Upper RPM miss?
Thread Starter
Team Owner
Joined: Oct 2007
Posts: 26,745
Likes: 694
From: PA
Upper RPM miss?
This is my fathers 1993 Corvette.
My dads car still has a miss in the upper RPMS. It is under WOT and at about 5300 rpm. It was worse but the coil was having some issues.
It is better than it was because we did a few things to it. The car has the LT4 hot cam, cut lid with K&N filter, MSD Opti-spark, MSD wires, MSD coil, NGK plugs, and a chip that is burnt for the modifications and we also had the shift point change to 5800 rpms.
Like I said, it seems to do it right before it wants to shift and then it acts like it catches and shifts perfect. The car runs great, has lots of power, but we cant get that hesitation/skip out of the car at around 5300 rpms. I was told that perhaps it is not getting enough fuel and that an injector swap should fix it. Not sure if it will or not but my dad and I have some 28 lb injectors to swap in.
Is this a worth while swap or is there something else a miss with the car? If you dont drive it like you are on a drag track the car is 100% perfect.
Any input would be greatly appreciated. Thanks guys.
My dads car still has a miss in the upper RPMS. It is under WOT and at about 5300 rpm. It was worse but the coil was having some issues.
It is better than it was because we did a few things to it. The car has the LT4 hot cam, cut lid with K&N filter, MSD Opti-spark, MSD wires, MSD coil, NGK plugs, and a chip that is burnt for the modifications and we also had the shift point change to 5800 rpms.
Like I said, it seems to do it right before it wants to shift and then it acts like it catches and shifts perfect. The car runs great, has lots of power, but we cant get that hesitation/skip out of the car at around 5300 rpms. I was told that perhaps it is not getting enough fuel and that an injector swap should fix it. Not sure if it will or not but my dad and I have some 28 lb injectors to swap in.
Is this a worth while swap or is there something else a miss with the car? If you dont drive it like you are on a drag track the car is 100% perfect.
Any input would be greatly appreciated. Thanks guys.
Former Vendor
Joined: Nov 2006
Posts: 2,482
Likes: 3
From: Carmichael ca
Swapping engines is not the way to fix this. You need to diagnose the problem first. I'd start by checking fuel pressure. Have you changed the fuel filter? Did you replace the valve springs when you changed the cam?
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
Could be fuel related. Hook up a fuel pressure gauge and make sure you are getting enough psi at that rpm. Could be valve float. Were the springs changed too? If you change to larger injectors, you will need a new tune.
Thread Starter
Team Owner
Joined: Oct 2007
Posts: 26,745
Likes: 694
From: PA
Like Paul asked, I am not sure about the springs. I believe its the .525 cam with 1.6 rockers but this having only the 1.5s it isnt as much. That being said would have me tending to believe that the springs were NOT changed. The cam was changed before we picked up the car so I have no idea.
We even had the tranny inspected. The shop stated that it was 1 step from being made in to a fully manual shift automatic so we had it backed off and put to just a little better than stock.
As for the reburning of the chip; when we had it burnt the guy put in there for more fuel so my thought is that the computer is saying it should have more fuel and because its not it wont shift correctly. Another thing that brings me to think it is fuel related is that anything less than WOT at that rpm shifts perfect. No hesitation, no rough spots, no slip anywhere.
The car is a nice car with just 50k miles on it so its worth investigating the issue further and making it correct.
Thanks guys.
Thread Starter
Team Owner
Joined: Oct 2007
Posts: 26,745
Likes: 694
From: PA
I cant recall what the fuel pressure was but we put an adjustable fuel pressure regulator on and I think at WOT it was (and this is from memory!) about 54psi? Didnt check it at that RPM though. Might have to do that. We have a few other MSD goodies to put on it and what not but I dont want to go any further till this issue is figued out.
