Without recounting the whole history of hunting down intermittent pinging/knocking all done by Vette tech at dealer who knows his stuff it has new fuel pump and filter, new o2 sensors now (I requested these more as a matter of age) and dealer cleaned injectors, throttle body. TPS, MAP and all systems check out. I have sat their with him on tech 2 scanner and watched it all.

All of this has resulted in a much cleaner idle and in general all around better performance.

It will now only ping and for extended time in what I will call bog type scenario. Slow cruise, light throttle acceleration before transmission kicks down. Give it a bit more gas and it will stop or stop when transmission kicks and RPM’s come up.

Before throwing any more parts at it we have to dyno it to know what was changed in stock program. Previous owner did the mods 7 years ago (1.6 rockers, corsa, shorty headers, bored throttle body) and all I know is he said "it was richened up" but who knows.

I think I have to put it on a dyno at a shop now. I have pulled datatmaster files but I am not sharp enough to spot a problem, though I have never pulled one while it knocked, now that I know exactly how to duplicate the knocking scenario, I bet I could record it.

Things in my mind still to consider.

1) Fuel Injectors, they are original but clean and car has had an easy 42,000 mile life.
2) Could throttle body play a roll?
3) Knock sensors are working, I can see them do their thing but can they be sluggish?

 

Did the dealer set the base timing correctly? Double check that. Maybe the distributor slipped, he was looking at the wrong mark, his light was off. If the initial timing is off significantly, no amount of knock retard will get rid of the ping.

 

Timing isn't adjustable in a 95 vette, unless the PCM program was altered. Assuming he is running premium gas and considering everything else he has done I would check the PCM for stock timing.

 

Premium gas for sure. Everything we have done so far I wanted to do anyways due to age. That said, I now have a new PCM on the way with good tune programmed correctly.

I know the previous owner said 8 years ago when he had the mods done, something was done to computer but who knows what or how at this point. With this new PCM now I can at least feel confident I am not fighting that before I carry on my quest.

What is stock timing by the way, the datamaster file I pulled at idle seems to show spark advance between 25-27 but usually 26.

 

You say it has 1.6 rockers, are they rollers? If so, was the LT4 knock module installed in the PCM? Your knock sensors could be seeing this and pulling timing if no upgraded module.

You should not hear or feel a knock or ping, if they system is working correctly, maybe only a lack of performance or knock counts on your scan tool.

That ECM can be reflashed, a Tech 2 can do this. Going back to stock flash might be a good diagnostic tool.

 

I think it has 1.6 rollers from lookng in oil cap hole? Do stock lifters have what appears to be needle bearings and a c-clip looking thing on the side. That is what I see when I look in there. I can't see that well. All mods were done by previous owner so I am not sure about LT4 knock module. I do remember him saying he wanted to make it "like an LT4"?

Madtuner.com will be sending me a PCM that is correct so I can now at least know that is not the issue and continue from there if necessary.

I have read that the low light throttle pinging I get can be optispark but what can't be blamed on that, and before I go down that path I wanted to handle what I would have done anyways on a car that spent most of the last 10 years sitting in a garage.

Either I will solve it or just press the go pedal a tiny bit harder which solves it.

 

It could be too much light throttle timing and too little non PE knock retard. I do not know how long you have had this car but if this issue has always been there I would look at the tune.

 

Yes I have only had the car since this summr. The previous owner had it for 7 or 8 years and only put 3000 miles on it in that time so I am not sure he would have noticed. He had the mods which included doing somthing to the computer done in 2004 or so. Hopefully my new PCM will solve it, or at least remove one unknown variable.

And yes last night I was thinking more, it is all light throttle. Like when you are driving and transmission is fully upshifted so say cruising at 20-45MPH and 1500RPM or so. Applying very light throttle just to gain a few MPH, not standing hard in it here comes the ping. A tiny bit more gas will make it go away or transmission downshifting or a combo of both. Heavier throttle application, or really romping on it the car runs like a champion.

Interestingly now that I think about it, it did not do it at all with the cruise control on. At least I have it fairly isolated at this point.

Does EGR come into play more at the type throttle application, I know my EGR is moving but I wonder, I have not done that yet.

 

The new computer may solve the problem but yours may have been tuned for the application and just needed a timing tweek at light throttle. The new computer is it tuned for your modifications? I would recommend a retune of your timing curve at light throttle on your existing computer.

 

I was reading thru some threads and came upon this one and though of your problem as I got to the last post #17. It might have some connection to your present condition. One line about engine knock.

Who knows

http://forums.corvetteforum.com/c4-t...code-28-a.html

 

Thanks for the help and yes the new computer will be tuned for the mods by madtuner.com. who has a great reputation on LT1's and is local to me. The old one was done 10 years ago by someone who is no longer around so I have no idea what was done to it or not. I suspect it was just a hypertech power programmer though.

I also have a new EGR on the way, at this point another $50 wasn't going to kill me.

