scotty64

In have a 1964 Chevy truck that has a TPI from a late 80's Camaro (I apologize, and have gone through the identification threads, and I still can't figure out the exact year of my TPI). I didn't do the engine swap--it was done by the previous owner. I'd be happy to provide other information or photos if needed.

In any case, when coasting from freeway speed and slowing, the truck will die at about 25-30 mph. There is no bucking or other drama--it just stalls. Kept below about 40 mph, it has never stalled. I have:
-confirmed correct operation of the locking torque converter (it unlocks with a touch of the brake pedal), and I have also disconnected the wire to the torque converter, and also tried downshifting into 3rd--no help.
-noticed that when coasting at about 50 mph, the engine RPM's are about 1300. When I apply the brake, the rpm's immediately drop to about 700 (this is just a brake tap--so me thinks the engine computer is dropping the rpm's).
-confirmed that there are no codes being reported by the computer.

If I shift into neutral and goose the gas while left foot braking I can prevent stalling.

My wiring harness has two labels on it that run to sensors at the front of the TPI manifold--one is labeled "throttle position sensor" and the other is labeled "idle air motor".

My research so far make me think that the idle air motor is not working correctly, and I'm wondering if pulling the intake boot off and spraying things with Seafoam, Berryman's, CRC MAF cleaner, etc. might work as a "first try".

I'd greatly appreciate any help, and would be happy to answer question.

Scott

Black88Z

Well, I think your on the right track. I would check/clean the iac and see if it helps any.

Id also check the tps, make sure its right at .54v, and check that its functioning correctly. ( while hooked up, slowly open the throttle all the way to WOT (should be @ or above 4v), and then back to idle. The number should climb steadily and by single digits, if its jumpy its flaking out on you and will need replacing)

Could you have a vacuum leak somewhere? Have you checked how much vacuum your pulling?

Im sorry, there was something else that stood out to me, but i forgot it while typing... Give me a few and im sure ill remember it

scotty64

Great info. I'm an electrical system moron (actually, I'm pretty much of an all around moron, but especially with electrical). Do I check the TPS by disconnecting the plug and hooking my meter to the leads on the wiring harness, or if I leave connected:
1) Does the truck need to be running, and
2) where do I put the leads from my meter

I'll check the vacuum and report back.

For cleaning the IAC without removing, does anyone have any good tips, tricks or suggestions?

Thanks again. Scott

scotty64

I know more now. System is a MAP system, and has no MAF. I pulled the throttle body, and things are a little gummy but not bad. Pulled th IAC and will clean it. Also found a cracked vacuum cap and will replace. I don't have a vacuum gauge yet. Two questions:

1) The coolant hoses that are supposed to heat the IAC are not hooked up--is this a problem, and could this be causing my problem?
2) If I clean the IAC, and chech the TPS voltage, and all is well but I still have the stalling problem, what's next?

Thanks!

S

Black88Z

lol

as far as how to test the tps, you leave it hooked up, you put the key in the ON position but dont start it. Look up how to check tpi tps on youtube and theres a few vids on it, a good one with a 3rd gen trans am.

The coolant hoses not being hooked up shouldnt make it stall, is it real cold where you live? If not, you should be fine. Ive done the bypass and never had any problems with it.

i would just spray the cleaner on it liberally, let it soak, and maybe use an old toothbrush to lightly scrub it to get it clean as possible.

Make sure to check your vacuum pressure, and if you can, check timing just to rule that out too.

If its map, you could possibly be having a problem with the map sensor. If you try these things and still no dice, i would look into getting/renting/loaning/whatever lol, a scanner tool. You can hook it up and run the engine and the tool will show you exactly what the computer is seeing.

scotty64

So I am heading back over to my Dad's house tomorrow to work on the problem, but I have been doing some reading. First, I had noticed that when I took my foot off the gas at 55 mph, the tach would show about 1300 rpm, then when I would just lightly tap the brake the idle speed would decrease to about 700 rpm. I now know this is normal operation--the ECU is shutting off the injectors (and they should come back on at about 40 mph, which is just around where the truck dies). What is not happening is that the injectors are not turning back on, and possibly the IAC valve is not operating as it should. But I replaced the IAC and that didn't help, so the IAC may not be getting the correct info it needs from the ECU, and the ECU makes that decision based on input from the VSS.

Based on my reading (and the fact that the speedo very occasionally gives some funky readings and bounces around at low speed) I suspect I have an issue with my VSS. While it could be vacuum, or the MAP sensor, or the TPS--the otherwise excellent drivability suggests the VSS. I read about testing it by monitoring A/C out when turning the rear wheels (even by hand).

I am taking the things necessary to check vacuum, the MAP, and the TPS. I'd appreciate any thoughts or suggestions as to whether or not I am on the right track. Thanks.

scotty64

Hi all. I thought I'd post the solution. It turns out the VSS (vehicle speed sensor) on the tailshaft of the tranny was hooked to the speedometer, but no the ECU. Once connected all of my problems were solved.

vetteoz

iTrader: (13)

Well documented problem
If the ECM does not know road speed it can not adjust accordingly
Also the fueling is locked in cell# 4

https://www.thirdgen.org/forums/post...740-post4.html
https://www.thirdgen.org/forums/diy-...cause-blm.html

https://www.thirdgen.org/forums/diy-...une-while.html