Hey guys,

I've been working on this 2003 AP1 S2K project for a few years now. It was dead in a driveway with a bad head for years. I replaced the head, spark plugs, upstream O2 sensor, timing chain, TCT, fuel pump, clutch, clutch hydraulics, and all fluids. The motor has stock headers, test pipe, and cat-back exhaust. The intake came on the car when I got it. It is just a generic pipe into the stock throttle body with a cone filter that's sucking in the engine bay heat. No current codes or faults found in scans. I also live in a state that does not have emissions tests, so we're not held back with that.

With all that said, here's the issue I'm having now.

The engine performance is very inconsistant. One day, it'll run flawlessly with great throttle response and power throughout the entire rev range. Other times, it's boggy and acts like something is dragging on it, similar to the A/C being on. I will also get a sudden drop in power around 7,500 - 8k RPM. The car will also not respond to even the lightest amount of throttle below 3k RPM when it's acting goofy. Things like this are almost always caused by something simple.

All of my performance experience comes from building roadracing motorcycles. I am still learning the 4-wheeled world. We will either add a piggyback ECU or flash the OEM ECU to simplify the system and remove the environmental-protection bits. I am hoping someone here has some input.

 

The best thing I ever experienced in making the engine perform more consistent throughout the day, and day to day was the installation of a intake snorkel. The car ran strong from morning to evening, before that it would bog down in mid-day heat. I don't know what intake you have but it might be making things worse, particularly if it is an underhood short ram intake. I just used the oem air box with a snorkel. I later ran a much more expensive Mugen intake and it performed similarly to the oem box with snorkel. I had the J's racing snorkel made for the oem hood.

When these engines detect high intake air temps they will scale back ignition timing and it will kill performance, the ecu can be very conservative with ignition timing once intake air temps get high. I am almost certain that is your issue. I'm not a fan of cold air intakes that are susceptible to water damage, but the Mugen style intake or a snorkel on the oem air box is one of the best mods you can do. Even the oem box sucks hot air from the radiator area due to the short snorkel that is on it.

My other favourite mod was a vented hood - J's Racing Type S hood, the engine bay was drastically cooler and the intake air temps were much lower, hence the engine performed very consistently all the time.

 

Thanks! I'll get a different intake on order. The hood will be an eventual update.

 

I took my S2k to the track this weekend and I think I found the source of my current problems........

Vtec would kick in only once in a while and the motor would lose power over 7,000 rpms with a mis-fire here and there. I short shifted the rest of the trackday below 7,500rpms. The car gave no codes or warning lights of any kind. The going theory was that the knock sensors were sensing detonation when I would reach higher RPMs.

The head on this car has been resurfaced twice and we think it's had too much removed. I mixed 1/3 of a tank of 91 octane with 1/3 of a tank of 101 octane pump gas. After a few miles of driving then 4 hard pulls from 1st gear through 3rd gear, the motor acts as it should.

Does anyone else think this is the solution, or is it disguising another problem?

 

I don't know if vtec engagement or rpm limit is based on knock frequency. If the engine detects knock it will adjust timing, same thing with high intake air temps. I think you have some other issues going on. Check your spark plugs, a misfire will affect top end performance, you don't have any check engine lights ?

 

No engine lights or codes at all. Spark plugs were replaced about 10k ago, but they're cheap to replace again.

No lights, no codes, nothing that would indicate a problem other than a drop in performance at high RPMs. There really isn't much more to replace in this thing other than a totally new engine. I've pulled every cover and there are no indications of anything coming apart in the top or bottom end.

 

Would, if you have not checked earths back of head inc all earths

 

If the car was sitting for so long, I would check for rust inside the gas tank.

 

The combustion chambers on these cars are very knock resistant, the head wouldn't be drastically affected by increased compression enough to cause massive detonation. Increased knock can be easily controlled in these cars, assuming your knock sensor is in place and plugged in ?

Were you experiencing issues in long high speed corners, or high-G corners in general ? If so it could be oil or fuel starvation under cornering forces if that is where you were experiencing issues.

I bought a fuel tank form a 2003 ap1 a few years back and I kept it in my outdoor shed for a couple years in storage with fuel in it. i eventually took the fuel pump and filter out and had a peek inside the tank, I was surprised how shiny clean the fuel tank was, it looked brand new inside. Even the old fuel in the tank came out clean, and the fuel filter was clean just a bit darker tinge to it, no deposits anywhere, and no a speck of rust to be found. I was really impressed with the tanks on these cars.

 

The fuel filter is in the gas tank with the fuel pump. Easy (for me to say ) to pull the pump and see what the filter looks like. Lots of rust in the tank can clog the filter.

OE VTEC switches over at 6000 rpm at WOT. Won't change unless you're pushing on the pedal.

-- Chuck