98 2.5 TL - Starts/runs OK when cold, stalls after reaching operating temperature
 

1998 Acura 2.5 TL, 137000 miles

Main symptom: Engine starts and runs OK when cold but stalls after warming up to operating temperature. CEL not lit, no OBD-II codes. Four mechanics worked on this problem for about a year before one finally solved it.

Other symptoms: In the months before the stalling problem became severe, the car began to show poor fuel economy, noticeable hesitation when accelerating and occasional losses of engine power at cruising speeds. Replacing spark plugs, wires and distributor cap did not help. EGR passages were severely clogged but cleaning them made the problem even worse. Compression on all cylinders was okay (minimum 175 psi, maximum 190 psi). Fuel pressure a little low at 35 psi instead of 38-48 but that should be okay. Cleaning throttle body and intake passages did not help. Checking the intake vacuum revealed an exhaust restriction (high exhaust back pressure). Replacing the exhaust manifold and its integrated catalytic converter improved performance significantly but did not solve the stalling problem. Replacing IAC, MAF and engine coolant temperature sensors did not help. Replacing the timing belt did not help. Cylinder head rebuild did not help.

The solution: The downstream (secondary) oxygen sensor had failed but the engine computer did not report the problem by throwing a code. The downstream O2 sensor output was stuck low. This falsely indicated to the computer that the fuel mixture was too lean. The computer tried to compensate by adjusting the fuel mixture to be inappropriately rich. This caused the engine to stall. However, when the engine was cold (as determined by the coolant temperature sensor), the computer apparently does not use the O2 sensor to control the fuel mixture, so the engine did not stall until it reached operating temperature.

Lesson: For unexplained stalling problems, look at the oxygen sensors very closely even if the CEL doesn't light and no OBD-II codes are thrown. We could have seen the problem early on if we'd used a diagnostic OBD-II scanner to monitor the O2 sensor outputs in real time. The O2 sensor outputs shouldn't be stuck low, they should be in the middle of their range and switching up and down.

Very informative post! :thumbsup:

Thanks, I spent a lot of time on this forum while trying to figure out this problem. Maybe it will help someone else.

Thinking back, maybe we could have diagnosed this specific problem by simply disconnecting the downstream oxygen sensor. This can be done by sliding the passenger seat as far forward as possible and pulling up the carpet. There is a 4-pin connector to the downstream O2 sensor that can be disconnected. I think the computer should recognize that the downstream O2 sensor isn't there and know to not pay attention to that measurement. I'd imagine the engine would run inefficiently and the CEL would come on but maybe it would stop stalling. I'm not 100% sure it would have worked that way but at least it's something that could be tried without needing a $2,500 diagnostic OBD scanner.

positive to all this you replaced a lot of things that are known to go bad. My intake in my 3.2 was clogged completely shut as welll! Good luck and may your TL reaxh 500k!

Yes indeed. At 137k that new car smell is just now starting to wear off!

I have a 96 Acura 2.5 TL
I've already read other post and tried those suggestions. This didn't work for me either
I've already replaced...
New battery, battery terminals, distributor cap, starter, fuel pump, ignition switch, temp sensor, both o2 sensors, main relay and removed the alarm.

Any other suggestions?

Still having the same exact problem as above after I changed...

new battery, distributor cap, main relay, ftw sensor, both o2 sensors, fuel pump, starter, ignition switch, disconnected alarm, terminals

First I want to be sure your problem is exactly the one I had: Engine starts and runs OK when cold but stalls after warming up to operating temperature. Check engine light not lit, with no OBD-II diagnostic codes.

The mechanic who fixed my problem explained what was happening. When you start the engine cold, it runs open loop. That means the computer runs the engine with fuel richness settings & spark timing settings that are inefficient but guaranteed to work. When the engine warms up, it goes into closed loop operation, meaning it starts using the emissions sensors to control everything.

So if the engine starts cold and runs okay, but then stalls after it warms up, it means something in the emissions control system isn't working right. It could be lots of things. The engine computer is supposed to detect when something goes wrong and throw an OBD-II diagnostic code. It just so happened in my case that there was an O2 sensor failure that wasn't detected. Maybe there are other kinds of failures that stall the engine but don't throw a code, I don't know.

One simple thing you can try is to simply disconnect the rear O2 sensor. I think the connector is under the front passenger seat. What might happen with the rear O2 sensor disconnected is the car still starts with the engine cold, but then does not stall when the engine warms up. If you hook the rear O2 sensor back up and the engine starts stalling again when it warms up, the O2 sensor is bad even if the onboard computer fails to throw a code. This is what I was seeing. Now, disconnecting the rear O2 sensor would not be a fix even if the stalling goes away. The engine would run rich all the time, fouling the plugs, ruining the cat converter in a few weeks and increasing fuel consumption way high. Don't ask how I know this, ha ha.

Correction:
The car usually starts everyday w/o a problem. Depending on the weather (if its not too hot), I can drive less than 10 miles (turn off the car for a while, time it takes to grab a few groceries) and the car will usually restart. Now if I drive from that location to another location (within 3 miles), do the same thing, the car will not restart.

It seems the car is overheating somewhere, and I can't start the car. I hear the starter click, fuel pump turn on but the engine does nothing, the radio comes on... If I wait approx 30 mins or more (depending on how hot the weather is), the car starts with no problem.

The car does not shut off on me, and I can run all errands as long as I don't shut the car off, regardless of how far.

Well, Lucky, that's a somewhat different problem than the one I originally described so I expect you'll need a different solution. What I'd probably do is to start a new thread and title it something like "96 Acura 2.5 TL - Won't restart after warming up" and hope it catches the attention of someone who's managed to solve that exact same problem.

Intermittent problems are hard to diagnose. The good thing is you seem to know how to set up the conditions for the problem to occur. The key will be to get the problem occurring in the shop while you or a mechanic is looking at it with all diagnostic tools at hand. It's hard to pinpoint an intermittent problem if you're stranded in some parking lot somewhere without even a toolbox. But it doesn't do much good for anyone to be looking at the car when the problem isn't occurring, either.

My 1996 2.5 TL drives fine in the winter but when it gets above ~60 degrees in the spring/summer, it will stall when you come to a stop. The good part is that it will start right back up again. Unless you keep your foot on the gas pedal and have the rpm's > 1000 it will stall again. I tried unhooking the downstream O2 sensor before re-starting the car but that didn't help. I have a $12 plug-in OBDII scanner that shows both two different O2 readings, the higher number one I'm guessing is the downstream one. when it's running, the graph for the 2nd one is relatively flat compared to the first so I'm guessing that my sensor might be bad.

If it's being caused by a defective ignition switch, why wouldn't it happen in the winter as well?

I'll look around for some other answers as well. Any input would be appreciated. Mine has 255,000 miles on it.