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Rough idle


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Ok -  you guys were a big help with getting the pathfinder fired up and home. Once I got it started it had a really rough idle. It’s also putting out a black substance that smells like gas from the tailpipe. It smells like its running really rich. I’ve sprayed some starting fluid around the vacuum hoses to see if it has a vacuum leak. Didn’t detect one. What else should I try?

 

Thanks!

Matt

 

 

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A vacuum leak generally causes a lean condition, not a rich one. Rich condition could be a leaky injector, bad coolant temp sensor, or possibly a borked oxygen sensor, though I wouldn't expect that to kick in at idle (idle's usually open-loop, meaning the computer isn't watching the oxygen sensor in the first place). Check your spark plugs. If one of them is wet, and the others aren't, that points to an injector leak. If they all look the same, I'd check the connector on the temp sensor. The computer adds more fuel when the engine is cold, so if the sensor is bad or the connector is corroded, it'll think the engine's cold as hell and inject way too much fuel for the temp the engine is actually at. There are two temp sensors on the nipple the upper rad hose goes onto. The one-wire goes to the cluster, the two-wire goes to the computer. There's a test mode in the ECU to monitor your oxygen sensor (via a blinking light on the back of the ECU), but that only tells you if the sensor is cycling, not why it isn't. EF&EC section of the service manual is your friend. You can get the '90 service manual free from cardiagn.com, or the '94/'95 (also free) from Nicoclub. The '90 covers my '93 more closely than the '94 does.

 

Also check the vac line to the fuel pressure regulator (back behind the plenum, should be able to reach it on the passenger's side). Make sure it's connected (not that it would cause this issue if it wasn't) and that there's no fuel in the vacuum line, which would indicate a failed diaphragm. I haven't heard of that happening on one of these, but it's an easy thing to rule out.

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Finally got the pathfinder to start reliably so I can concentrate on this. I replaced the coolant temp sensor - the two wire sensor - and it didn’t change the condition from what I can tell. I will pull the plugs and see how they look.
 

I noticed that the temperature gauge in the truck never goes above 1/4 and that’s after a lot of driving. Could that be related to this issue?  Stuck thermostat?

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