Hello all,

Please help me out here. I have a 98 with the 3.4L and 180k miles. Automatic. It's all tuned up but is ready for a Timing belt. The problem is that it has a random misfire at idle; will shake the truck. Seems worse when hot. It also has a pending code of P0420---cat below threshold. It averages 19 mpg, but doesn't like to go above 65mph and forget about passing anything quickly.

The o2s are about 60k miles old and the pre cat is appears to be working fine. The post cat sensor voltage is bouncing around...bad sign. It's a new Catco converter, could the sensor be failing?

Another observation: The idle is 600 rpm but the timing varies between 9 and 16 degrees!?

I checked the resistance on: coil packs, TPS, cam position sensor.

Any ideas?

thanks,

Jon

Why was your cat replaced? Do you know what brand of O2 sensors got put in? Bosch/Denso...?

I used Bosch O2 sensors.
I replaced the cat because it was behaving like it was getting plugged up and the exhaust needed to be replaced. It was just as easy to take the cat out with the exhaust.

I don't have experience with after market cats. Some people say they work great some say you'll always get a CEL with them. If you're bosch's have 60K they are probably at fault. I'd replace the downstream sensor (with a Denso).

Let's forget the P0420 for now. I think that it's a separate issue.

What would cause the Spark Advance to wander between 9 and 16 degrees. I think that's my big clue but I don't know what to check besides the cam position sensor; the ohms were ok.

Thanks,

Jon

By "ready for a timing belt" do you mean the second one or has it ever been changed? I believe the ECU controls the timing on these based on what the sensors tell it so if the cam sensor checks out maybe the crank sensor is off..? If your belt is really old maybe it's streched just enough to affect the timing....? I'm totally guessing

I was being lazy last night but I looked it up today and your timing is fine.

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Wow, I appreciate you looking that up. I thought I really had something when I saw my timing hunting from 9-16 degrees over a period of seconds. It is on its second T-belt so it's due again. So where does this leave me and my random misfire?

I can double check the cam and crank sensor resistances and clean the ICV. Anything else you can think of? Could the current T-Belt have excess stretch/slack causing an issue?

I doubt that it's your timing belt causing any misfires. So it just misfires at idle? What does it idle at? 600? How long since the last tune up, plugs, wires? Clean the MAF (easy and free). Are you getting a P0300 code for the misfires or you just feel it?

Here is the link to the FSM so you can do your own late night searching

1996 to 2002 Toyota 4Runner OEM Repair and Wiring Manuals

No other codes. I just feel what I think is a misfire. It can be felt thru the whole vehicle and does it in park too but it's much less noticeable. The idle is right at 600rpm and doesn't dip unless I get a few misfires in a row.

I did a complete tune up about 10k miles ago. Plugs, wires, oil change, fuel filter, air filter, cleaned MAF, checked the PCV valve. I'm waiting for a code but I'm starting to think its something like a fuel injector or torque converter or something weird.

Did the misfiring just start recently? If it was an injector or coil/boot problem you would probably get a CEL right away pointing to that cylinder. What kind of plugs did you use? Dual ground?

The misfire has been happening for about a year now but seems to be getting more frequent and more severe. I used dual ground plugs; NGKs if I recall correctly.

My plan at this point is to run a compression test to make sure the engine is sound and then just drive it. I'll probably have to give this to a pro to get fixed.

I was going to suggest swapping the coils around but without a CEL you can't tell if it's a specific cyl or random misfire. How old are the plug wires?