Hey guys and gals. So have a bit of an issue and so I wanted to give the group a chance to give me their thoughts and recommendations. To start with, for the past several weeks I have been getting a rather strange "miss" when in overdrive, I would get some serious missing, hiccups and rough running but the misses were rather random, This would only happen when the engine is under load (slight acceleration) but not when coasting down hill. The CEL will come on and stay on. If I press the accelerator and get the tranny to drop down out of overdrive, the miss seems to go away, I have recently had the CEL to flash, and even at times go out. This has been going on like I said for quite some time now. In the last few days I have been getting rough idle and it definitely sounds and feels like it is missing. When I accelerate and get out of overdrive and the vehicle accelerates, the CEL will flash and then goes solid on, rough running but worse then before when it was just a random miss. Checked the codes and got the read out of P0303-cylinder 3 misfire detected, P0304-cylinder 4 misfire and P0300 random misfire. The P0304 indicates too many misfires within a predetermined period of time, so the machine says. Recommendation is to replace the ignition coil. As this truck has never had them replaced, over 210,000 miles so maybe a good idea to change out all three? So before dropping $220 at the local Auto Zone, went online to check on these and there seems to be quite a variety of these. My question is there a particular brand that is recommended? I have seen 3 paks for as little as $35 and even OEM Toyota paks for under $80 (for 3).

So with the codes that have been thrown, replacing the ignition coils seem like a reasonable determination of the issue? Should I replace them all and start fresh? The best recommendation of replacement coils, Toyota parts or after market ? Thanks for the inputs

1998 SR5 3.4l V-6, only engine modes are electric fan cooling

 

Use Denso coils, buy 1 and keep trying it in each location until it resolves the problem, these coils rarely fail ever, at all.., make sure your running NGK or Denso dual electrode plugs they should be gapped and no anti-seize. If plugs wires and coils are good I would suspect its an injector, seems to be the trend after 20 years.

 

So what is your opinion of Toyota branded coils? Should I replace all of them and start fresh? Does a single coil supply cylinder 3 & 4? Does Denso make Toyota OEM coils or find it as a "Denso" branded coil? I replaced the wires about 8 years ago, only put about 15K miles on it in that time. Will look into new plugs but had changed them out when I did the wires, can't remember the brand plugs, they may have been NGK. Thanks

 

Toyota branded coils are made by Denso Part# 673-1201, again they are long living and rarely fail, so I would only buy 1 and try it in each of the 3 locations over a few days to see if it resolves the problem or better yet try the inspection toyota advises in the FSM if you have the equipment, as it is very unlikely the problem.., the rubber boot can fail causing issues though and are replaceable separately. On the waste spark system on the 5vz-fe the coil on cylinder #1 fires cylinder #4, the coil on cylinder #3 fires cylinder #6. Only use NGK or factory wires and insure you used the correct dual electrod plugs, as not doing so can damage the coil as per the picture on the timing belt cover.

I should add that the wire connections on the coil can corrode, and the plastic holding clips are getting fragile at this age and exposure to heat, I use di-electric grease at these locations after insuring it is clean.

EDIT: I doubt the autozone guy ran the freeze frame and diagnosed the coil as the problem as per the FSM, he guessed coil for no reason except misfire (I'm guessing now too lol), he could have guessed plugs wires or coils aswell, personally before I spent money I would check the connections on the coils, check plugs and plug wire connections, if all looks good and you cannot test the coil buy 1 replacement coil, if that doesn't resolve the problem start leaning towards an injector issue as it is a VERY common issue at this age.

Coil inspection
http://www.teamtoyota4x4.org/archivi.../ignsy/ovi.pdf
Injector/ missfire inspection
http://www.teamtoyota4x4.org/archivi...e/cip0300p.pdf

 

Before you buy a coil, take the one under suspicion out and and trade it for one of the others. Then drive around and see if your miss moved with the coil.

If it did… bingo, problem solved.

if it didn’t, try the same with the plug wires. I realize the wires are each different lengths, but usually if you reroute them straight over the intake they’ll fit where they weren’t intended. Again if THAT makes the miss move… problem solved.

You can swap spark plugs to narrow down the same concerns.

The final piece of the puzzle is the fuel injector. These do die from time to time, normally the work well enough when cold and then start to act up as they got hotter. You can swap injectors around too for testing for the cost of a plenum gasket.

 

Well decided to just clear the deck on fixing this 300, 303, 304 CEL codes. Found really good deal on a set of three coils, 6 denso Toyota OEM plugs and NGK spark plug wires. Seems that part of the issue was both plugs and wires. one of the wires going to #4 had cracked edge at the base of the stem where it connects to the plug, could see what looked like little gray "lightning" streaks. All of the plugs (dual tip NGK) showed the electrode being oval shaped around the tips. Clean no burn marks or oil. I will attempt to take some pics for reference. As for the coils, I just wanted to start fresh. She is purrin now! Thanks to all of you who chimed in and offered advice and recommendations. Took me a bit longer ten I expected getting stuff out of the way, so once again a successful day wrenchin on my Yota!