89 Nissan Pathfinder Misfire

nissan

#1

1989 Nissan pathfinder. 6cylinder - VG30i - Manual trans - Throttle body injected. No CEL.

I’ve been trying to diagnose a misfire on my 89 Pathfinder during my lunch breaks. I just changed out the the plugs, dist. cap, dist. rotor and wires. Still misfiring.

When it starts-up the misfire “putt” is random, but once the idle lowers to about 1k it gets very rhythmic and just sounds like part of the idle and the car vibrates a little too much. Stinky exhaust (obviously) but no blue smoke or anything.

Finally bringing the car home to do some diagnosis over the weekend. Any ideas would be welcomed. Next I’m going to do some cylinder compression checks and vacuum checks. Probably check fuel pressure too. Is it odd that I’m not getting a cylinder misfire CEL code? The manual lists the code type.

I’m guessing maybe a bad valve… I hope not, I haven’t done a valve job since I was a kid in high school.


#2

It has to be among spark, fuel, timing, or compression. With no codes, just make an educated guess and test for it is probably the best method. Changing out the plugs, wires, cap, and ignition rotor was a very good place start, and will often fix this. hmm … well the compression and vacuum tests you are planning make complete sense for next on the agenda. On an 89 you sort of have to consider things like a bad distributor or bad ignition module too. Might pay you to have it tested on a pro ignition analyzer machine.


#3

Check for a stretched timing chain

Remove the distributor cap. While someone turns the engine over in the normal direction at the crankshaft bolt watch the rotor in the distributor. Now have that person turn the crankshaft bolt in the opposite direction while watching the rotor in the distributor.

If the crankshaft can be rotated more than a few degrees before the rotor in the distributor begins to rotate, the timing chain is stretched.

Tester


#4

Just did a compression test on the cylinders. The shop manual spec is 180lbs. I got close to that on a few and ~150lbs on the others. I’m not a pro but that doesn’t seem like anything out of the ordinary for a 26 year old engine. Most importantly doesn’t seem like it would be the source of a misfire. Could it be a valve is stuck shut thereby getting get good compression but still misfiring ? Fyi, this car has hydraulic lifters (and two OVC) so there is no valve lash.

Also I never pulled the coil to test with the multimeter before but I did a spark test on #1 wire and it’s nice blue spark, that said I’m ruling out spark, and probably compression.

Next up to test: timing, vacuum and fuel.

Timing:

  • This car has a dizzy cap so I’ll borrow a timing light and check that.
  • I’ll test the the belt per ‘Tester’ above. I found a note under the hood that says the belt was changed at 120k an I’m at 180k, so that’s mildly encouraging. The previous owner would 4x4 in this truck so maybe he skipped a tooth or something while offroading.

Fuel:

  • Test fuel pressure. I can see the gas on fins of in the manifold so I’m doubting it’s that, but what do I know. Maybe it’s running rich. I’ll replace the Fuel filter and test it anyway, sure it needs it.
  • I’ll also inspect evap. canister. Maybe the evap can is dumping extra fuel in the intake, Dunno. I’ll get vacuum pump and test that think too. Probably needs a new breather filter anyway.

Vacuum:
Yeah… Old hoses and stuff. Gotta test that one. Mentally I’ve been kinda ruling out some parts like EGR because the miss seems consistent idle through throttle. Although I’ll probably just test everything since I’m dinking around under the hood.

I’ll need to find a place with an pro ignition analyze to GeorgeSanJose’s point above.

It’s nice when a pro mechanic can just tell you what’s wrong and then you can go do the fix but I feel like my very rusty home shop skill are getting quite a polish, especially if I can ultimately fix the dang thing.

I’m really starting to appreciating the OBD II engine information. Mechanics must love that the car can tell them what’s wrong and then just fix it. Wow! I’d like that right now.