Hey there.

I own a 98' 3.4L that has been having reoccurring issues with the spark plugs and ignition coils.

I'm not sure which is occurring first - the ignition coil or the spark plug, but I'm fairly certain it's the spark plug. The spark plug will either explode out of the engine or stay in the threads and not work any more. Usually it strips the threads with it and 3/5 times its happened it destroys the ignition coil. It happened again today for the fourth or fifth time over the last 5 years. 3 times it has stripped the thread and destroyed the ignition coil along with it and 2 times the spark plug has stayed in the threads, but the electrodes on the spark plug have squeezed together to the center electrode making the spark plug useless.

I've had all the spark plugs replaced twice. The wires have been replaced within the last year. I've had 3 different mechanics look at it and none of them have told me that they've seen this as a common problem on any 4runners they've worked on, but have seen it happen a couple times before. All of them have said they've only had to helicoil a 3rd gen maybe once if at all.

I would normally change spark plugs and wires myself, but due to the fact that its been a reoccurring issue, I've had any issues with the spark plugs/ignition coils done by a mechanic.

I'm the second owner and I bought it from a dealer, so I'm not sure what kind of work the first owner did to it beyond the basics, but I'm at a loss as to what it could be and I'm hoping someone here might have an idea of what could be going on.

Thanks for your time.

For a "common problem" this is the first time I have ever heard of it. When you say it is stripping the threads, are you referring to the plug or the head?

If the head is stripped it won't hold a plug long no matter how snug it feels.

It's definitely not a common problem on 4Runners, except for mine seemingly. It strips the head. I've had to have it helicoiled twice.

Here's a picture of what it does.

I had 1 plug do the same thing, try to blast its way out the hood, took the coil with it. The only symptoms prior to this was a strong miss about 150 miles before the final destruction occurred - the miss went away after about 20 miles and was smooth until the last moment. I asked mechanic if he'd seen that before, he said several times. He repaired it and that was 50K ago, all good since. He said it's weak metalurgy, don't know what else would explain it? So its not uncommon but probably not pervasive to all runners, just the unlucky ones.

If it only happened one time, I wouldn't be as concerned, but this is the fifth time it's happened.

Bummer man, sorry to hear that. So each time its a different plug? When was the TB done?

Seek out 'The Durk' on here, he'll have a clue on this. Luck.

Im going to stay tuned to this thread. I have not heard of this in a 4Runner, early Expeditions, but not a Runner.

Have you considered replacing the head, I have seen them go for about 150.00 completed.

Awww.....

Plug blow-outs are not common, but it does happen--usually only once when fixed with a Time-sert. We see it once or twice a year on here. I assume the original cause is someone overtorqued a plug in the soft aluminum head, damaging the threads. The key question was already posed: Was it always the same cylinder, or five fails in five different cylinders?

If it's just one cylinder, the fix is a helicoil or Time-sert (preferred). If that's been done and then failed again, it's new head time. If it's always a different plug, I have to assume some musclebound moron overtorqued all of them, and they are all going to fail--two new heads or six Time-serts/helicoils is the fix.

The real headscratcher is the 'stays in the thread but compresses the electrodes together.' I have no idea how you accomplish that on a non-interference engine like the 3.4L V6 and I've never seen a report like that. Maybe @Singtoe or @EatonKyle can shed some light.

This time the blow out was on an already helicoiled slot. It was done about two years ago. Most of the other times I believe it's been on different cylinders. I've had to have it helicoiled on the 1 and 5 cylinders and this will be the second time on the 5 cylinder. It's sounding like my best bet at this point is to replace the head. I'm wondering it's going to continue doing this though without the problem diagnosed.

The timing belt was done 10k miles ago by the way.

That pretty much sums it up. Cylinder heads can be repaired, but it would probably be cheaper to buy some good, used, already serviced heads.

The plug gap closing up, is usually a sign of detonation. So what is causing the detonation? A likely possibility, and a scenario I've seen before, is an improperly installed heli-coil extending into the combustion chamber. This causes a hot spot, and subsequent detonation. Once the spark plug gap is closed, the gasoline being continuously injected eventually hydro-locks that cylinder, since the "repaired" plug threads are already weakened, the path of least resistance is the plug shoots out of the head.

I would suggest pulling the heads, and have them fixed properly, or replaced.

What kind of detonation? I mean, in normal life, there is detonation in a cylinder from 300 to 2500 times a minute. That's what we want. Do you mean premature detonation as in a backfire?

How do you hydro-lock a reciprocating cylinder with a working exhaust valve? Won't the air/gas mixture be expelled, even if unburnt?

Not saying you're wrong. Just trying to understand.

I thought it was obvious, but yes, detonation as in pre-ignition, AKA Engine Knock. As in ignition of the air/fuel mixture which takes place too early, while the piston is still traveling upwards in the bore, which causes extreme cylinder pressures.

Liquid doesn't exit the cylinder as readily as a burning gas vapor. It doesn't take much liquid gasoline to hydro-lock a cylinder.

I didn't say it and just assumed it would be fairly obvious, but the extreme cylinder pressure from Detonation/ Pre-ignition/ Knocking which can deform the spark plug electrodes, and severely damage the piston, can also shoot a plug out of the head, particularly if the threads have already been compromised.

I figured that's what you were saying but I've never heard pre-ignition/knock/premature detonation referred to as simply detonation. It just didn't make sense to me reading it. But a little Googling reveals people do call it that, so I just learned something. It still sounds silly to me if you leave off the premature.

As to the hydrolock, I've heard of fuel hydrolock due to a malfunctioning injector/ECM (or carburetor if there was one) dumping LOTS of fuel but not just from lack of spark into proper ratio air/fuel mixture. People have malfunctioning plugs and coils all the time and don't blow their engines apart. So that part doesn't make sense to me and still doesn't.

But I had no idea that pre-ignition could collapse plug electrodes. That does make sense and I'm glad to learn that. I have to believe that's the only thing that COULD do that. Thanks.