Hey folks - this is a tricky one for me, I appreciate any help you can offer.

The problem:
I have an 2003 Accord EX, V6, 6-speed Manual. When not driving, the car seems to run pretty well, revs smoothly through the range.
BUT
On the road, in any gear, the engine feels a little underpowered up to between 3500-4000 RPM, but at that point it starts to struggle badly and loses power. Feels like multiple misfires, cutting in and out, worse if I apply more gas. The Check Engine light stays off (mostly - see below).

The first time:
The first time it happened, I had just hopped on the highway about a month ago, accelerating up the on-ramp, the problem occurred as described above, but I figured I'd try to push through it and see if it was just a fluke, so I hit the gas, the problem got worse (as above) and the CEL started flashing. I let off the gas immediately and the light went out again. I babied it back home.

I've test driven it around a couple times as I have tried to isolate the problem. I only pushed it really hard one other time, to get the codes to drop with a scanner, and the flashing CEL registered as P0300-P0306 (random misfire and a misfire for all cylinders), with no other codes present.

Background:
Some (probably) important points. I recently swapped the engine and heads. It was supposed to have been a direct replacement, but there was an extra intake port, some differences in the intake, and 2 extra coolant tubes (heater hose size). I blocked off the extra port into the intake by using the old intake (I would have used the other intake, but it didn't have the flapper valve and some other odds and ends were different). I DID keep the 'new' intake.
After the swap, the car never seemed to engage the VTEC properly, but ran fine otherwise. It had a few thousand miles on it before this problem cropped up.

A few things I've checked:
The timing looks to be correct (I did replace belt and WP)
I have a BlueDriver scanner, however, the data is a little sketchy on this car (it does better with newer cars), but I can check codes etc anytime.
I have double checked every wiring connection and hose I can think of to make sure I didn't leave anything unplugged/loose.

Thanks for any help, I'm happy to provide any other info you need.

 

Your scanner should be able to access LT & ST fuel trims. This may provide additional clues.

An intake air leak might produce your symptoms.

good luck

 

What should the fuel trims look like? I do have access to the values.

 

Post the values of the STFT and LTFT when the engine idles and is warmed up. Then post the values when it runs poorly at higher RPM.

Its hard to give exact numbers since it depends on the condition of the engine, running conditions. The values should be

 

High values (> 10% both + and - ) of LTFT indicate problems. Post your numbers.

good luck

 

Will do, I'll take it out tomorrow during daylight and see what they say - then report back.

 

Long term fuel trim was minimal - but the battery died a few days ago and I haven't been driving it.
Bank 1 LT: .8%
Bank 2 LT: 0%


Short term Fuel trim though...
Idle:
Bank 1: 7-14%
Bank 2: 0-1%

2k RPM (Light throttle):
Bank 1: -5%
Bank 2: -10%

3500: Starts out at these values
Bank 1: -20%
Bank 2: - 25%

And THEN - it drops off to 0% on both banks until I reduce throttle.

Any idea what that means?

 

I would also add that if I rev it through the range while parked, it seems to run fine as well. I can check the STFT values in that situation as well.

Another question - is there anything that engages/disengages ONLY during active driving around 3500 RPM?

 

ST should go rich (negative ) on rapid throttle advance. No trouble indicators w/ LT or ST Fuel trim.

You could have a Coil-On-Plug/s (COP) that is breaking down under load causing the missing under load and CEL. Suggest reproducing problem and attempting to capture code from scanner.

good luck

 

Ok - no replies in a while, so I'm bumping this - I have a few other specific questions to help get things rolling:

1. The engine basically loses ALL power around 3500 rpm - it simply won't accellerate beyond this. Are there any likely general causes.

2. The engine that was swapped was slightly different from the original - notably the intake, which was missing the actuator flapper and had an extra port (coolant
I'm fairly sure). There were also 2 other heater hosed sized connections in the valley which I capped off. Is there any chance that these items could be causing problems, and also, what car might this engine have come from? The place I bought it showed it as a direct replacement.

3. Looking at the Short term Fuel trim, it drops to a fairly large negative number at about 3500, then when the severe misfires start it drops the STFT to 0 (according to the scanner). Is there anything that might cause that - or could it point me in a direction?

4. I've been scouring the internet for ideas...TPS has come up, as well as coil pack (though the fault feels like way more than a single cylinder miss on a v6). Any thoughts on how to check out the TPS/MAF?