Engine Conking Out When Hot

[Mister Rupe]

I have a '92 LS. It has 173,000 miles on it. For the past couple of years, it has developed a really annoying habit. When I have been driving it for a good while or on a highway trip, the engine seems to start chugging and acting like it's not getting any gas. It acts like it wants to cut off, and often does. As long as I remain at highway cruising speeds, this does not occur. But at city traffic speeds, and more frequently on inclines, this chugging and engine cutoff seem to occur. I try to stomp the accelerator to try to give it more gas, but this only makes things worse - the car will then start lurching violently and the transmission slams hard. The only way I have found to keep it running when it does this is to put the car in neutral and punch the accelerator, and the engine will always rev that way. Then I have to put it right back into drive and force the car to move forward. Obviously this is very hard on the transmission. I have taken it to Lexus. They say they can't diagnose it unless they experience it. And they can't test drive it long enough to make it happen. I have also taken it to three other mechanics who have said the same thing. My Dad experienced the same thing in his Toyota Camry, and it turned out to be the EGR valve. I have had people tell me that's what they think it is. But I don't want to shell out who knows how many hundreds of bucks to replace that if it isn't the root of the problem. Has anyone else experienced this, or does anyone know what might be causing this? It's making me CRAZY! And it's a dangerous problem in busy traffic.

Thanks!

[curiousB]

Well if it is engine related it could be ignition or fuel starvation. It might be fuel starvation because you say at constant speeds and when you go into neutral it runs fine. This is when engine has little load so fuel consumption is more moderate. I guess next time it happens double check to see if there is any acceleration.

Causes of fuel starvation could be dirty or clogged injectors, plugged fuel filter, or weak fuel pump.

Maybe try a couple of tanks of gas with a fuel injection cleaner additive as first try. Can't hurt and maybe you get lucky? Failing that then maybe change the fuel filter as it’s a normal maintenance item and not too expensive. Last is to check the fuel pump. Normally I would recommend connecting a fuel pressure gauge to check fuel pressure but since your problem is intermittent that may be more difficult. Maybe take a flier and put in a new pump and see.

If its not fuel it could be intermittent ignition due to one of the coils failing as it gets hot.

Not sure any of these will fix it but they are reasonable places to start with.

I haven't really considered transmission issues because at highway speeds the torque converter should be locked up and basically a fixed gear path to the wheels.

So if you are able to do the above your cost is two bottles of injection cleaner, one fuel filter, fuel pump, and ignition coil. Try one for a few days before spending on the next. If you end up doing them all and still not fixing the problem you’ll have a lot more detailed story for your dealer and only be out $200 or so.

[fsuguy]

Mister Rupe,

I have a '92 with a similar problem, but more mild in that the car still runs reasonably, although it is obvious that performance is not at par. I have suspected the EGR temperature sensor, but have not as yet tested it - there is a test for it in the manual. I am not sure, however, since I also suspect the fuel pump, the fuel filter, or the fuel pressure regulator, since I find that sometimes the doggone car runs fine anyways, even if I have been driving for a while.

Unfortunately for me, there are no Lexus dealers within at least 200 miles of where I live, and the Toyota dealerships have bumbling !Removed! for mechanics - I say this based on direct experience with them !Removed! up things with my LS such as ripping out the wiring from Speed sensor #3 when I had asked them to replace #2 and even provided the OEM part!!!

Anyway, would surely like to know what you find out is the root of your problem.

fsuguy

[sedanman]

I have been experiencing the same thing on my 2004 LS430. It happened for the first time about a year ago, but the check engine light would never come on and the repair shop told me the same thing: they could not diagnose it unless they could experience it or get a check engine code. Just last week, it became impossible and dangerous to drive and the check engine light finally came on and I brought it in to the repair shop.

They said that there were several codes recorded but that they thought the air flow sensor was bad.

They replaced it, but it stalled again on me yesterday so I am back to the drawing board.

