Odd case of Rough Idling and Misfires on my 96 FZJ80 (1 Viewer)

[Miilkman]

For the past month, my LC has been running a bit rough on idle, and I have been doing my detective work in trying to figure out the cause.

I've replaced the spark plugs, spark plug wires, dizzy cap, rotor with OEM parts, but no luck. I checked the injectors, wiring harness, checked if there were any combustion gases in my coolant system, did a compression test and all came back good... Brought it to 2 local shops to get a second opinion and they could not find the cause of the issue.

The weird thing is that it only idles poorly and misfires when my idle RPM hits 650(warmed up) or when my drive idle(at a light or stop sign) hits 600. If I turn the AC on to bump up the idle RPM greater than 800, it runs perfectly without any issues--no hesitations, rumbling or misfires. Other than this issue, it runs totally fine on the road.

I'm starting to think it's a vacuum leak and ordered some new parts to replace those, but is there anything else that I should look at? Possibly the IAC or something else? This rabbit hole is getting deep so any and all thoughts would be greatly appreciated. Thanks

 

[BILT4ME]

For the past month, my LC has been running a bit rough on idle, and I have been doing my detective work in trying to figure out the cause.

I've replaced the spark plugs, spark plug wires, dizzy cap, rotor with OEM parts, but no luck. I checked the injectors, wiring harness, checked if there were any combustion gases in my coolant system, did a compression test and all came back good... Brought it to 2 local shops to get a second opinion and they could not find the cause of the issue.

The weird thing is that it only idles poorly and misfires when my idle RPM hits 650(warmed up) or when my drive idle(at a light or stop sign) hits 600. If I turn the AC on to bump up the idle RPM greater than 800, it runs perfectly without any issues--no hesitations, rumbling or misfires. Other than this issue, it runs totally fine on the road.

I'm starting to think it's a vacuum leak and ordered some new parts to replace those, but is there anything else that I should look at? Possibly the IAC or something else? This rabbit hole is getting deep so any and all thoughts would be greatly appreciated. Thanks

This could be a serious rabbit hole.

Is the juice worth the squeeze?

Mine idles similar and I pretty much ignore it. It is my DD and I run about 15-20K per year. That is the least of my worries.

It could be:
Ground wires not making clean connection
Wire connectors needing pins cleaned on all engine harness connections
Fuel injector wires
Temp sensor wires
IAC valve
Vacuum leaks
MAF sensor sensitivity issues and buildup on the wire
Clean the throttle body
Change the fuel pump
Change the fuel filter

So many possibility and VERY expensive to diagnose and swap parts because by the time you're done, you've touched everything and may make something worse by messing with it.
Fix it until it's broke.

 

[Malleus]

It's the IAC

 

[Miilkman]

This could be a serious rabbit hole.

Is the juice worth the squeeze?

Mine idles similar and I pretty much ignore it. It is my DD and I run about 15-20K per year. That is the least of my worries.

It could be:
Ground wires not making clean connection
Wire connectors needing pins cleaned on all engine harness connections
Fuel injector wires
Temp sensor wires
IAC valve
Vacuum leaks
MAF sensor sensitivity issues and buildup on the wire
Clean the throttle body
Change the fuel pump
Change the fuel filter

So many possibility and VERY expensive to diagnose and swap parts because by the time you're done, you've touched everything and may make something worse by messing with it.
Fix it until it's broke.

Great point... I think what makes its most frustrating for me is that the fact that turning on the AC fixes it. Seems like it should be an easy fix but here I am

 

[Miilkman]

It's the IAC

For sure, I'll give that a check later today.

 

[Malleus]

Make sure you use the FSM and perform both tests.

 

[LongDuck]

My money's on a blown head gasket. How many miles are on it?

 

[Miilkman]

My money's on a blown head gasket. How many miles are on it?

Compression test came out okay and I didn't find any combustion gases in my coolant system. 200K Are there any other tests that I could do to check this?

 

[Roamer]

For the past month, my LC has been running a bit rough on idle, and I have been doing my detective work in trying to figure out the cause.

I've replaced the spark plugs, spark plug wires, dizzy cap, rotor with OEM parts, but no luck. I checked the injectors, wiring harness, checked if there were any combustion gases in my coolant system, did a compression test and all came back good... Brought it to 2 local shops to get a second opinion and they could not find the cause of the issue.

