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Matt, Engineer
Category: Australia Car
Satisfied Customers: 21631
Experience:  Automotive Engineer for 20 years, BEng hons degree,
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Jay I have a 2005 Nissan Navara v6 petrol VQ40 280000ks. I

Customer Question

Hi Jay
I have a 2005 Nissan Navara v6 petrol VQ40 280000ks. I had an issue with it running rough at idle and wouldn't accelerate. I kept pulling a Po300 code. I found the timing chain & guides well past there used by date to the point that the chains were loose. I replaced all chains, guides ect as per the workshop manual. Now I've got it all back together it still runs rough and keeps showing Po300 code. Runs better then before as the cams are in timing now but still no good. Your thoughts?
Submitted: 1 year ago.
Category: Australia Car
Expert:  Matt replied 1 year ago.


if you haven't already then replace the spark plugs if any of them fail this checklist:

You can also try spraying some water on the coil pack with the engine running. It is possible the coil is faulty and causing the misfire. If you see sparks arcing with the water, then replace the coil.

I'd check the condition of the injector plugs / connections as this could be a lean misfire if an injector isn't firing properly - the best way to check this is with a 'noid' light

Then check all the connections to the engine sensors one at a time, remove each one and examine for signs of corrosion or damage and a squirt of contact cleaner ( not WD40) will also help.

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and leans the engine out causing rough running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattenedpiece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Expert:  Matt replied 1 year ago.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Customer: replied 1 year ago.
Plugs have been changed with Autolite, does that matter ?
Customer: replied 1 year ago.
I've ran a smoke test for air leaks and no leaks were found
Expert:  Matt replied 1 year ago.


autolite aren't the best plugs but if they're quite young they should be fine

an as there's no air leak then the next step is to measure fuel pressure

Customer: replied 1 year ago.
I felt that fuel pressure was fine so I skipped that test. After further inspection I found one of the catalytic converters blocked, all fixed and working fine now.
Expert:  Matt replied 1 year ago.

glad you got it sorted

however 1 thing to bear in mind is why did the cat block up in the 1st place?

as they should last 150K miles

so it may be worth checking that its not using oil and that the breather system is functioning correctly

Customer: replied 1 year ago.
the timing was out due to worn timing chains and guides causing the engine to run rich. Also the plugs were well out of spec and have know to throw a misfire code. Fixed now
Customer: replied 1 year ago.
it has 280 thousands k's
Expert:  Matt replied 1 year ago.

ah well that would explain it

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