Misfire fixed by increasing fuel pressure?

[prelude_alex]

Hi Everyone!

I have a 1996 Prelude Vtec that developed a misfire a few months ago. It did produce a code for cylinder 1 misfire. I did everything in the book to correct the problem. Nothing helped and I took it to a local race shop, they went through everything, determined it was running too lean but couldn't find the cause. They ended up using an adjustable fuel pressure regulator to increase fuel pressure. The car runs good but I'm still curious about the root cause of this problem. The fuel pressure was in spec before (30-39 psi). The shop put it on a dyno and tuned in to a higher fuel pressure, in the 40's i think, but this is way above the spec specified in the manual.

Here is the list of everything that has been done:

- new spark plugs/wire/rotor cap/ignitor
- new fuel injectors/filter
- egr valve & ports cleaned
- cleaned manifold & throttle body
- check for vacuum leaks
- adjusted valve lash
- check all sensors at the ECU (voltage drop tests)
- check timing belt
- fuel pressure was normal before the fix
- check injector and spark pulse with oscilloscope

[spiffyguido]

Wow....lots of good checks done already. I'm a bit stumped.

You've addressed fuel and spark. Have you checked the air supply of that cylinder and done a compression test?

[prelude_alex]

Yes the compression was checked, checked for vacuum leaks, the guys at the shop said it has a lack of fuel rather than too much air. Strange problem this is, but the engine does have 250k miles on it.

[spiffyguido]

I assume the fuel supply rail is undamaged.

Did you check the wiring harness for the #1 fuel injector with a multimeter?

[prelude_alex]

the tech at the shop checked ignition and injector pulse with an oscilloscope and there were talking to Honda tech line about the data, everything came back ok.

[spiffyguido]

If the fuel rail pressure checks out (and you've stated it does) and the injector wiring is good, and you have good air supply, and the spark plug and wires are good, you really only have two potential problems.

-A mechanical problem, for example the compression test shows a leakage in that cylinder. This was done though, and it's thought to be a fuel problem.
-A fuel supply problem caused by the injector itself.

If the cylinder is really getting starved for fuel, the injector might be to blame. I know these have been replace already, but assuming the wiring is good (as you say it is) and the connection point to the injector itself is good, the injector should open enough to let in sufficient fuel when asked. Fuel injectors can be tested on a test stand, or you swap a known good one to that cylinder and see if the misfiring problem migrates to a different cylinder.