Slight engine miss at idle???

[BrettinSanAntonio]

1997 LX450
121K miles

I have a slight miss at idle. This seems to be the only point where the miss is noticeable. The engine is smooth at RPM’s above idle and my fuel economy is right on target. All the tune-up items have been changed with Toyota parts minus the fuel filter and O2 sensors. I’ve also changed all the vacuum lines on top of the engine including the PCV valve, grommet and hose. I’ve put BG44K through the gas tank a few times but it had no effect on the idle miss.

Does anyone have an idea as too what might be causing by problem?

 

[DTAYLOR]

1997 LX450
121K miles

I have a slight miss at idle. This seems to be the only point where the miss is noticeable. The engine is smooth at RPM’s above idle and my fuel economy is right on target. All the tune-up items have been changed with Toyota parts minus the fuel filter and O2 sensors. I’ve also changed all the vacuum lines on top of the engine including the PCV valve, grommet and hose. I’ve put BG44K through the gas tank a few times but it had no effect on the idle miss.

Does anyone have an idea as too what might be causing by problem?

Brett,

Did you notice this after doing all these things? Did you do them trying to fix the miss?

 

[BrettinSanAntonio]

Brett,

Did you notice this after doing all these things? Did you do them trying to fix the miss?

I did these things trying to correct the miss.

 

[hkimsoft]

1997 LX450
121K miles

I have a slight miss at idle. This seems to be the only point where the miss is noticeable. The engine is smooth at RPM’s above idle and my fuel economy is right on target. All the tune-up items have been changed with Toyota parts minus the fuel filter and O2 sensors. I’ve also changed all the vacuum lines on top of the engine including the PCV valve, grommet and hose. I’ve put BG44K through the gas tank a few times but it had no effect on the idle miss.

Does anyone have an idea as too what might be causing by problem?

I have exact same problem... haven't found a solution yet...

[EDIT]My next thing on the list is a fuel filter.

 

[Beowulf]

I have exact same problem... haven't found a solution yet...

Please elaborate.

Is this just an occasional "stumble" while idling?
Are you getting a cylinder misfire MIL?

Mine exhibits an occasional stumble at idle and has since I got it almost 6 years ago (at 96k miles.) I attributed it to the intake needing cleaning but you've done that. Try running 5 tanks of premium with Red line fuel treatment and see if it improves. Mine does but it is temporary and the miss is so minor that I don't worry about it.

-B-

 

[NaterGator]

Out of sheer curiosity, do you have very slight predetonation around 1250-1400 rpms?

 

[corsair23]

Mine exhibits an occasional stumble at idle and has since I got it almost 6 years ago (at 96k miles.) I attributed it to the intake needing cleaning but you've done that. Try running 5 tanks of premium with Red line fuel treatment and see if it improves. Mine does but it is temporary and the miss is so minor that I don't worry about it.

-B-

Maybe we all have the equivalent of the infamous "FJ40 Cruiser Lean" but only in the 80 series. My '97 LX450 exhibits the "stumble" still after new plugs and wires etc. Lexus dealership looked it over while in for a purchase inspection and stated there was nothing that they could discern wrong. It is just a little burble, only at idle (AFAIK), and somewhat annoying.

 

[BrettinSanAntonio]

Yes, it is an occasional "stumble" that occurs every three to five seconds and lasts for a second or less.

I've not scanned the computer yet but there is no CEL so I thought there wouldn't be a code stored without a light. Is this correct?

Yes, it's a minor irritation but I thought I'd ask to see if anyone has found a solution.

Please elaborate.

Is this just an occasional "stumble" while idling?
Are you getting a cylinder misfire MIL?

Mine exhibits an occasional stumble at idle and has since I got it almost 6 years ago (at 96k miles.) I attributed it to the intake needing cleaning but you've done that. Try running 5 tanks of premium with Red line fuel treatment and see if it improves. Mine does but it is temporary and the miss is so minor that I don't worry about it.

-B-

 

[BrettinSanAntonio]

No, the throttle response is smooth above idle.

Out of sheer curiosity, do you have very slight predetonation around 1250-1400 rpms?

 

[BrettinSanAntonio]

A friend in Dallas with the same issue just told me he removed his injectors and sent them to be cleaned and tested. He claimed after reinstalling the injectors, the issue went away.

However, I'm wondering if all the cleaning in the throttle body and EGR system may have also contributed to correcting the problem.

Maybe we all have the equivalent of the infamous "FJ40 Cruiser Lean" but only in the 80 series. My '97 LX450 exhibits the "stumble" still after new plugs and wires etc. Lexus dealership looked it over while in for a purchase inspection and stated there was nothing that they could discern wrong. It is just a little burble, only at idle (AFAIK), and somewhat annoying.

 

[RavenTai]

I used to have this, a slight miss at idle that you feel as a vibration though the truck more than hear. About every 3 to 5 seconds sounds right. It came and went, did not do it all the time.

