1999 Pathfinder Multiple Random Misfires

[LegioCorpus]

Okay, need some thoughts on this. My wife bought this 99 V6 Patherfinder used, and it ran smoothly, but with some exhaust, electrical and recall problems, and a massive oil leak. I repaired the electrical and exhaust issues without problems. Vehicle still ran smoothly without codes. Then I addressed the oil leak. It turned out to be an 1/8" crack in the passenger side valve cover gasket. After replacing the gasket, O2 sensor codes appeared along with a bad knock sensor and multiple random misfires. I replaced all four O2 sensors, knock sensor (what a crazy location), driver's side valve cover gasket (another oil leak), air filter (completely caked with oil and dirt) and the intake manifold gaskets. All codes disappeared except the multiple random misfires. The misfires are noticeable when driving and the engine hesitates on initial acceleration but then takes off. Replaced spark plugs, distributor cap and rotor and reset ignition timing. No improvement, noting before replacement it still had two original spark plugs (4 and 6) and original rotor and cap. During test drive I tried slightly advancing the ignition timing. While listening to the exhaust I noticed the engine ran smoothly for about 30 seconds, then had a couple of sputters, then flat multiple non-cylinder specific misfires, only to cycle back to 30 seconds of smooth operation and a repeating of the previous sequence. I have now replaced the fuel filter and pressure tested the fuel lines. The return line operates at 24 psi and the deliver line operates at 32 psi. When I pressed the pressure relief valve on the return line there was no affect on the operation of the engine, but when pressed the pressure relieve valve on the delivery line, I recreated the the flat random misfires consistently. It would appear that there is a constant pressure before the injector rail and after the injector rail, but by recreating the drop in pressure (although not noticeable with pressure gauge reading) I'm able to recreate the misfires. Any ideas? Fuel pressure regulator? Fuel pump? Fuel pressure relay? Vacuum leak?

[4xq]

Welcome to NICO! Odd that the misfires began appearing after you did the valve cover gaskets. I am not sure what your O2 codes were, but lean misfires (vacuum leak or low fuel pressure) can cause codes to be thrown by the O2 sensors. Look at p0300 here: http://engine-codes.com/p0300_nissan.html. So, making sure your PCV hoses are on tight, oil cap / oil dipstick are not allowing air into the crankcase (that is a vacuum leak), and that the intake air tube is back on tight would be good things to check. Also check the air tube for cracks - that happens from time to time.

The service manuals for your truck are here: http://engine-codes.com/p0300_nissan.html. Look at the EC section, on page EC474 specifications for various items are given.

Your fuel pressure seems a bit low ( 34 looks like the minimum with the vacuum hose connected to the regulator) so that is probably worth checking.

Lets use a little trick, and see if we can make your misses at idle magically disappear. Pull the hose off the pressure regulator - you want to see at least 43 with the engine idling (make sure you are warmed up here). Now, with the fuel pressure at about 43 lbs pressure, you have increased your fuel pump pressure by 10 lbs or so. Listen to the engine idle for awhile and see if the extra pressure makes your misses go away. If that works, I'd say replace the fuel pump.

If the fuel pressure increases like it is supposed to when you pull the hose, the regulator is probably ok - just check the vacuum hose for gas when you pull it off

Spike753 did a very good post a while back on replacing the fuel pump - lots of pics. You might search for that if you need it..

Post back and let us know how it plays out!

[LegioCorpus]

Thank you for the welcome.

Been busy with a few other things, but finally disconnected the fuel pressure regulator vacuum hose... no effect. Capped the vacuum connection to make sure the vacuum leak didn't effect the issue, and there was absolutely no change. No gas either. Replaced the fuel pressure regulator... no effect. Advanced and retarded the distributor, which completely replicated the misfires, making them constant random misfires. Disconnected the #2 cylinder and the multiple misfires appeared to disappear, just the constant #2 cylinder misfire. Also took the garden hose to the engine to check for vacuum leaks... no effect. There does not appear to be any vacuum leaks, if so I should have noticed it with the water. Still perplexed. Maybe bad distributor assembly? I was considering bad module or MAF sensor, but one would thing that if these were bad, they would be complete failure.

The O2 sensors were apart of the possible fixes for the random misfires. However, I definitely had two bad O2 sensors. One was running lean and the 2nd was contaminated with oil from the valve cover leak.

