread Starter
Heel & Toe
Joined: Nov 2015
Posts: 24
Likes: 0
I haven't tried that yet but its extremely rare for it to pop/bog/jump in park no matter how much I rev the engine. I don't even think I could get it to act up in park. It's very rare. 98% of the time it happens going down the highway somewhere between 2000-3000 rpm. It has the slightest rough idle sometimes once warmed up but like I said not very noticeable most of the time at idle and in park.
Last edited by Torched94; Nov 30, 2015 at 10:18 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
*I* don't think that the vacuum signal is making it jump THAT violently...after all car is still in gear, turning a relatively consistent RPM, at a relatively consistent throttle setting. BUT, unhooking it will eliminate that influence and variable.
Drifting
Joined: Aug 2006
Posts: 1,326
Likes: 18
From: Los Angeles California
I had a similar symptoms with my LT4. It would idle rough and buck at part to 3/4 throttle. I found the issue when we put the car on the dyno. The issue ended up being the connector at the coolant temp sensor in front of the block just under the air intake piping. Try to wiggle yours and see if it clears up. It is not too difficult to splice a new one in. You can find them at the Corvette specialty stores. Mine has been fine ever since.
Really in my pseudoscientific mind my first instinct with this thread was Opti, but the erratic fuel pressure thing really throws a twist in it. Could the injectors, even if stuck, ever do that? Seems not.
Thread Starter
Heel & Toe
Joined: Nov 2015
Posts: 24
Likes: 0
I had this exact thing happen to my LT4. One of the first successful diagnoses I made. After triggering it enough times I noticed during the bucking that my digital coolant temp was what was wigging out. Whatever plastic that pigtail connector is made of cannot take the heat from its place right there.
Really in my pseudoscientific mind my first instinct with this thread was Opti, but the erratic fuel pressure thing really throws a twist in it. Could the injectors, even if stuck, ever do that? Seems not.
Really in my pseudoscientific mind my first instinct with this thread was Opti, but the erratic fuel pressure thing really throws a twist in it. Could the injectors, even if stuck, ever do that? Seems not.
Melting Slicks
Joined: Jul 2011
Posts: 2,605
Likes: 202
From: Trinity FL
Is this a 6-speed or an auto? If it is a 6-speed put it in 4th while going 20mph and put the pedal to the floor. If it starts bucking the answer is almost certainly your optispark.
Also, I think a fuel pump and pulsator replacement is good advice especially at 107k miles. It is a wear and tear item and is very easy to replace on these cars. I would recommend the Racetronix kit with the Walbro pump and the submersible fuel line in place of the Pulsator.
Also, I think a fuel pump and pulsator replacement is good advice especially at 107k miles. It is a wear and tear item and is very easy to replace on these cars. I would recommend the Racetronix kit with the Walbro pump and the submersible fuel line in place of the Pulsator.
Last edited by TorchTarga94; Dec 1, 2015 at 09:57 AM.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
I haven't tried that yet but its extremely rare for it to pop/bog/jump in park no matter how much I rev the engine. I don't even think I could get it to act up in park. It's very rare. 98% of the time it happens going down the highway somewhere between 2000-3000 rpm. It has the slightest rough idle sometimes once warmed up but like I said not very noticeable most of the time at idle and in park.
But on this one I will stand on my soap box and give my opinion as opti.
Last edited by pcolt94; Dec 1, 2015 at 10:00 AM.
Melting Slicks
Joined: Jul 2011
Posts: 2,605
Likes: 202
From: Trinity FL
These are more of the same symptoms I had also with my opti problems. I lived with the problem for a while the first time and spent weeks on and off making voltage measurements and checking everything you could name. In the end it was the opti. I'm not saying don’t do your homework and check all you want.
But on this one I will stand on my soap box and give my opinion as opti.
But on this one I will stand on my soap box and give my opinion as opti.