I have a '92 LS. It has 173,000 miles on it. For the past couple of years, it has developed a really annoying habit. When I have been driving it for a good while or on a highway trip, the engine seems to start chugging and acting like it's not getting any gas. It acts like it wants to cut off, and often does. As long as I remain at highway cruising speeds, this does not occur. But at city traffic speeds, and more frequently on inclines, this chugging and engine cutoff seem to occur. I try to stomp the accelerator to try to give it more gas, but this only makes things worse - the car will then start lurching violently and the transmission slams hard. The only way I have found to keep it running when it does this is to put the car in neutral and punch the accelerator, and the engine will always rev that way. Then I have to put it right back into drive and force the car to move forward. Obviously this is very hard on the transmission. I have taken it to Lexus. They say they can't diagnose it unless they experience it. And they can't test drive it long enough to make it happen. I have also taken it to three other mechanics who have said the same thing. My Dad experienced the same thing in his Toyota Camry, and it turned out to be the EGR valve. I have had people tell me that's what they think it is. But I don't want to shell out who knows how many hundreds of bucks to replace that if it isn't the root of the problem. Has anyone else experienced this, or does anyone know what might be causing this? It's making me CRAZY! And it's a dangerous problem in busy traffic.

Thanks!

[AndyLowe]

You mentioned that during Inclines, the chugging is present.

This is a classic description of a defective Ignition Coil.

Some coils are more erratic as they warm up, and then you feel the tremendous surge of power intermittently to the transmission, which makes you think the transmission is "going out".

It's not!

You need to get on Ebay and buy a coil or two; I've bought them, with shipping, for $22 each. You need two and when they come, replace both.

It's $44.00 to get rid of a Coil that is electrically breaking down UNDER LOAD.

UNDER LOAD is what you're describing and the 30,000 volts of spark is no longer going off to the Distributor, across the Rotor's air gap, and then onto the Spark Plug's Air Gap.

No, your high voltage is jumping internally within the old coil, and it is jumping to the path of least resistance for 30,000 volts; the ground on one of the coils four phillips head screws.

Change these two coils out for half the cost of one hour of Service Dept guessing and you will enjoy your wonderful car again.

One other item to also address is the two wires that are bolted to your + Positive Battery Clamp.

Take a 12mm socket and remove that nut, which holds these two flat terminals to the side of the + Battery Clamp. Now take some 80 grit sandpaper and clean the top and bottom surfaces of both flat terminals. Now re-install this critical junction.

I found this out while monitoring both O2 Sensors; my voltages were erratic and the car was having trouble responding to me giving the accelerator pedal a fast, large input. The engine was bogging down!

When I cleaned these, the correct response came back.

Now keep in mind, my condition was NOT under load, but with gear selector in Park, and car in my driveway. This non-load condition will NOT identify a weak Ignition Coil.

The Load of going UP A HILL, adds extreme pressure to the combustion chamber. This is the same thing as adding more resistors to a circuit, and so the high voltage and current simply opt for a more easy path. That's why you're getting this problem when you have warm coils (they've expanded closer to their close by phillips head screw ground) and the under load is the "Road Work Ahead, Next 2 Miles" sign, so your coil says, Let's take another route and still have a nice trip!

Good luck and let us know!

[curiousB]

The Load of going UP A HILL, adds extreme pressure to the combustion chamber. This is the same thing as adding more resistors to a circuit, and so the high voltage and current simply opt for a more easy path. That's why you're getting this problem when you have warm coils (they've expanded closer to their close by phillips head screw ground) and the under load is the "Road Work Ahead, Next 2 Miles" sign, so your coil says, Let's take another route and still have a nice trip!

I agree with replacing the coils but not with this theory of resistors and pressure in the combustion chamber. The ignition coil doesn't understand what is going on in the combustion chamber. It only knows how often it is being cycled to spark. It can and does heat up with a warmer engine and higher rpms (more sparks). That can cause items to expand and tiny cracks to open up. That becomes a discharge path to ground on the high voltage side. Rather than the spark plug gap reaching flashover (electrical breakdown, the point an arc is passed) it happens earlier in the coil (IE at a lower breakdown voltage). As such the spark never happens in the cylinder and you get a mis fire.

[AndyLowe]

The Load of going UP A HILL, adds extreme pressure to the combustion chamber. This is the same thing as adding more resistors to a circuit, and so the high voltage and current simply opt for a more easy path. That's why you're getting this problem when you have warm coils (they've expanded closer to their close by phillips head screw ground) and the under load is the "Road Work Ahead, Next 2 Miles" sign, so your coil says, Let's take another route and still have a nice trip!