It should be noted that the timing fluctuates down to 9 degrees advanced with the both sides of the fuel circuits are closed. We have the random misfires then we jump back up to 15 degrees advanced and smooth running. Noting that the engine lacks power upon acceleration then it takes off. Also noting that the misfires become nearly constant when cruising throughout all gears, but most noticeable in overdrive.

[4xq]

What are fuel pressures now with vacuum hose connected / disconnected?

I see I screwed up my link to the service manual which is here: http://www.nicoclub.com/FSM/Pathfinder/1999_Pathfinder/

You should clean the maf sensor with maf cleaner. I don't really think it is the cause, but it is a good time to do the maintenace stuff.

May as well finish making sure your fuel pressure is ok before moving on to the electrical stuff.

[LegioCorpus]

I'll need to rent the fuel pressure gauge again.
No worries on the service manual, I'll take a look at it.
Already cleaned the MAF before I posted on the 3rd of April. As I thought that might have been the culprit too.

[LegioCorpus]

Okay,
Ran the tests today.
-Ignition on first 5 seconds - 41.5 psi. (Cold)
-After 5 seconds - 34.25 psi. (Cold)
-Started Engine - 34 psi. (Cold)

Started and ran vehicle.
Re-ran procedure.
-Ignition on first 5 seconds - 42 psi. (Warm)
-After 5 Seconds - 34 psi. (warm)

While engine running - Fuel Regulator Vacuum Connected
-Circuit 1/2 Open - 34 psi (Ignition Timing 13 - 15 Adv.)
-Circuit 1/2 Closed - 34 1/16 psi (Ignition Timing 15 - 17 -15 Adv.)
-Circuit 1/2 Open - 33 15/16 psi (Ignition Timing 13 - 15 - 9 Adv.)
-Circuit 1/2 Closed - 34 1/16 psi (Ignition Timing 21 Adv.)
-Circuit 1 Open / Circuit 2 Closed - 33 15/16 psi (15 Adv.)
-Circuit 1/2 Closed - 34 psi (15 - 17 Adv.)
-Circuit 1/2 Open - 33 15/16 psi
-Circuit 1 Open / Circuit 2 Closed - 33 15/16 psi

Fuel Pressure Regulator disconnected 34.5 psi
Fuel Pressure Regulator disconnected/vacuum line on air intake sealed 35 psi
Fuel Pressure Regulator disconnected/vacuum line and vacuum hose to FPR sealed 34 psi
Fuel Pressure Regulator reconnected 34 1/16 psi.

All these numbers at idle or greater than 1500 rpm without load.
Still has numerous and near continuous random misfires while cruising or under load.

[rgk]

You said you received a #2 misfire, and not a random misfire, after disconnecting the #2 cylinder. This is an excellent clue, and one you should pursue further.

Have you tried swapping a cable from another cylinder to the #2? Maybe there is a problem with the #2 cable itself.

[4xq]

Well, you should be getting 43 psi or so with the fuel pressure regulator hose disconnected. Your tests say about 34 to 35. You said you replaced the pressure regulator, so I can only conclude your fuel pump is weak.

Fuel pressure should be 34 or so in high vacuum situations (idle, Deceleration) where fuel demands are low. When vacuum drops (acceleration, medium and high throttle) fuel pressure should increase to 43 +.

[LegioCorpus]

Okay, replaced the fuel pump with no change. Replaced the spark plug wires with no change. Replaced the distributor with no change. Then I took another look at the data stream. Noticing that O2 sensors on the both bank 1's had odd readings: One read .9v and the other read .03v with slight variances within both. So I unplugged both, and the engines frequency of misfires diminished greatly. I then swapped the sensor plugs, and the engine ran perfectly. However, the plugs are now crisscrossed according to the manual. So the best I can figure is there is some mis-wiring somewhere or there is a computer glitch. Nevertheless, it is now running smoothly. My guess is the O2 sensors were fouled out because of all the oil being burned, and they were giving bad readings before fixing the valve cover gaskets, as the O2 sensor connections were correct when I started. I just cannot think of another explanation.

Of a separate note, the distributor manufacturer did not tighten and/or screw in the set screw to the rotor and the engine died after about 60+ miles. Had spark, fuel and air, but the engine would not start. Took me a tow truck, and almost 4 hours to figure it out. Only after taking off the distributor cap to see a 3 cent screw lying next to the rotor, did I realize what had happened. Realigned the rotor and installed the set screw. Tada! We're back in business.

So thank you for your help, and remember...thoroughly inspect all new parts, even if they are plug and play, and cost $140 or more. Ugh! LOL!