Rickgosk8

Stoked to finally be apart of this forum instead of just reading them all the time lol, so this is my ice breaking post here, unfortunately it has to be a problem. Anyways let's cut to the chase, I have an 86 300zx non turbo, and amongst a million problems I've encountered since my recent purchase I now am having this weird sputtering loss of power problem almost as soon as I get it in fifth gear,. Recent things I've done include: putting in New tps pigtail,(double checked wiring) did a complete timing job as soon I got the car but it ran fine for a while afterwards so it's not timing related I don't think, replaced pvc valve with oil change, and that pretty much wraps it up as far as engine components go. Now here's the kicker, the problem started right after I sea foamed my car, which I poured half a can through gas tank and rest through brake booster vacuum hose,,,, so a week after that I did more through booster thinking it clogged some stuff up, I added a lot more and let it sit longer,, and still the same exact problem,,, the sputtering doesn't exist in first 4 gears no matter what rpm but as soon as I hit 5th, it starts acting like it's trying die out, and every gear below does the same thing after it starts sputtering until I cut the car off and try again, most of the time it runs fine back up until 5th after I cut car off and on,,,, so any help would be sooooo frickin awesome and greatly appreciated, let's get this solved for anyone else having similar problems,,,,, -Rick

86tcoupe

sounds like a load issue, does it do it lugging some in 4th ,3rd up hill ?
It's possible the clutch is slipping too.
5th is more of a rpm reducer than power gear
the chemical additives , like SEA foam are known to dry out the old fuel inj hose causing it to granulate and clog the screens also.