I’ve been looking everywhere for anyone who may have solved this issue for themselves and can’t find it. We have an 86 Z28 305 TPI. Origjnal everything as far as we can tell. We have had it a week and going through it now for my son. The tranny does have a small leak and finding that is next on my list but before I dive in, what would cause the car to severely jump when put into D or R? In some cases it will actually cause the rear tires to squeak. Tach doesn’t work but I used a timing light to check it in P and it leveled around 900 to 925. In D or R it dropped to about 700 or less. I’m not sure if thats too high but being I’ve never worked on a 700r4 I’m not sure what to check next. Thanks for any help.

 

Check your brakes, they might be in need of a rebuild. They should bite hard enough to prevent any movement when you shift into gear, sounds like the rears might not be working.

When mine was an automatic it always had a good amount of "oomph" when shifted into gear, since these cars don't require the brake to be depressed to shift from park I tried it one day and it lurched quite a bit, something that it did not do with my foot on the brake. That's why I think you make have some brake issues.

Those RPMs may be a tad high but not terribly so.

 

Sounds like the idle is WAY high. Should be around 650 or thereabouts, and shouldn't change much when put into gear, because the ECM should adjust it. Might want to check THOROUGHLY for vacuum leaks, verify that the IAC works properly and isn't stuck, and then go through the minimum air adjustment. Although, that being a MAF car, and a crude one at that (just think back to what you were using to play CoD and GTA on back in 1986), a vac leak would probably cause far worse problems than just a high idle, so most likely it'll end up at the IAC.

Not a transmission problem in any case. The trans behavior is just the symptom of whatever the problem is, not the problem itself.

 

This is very helpful. Thank you

 

Much appreciated. I read all about these tips before posting but read conflicting information the idle in park. Good to hear it from a reputable member profile. I’ll check brakes first then go into all this computer stuff. But first, I’m going to find the leak in the transmission and get that repaired.

 

Make sure that the TV Cable is not adjusted to full Line Pressure all the time.

 

any quick tips on how to do that? First time working on automatic. I checked that line yesterday at the tb and had moderate slack. Is that normal?

 

Somewhat, yes.

The problem Vortec was referring to, is having the cable pulled partway out all the time. That will make the line pressure go high when it's not supposed to, and everything the trans does near idle, will therefore be harsh. If that was the case, the shift points would also be STUUUPID high with very light throttle; when it should shift up to second at 15 mph or whatever, it would hold on to 1st until some MUCH higher speed. If it's not doing that, then the TV cable is probably OK, at least in that regard.

​​​​​​​Have you done the TV cable setting procedure?

 

I have not. But I will see what speeds it’s shifting each gear at.

 

The 1986 does not have the forward accumulator and the reverse clutch pack does not use a waved steel. This will make forward and reverse engagement firmer. 1987 and later units had the above mentioned, which made both engagements much better. As stated, the idle being high will make the engagement even firmer. You might want to put a pressure gauge on this if the lower idle still makes the engagement too firm. Pressure should be around 70 and 80 psi. at the most at an idle when warm.

 

Well I appreciate the advice here. Good to know about the 86 tranny differences. We are still chasing down a leak on the tranny. I’m hoping it’s not from the pump seal and not have to pull it out. We are at the least going to change oil, pan seal, change filter, test synchronizer, and put in new servo. If we have to pull the tranny we will also replace the pump seal and inspect the converter. Any other recommendations that would help with the shifting in either scenario?

 

100% its the idle, he said its idling at 900-925, that's wwwaaaayyyy too high. it should be between 700-750 neutral/park, and drop around 50 in gear, get the idle fixed and go from there., its also going to shock the entire driveline every time its put into gear and will eventually break something.

 

thanks for your reply. We got the tach fixed and now I can read it in car. It’s around 950 cold. Definitely too high. I’m not used to computer cars as I’ve worked on older cars but I’m learning. Anyone have a reference in the correct procedure for setting idle?

 

I've never messed with a TPI, I went right from carb to aftermarket EFI, but, the first thing I would do is search for vaccume leaks that would cause a high idle, the way I used to do it was to pull off each vacume hose 1 at a time at the intake and temp plug it, the one that drops the idle down when pluged is the leaker, then trace it from there, once that is done see if there is an adjustment on the throttle body to lower the idle, im kinda at work so I can't look in my hays/Chilton manual right now...either one is a good investment
Also....is your heating/AC vents stuck in defrost? its vacume controled and it defaults to defrost no matter what is selected when it looses vacume, and the hard lines in the system tend to get brittle and crack/break causing a vaccume leak and the sys to loose vaccume