The weird thing is that it only idles poorly and misfires when my idle RPM hits 650(warmed up) or when my drive idle(at a light or stop sign) hits 600. If I turn the AC on to bump up the idle RPM greater than 800, it runs perfectly without any issues--no hesitations, rumbling or misfires. Other than this issue, it runs totally fine on the road.

I'm starting to think it's a vacuum leak and ordered some new parts to replace those, but is there anything else that I should look at? Possibly the IAC or something else? This rabbit hole is getting deep so any and all thoughts would be greatly appreciated. Thanks

I have a 97 LX450. I chased a similar problem for 8 months it was intermittent misfire that eventually got worse. I kept getting PO306 misfire #6. I did the exact same trouble shooting you did. It ended up being a burnt valve on #6 cylinder. I had the P0140 EGR code when i bought it 13 years ago and put a 4 ohm resister in the EGR temp connection and it had run like a dream for the last 120K miles currently 275K. The Mechanic explained to me that this may have caused it to run lean for a long time which caused it to burn the valve. A compression test was done and all cylinders are around 165 except #6 which was 40. Head is coming off in a few weeks and being completely refreshed. Hope this helps and hope its not this problem just something simple and cheap.

 

[LongDuck]

Alright, man - I should have noticed your post count before going with the age-old "It Must Be Your Head Gasket Because It's An 80" joke,...

Seriously, it's not your head gasket. What you describe is most likely your first inclination - a minor vacuum leak. The most likely - and often overlooked - is a vacuum leak at the accordion tube going from your air cleaner to the intake manifold. Given age and heat, slight cracks can develop in this intake tube that will cause a hesitation or poor quality idle. Specifically, remove the intake manifold side and stretch it out at the bellows, paying close attention to the folds of the bellows. Be careful doing this - if it's fine, you don't want to cause a problem by breaking it!

If its not this, can you get it on a diagnostic reader and look for codes? Good luck, I'm betting it's something simple, and please pardon my joke. If youve hung around here awhile, it would have brought a laugh,

 

[Miilkman]

Alright, man - I should have noticed your post count before going with the age-old "It Must Be Your Head Gasket Because It's An 80" joke,...

Seriously, it's not your head gasket. What you describe is most likely your first inclination - a minor vacuum leak. The most likely - and often overlooked - is a vacuum leak at the accordion tube going from your air cleaner to the intake manifold. Given age and heat, slight cracks can develop in this intake tube that will cause a hesitation or poor quality idle. Specifically, remove the intake manifold side and stretch it out at the bellows, paying close attention to the folds of the bellows. Be careful doing this - if it's fine, you don't want to cause a problem by breaking it!

If its not this, can you get it on a diagnostic reader and look for codes? Good luck, I'm betting it's something simple, and please pardon my joke. If youve hung around here awhile, it would have brought a laugh,

ahah No worries at all--I was bound to hear the joke sooner or later. I def gotta get accustomed to the insider jokes. I'll take a look at that, I actually bought a cheap Ebay intake vacuum hose right around the time that this issue popped up, could be correlated.

 

[LongDuck]

Its always the last thing you did to it!

 

[Miilkman]

@Malleus @LongDuck

Some updates: So I replaced the IAC, cleaned the throttle body, changed majority of the vacuum lines with OEM parts, changed the pcv valve, replaced the intake vacuum hose with an OEM one.

It's still idling a bit rough and I am stumped. I still see the engine shake a bit, stutter and the exhaust puffs on occasion while in idle. I do notice when the engine hesitates, the headlights and dash-lights dim for a millisecond and then go back to normal.

At one point--maybe a month before I started fixing things, I was consistently getting P0304, P0300, P0305, but checking my monitor, i don't see any misfires. so no apparent misfires or anything--i think, although it feels like one?

What else can I check before I go crazy?

 

[Malleus]

The FSM troubleshooting matrix is your best tool

 

[Zhowardfzj80]

Any update on this situation? I'm having a similar issue that has actually got worse in the last few weeks and I've been replacing parts as I can hoping for the cure.

This all started a week affer replacing plugs and wires. Drove like a champ for the next week then the symptoms started as a sudden quick pulse of rpms. It would be fine and at like 650 rpms and then it would very quickly hit fluctuate just enough for the needle to jerk and the entire vehicle would jerk. But the needle didn't really go up or down. It just pulsed. And it would originally only do it in reverse or drive. After replacing fuel filter, charcoal canister, o2 sensors, air filter, running lucus in gas tank, cleaning throttle body, cleaning IAC, and and replacing/fixing a few vacuum hoses it still pulses and even more often now and also drivability has declined. It sputters at random times now but with no consistency. Might be at take off or might be at 60 mph. Cuts out so bad it wont hardly move. It won't do it the same ever. I also noticed puffs from the exhaust occasionally. Curious if you have come to a conclusion.

