MARKTHOMASBRAND

Hi,
My car is a 95 Z28 M6 and the engine is stock apart from a K&N panel filter.The car has covered 58,000 miles and runs very well,but when accelerating it will not rev past 4500 rpm.Popping and banging occurs at the 4500 rpm point and I have to shift up,in my opinion,way too early.
This is my first car with the LT1 engine in,and have never driven another LT1 M6 to compare it with,but surely it is capable of higher rpm's.
The car pulls fine right up to this point,so I know it's nothing too serious,it's just a bit frustrating when overtaking,just when the car is really getting into it's stride I have to change up.
When does your rev limit kick in?

Many thanks in advance,all the best,Mark.

Javier97Z28

Sounds like you have an ignition problem to me. Doubt the rev limiter set itself low randomly.

Razor_Blade

You have a problem. Unless someone lowered the rev limiter, there is no way you are hitting it at 4500 rpm's. Try reving it in neutral and see if it goes past 4500 without a load. You can check the fuel presssure, but It could be ignition related. If the fuel pressure checks out OK, I would try new plugs and wires. If that doesn't help, you could be having peoblems with the dreaded Optispark.

MARKTHOMASBRAND

Hi,
Just been outside and revved the engine in neutral,no problems with that,it revved without hesitation well into the red ,close to 6000rpm.I didn't hold it there long,just gave it a quick bootfull,revved clean and strong.
Does this rule out any of the above mentioned possibillities?I don't have a fuel pressure tester so I can't comment on that, other than to say WOT response seems fine under a load right up until the needle reaches 4500 rpm.
Sure hope the opti-spark isn't dying on me,I only fitted a new water-pump 3000 miles ago!
Thanks for the advice so far,any more ideas now it's been established that the car revs freely in neutral?

OBE1 95Z28

Your mostly likely candidates are still fuel and spark. Reving it in neutral doesn't have the same demand on either system as under load. It could be as simple as a clogged fuel filter that just can't keep up with the fuel demands under the loaded conditions. Taping a fuel pressure gauge to your windshield and monitoring the fuel pressure when you hit 4,500 rpm is a good diagnostic approach. A gauge is only about $35 and connects to the Schrader test port on your fuel rail.

MARKTHOMASBRAND

Hi,
A big thankyou to all the guys who came to my aid.
I will definately change the fuel filter and the spark plugs as soon as possible,will also look into purchasing a fuel pressure tester in the near future,hopefully it'll be as cheap to buy in England as it is for you guys in the U.S.
Parts over here for American cars are pretty damn expensive,the water-pump I fitted back in february cost over £200,still if it keeps the old Z28 on the road it'll be worth it.It's a tough job blowing the doors off Euro-Box cars,but somebody has to do it. L.O.L
All the best,Mark.

Razor_Blade

Reving it in neutral ruled out the rev limiter. If it were the limiter, it should hit at the same RPM regardless of the load on the engine. You can now rest assured that you have a spark or fuel delivery issue.

MARKTHOMASBRAND

Hi All,
About a week ago I changed the fuel filter on my car to rule out a partially blocked filter causing it to cough and splutter at 4500 rpm under load.I used the proper AC DELCO part,changed it with no problems,primed the fuel pump 4 or 5 times and the car started straight up as it should.
So I took it out for a test drive and did some WOT runs and the damn thing still would'nt rev past 4500 rpm,the annoying thing was, not only had I not fixed that problem,another one had developed.Ever since I fitted that new filter the car will bog down if I floor it from low revs.It is most noticabled if I sit in the car whilst it's idling and quickly stab the throttle,it just stumbles,but If I press the pedal in a more controlled manner it revs no problem.
Today I managed to fix the 4500rpm problem by installing a new coil wire,the car now accelerates cleanly up to the proper redline,I was amazed at how quick it was,that last 1000rpm really makes a difference whilst overtaking.
So I'm happy I have one problem sorted but dissapointed by the low rpm stumble that's been occuring ever since I changed the filter.
Any ideas on my current problem?
All the best,Mark.

MARKTHOMASBRAND

Anyone have any ideas?