Unsolvable random misfire code.
 

2004 MSM that I had for sale in another thread until a missfire code could not be solved. The car had been modified with a FM downpipe, wideband, begi intake and FMIC and passed emissions testing fine. Added a megaquirt, injectors and boost control for lapping days which ran well. Reverted back to the identical modified configuration after two years and got the missfire code. Tired to fix it with the simple stuff and gave up, turning it over to the professionals and now we are at the place of scrapping it or going wire by wire in the whole harness to try and find the issue.

Below is what has been done by a pro-race shop and if any of the members that work there chime in you will know they are not hacks (like me). My question for the community is this: can you think of anything that was missed short of a wire by wire search that should be done before I do something rash with the car?

"We've put it all back together and at this point I don't know if there is anything else that we can look at.

The list of things we have checked/replaced to verify proper function:
-Boost/vacuum leak tested for and verified with a smoke test.
-Leakdown and compression were fine.
-MAP sensors
-MAF sensor
-Boost solenoid
-Plugs/Coils
-Cam and Cranks sensors
-ECU swap
-Mechanical and Electric timing
-Fuel Pressure Verified
-Injectors cleaned and flow tested
-Catalytic converter inspected for clogs
-Primary O2 sensor replaced which did fix the fuel trims because the old sensor was not responsive
-Throttle position sensor and Idle Air Control Valve
-Bypassed the jumper harness to the ECU and plugged it straight in.

All the parts mentioned above have been verified to be working. The car will drive 5 miles just fine, and then randomly it will come back. It could be that there is a wiring harness issue somewhere in the dash or in the engine bay. We've spent roughly 35 hours of diagnostic time on this but everything seems to check out. At this point I can get the car back to you if you'd like or we can start pulling the wiring harness apart and inspecting it for damage."

Thank you.

Harmonic damper? Wonder if the crank timing wheel gets squiffy.

Not sure what squiffy means, but we did check the wheel. No obvious signs of damage, but it was never replaced.

do you actually feel the missfire? What is the exact code you are getting?

P0300? I scratched my head a long time battling a random misfire on my 99.

Things I tried:
New secondhand coilpack
New NGK wires
New NGK plugs
New Crank Position Sensor
New Cam Position Sensor

What ended up being the ticket was the CPS.

From what I read, these seem to go and cause some gremlins but not necessarily flat out stop working.

On this note, if you're having a hard time actually feeling the misfire and your CPS hasn't been flat out replaced, I'd replace it. It's cheap, like 20$ cheap and for the headache I went through I'd say try it.

Thanks for that feedback. Is that CPS you are referring to the Crank of Cam?

Cam position, sorry for the lack of clarity!

On the msm if the boost solenoid can cause all kinds of problems. I periodically had to take mine out and spray it with wd40 because it would randomly get stuck.I would get some random issues from that.

We replaced both cam and crank sensors, no effect. We fixed a lot of little things along the way, at this point it wouldn't be a horrible idea to try the cam sensor again as easy as they are to replace.

We did clean, eliminate, and replace the boost valve.

And yes it's a P0300, after about 5-10 miles/minutes of driving. We've gone as long as ~20 minutes without the code. Never feel anything. Little boggy at lower RPMs but pulls great above ~4000. Typical MSM from what I've read.

Hmmm. I never distinctly felt a misfire either, but I figured out a rhythm for when the code would come up. I would drive on the highway no problem and get off an exit, drive at 25-35 city speeds and it would trip after a couple minutes.

It seems like this P0300 is coming up almost entirely randomly?

If you aren't able to feel the misfire at all, I'd lean towards some flukey wiring or sensor failure and that the car isn't actually misfiring. I am by no means versed with MSM's in particular though.

I would start scoping things until you find the problem. Sensors and outputs until you find what is wrong.

Squiffy - slightly drunk.

P0300 is basically the same as sync loss on megasquirt. The way misfires are detected - very simplified - the last 4 crank sensor passes happened 2.2ms, 2.1ms, 2.0ms, and 1.9ms apart (RPM is increasing). We can expect another pass in 1.8ms - when that doesn't happen a misfire is assumed. If the occurrence happens often enough to pass a threshold for a certain cylinder, a P0301-4 code is thrown for the cylinder. That usually means some badness there - injector or valves. You've already done most of the things I would have tried and can't feel/hear a misfire, but my next thing would be to pull the pulley, inspect the keyway, and take the timing wheel off to see if the mounting holes oblonged at all. In a similar line of inquiry - check that the timing belt idler pulley is tight - slack in the timing belt could throw off the intake cam signal.

Could the timing wheel be on backwards?

I like that as a possibility.


Maybe the distance or positioning of the Crank Position Sensor? Odds are the car wouldn't even start then, but potentially worth double checking.

Been done.

I don't think it would run if this was the case, and see above.

If the 2004 MSM has the connectors hanging on the front of the engine by the throttle body, open the connectors and inspect for corrosion.

They are super clean and I think all NBs have those same connectors?

Yeah, the car overall feels great...as great as it can feel with the stock, laggy-ass, ECU.

Clean all the grounds, wire wheel and scrubby pad, a loose/finnicky ground could make this happen. Also, battery connectors. I would get sync losses when my battery terminals were loose/corroded. Also try a different MSM ecu.

Yeah, they did the whole ECU/immobalizer/lock swap and the same issue.

Really, I think it is some finicky-ass wire some place and more than not likely related to the timing wheel.

I wonder how hard it would be to trace and replace the entire line for the cam and crank angle sensor.