verano29

I'd appreciate if you guys could help me diagnose a strange issue I get at WOT

Let me set the stage:

Car
98 Corvette A4
47k miles

Mods
Engine
Dart Pro 1 205 heads
LS2 Head gaskets (heads have a 4" bore)
228/230 .571/.573 112 Comp Cam
Comp Cams Oem lifters
Comp Cam Hi-Tek push rods
LS2 Timing Chain
LS6 Oil Pump
BBK Shorty Headers
BBK SSI Intake
Ported BBK 80mm Thottle Body
Hi Flow Air bridge/Smooth coupler
Vortex CAI
160 Stat
GNPP Red Spark Plug Wires
NGK Iridium plugs
Stock (original) injectors (I think for 98s they a 28#)

Trans
Yank SS 3600 Stall
3.42 Z06 rearend

Exhaust
Stock w/ z06 Ti

The car was tuned last year with the stock TB and air bridge. Tuner left the AF ratio a little rich (WOT) to account for the unknown of the ram-air effect of the CAI.

Possibly related symptoms:
Car is a hard start. That is, it doesn't pop to life the instant I turn the key. It takes a few tries to start it, unless I just stop it then it restarts instantly. The tuner thought it might be a bad injector(s) (leaking) making it difficult to start the motor.

WOT issue:
This is kind of hard to describe. Under WOT 4500 rpm and up the motor seems to be, well the best way I can describe it, pinging/fluttering/misfiring. That is, there is a very mild (but noticeable) flutter/bucking sensation. In addition, it is accompanied by a noise that I would describe as a mild pinging (very similar in sound (but on a lower scale) to how the motor sounds when it hits the rev limiter (fuel shut off). This does not seem to be happening, or at least its not noticeable, at partial (say 3/4 throttle). In addition, there is only a very minimal difference in acceleration between 3/4 throttle and WOT.

It doesn't seem to matter if the engine is hot or cold. Although its difficult to say as it is very difficult to get the car somewhere where I can safely get on it before the motor (and that darn aluminum bbk) heats up. I've tried running NOS Octane boost (which brings the octane rating to 98) and it appears to have no effect.

At this point I'm kind of leaning towards the injectors. If one (or more) of the injectors is having trouble keeping up at WOT I would tend to believe it would produce the experience I'm describing.

Question
Since I've replaced the stock TB and airbridge with a ported 80mm and high flow air bridge since the tune do you think I will be safe to replace the injectors with 30# injectors? The thought is that with the upgraded intake components I'm probably running a little lean at WOT and the 30# injectors (since the PCM doesn't know about them) would richen up the mixture at WOT.

So, any thoughts?

Thanks for the help.

verano29

405HP Z06 any thoughts on this?

405HP_Z06

iTrader: (11)

Do you have any DTC's? Have you checked your fuel pressure? Do you have a logging tool? How do you have your PCV system configured?

verano29

I do not have any codes. However, occasionally I've seen a "right bank rich" historical code.

I have not checked my fuel pressure.

Here is a stupid question, what is a PVC system?

Here is some new info. Today after work I was able to go WOT when the engine was still quite cool. It didn't seem to be exhibiting any of the symptoms that I've described above.

405HP_Z06

iTrader: (11)

PCV - Positive Crankcase Ventilation

Can you tell that it starts to 'act up' after a certain water temp? When it's cold your in open loop. The PCM is using the OLFA table for fueling, ignoring O2 sensor feedback.

verano29

As far as the PCV is concerned it's pretty much stock. I say "pretty much" because the BBK TB and Intake do not same opening as the stock setup and I'm probably losing a bit of vacuum as the only opening in the TB is pre TB rather than post. With that being said, I've noticed these symptoms when I was running the stock TB as well.

Yes, today when the engine was quite cool it didn't seem to exhibit any of the symptoms.

405HP_Z06

iTrader: (11)

Under normal conditions the PCM will transition to CL mode at 189F. Have you cleaned the MAF sensor? How new are your O2 sensors? You need to scan/log PCM activity to pin point the problem.

verano29

That's interesting. I have a 160 degree thermostat and the fans are set to come on at 170 and 180. I would have to be sitting in traffic to exceed 189. The tuner was aware of this so I assume he has compensated accordingly.

I have no cleaned my MAF. But one thing I did do to the MAF is to remove the screen.

My o2 sensors are the originals. I have approximately 47k miles on the vehicle.

I'm probably going to be purchasing the HP tuners software soon. The car was previously tuned with ls1 edit if that matters.

Without the benefit of hard data, what is your gut feel for this problem?

405HP_Z06

iTrader: (11)

Without data we are flying blind. If you have checked all of the mechanical possibilities and ruled them out then the next step is to log data. I would recommend EFILive.

02BLKWS6MD

iTrader: (22)

This might sound a little stupid but whens the last time itas had a fuel filter? I se your leaning towards the fuel system I dont see that youve replaced that yet. I've had a few cars get goofy with a nastified fuel filter. Since its a 98 I believe they had the 26lb injectors I think 01-02 got the 28lb injectors. But I'm sure those injectors are working to the full extent being that the car has heads and a cam.

verano29

I'll replace the fuel filter...its relatively cheap and to my knowledge it has never been done on the vehicle.

Another thing I'd like to mention is that the car has always been a hard start and it has been ever since I've bought it with 30k miles on the clock. It never starts on the first try (unless I just shut it off). It usually takes several tries to start it. Do you think this is related?

Most ls1 cars virtually start the instant the key is tuned....mine is not even close.

muayyadf1

fuel pump..if it's injectors it would misfire.