Cylinder Misfire



I have a 1999 Toyota Forerunner that has been misfiring. Mechanics replaced all three ignition coils and it’s still misfiring and runs between 5-10 mph when misfiring. They then replaced an air sensor, and still having misfire problem. The car seems to start misfiring when ascending a hill and in damp weather. Please help! Thanks!


I would have hoped that if they went to the trouble and expense of replacing all of the coils the plug wires and plugs would also have been replaced.

While it’s not often done, unfortunately, my opinion is that Step One in diagnosing an engine performance problem is running a compression test. One always weeds out any potential mechanical faults that no amount of parts replacing may cure.

From memory here, I think your vehicle also has mechanical valve lifters and with mechanical lifters the lash should be inspected on a regular basis. Tight valve lash on one or more valves can cause an engine miss and top end engine damage if it goes on long enough.
If this were the case, the engine should have a slightly rough idle or stumble at idle now and then. (Usually, but that’s not etched in stone)

There’s a number of things that it could be but not enough info is known (manifold vacuum, compression, engine mileage, codes, etc.) to be real precise.


If it truly is related to dampness then, as ok4450 mentioned - the plugs/wires would be prime suspects - especially the wires. Take a spray bottle with water in it and have someone rev up the engine as if it is under load while you mist the spark plug wires. See if you can reproduce the miss.

What do you mean by: “runs between 5-10 mph when misfiring” ?

Is or was the check engine light on? If so, what were/are the codes? If its not on your invoice your mechanic may be able to tell you. Alternatively, many auto parts chain stores will read them for free. If there are any post the exact codes (format: P0123)


V6? I show this as a distributor based ignition system.

Based on yuor symptoms, and assuming as OK did that they must have changed the plugs and wires, I’d look to that first. Puttng the ignition traces on a scope should show the problem clearly. Especially with a spray bottle of water handy.

In essence I agree with ok4450 that not enough diagnnostic work seems to have been done. Perhaps a better shop would be able to get to the root of the problem.