 

[BILT4ME]

Any update on this situation? I'm having a similar issue that has actually got worse in the last few weeks and I've been replacing parts as I can hoping for the cure.

This all started a week affer replacing plugs and wires. Drove like a champ for the next week then the symptoms started as a sudden quick pulse of rpms. It would be fine and at like 650 rpms and then it would very quickly hit fluctuate just enough for the needle to jerk and the entire vehicle would jerk. But the needle didn't really go up or down. It just pulsed. And it would originally only do it in reverse or drive. After replacing fuel filter, charcoal canister, o2 sensors, air filter, running lucus in gas tank, cleaning throttle body, cleaning IAC, and and replacing/fixing a few vacuum hoses it still pulses and even more often now and also drivability has declined. It sputters at random times now but with no consistency. Might be at take off or might be at 60 mph. Cuts out so bad it wont hardly move. It won't do it the same ever. I also noticed puffs from the exhaust occasionally. Curious if you have come to a conclusion.

Since this occurred shortly after doing the plugs and wires, start there.

OEM or aftermarket?

Closely inspect the plug on the top of the distributor as those are known to fail after being bumped or moved.

You are describing an electrical issue. The fast cut outs are not fuel or mechanical related.

As @Malleus said, the FSM trouble shooting list is there for a reason.

Or you can keep shooting the parts cannon at it.

 

[Miilkman]

Any update on this situation? I'm having a similar issue that has actually got worse in the last few weeks and I've been replacing parts as I can hoping for the cure.

This all started a week affer replacing plugs and wires. Drove like a champ for the next week then the symptoms started as a sudden quick pulse of rpms. It would be fine and at like 650 rpms and then it would very quickly hit fluctuate just enough for the needle to jerk and the entire vehicle would jerk. But the needle didn't really go up or down. It just pulsed. And it would originally only do it in reverse or drive. After replacing fuel filter, charcoal canister, o2 sensors, air filter, running lucus in gas tank, cleaning throttle body, cleaning IAC, and and replacing/fixing a few vacuum hoses it still pulses and even more often now and also drivability has declined. It sputters at random times now but with no consistency. Might be at take off or might be at 60 mph. Cuts out so bad it wont hardly move. It won't do it the same ever. I also noticed puffs from the exhaust occasionally. Curious if you have come to a conclusion.

Hey, not yet... I brought it into a local shop that reputable in land cruisers(A+ Japanese Auto Repair) the other day and they said that my spark plug and spark plug wires are worn... He said the spark plug ceramics were spinning and the spark plug wires were arcing. I was pretty surprised hearing this cause i just had these replaced with OEM parts 2ish months ago. I'm going to go get new parts and see if that fixes the issue.

I also found that my uncle who owned this prior, had the back O2 sensor welded on for some ******* reason... The voltage on that o2 sensor was everywhere--ranging from 0.01-.9 which doesn't sound right to me. So after I replace the spark plug and spark plug wires again, I am going to tackle this o2 sensor issue, and if that doesn't work, I am going to replace my distributor--which was recommended by Georg.

 

[Malleus]

I'd doubt OEM wires were arcing if they were two months old. Maybe the plugs were, if the ceramic was broken, but then, I'd be surprised if they even fired.

 

[Miilkman]

I'd doubt OEM wires were arcing if they were two months old. Maybe the plugs were, if the ceramic was broken, but then, I'd be surprised if they even fired.

Yeah I felt like they were trying to hustle me out of money. I cant imagine my car running perfectly fine under load with these issue, but i'm going to see if replacing these with newer OEM parts directly from Toyota will help.

 

[AgentOrange]

Since this occurred shortly after doing the plugs and wires, start there.

OEM or aftermarket?

Closely inspect the plug on the top of the distributor as those are known to fail after being bumped or moved.

You are describing an electrical issue. The fast cut outs are not fuel or mechanical related.

As @Malleus said, the FSM trouble shooting list is there for a reason.

Or you can keep shooting the parts cannon at it.

I'm a firm believer that every technician and home mechanic should have this diagnostic tool in their shop...