The car: 1986 Fiero GT 4 speed with 2005 Chevrolet Equinox 3400 V6 (LNJ) swap using 1998 3400 (LA1) controls. Swapped to 24x crank trigger wheel (harmonic balancer) and installed external 7x crank trigger wheel. 7X, 24x, and cam position sensors were all brand new when I did the engine swap in 2011.

Car has been slowing misfiring worse and worse since before winter. Did a clutch and exhaust job over winter with the engine out, finally got it back together but now after about 20-30 miles of test drive it is misfiring bad enough to barely be able to cruise when below 3k rpms. Car starts bucking when given throttle. Anything over about 3k no big deal, car runs great.

Based on past experience the issue is with one of the crank sensors.

codes are:
P0300 (multiple cylinder misfire...no surprise here)
P0336 (crankshaft position sensor A circuit Range/Performance)

So my first thought is, have I messed up the timing by accidentally adjusting the 7X trigger location? Picture below is the best I have right now of the trigger and you can see it has +/- 10-15 degrees on it. Because this is a DIS engine, should I just check the gap and make sure this is set to 0*, call it done, and move on to the 24x sensor?


If the 7x sensor isn't telling the computer that the crank is out of sync with the 24x and cam sensor, and the sensors are all new, I think I'm leading toward a wiring issue with one of these sensors. The 7x wire is just a plug and play twisted wire from the sensor to the coil packs (ICM.) Getting into the 24x and cam sensor wires is going to be a chore so I thought I would ask you all for opinions on what might cause this misfire and crank code.

Thanks

Edit* I adjusted the 7x sensor timing. It was 1 degree off, set it back to zero. Cleared the codes, let it idle for a bit while I put the wheel back on and moved cars around the driveway before going for a short test drive. Car doesn't really drive any differently. This leads me to the 24x sensor?

[This message has been edited by KaijuSenso (edited 04-11-2016).]

If I understood correctly, the problem gradually gets worse the longer you drive it. That is characteristic of an ignition part in poor condition deteriorating as it heats up from use. I'd be suspicious of the ignition module also and in this case it would be first on the list. There should also be a diagnostic procedure for narrowing down the cause. I haven't worked with the timing cover located sensor enough to recall specifics but have had block mounted CPS sensors quit in an instant around the time I was working on the car causing a lot of confusion since the motor was running fine before I started and suddenly not after I finished. The ignition module failure is the closest I've experienced to what you describe where fuel economy starts dropping and performance gradually declining to limp mode character. I'd suspect the misfire is still there at 3k but not as easily noticed since the motor smooths out with higher rpm with that problem, but then again I believe that's the point where it switches from sequential injection to batch injection. Not sure anymore, its been a while since I read up on that stuff.

[This message has been edited by Joseph Upson (edited 04-13-2016).]

I have a 3.4 dohc swap with the misfire below 3k. From what i experienced it was the crack position sensor and maf sensor, not sure if it helps for your engine but my two cents.

Well I broke my collar bone so have not been able to work on the car let alone drive it. There goes my Fiero summer...

so because the car was throwing a crank sensor code, is there a way to tell which of the 2 sensors it is? Like Joseph said, the ecm switches from SFI to batch fire at around 3k rpms (if memory serves me right) and batch fire only needs the main 7x crank trigger?

is there a way to test the 2 crank sensors for defects? again they are both new as of the engine install 5 years and 7k miles ago.

I think it might be better to bring this question up on 60degreev6

Just to close out this problem. It turns out the crank pulley bolt was loose. I was able to spin the external crank trigger wheel a few degrees, that is why the issue got progressively worse and wasn't consistent. Torqued the bolt properly and car drives normal again. I will replace the bolt if the problem comes back. This crank pulley has been off a few times for engine upgrades and fixes.

quote Originally posted by KaijuSenso: Just to close out this problem. It turns out the crank pulley bolt was loose. I was able to spin the external crank trigger wheel a few degrees, that is why the issue got progressively worse and wasn't consistent. Torqued the bolt properly and car drives normal again. I will replace the bolt if the problem comes back. This crank pulley has been off a few times for engine upgrades and fixes.

You are fortunate the crankshaft woodruff key did not shear and ruin your valves. The late model V6 engines have a torque and degree tightening sequence, I'm not sure if it started with the 3500 V6, or the early 2000 3400 V6. It's not easy to do with the engine in the car. Your bolt does not look O.E. with that shine on it, perhaps that has something to do with it backing off.