I have not noticed it since I replaced my O2 sensors. they were original w/ 160k some odd miles, I'll try to remember to listen for it again next time I drive to make sure.

Nator I do sometimes get sight detonation off idle in hot weather, especially sitting at a light with the AC on. One possibility may be heat soak of the air in the intake system. Once the RPM's come up the air will pass through faster and should be cooler.

Been thinking about trying to insulate the air filter box to possibly help with this.

 

[NaterGator]

I'm thinking of installing hood vents because my under-hood temps in Fl are utterly unreal. After running my engine there is absolutely no way I could touch my valve cover even 30 minutes after shutting the engine off... it is like a giant oven in there.

I also think injectors may be my problem. I have the same occasional and sporadic idle-stumble, but the pinging bugs me alot more.

 

[BrettinSanAntonio]

Yep, that's a very good description. It's more of a vibration through the truck. Interesting thing is before I fixed my exhaust leak (a big one) you could hear it via the exhaust system.

Hmmm...now O2 sensors is something I've been thinking about doing since they've not been changed. Come to think of it, I don't notice the miss at startup. It's only after the engine is warm and it drops down to normal idle. Isn't this when the O2 sensors become active? I don't believe the O2 sensors are doing anything until the engine is at normal operating temperature.

I used to have this, a slight miss at idle that you feel as a vibration though the truck more than hear. About every 3 to 5 seconds sounds right. It came and went, did not do it all the time.

I have not noticed it since I replaced my O2 sensors. they were original w/ 160k some odd miles, I'll try to remember to listen for it again next time I drive to make sure.

 

[RavenTai]

Come to think of it, I don't notice the miss at startup. It's only after the engine is warm and it drops down to normal idle. Isn't this when the O2 sensors become active? I don't believe the O2 sensors are doing anything until the engine is at normal operating temperature.

correct

 

[BrettinSanAntonio]

We've got pretty brutal summers down here in southern Texas and as long as my engine temp is good, I've never worried about the engine temp. July and August is usually around 100 degrees every day, not to mention the drives through west Texas, 110 degrees plus. Most of the summers I can't touch my hood to open it and working on the engine is something that can only be done comfortably in the morning.

I'm not sure how much an exhaust fan would matter. As long as your cooling system is functioning properly, I wouldn't worry about it.

I'm thinking of installing hood vents because my under-hood temps in Fl are utterly unreal. After running my engine there is absolutely no way I could touch my valve cover even 30 minutes after shutting the engine off... it is like a giant oven in there.

I also think injectors may be my problem. I have the same occasional and sporadic idle-stumble, but the pinging bugs me alot more.

 

[NaterGator]

The 80 cooling system is generally considered one of the more problem-prone areas, and after doing the temp gauge mod (kudos to RT and the gang ) I can tell there are situations where my cooling system is saturated and cannot sufficiently dissipate heat... the factory gauge's dead spot hid this behavior though. Perhaps it is normal, but there is nothing wrong with lower under-hood temps!

 

[Beowulf]

The 80 cooling system is generally considered one of the more problem-prone areas,

Nate,
I'll agree with the statement that there are problem areas with the cooling system but it is a FACT that the cooling system is easily capable of keeping the engine temps where the designers intended.

The caveat here is that the cooling system must be properly maintained and in good operating condition; radiator, coolant, thermostat, water pump, belts, hoses, temp sensors, radiator cap, fan, fan clutch. It is a mechanical system and all these bits need to be functioning 100% and if they are, the system will work very, very well.

-B-

 

[NaterGator]

Ok, I'll agree with that -B- for sure...

However, I will also say that the 80's cooling system needs attention much more than any other vehicle I have worked on.

I think the margins are so slim that once one component of the cooling system starts to fall out of spec it shows up pretty quickly and is not easy to pin down. My cooling system shows zero signs of sludge or buildup, my rad is clean and unobstructed, my fan seems to work properly, etc. Sorry though, not to hijack the thread!
-----Nate

 

[landtank]

Nate,

Some of us have tweaked the blue fan clutches with good results. Not only seeing better cooling but better under hood temps. One guy is in Texas noticed a drop in detonation as well.

 

[BrettinSanAntonio]

Hey, no worries. I'm not saying it's a bad idea. I just don't think it will make that much of a difference if a component of the cooling system is not functioning properly. Hell it certainly can't hurt. If the under hood temps are lower, it may help hoses and such last much longer.

Ok, I'll agree with that -B- for sure...

However, I will also say that the 80's cooling system needs attention much more than any other vehicle I have worked on.

I think the margins are so slim that once one component of the cooling system starts to fall out of spec it shows up pretty quickly and is not easy to pin down. My cooling system shows zero signs of sludge or buildup, my rad is clean and unobstructed, my fan seems to work properly, etc. Sorry though, not to hijack the thread!
-----Nate