LT4 High RPM Miss - Help!
04-06-2006, 04:17 PM
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LT4 High RPM Miss - Help! I have a '96 Vette with an LT4. It's got 78,000 miles on it. I've had the car about 3 months and I am really struggling to find out why it misses at high RPM. It just noses over a few times on its way to the rev limiter anywhere between 5,200 and 6,400 RPM. This is very annoying since it’s where the LT4 really shines.
I’ve replaced the following with new GM parts
Opti-Spark
Plugs
Wires
Coil
Fuel Pump
Fuel Filter
When I first got the car the water pump had a small leak So, I replaced the pump and did the Opti Spark, plugs and wires at the same time. After that it ran a little better but still had something wrong. It turned out it was lean. I saw this on the Dyno and replaced the fuel filter and then the fuel pump. That made the car much better. Now, it’s perfect up to almost 5,500 RPM and the air/fuel ratio looks great. At about 5,500 it starts to miss and does not gain RPM like it should. It will stumble 2 – 4 times on the way up to the rev limiter. This is really only noticeable at WOT and it seems worse when the engine is hot.
What the heck is it? I have spent $150 bucks at the dealership only to be told I’m driving the car super hard and that I must be hitting the rev limiter – crazy. Then, I spent another $200 bucks in Dyno time with scan tools and everything else hooked up trying to find it. No codes, no obvious failure of ignition, air / fuel looks good on the dyno sniffer but you can sure see the HP and TQ curves break up.
I appreciate everyone’s patience with the long story. This one is starting to drive me a little crazy. At this point I’d rather buy parts than pay for diagnostic time. At least I have new parts and the fun of putting them on when I’m done. Fuel injectors? Intake valve spring?
Thanks,
Ryan
I’ve replaced the following with new GM parts
Opti-Spark
Plugs
Wires
Coil
Fuel Pump
Fuel Filter
When I first got the car the water pump had a small leak So, I replaced the pump and did the Opti Spark, plugs and wires at the same time. After that it ran a little better but still had something wrong. It turned out it was lean. I saw this on the Dyno and replaced the fuel filter and then the fuel pump. That made the car much better. Now, it’s perfect up to almost 5,500 RPM and the air/fuel ratio looks great. At about 5,500 it starts to miss and does not gain RPM like it should. It will stumble 2 – 4 times on the way up to the rev limiter. This is really only noticeable at WOT and it seems worse when the engine is hot.
What the heck is it? I have spent $150 bucks at the dealership only to be told I’m driving the car super hard and that I must be hitting the rev limiter – crazy. Then, I spent another $200 bucks in Dyno time with scan tools and everything else hooked up trying to find it. No codes, no obvious failure of ignition, air / fuel looks good on the dyno sniffer but you can sure see the HP and TQ curves break up.
I appreciate everyone’s patience with the long story. This one is starting to drive me a little crazy. At this point I’d rather buy parts than pay for diagnostic time. At least I have new parts and the fun of putting them on when I’m done. Fuel injectors? Intake valve spring?
Thanks,
Ryan
04-06-2006, 04:26 PM
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Sounds computer related. If you've done all the ignition and fuel stuff. Like it's hearing knock and retarding spark.
When I put roller rockers on my LT1 it did the same thing until I put the LT4 knock module in the pcm. It still does it sometimes if the car is hot and at wide open throttle.
Take out any Hypertech tune if you have one installed.
When I put roller rockers on my LT1 it did the same thing until I put the LT4 knock module in the pcm. It still does it sometimes if the car is hot and at wide open throttle.
Take out any Hypertech tune if you have one installed.