Poor acceleration and slight idling issue

1998 Sienna XLS, California vehicle emissions.



Initially, I began with an check engine light (CEL) code requiring a check/replacement of my A/F (not O2) sensor. I managed to get by a smog check/registration renewal by cleaning the sensor with some Seafoam and clearing the engine code by temporarily removing the EFI fuse. Later, I get the same CEL code but ignore it for a while. Within the last several weeks, my CEL begins blinking and I experience sluggish acceleration (van struggles to get up to 60mph@4500+rpm), hesitation during acceleration and slight hesitation at idling. I had my catalytic converter (weld-in unit) and all my spark plugs replaced with NGK iridium units. This significantly improved the sluggish acceleration but now I am now experiencing slight hesitation during acceleration and during idle. According to my mechanic, the engine is misfiring on the 6th cylinder. Suspecting that it may possibly be a bad coil, I swap the 6th coil with the one to it's immediate left (4th cylinder, I believe). I then clear the CEL via the EFI fuse. At this moment, I have yet to trigger the CEL and read the results in order to determine whether I have a bad coil.



Where should I look to next?

Reply 1 : Poor acceleration and slight idling issue

I believe the fuel filter is replaceable on the 1998 Sienna (my 2003 has it inside the fuel tank).

Also a fuel system cleaner......cleaning the fuel injectors.

You can get your code(s) read for free at many auto part stores, but you will need to ask for the specific code.

A misfire on cylinder #6 would be P0306.

A blinking CEL is indicating a condition that can cause damage to the catalytic converter if you continue to operate the vehicle.

A misfire is a code that will cause the CEL to blink.

If the misfire is caused by a bad coil or spark plug....or other spark issue.....raw fuel is then going to be exhausted into the catalytic converter, causing it to overheat.

No spark means no combustion......so the fuel entering the cylinder does not burn, and it will be pushed out in the exhaust cycle.

If the misfire is caused by a clogged fuel injector, then the misfire is caused by no fuel being present when the spark takes place.

This should not cause raw fuel to enter the catalytic converter.

However a fuel injector that is stuck open (less common) can cause raw fuel to enter the catalytic converter.



One code can cause other codes to show up.

This is because the computer is trying to compensate for the initial fault.

Such as.....misfire can cause a rich indication.....and the computer will try to spray less fuel into the cylinders to correct for that.....causing a lean condition also.

Quite often the first code is one that a mechanic will focus on.

But.....interpretation of the diagnostic codes can often require experience with a particular vehicle.

A internet search on the code and your vehicle/year can yield some common causes of that for your particular vehicle.



At this point, I would see if the code comes back and see if the problem moved with the coil.

Also, making sure that you are up on ALL the routine maintenance items can save you from a lot of chasing around trying to track down a problem.



One common problem with a vehicle not idling properly......such as stalling when you remove your foot from the accellerator or the idle speed going up and down, is a sticking Idle Air Control Valve (IAC).

This is located on the front end of the throttle body.

The IAC is like a mini throttle body....the computer controls the idle speed when your foot is not on the accellerator, using a motor to open/close a valve that permits air into the motor in the correct amount to maintain the correct idle speed.

This will NOT have any effect when you are driving with your foot on the accellerator.

Reply 2 : Poor acceleration and slight idling issue

I purchased a new coil from my local Autozone and installed it right away. My van is now functioning so much better.



Thank you TN members for all your help!