Hey everyone, my mind is running in circles and I am in need of some help. A few days ago my 98 T100 4x2 A/T with the 3.4 v6 230k (I know not a 4 runner but same motor and tbh there are alot more people on this forum than the t100) developed a misfire read by the obd as misfire in cylinder #4.. only. No other codes. I have had this issue once before, ended up being the plug wires, changed the wires easy peasy done. This misfire is not so simple unfortunately. There are numerous misfire posts on this and other forums but none that I have read have the same symptoms, or the fix for many of their misfires did not work for me.

The first thing I did as many others is focus on the ignition/spark. I am 99% sure this is not where my problem lies, I have changed ALL the plugs NGK dual ground plugs. (old plug looked fine BTW, as did all others) I have moved the coils (twice) the misfire did not move. I have moved the plugs, (even though they are practically new) no change.

Next I decided to check compression on that cylinder and others. No issues, compression was not only good it was great 190-195.

So good compression, good spark, has to be injectors right? I buy tested re-manufactured denso injectors and replace not only cyl#4 but all the cylinders since its probably never been done and has 230k. The install went smooth no problems. I go to start the truck, Truck runs great no stumble no miss no hesitation... for the first 5 min. Almost as if once it got warm it would begin to buck. Same code misfire #4cyl.

The misfire is odd as well, first off it is intermittent. It misfires in park at idle (when warm) but when i rev the motor (in park or nuetral) the misfire disappears but promptly returns when returns to idle or placed in drive under load. Now to the intermittent part when i test drove it down the street it would be missing the whole time then would have periods where the misfire would resolve, I could actually feel the cylinder kick in and fire, that would happen for a few seconds then misfire again. this would happen randomly or sometimes not at all.

I am racking my brain trying to think of things i missed.

I've heard of the crank pulley coming loose causing the knock sensor to create random misfires, however this affects many cylinders and not just one. I also haven't ruled out vacuum lines but it is not high on my list of possibilities.

I would next like to check the electronic system of the injectors, as well as the ECU. Any one have any experience with this as I really don't know where to start other than testing resistance on the injector wire for cyl #4. I would live to swap out an ECU to see if that resolves my prob but they are expensive and I feel like that should be my last resort.

The only other thing I cant stop thinking about is the exhaust valve. could i have a bad exhaust valve even with great compression? I plan on taking the plenum of again to reach the injector wire, and taking the valve cover off while im down there to check the valve clearance.

So that's about it! any ideas? BTW ive posted this in the T100 forum as well

 

Does it only happen when warm? Or are there times it will miss when stone cold? Have you tried spraying ether around the surfaces while running to see if you have an intake leak?

since your issue is single cylinder specific, that leads me to rule out systemic things like fuel pumps, clogged catalytic converters, o2 sensors, or even the crank key because that would cause random misfires all of for no reason.

Asking about the heat vs. cold issue brings up the idea of heat soak, where electrical components malfunction when they are hot but not cold. This is primarily the case when 5vz injectors die.

If it were a bent, stuck, or leaking exhaust valve it wouldn’t really come and go, it would be better cold and than get progressively worse as it heated up and stuck open. It wouldn’t come and go as you were driving.

I would try 2 more tests with the injectors. First, while it’s running, jiggle and mess with the injector wiring to see if it is a bad connector or chafed wire or something. Second, swap out #4 injector again. I know you replaced them and the likelihood of having a bum injector in the new batch that coincidently got put in #4 is highly unlikely, but you have to rule it out.

 

So far its definitely worse when warm, but I live in FL so it doesn't take long to warm up. I have yet to start it when cold and have it start misfiring immediately, as it would when warm. I have not tried spraying the intake fro leaks but I will add that to the list.

Tomorrow I plan on running the truck and seeing if I can hear the injector clicking open with a screwdriver. hopefully I will be able to because the #4 injector is in the middle right underneath the plenum. If the the injector is not firing Ill start chasing wiring to the ECU, always worth a shot to wiggle some wiring as well.

Funny you mention heat and electronics... The bundle that holds the injector wiring used to have a plastic housing over it that I suppose protected the bundle form heat... that housing is gone.. non existent. the only reson I know its supposed to be there is because the passenger side injector wiring harness has one.. its falling apart but is there. That could be the cause for some of these problems. I will tear into it tomorrow morning and post the results.

 

So I put a screwdriver up to the injector of cyl 4 while running and I can hear it clicking. I wiggles the bundle no change. I’m stumped. Bout to take it somewhere for a leak down test.

 

Do you have access to a good scan tool that can read live streams of the sensors? Not just a code reader.

With a good scanner you can look at fuel trims, o2 outputs, etc. If you had a leaking injector, your o2 stats should be off and the fuel trims should be low. If the injector isn’t firing and delivering fuel the way it’s designed your fuel trims should be really high and your o2 high and lean.

In any case, you should be getting more than just a misfire code from the computer.

 

It may be worthwhile to spend $20 and get the 2day subscription to TIS and download everything for your particular VIN.

https://techinfo.toyota.com/techInfo...tis&_nfpb=true

 

I don’t have access to a decent scan tool I agree though. Will take it to a shop that does. What is TIS?

 

Nm just saw the link. Thanks. I’m bout to do a second compression test just to confirm the basics

 

190-200 in all cyl. Just bought leakdown test for$30 (cheaper than the shops hourly rate) will post results.. got my eyes set on head gasket. Could be small enough to not register on compression check

 

This is happening while warm and acting up?

 

Exactly. The misfire is occurring while I’m listening to the injector for cyl # 4 (which is a PITA to get to btw. The injector responds with increase in throttle as well.

I personally thing the head has a very small crack or there is a HG prob. One that is so small compression check won’t show it

 

Leak down test performed with no results

 

No negative results

 

You don't mention plug wires other than changing them before- but, though it seems unlikely, you've probably also looked into that as well since the wires never move even if the coil is moved.

 

I have tried switching around the plug wires. Im going to take it somewhere to get it hooked up to a computer and diagnosed while
running To see if there is something I’m missing

 

Is there white smoke when the engine is warmed up and starts to misfires with white smoke the. it’s a bad head gasket. I just completed a whole top end rebuild on my 97 4runner with the 3.4 and I had both cylinder heads rebuilt. It’s a pretty big job if like the challenge and when I removed the driver side cylinder head the gasket failed in cylinder 6 with coolant inside the cylinder. I done compression and leak down test and it all passed but when I saw the white smoke it was a good indication it was a bad cylinder head. Another thing when performing the leak down test make sure to remove the radiator cap to see if you see bubbling which is another sign of a bad head gasket.

 

Did you spend the $20 for a subscription to TIS (Toyota Tech Info System)? Might be worth it to download schematics (in fact, any stuff for any year). If you do the heads, you can fetch the mech side too.
I'm going to venture out and say I bet its a wire. Might test that whole harness (that plug on ECM goes to injector, but then also is shared back thru the harness on the ground side).

Your Money back if its a gasket!

 

My 5vzfe did that same exact thing, missed when warm and at idle but went away at higher RPM's up. Went through the entire system (plugs, wires, tested the igniter, tested the coils, cleaned the injectors) and narrowed the issue down to the coil boots/springs.
My boots where cracked and causing the spark to ground out on the spark plug tube and caused a miss, the spring inside the boots where also not contacting the tip of the spark plugs and that made the issue worse. Hope this helps you out in some way.
You can get just the boots off of Rockauto.com without having the replace all the coils.