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post #1 of 2 (permalink) Old 06-25-09, 07:52 AM Thread Starter
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troubelshooting rough running 3.0 v6

As a automotive tech, I rarely work on mazda's but the customer was insistant in having me troubleshoot a run rough/low power issue after he changed the spark plugs. This is the same engine as the ford escape.

The customer changed the front three spark plugs. He removed the coil packs, replaced the plugs and reinstalled. The engine has the well known 1 cylinder thump miss characteristic of vacuum leak,bad spark misfire or just plain dead cylinder from no compression. I had noticed that he also did not tighten #4,5 spark plugs.

So I attempt to isolate the issue. I inspected the plugs electrodes, re tightened and engine showed the same issues. I installed the other unused set of plugs "in case he broke the ceramic" and reinstalled into plug numbers 5,6. I pulled all three coil packs and checked the primary and secondary resistance. Primary side of coils 4,5,6 were less then one ohm each. Secondary side was 19k ohms on all three coils so all are good.

I would pull the coil on #5 coil pack. 2002 Mazda Firing Orders
Had a very noticeable engine rpm drop so that cylinder was good. Pulled the coil pack connectors to #4 and #6 and got almost no change in cylinder rpm drop. I swapped 5 and 6 coil packs and noticed that #5 has little rpm change when disconnecting it.

Cylinder bank with plugs 4,5,6 is much hotter then the rear cylinder bank and getting the rotten egg smell out the tail pile "hydrogen sulfide

Now, the owner did overtorqe the intake manifold bolts pictured here with the white arrows. The nuts, or plastic that hold the nuts are broke. I suspect a a vacuum leak as a result of this. I put my propane bottle to these points over and under the manifold. Got little if any rpm change. I tried my starter fluid but the nozzle broke before I had any change to use it.

Catalytic converter is 500F at the front and 700F at the rear.

So, what could be the issue? I highly suspect a vacuum leak but it was not evident. All Spark coils on 4,5,6 Ohm readings are good. I inspected the spark on #6 coil with a tester and got sufficient spark.

Do modern day scanner read the Kvs or general health of these coils while in operation? How do I pull codes with the onboard computer?

I know the snapon modis is a very pricy scanner and have thought of the Launch 431 scanner that is so popular to technicans over seas at a big cost savings over the snapon.

Your thoughs much appriciated.

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post #2 of 2 (permalink) Old 08-05-09, 11:51 AM
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Re: troubelshooting rough running 3.0 v6

I've had a terrible time with the same types of problems. Here's what I've done to this point.

Cyls. 5 and 6 new injectors
O2 sensors
new IAC
cleaned throttle body
Replaced rear catalytic converter.

Still idles rough and misses at 4,000rpm

Found out a couple days ago there are two more catalytic converters inline right off of the exhaust manifold. (you have to buy the whole assembly). There not resonators they are known as pre-cats and they are definitely cat converters.

I cut the middle pre-cat out seeing if I could blow anything out of the first one. She definitely blew, It looked like I emptied my coal furnace.

Problem seems to have improved but it still isn't at 100% with the idle and tends to still miss when revved to 4k in park.

Hope this helps a little bit.
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