98 4Runner with a miss under load

We have a 98 4Runner with just over 250,000 km that has developed a miss. The car starts fine & backs out of the drive uphill OK but as I accelerate it starts to miss. When I give it full throttle & the passing gear kicks in it will get up to full speed & then will run fine at speed above 50 to 60 km/hr. A full tank of gas seems to help a bit. The odd time it will run smoothly at a low speed but then will start to miss again when I'm putting a load driving uphill. I've added some STP to see if it has any water from condensation that might be causing it. I did a tuneup last spring plugs, wires, oil & the usual.

When I took it into the garage they hooked it up to their machine but couldn't find anything wrong. The mechanic diagnosed it as having a blown head gasket but I'm not 100% convinced. Would anyone have a suggestion to try before I drop $3000 plus I don't really have at the moment?

Are you getting any trouble codes? Check engine light on?

Hi Brian

The check engine light is on but has been for the past 15 years, regarding the codes I'm not sure about what the mechanic found but he said it wasn't anything out of the ordinary. I just went out for some short drives the past few days & the truck ran smoothly this time with a slight miss. The only difference is the outside temperature which is close to 50 F. This is mind boggling to me trying to sort out why it runs terrible for over a month & now it is running smother again although they were only a short trips.

Pull the ECM fuse (Under hood by battery) to delete all the codes. Drive it for a few days until the light comes back on and check the codes. Only the most recent code will show and post the code.

Will the Engine Control Modul fuse be clearly marked in the panel under the hood? I'm in a wheelchair so will have to ask my wife to reset it. Thanks Brian.

Yes, the cover has a diagram showing the fuse identities.

When you did the tune-up, what brand of plugs and wires did you use? DENSO and NGK are the only brands which work reliably.

I had the garage do the tuneup & not positive of what was used. It's been a year since it was done.

To me, it definitely sounds electrical. It is impossible to be sure without an error code. A blown head gasket would exhibit some pretty radical problems, antifreeze in the oil, cooling system overflow and boiling, white smoke in the exhaust, anyway, something pretty difficult to ignore. I think your mechanic was remembering the previous model of 4-Runner which had a problem with head gaskets. Not your engine. It is a tank. Just to be sure, you have the 5VZ-FE six cylinder, correct?

yes the 6 cylinder

Sorry that it has taken me so long getting back to this thread. I added some carb cleaner & there has only been a slight miss up until a few days back when it started to act the same way again. I added another bottle & now it's running fine again Do the fuel injectors need to be rebuilt every so often? I had a feeling that there was water in our gas since I've filled up at PetroCan both time when the trouble started. I've always used the premium grade in our old junker since day 1 & I don't believe that there is any difference in PC's gas besides using Tactrol in their fuel.

No, you don't have to clean the injectors periodically. Only if they get gunked up. If it's running fine, don't mess with it.