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Engine stumbles under load


bcti
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Hello!

 

I've a 95 Pathfinder, V-6, auto transmission, 189,000 miles. About Christmas time last year, I noticed that at about 70 mph, the engine would start to stumble. It rapidly got worse, and soon it was doing it at any speed. BUT it doesn't do it when the engine is cold.

 

I've replaced the fuel filter, distributor cap and rotor, spark plugs, and air filter. All of the old parts looked terrible; I wouldn't doubt that they were original parts. The gap on the old spark plugs, which should be between .031 and .034, was over .055! And you could barely see a tiny line down the middle of the electrode where the V-gap once was!

 

While giving it the tune-up, I noticed that two vacuum lines were dry, hard and cracked. One was on the charcoal canister, and the other was on what I believe is the fuel pressure regulator. I only found that hose was split while feeling around for the spark plug that's between the engine and the firewall. I looked around, and didn't see any other questionable vacuum lines. So, I replaced the two bad ones.

 

Let me see...what else...oh yeah! At about 40 mph, if I mash the accelerator pedal to the floor, it will backfire out of the exhaust pipe. Also, it starts and idles wonderfully, regardless of engine temperature. And if I rev the engine while the transmission is in Park or Neutral, there is no stumble.

 

And I'll say it again, it doesn't stumble when the engine is cold.

 

I've not checked for error codes. Since the 'check engine' light hasn't come on, would there be any error codes to retrieve?

 

I'm going to look over the repair manual and see if there are any troubleshooting suggestions for this problem.

 

If anyone has had a similar problem, or if anyone with more experience with this engine than me has some advice, I would appreciate hearing from you.

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MAF wiring, poor ground. There's a fix for this that's worth a shot.

 

That was a quick reply! Thank you for the input. That sounds like an easy solution, I'll give it a try.

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I referred to my pdf copy of the "Nissan Truck and Pathfinder" factory service manual for 1995, (Actually, I just noticed it says © 1993 Nissan Motor Co., LTD., but I'm using it anyway.) and under the troubleshooting page, i decided to go with Diagnostic Procedure 9 - Hesitation under normal conditions on page EF& EC 59.

 

It is the bottom part of page 59 and the top of page 60. I used the "uneven, jagged line" option in Photoshop to outline the appropriate sections.

 

The first step is to inspect the spark plugs. I already replaced them, so ahead to step two.

 

Step two is just as adamzan suggested, which the service manual calls "check heated oxygen sensor."

 

I'll post the results when I've completed that step.

 

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That or the o2 sensor if you haven't changed it.

 

You and the factory service manual agree. I'll post the results after I've completed that step. Thank you for your suggestion!

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  • 4 weeks later...

Ok, so I checked the O2 sensor, and it was ok. I checked the EGR and it was ok. I looked for vacuum leaks again, and still found none.

 

At that point, I decided to bite the bullet and take it to a professional. He retrieved the trouble codes, and there were none. He checked the fuel pressure, looked for vacuum leaks, and looked for slop in the timing belt, and he found no problem in those areas. He also checked the ground for the MAF sensor, which wasn't bad. He attached some sort of engine analyzer to it, and (I'll edit this part later) either the camshaft position or crankshaft position would fluctuate (on his meter) whenever the engine mis-fired. Whichever it was, it led him to feel that it was either the pickup in the distributor, or a bearing in the distributor. I ok'ed him to remove the distributor and check it out.

 

Once the distributor was out, he said you could spin it, and it would suddenly stop spinning. Indicative, in his experience, of a bad bearing/bad bearings in the distributor.

 

New distributor, about $600. Rebuilt distributor, about $430. As much as I'd rather go with a new distributor, well, money, economy, recession, etc.

 

I'll post the final bill once I have it, and clean this post up a little bit when I'm less rushed.

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