# 2004 Frontier (VG33 V6 3.3L) idle stumbles periodically



## Capt_Tight_Pants (Jul 19, 2017)

I have been scratching my head about this one for a while now. When the truck is idling it feels like the engine skips a beat which sends a considerable bump you can feel through the whole truck. I guess you can call it rough idle, only it doesn't happen all the time. It can idle fine for five minutes in a row and then start doing it when it would start stumbling every two or three seconds. It usually gets worse when the truck warms up. Loading the engine seem to affect it for the worse as well (putting it in gear, especially turning AC on). However, it will still do it in park with nothing else on, just not as often. When I stand in front of the engine and this "stumble" happens I could hear a click (sounds almost like a relay click) coming from somewhere in the area of PCV valve. Not sure if it is cause or effect of stumbling though. When it starts happening while I'm at the light and I start accelerating, you can feel it through the initial stage of acceleration as intermittent hesitation. Other than that it feels normal while driving at higher speeds. I did feel it happen one time going on the highway though.

Check engine light is off. So far the things I've done are: clean the throttle body, new air filter, cleaned MAF sensor, cleaned grounding wire from the battery, ran a bottle of seafoam trough it, changed the distributor, verified correct timing. Nothing seems to affect it. Any ideas what else may be causing it?


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## Capt_Tight_Pants (Jul 19, 2017)

Since I've received such an overwhelming response to my post, I figured I would post an update for those interested. Over the weekend I hooked up an OBDII dongle. There was a knock sensor high code sitting in the memory, which is to be expected, since my main problem is idle backfire. Everything else looked good. Then I hood up an oscilloscope to the O2 sensors. Those checked out as well. Nothing out of the ordinary. Then I decided to do a piece of paper to the exhaust pipe test, just because it seemed like there was nothing else left to test. Sure enough, when the engine would stumble, the paper would momentarily get suctioned to the tail pipe. So it seems like I've got a sticky valve. I'm going to try to put Seafoam in the oil and spray it into the intake, to see if it can fix the issue at least for a while. If nothing else, it will confirm that it is a valve issue. Next stop, I would have to pull the heads and have them rebuild.


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## smj999smj (Jan 1, 2006)

Do you think there may be a chance it might be a weak hydraulic lifter? They do go bad a lot a VG motors and are a lot easier to change than R&R the cylinder head.


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## Capt_Tight_Pants (Jul 19, 2017)

smj999smj said:


> Do you think there may be a chance it might be a weak hydraulic lifter? They do go bad a lot a VG motors and are a lot easier to change than R&R the cylinder head.


Can you elaborate please? The way I understand it, a weak lifter wouldn't open a valve all the way? If so, would it really cause a backfire condition? How would you test to check if a lifter is weak?


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## Capt_Tight_Pants (Jul 19, 2017)

Just an update. Pulled the heads off the engine. Haven't found a smoking gun, but there are couple things that look weird. First, the lifter for the intake valve on cylinder 3 was pretty badly chipped on the edges of the face that contact the rocker arm. It still had flat surface left in the middle though. The rest of the lifters looked fine. When I took the spring off that valve it didn't feel any binding, doesn't seem to have excessive sideways lash either, around 0.004" at the tip of the stem. Not sure if I should replace valve guide and the valve anyway.

Another thing that looked weird was discoloration of the rockers and rocker shafts for cylinder 3 and 5. It was deep brawn color, like it was heating too much. All the valve train on the driver side looks nice golden color, the same for cylinder 1, but number 3 and 5 are dark brown, almost black. Cam shaft turns easy by hand. All valves seem to be ok as well. Rockers turn pretty easy on the shaft as well. Maybe the rocker for the cylinder 3 was not getting any oil? Couldn't find any info on oil routing in the shop manual. It seems that oil is being supplied the the channel under the rocker arm, around the holding screw in the rocker arm shaft and then from the bracket that holds hydraulic lifters. I will try to trace it and see if there is any blockage there.


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