Car wants to die stopping abrupt or slowing down

[Teds89IROC]

LT1 and T56 swap into my IROC. The car starts fine and idles fine but I noticed when I'd be driving and come up to a stop I'd push in the clutch the rpms would drop to about 1000 like normal and then fall below and want to die. If I see the rpm's fall below and tap the gas it bumps back to 1000 and idles perfect. I have a fuel pressure gauge and that stays constant as well as my volts while it's trying to die.

I tried cleaning the IAC valve and no change so I bought a new one, did the reset process and no change. Then a few stops later it worked perfect, but has happened again. Is there a learning curve with the computer for this? Any other ideas?

[ThoR294]

did you fully clean the throttle body?

[Teds89IROC]

I used the parts washer on the intake manifold and valve covers when I swapped cams but I can't remember if I did the TB, I think it looked clean so I didn't bother.

[WildBillyT]

What happens in neutral if the car is parked?

[sweetbmxrider]

I'd use data logging software or a scan tool and watch the IAC counts when the issue occurs. Stock throttle body?

[Teds89IROC]

nuetral parked, it idles beautiful. Scan tool can't happen, I didn't get to wiring the diagnostic port yet lol. and yes, stock TB.

[WildBillyT]

Just to confirm, if you have the trans in N, go under the hood, and blip the TB it comes back to normal with no issue?

[LTb1ow]

IDK the history of the car but was the PCM setup for an automatic and now its running a T56?

[Teds89IROC]

Just to confirm, if you have the trans in N, go under the hood, and blip the TB it comes back to normal with no issue?

yup, no issues. Only while driving if im at like say 2500 rpms and i push the clutch in and brake, sometimes the rpm dips below 1000, other times it didn't after changing the IAC.

IDK the history of the car but was the PCM setup for an automatic and now its running a T56?

the PCM for the LT1 was orginally for a 6-speed car, and when I spoke to the tuner I gave all the details that it was still a T56 and being swapped into an 89 IROC.

[ThoR294]

I'd try just cleaning out the throttle body... might as well since you haven't done that. It sounds like there is gunk somewhere randomly getting stuck lol.

[Teds89IROC]

Yea, I'll take it completely apart and clean it in the parts washer and see what happens. The fact that it doesn't always want to dip below 1k rpm coming to a stop and idles perfect just revving it in N makes me think its nothing too serious

[deadtrend1]

Sounds like a tuning issue, not the throttle body. For one try and get your obd plug wired. What is the actual spec on your car. Its a lt1 swap? Or an lt1 pcm? What year? Who did your tuning? Seems like is something with the vss in relation to the idle decay

Also, 1000 rpm idle is high. No massive vacuum leaks like the egr at the back of intake?

[Teds89IROC]

Sounds like a tuning issue, not the throttle body. For one try and get your obd plug wired. What is the actual spec on your car. Its a lt1 swap? Or an lt1 pcm? What year? Who did your tuning? Seems like is something with the vss in relation to the idle decay

Also, 1000 rpm idle is high. No massive vacuum leaks like the egr at the back of intake?

95 LT1 with a cc306 and T56 swap. PCMforless did the tuning. I explained it was a 95 LT1 ECM with a T56 but in an 89 car with a mechanical speedometer.

could be 900, hard to tell on the gauge. No leaks I can hear or tell. If it was the tuning then why would it act normal occasionally?

[WildBillyT]

95 LT1 with a cc306 and T56 swap. PCMforless did the tuning. I explained it was a 95 LT1 ECM with a T56 but in an 89 car with a mechanical speedometer.

could be 900, hard to tell on the gauge. No leaks I can hear or tell. If it was the tuning then why would it act normal occasionally?

What does the coolant temperature look like?

Does the problem show up right when you first start driving or not until you've cruised around a while?

[Teds89IROC]

It was tuned for a 160 t-stat, fans come on not much above that.

It happened the first block i turned onto last night after leaving the driveway. After a few minutes the next couple of turns and stops it acted normal, so I thought it was just a learning period for the new IAC. Got to my destination, pushed in the clutch at about 2500 rpm and they started to dip low like it was going to die but I couldnt tap the gas. It caught itself and rpms shot back to idle speed. On the way home the first stop i made it acted normal. A few minutes later approached a traffic light and it wanted to die.

