Torched94

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.

antfarmer2

look and see if that poping blew off some vacumme lines.

antfarmer2

this one has me stumped. do you have something else to drive?

Torched94

Boy do I!
2013 GTI stage 1 ftw

antfarmer2

SJW

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

l98tpi

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.

pcolt94

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.

antfarmer2

I sure hope not did yours set a code when this happend?

Torched94

im not too sure, I'll replace the O2 sensors and then if that doesn't cure it I'll find a reputable mechanic and pray it isn't the Opti.

pcolt94

I never did have a code probably because the low resolution pulse was OK. I assume it was a cap problem which is just a feeling or guess.

ex-x-fire

Unhook the vacuum line to the pressure regulator & rev the engine, does it still jump?

Torched94

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.

Tom400CFI

Then run that test going down the highway at 2000-3000 RPM.

*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.

LouisvilleLT4

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.

Torched94

Ohms on all eight injectors read between 12.5-12.8 hot and cold.

antfarmer2

I believe they should be between 12 and 16 and all close. talk to Jon at FIC for good deals on bosch 3s

still waiting on 02s?

TorchTarga94

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.

pcolt94

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.

TorchTarga94