 

I would not rule out the KS, You can check sensitivity by VOM set to MV red on terminal black to KS body (out of engine) tap on end of KS and check readings. The problem is having another KS to compare to

 

Yes, once the new PCm, is in and EGR there will be only a few things left. The knock sensors are working, but have not been measured for detailed specs. IE, I have watched them do their thing on a scanner when you tap the block.

 

The new EGR is in, good news significantly improved, bad news not totally fixed but the range where it does it is getting very small and at very specific throttle application. Say 35-48 ish MPH, RPM’s hover around 1500 once torque convertor locks up. Real light throttle and slow climb it’s fine. Just a teeny bit more throttle as you climb through 40 up to 50 ish slowly it will do it, give it just a bit more, transmission kicks and it is fine. But will not even do it every time, but I would say I can duplicate 50% of the time in this scenario.

I used to get a quick few pings on heavy throttle downshift, those are now gone. Once you are over 2000RPM say at around 50MPH or so it seems to have no issues, even with very light throttle and trans kicks down faster at that speed as well.

The EGR I took off was not the original AC Delco and was very cruddy on the inside. Additionally the gasket that was on it blocked off two of the three holes. There is one big hole in the center and two small ones on each side of the big hole in the center where it mates to intake manifold and the two little holes were blocked. The replacement gasket that came with the AC Delco EGR and one extra gasket I ordered (did not know EGR came with gasket) both seal off the two little holes. Hmmmmm?

So we are getting closer, hopefully when my new PCM shows up with a proper tune I will be there finally.

Secondly, what causes or controls the transmission to kick down in the scenario, it seems like mine should easier. I am wondering since I know this PCM also had the shifting firmed up if that affects the kick down, like does it need more throttle to kick than stock would? My new PCM coming is going to be set to be 50% firmer than stock but not the full 100% a lot of people select. I just had the pan off the trans and it is spotless with brand new fluid now.

 

I realized that since the new fuel pump, injector and plenum cleaning, new o2's and EGR I had not rest my PCM. So for giggles I unplugged negative battery cable for a bit to see what would happen and by god, 97% almost there. It did make a big difference.

I only got it to it twice despite trying 35 times and duplicating the scenario that pre computer reset, I could get it to do it almost every time. The two times were at top of hill (such that it is in downtown Chicago, think short bridge overpass) after bogging it the whole way up the hill. I am sure I was driving everyone crazy on the road slowing down to 30MPH and then bogging my way up to 50 over and over.

IE, transmission is fully downshifted and locked at about 35MPH. Give it slight gas such that it won’t down shift but gain speed slowly by top of hill at about 50 get it a bit but then it kicks to next gear and done. Pre reset of PCM it would do this on flat surfaces as well, I could not get it to do that once.

But there is no doubt it is better and I have to wonder if now I am really creating a situation I won’t encounter that often. I still think the car should be able to compensate but it is difficult now to create at least.

Also interesting that if transmission is not downshifted so say I start out on a parking ramp and feather throttle ever so lightly up the hill nothing. Its only in that cruising low RPM scenario. Whast does the PCM see different at slow speed 1500-2000 RPM light throttle than it does at 40MPH same range I wonder.

Basically now its only happening on the ragged edge of where the transmission should have shifted anyway if I give it just a bit more juice at low RPM thorttle at speed (35-50MPH) bogging it intentionally scenario. Maybe the shift-points have been moved by the last guy who did something in the PCM. Perhaps the PCM on the way will have the answer.

 

For the love of God and country would you just step on the pedal that makes the loud noise and drive it like a Corvette. Around town just leave it in Drive rather than OD. Who ever did the chip previously optimised it to be a Corvette and have sharp acceleration not see how slow it could climb a hill in OD why would you even want to try. The car has no problem at all you are asking it to be a KIA and just like a KIA can never be a Corvette a Corvette does not want to be a KIA it is complaing to you and the message is if you wanted a plow horse go buy one but i want to run.. I have a 4 door Pontiac v-6 sedan it gets 30 something mpg on the interstate and 20 something in town I have a 91 coupe it gets undetermined mpg and just say it gets thirsty often just like the owner, two different horses for 2 different courses.
Dave

 

Very funny. I understand what you are saying. Before I started the work it would do it in various normal driving situations too often to be ignored. ALmost there now and once fixed I will hold the throttle to the floor 24/7.

 

Maybe just reprogram the trans to shift a bit sooner might be the answer or to leave the converter unlocked below a certain rpm might help. For fun try leaving it in D rather than OD in town or wherever this happens and see how you like it. It might give some insight to shift points.
Dave

 

Both very good ideas. My new PCM is on the way and will include a sheet so I know where my shift points are. If I don't like it I can have madtuner.com play with it. I left the shift points up to him and did not specify.

 

What i am suggesting is OD is not your friend if the engine is not ready and willing to use it. Better to change that than make the car sluggish..
Dave