EDIT: Unless the tune is to run in constant closed loop, bypassing a VSS, which from what I read I think it uses the 02 sensor reading. I'll clean the TB regardless and get in touch with the tuner to find out what was done regarding a VSS signal. I think I see a replacement VSS with a cable output as well for a mechanical speeodmeter which might be what I need

[Teds89IROC]

PCMforless is telling me I need to drill a small hole in the TB blade 1/8"-1/4" and that it's a common problem. I'm trying to get more info from them.

[sweetbmxrider]

Don't do that. For the love of god buy an aldl cable, download free software, and hook up your obd port! You could check your tps voltage but I doubt its an issue. Should be the blue wire. You want around .67 closed throttle and about 4.5 volts WOT.

[WildBillyT]

Don't do that. For the love of god buy an aldl cable, download free software, and hook up your obd port!

This. 1000x this.

Datamaster might tell you right away what's wrong.

[LTb1ow]

Leave drilling holes in TB for half ass perf shops and idiots.

If its not a stock TB you can make the IAC passage correct for an LT1 intake with some skill.

[BonzoHansen]

PCMforless is telling me I need to drill a small hole in the TB blade 1/8"-1/4" and that it's a common problem. I'm trying to get more info from them.

why do you have a big by large cam? that is an old carb trick which also was sometimes not really needed!

[Teds89IROC]

From what I've read its been done by other 306 cam owners but its been described as a crutch. Unless it's an aftermarket TB where the IAC port is different than stock.

I already have an ALDL connector from my TPI tuning software. If I wire the pinouts on the 89 diagnostic port with the LT1 ECM I can still read the car with the same cable? I'm also using a macbook, will this be an issue and what is the free software?

[ThoR294]

This. 1000x this.

Datamaster might tell you right away what's wrong.

+2 to this

[sweetbmxrider]

From what I've read its been done by other 306 cam owners but its been described as a crutch. Unless it's an aftermarket TB where the IAC port is different than stock.

I already have an ALDL connector from my TPI tuning software. If I wire the pinouts on the 89 diagnostic port with the LT1 ECM I can still read the car with the same cable? I'm also using a macbook, will this be an issue and what is the free software?

I have a 238/246 cam in my car with no issues. I do not know about wiring the diagnostic port.

[deadtrend1]

Funny how LS1's have their blades drilled from the factory...

[sweetbmxrider]

Funny how lt1's don't....

[Blackbirdws6]

Shame you aren't using a OBD-II PCM. My computer is in a box with a tune for a CC306 6spd which I could lend to see if that would make any difference. I think your issue may be the tune but PCM4Less has done those before so I would imagine they are using default values. As mentioned, start data logging and report back. No need to drill a hole in the TB.

[LTb1ow]

Funny how LS1's have their blades drilled from the factory...

Funny how LT1 intakes have IAC passages, and LS1's don't.

Funny how chasing split BLM's for all eternity from a hack job is fun.

[deadtrend1]

Yea, youre all right. Hope all you experts can help Ted out then. Good luck

[Teds89IROC]

I found the schematics to cross reference the LT1 to my 12 pin diagnostic port so I'll be all set. Even have a windows laptop to borrow for logging. What software do you guys recommend I dl?

PCMforless told me it's a very common setup so they already had base figures they used, supposedly...

[WildBillyT]

http://www.ttspowersystems.com/DataMaster_downloads2.html

[sweetbmxrider]

Yea, youre all right. Hope all you experts can help Ted out then. Good luck

Stay classy. What are you accomplishing or improving by drilling holes into the throttle body?

[ThoR294]

Need some datalogs before we just start throwing parts or drilling stuff.... :)

[Teds89IROC]

Update - The car runs perfect!

I wired up the diagnostic port and tried to datalog but I couldn't get a signal (which could be security on the work laptop) At the same time I was doing research and the problem sounded like a VSS issue. Years back when I swapped in the T56 I had the tailhousing machined to accept a mechanical speedo drive. I assumed this replaced the VSS. Under this assumption, when I swapped in the LT1, I asked PCMforless if the VSS could be tuned out along with other things on the build sheet. After reaching out to the tuner, more research, and actually looking under the car, i saw the vss right next to the speedo drive. So I bought a connector, wired it up, and it ran perfect tonight. I guess I thought the VSS was the skip shift or something when I put the trans in :doh:

I also got my wipers working too, great night!

[sweetbmxrider]

Good stuff, thanks for the update.

[Blackbirdws6]

Not bad!