so i came back from my first Testdrives with my new build 305 TPI Auto. IROC. Nearly Everything is fine, runs great ( i take it slow for the first cuple miles, she is a Flat nonroller)

But one thing makes me confused.

Car idles perfect and goes into closed loop in park....goes around 600 rpm ...no bogging just smooth.

but in Drive at cruising speed you take your foot of the gas and it just stays at 1500rpm and drives on its own whitout decelerating or engine braking. you have to step on the brake and force it to slow down .

As soon as you came to a complete stop engine RPM goes back to normal idle. Throttle valve linkage is free an blade shuts close like butter.

It feels weird and its very annoying to use the brake to "battle" against the engine every time you approach a red light in the distance.


checked a lot of things, tps, minimum air, spark timing is set , Cruise controll Actuator seems to be asolutely free and works if you switch it on

only thing i found so far is a mildy worn throttleblade shaft, i dont know if a leakage this small will have such a big effect.

please any ideas ?

thank you in advance

from Germany whitout any Translator tool :-)

 

With the smooth lower idle speed I would check for throttle shaft wear. With the engine off turn the throttle shaft the 1/8-1/4 throttle . Feel if any play side to side movement is felt. A temporary test would be to add a throttle return spring from the throttle arm to the cable bracket.

 

There is a bit play back and forth ( like shaft hole is worn out) but no play side to side ( in direction of the shaft itself)

Do you assume a vacuum leak or the shaft binding in a certain position and gets stuck ?

 

I think the shaft is physically binding. Even when new the shafts leaked slightly. Spr performance might help you with a rebuilt unit. But might try a return spring to confirm it’s worn out causing the issue.

 

I don't think the TB gets stuck at speed and unstuck at a stop. Weirder things have happened.

But, would a 5spd tune have the opportunity to hold a higher idle (avoids stalling) and the IAC just be allowing too high a cruise idle?

 

So i just did a smoke leak test an the shaft is leaking but not horrible. Also checked for binding but its absolutely free an the stock spring has plenty of power.

The 5 speed tune could be a Thing since the last owner has trown 2 cars together.
Is there a posibility to prove this ? Memcal number maybe ?

 

https://www.3rdgenformula.com/87/promcodes.htm

I don’t remember a t5 305 in 86 or what the chip code would be. This is 87 list . Don’t think I’ve ever seen a iac hang and cause acceleration. Can always watch data with winaldl, tunerpro rt or a old mr2500 scanner but seeing your location winaldl might be the best option.

 

The '86305 A/T should have an original broadcast code 8993AUM, which was subsequently replaced with the 8291AUM (1228291) if the vehicle was serviced under the TSB for driveability issues.

I don't know the M/T code(s), but if it is not one of the above, that could cause problems. My '86 TA has a similar issue where it would not idle along the street and stay below 30-35 MPH. The later PROM fixed that, among other issues. The 2.77 axle and TCC would lock at 28 MPH and never let it run below 30-ish.

 

I checked the prom and its a ABUN. Its not in that list but google says could be a 87 305 tpi . Correct?

I also rebushed the throttle shaft but no improvment.
It seems the ecm commands a higher idle while driving.

 

https://www.gearhead-efi.com/Fuel-In...nformation-32B

do you have a way to burn a prom ? Your right the abun is a 87 automatic

 

I got news
i think i have found the problem. I was going trought other troubles like gas smell and random engine cut off at cruising speeds. Found a stuck tank breather valve and a stuck charcoal canister solenoid.
the engine was sucking constantly additional fuel vapors trough the canister leading to high driving idle.

Damn that was a hard one. I would have never searched in that direction

 

That’s a crazy find, glad you figured it out