I agree with replacing the coils but not with this theory of resistors and pressure in the combustion chamber. The ignition coil doesn't understand what is going on in the combustion chamber. It only knows how often it is being cycled to spark. It can and does heat up with a warmer engine and higher rpms (more sparks). That can cause items to expand and tiny cracks to open up. That becomes a discharge path to ground on the high voltage side. Rather than the spark plug gap reaching flashover (electrical breakdown, the point an arc is passed) it happens earlier in the coil (IE at a lower breakdown voltage). As such the spark never happens in the cylinder and you get a mis fire.

Snap On offered advanced diagnostic classes to my employer, about ten years ago. One of the courses I attended was Ignition Principals. The instructor was very knowledgeable and taught theory for several brands and we learned very helpful tips.

The fact that compression chamber pressures increase along with cylinder head temperature, (when under load,ie; going up a hill or accelerating to enter a highway,) is the reason for the breakdown of 30,000v to a path of lesser resistance. Normal, non-load combustion temperature and pressure do not require 30,000v, but more close to 18k to 24k volts.

It's the added resistance of the spark plugs environment that causes the weak coil to give out. Watch a spark line on an Oscope when under load; it grows into the 30kv range.

I won't argue with you, but thought I would pass on to you what the Snap On Instructor taught. I thought this particular fellow was very bright. He shared a few war stories that helped us know more.

I like this forum and respectfuly hold my point of view, but also respect those that disagree with me. I want to learn more~ Perhaps I'm wrong? There are some very bright folks in this forum.

[curiousB]

No you are correct, I was incorrect. Spark firing voltage increases with engine load. It is at its maximum when engine is at max torque as that is when the engine is at maximum temp and pressure inside the cylinder.

So at lower speeds (lesser engine load) a weak coil can chug along fine but as you load up the engine it misfires. The breakdown within the coil is because of some type of failure (insulation breakdown or tiny crack that builds up with carbon over time) so once it starts to happen it generally will get worse over time.

These factors can be why it is tricky to diagnose. At idle is least likely to happen (IE the time you are looking at it under the hood).

I stand corrected. Sorry for the misinformation.

[landar]

Andy, interesting info...thanks. Curious, I did not see where you were necessarily wrong. The coil does not know what is going on in the combustion chamber. It's output voltage rises as the coil's magnetic field collapses and whenever the plug gap ionization point occurs, it occurs. Higher cylinder pressures require higher ionization points which results in higher energy/voltage levels. No smarts in there.

I would think that with a compromised coil(insulation breakdown), anything from no spark to spark, but not complete combustion, would occur when under high pressure. The fuel/air mixture may indeed get "sparked" but may get blown out prematurely by the excessive cylinder pressure. In other words the spark was not hot enough (voltage too low) because of insulation breakdown at the coil/rotors/wires/plugs. Any component along that series chain could compromise the combustion spark.

[curiousB]

I think the key point is spark ionization voltage rises with temp and pressure. I was assuming more of a steady state due solely to gap geometry of plug. If anything I thought gas mixture would reduce flashover voltage relative (volatile conductive mixture) to just air when in fact it is the opposite. So I learned something new thanks to Andy.

That said you still need a problem in the coil or wiring for this to be a problem. Sound, in-tact wiring and coil won't have leakage paths to arc over before the plug fires so it doesn't matter that firing voltage wanders around a bit. That voltage rises quickly as you note because of the field collapse of the coil primary (an L di/dt phenomenon) The point here is the marginal case. If the compromise on the coil (crack or HV wiring insulation failed) is below 18kV but arcs >18kV then you can see why at light engine loads plugs will fire fine (say around 12kV) but as you load engine and spark rises to >20kV then the coil problem will arc first.

So moral of the story is if you have bad coil or wiring it doesn't matter much exactly when it fails. It might be more annoying at heavy acceleration but anyway needs to be replaced as it is a defect scenario and not something to live with.

[sedanman]

I brought my lexus back to the shop this week. They found that my fuel filter was completely clogged up and that there was particulate/trash in the gas tank. They replaced the filter and sucked out as much trash as they could. It is running great now.

I always use a good 93 octane gas (Shell or Exxon or Chevron)so i am not sure where the trash came from. I bought the car used so I will never know.

You might have someone put a fuel pressure gauge on your car to see if the pressure is declining as it idles.

[curiousB]

Usually a plugged or restricted filter will result in fuel starvation at higher engine load and might not even be noticeable at idle (least fuel draw). The flip side of this is if you have fuel starvation at idle it must be very bad at all other throttle positions.

[k_tieu]

very odd

in neutral ur rpm is abit higher then in drive could be ur fuel system