94 ES300 Misfiring
 

I have a 1994 ES300 V6.

The Misfiring is Intermittent.
The front center cylinder appears to misfire (not fire at all). Misfiring then occurs at idle, under acceleration (most notable due to engine vibration), and at constant speeds.

Once the misfiring occurs, at idle, I have pulled the front center low voltage lead to the coil pack without a change to the engine. Pulling the front driver or front passenger low voltage lead to the coil pack causes the engine to run worse.

I have swapped coil packs and spark plugs. The problem stays with the front center cylinder.

Trying to fix the misfiring issue:
- Ran one can of BG44K and have a second can in the tank
- Replaced fuel filter
- Replaced all spark plugs (iridium tips replaced Bosch platinum)
- Played with electrical connections to front center cylinder with no apparent effect

NOTE, possibly contributing to the misfire is a failure of the spark plug in front driver cylinder (not the current front center problem cylinder). The front driver sparkplug ‘lost’ the ground electrode and started to melt the plug. After replacing all the plugs, the car ran well. Then the misfiring seemed to begin intermittently. I did not pull the head to get to the cylinder, but carefully probed the cylinder for the electrode tip, which I could not find. I suspect it either burned up or was ground down.

Recently did some maintenance to successfully fix an idle problem which appeared to have been a sticky idle throttle plate. I am unsure of the correct name, but for this car a small butterfly throttle plate is located below the main throttle plate and driven by a solenoid.
- Cleaned MAF
- Cleaned throttle plate, both large for driving and small solenoid driven one for idle. For smaller idle throttle plate, flooded with carburetor cleaner and blew out with compressed air (several times).
- Replace air filter

Note, if the injector is bad, one mechanic who has worked on Toyota & Lexus said that replacing one bad injector while keeping 5 old injectors still in the car can cause poor performance because the O2 sensors can’t get one measurement that works for both the new and old injectors.

Anyone have any insight on what's going on with my car?

I have the same thing going on with my RX300 (basically the same engine). I've done all the same diagnositcs you have and, by elimination, I've targeted the problem to the injectors. I've got new ones on the way and will replace them tomorrow.

Did you ever find what the root cause of your problem was?

Id check compression, especially if it runs well when the engine is cold. How many miles are on it? What was the maintenance history of the car prior to the problem?

Bump.... any updates?