Hi all! I just joined due to some issues I'm having with an '89 T/A I bought.....one of my very good friends owned this car and was playing with it, until he had a sudden accident and passed away. I bought the car from his brother with the hopes of fixing it up and finishing it off in his memory. Part of the problem is that I don't know what exactly he did or didn't do, so heres the vehicle details and symptoms...

1989 Pontiac Trans Am 5.0 TPI.....when I hauled the car home, it wouldn't run...stuck a new set of plugs in it and it fired and ran, but VERY rich. There was talk of things being swapped and modified, so I did some checking, and found that it now has a '91 350 TBI pickup engine in it....no biggie,and may make a slight difference in how it runs, but shouldn't be making it run pig rich...further inspection showed 350 injectors in it....swapped in a set of 305 injectors from an '86 I had sitting around for parts. Runs better but still very rich....black smoke out the tailpipes at idle and especially during accelleration. Got my scanner on it and am noticing some odd things....when not running I can only get a few intermittent lines of data....what I did find odd is that it's showing the coolant temp sensor and air intake sensor at -40 degrees...unplugging both has little change. I'm beginning to suspect a problem with the PROM..I swapped another good ECM into it and have no change.....the other odd thing is that once I start the car that I lose what little datastream I had...the other thing that's strange is that I get NO check engine light at all when it's running rich, and can't retrieve codes either...I do get a check engine test cycle when keying on, but it goes off once it's started....also, it runs rich right from the get go, which tells me something is screwy, because at startup it should be in open loop, running off the computer presets...the PROM is the correct 16072667/APYR3506 for the car, while the ECM is a reman...tried another ECM(correct P/N)with the original PROM and no change...fuel pressure is actually a touch on the low side at 36 psi, but does spike over 40 psi during a snap throttle....

So where in the heck does a guy find a PROM anymore? GM has discontinued most, and I can't swap an earlier or later PROM in due to the VATS system on the '89, and speed density computers in the '90 and up models.....I DO have an '86 ECM and PROM which I could use if I could get around the damned VATS system...tried that already and no start obviously. So otherwise how does a guy get around the VATS to try a pre VATS ECM? I could give a darn less about the VATS...I just want to get this car up and running to honor my friend and let his family know that someone is trying to do something for him :S

ANY advice or suggestions would be welcome!!

 

you would need a VATs bypass kit and Maybe even a 91' ECM out of another car that is TBI'd!....I had a friedns who had a problem pretty simular to yours and the it never hurts to check the Distributor and see if it has anything do with the running rich issue?

running an ecm of a different year will cause problems the engine because the computer/ecm runs the injectors and the ecm is set for the that or you would have to find someone that can custom make you PROM for the cars set up.

 

It just accured to me that you might want to check the V.I.N. tag and see if the car had a TPI engine?...if you used another ecm from a tpi or carburated car that would make a huge difference there and even if you used the ecm from the 86' was it a TPI car?...the TBI would require the correct ECM set up!...just food for thought*

 

Start by replacing both ($14ea ) and checking the wiring to them.
Confirm on scanner that correct engine temp is being seen by the ECM

Your scanner should have shown DTC 15

Code 15 indicates a defective coolant temperature sensor or an open in circuit
Unplugging obviously has same effect as them being open circuit (indicating -40 )

CTS is a crucial sensor ;feedback from it controls all fueling.
If ECM "thinks" engine is cold it will add more fuel and if ECM doesn't see the engine
warm up it will maintain that over rich cold start fueling

The problem is the ECM is not seeing the correct engine temp input from the CTS

 

You just get one of the many online tuners to program you a custom chip with whatever you want on it ;
no VATS , no EGR , different fan on temps , different size injectors
Anything that is on the program can to changed

 

No such thing
All 86 - 89 MAF TPI ECM's are identical ; 305 or 350.

The programming on the prom inside is what determines it usage and if it is VATS enabled
The " non VATS " memcal / chip / prom , call it what you will
from your '86 ECM should work in your 89 ECM ( so long as it actually is a '86 memcal )
In any case you have to manually bypass the starter enable relay also controlled by VATS
to have a completely VATS free car

 

Ok....I feel I need to clarify a few things..

shelton, it IS a TPI car, which I stated in my post.

Vetteoz.....with the '86 ECM and memcal installed in the car, it will crank but not fire...I believe this is because the VATS sensor is not seeing a signal from the earlier ECM or PROM to fire the injectors..I don't see how installing a resistor bypass in the VATS will make any sort of difference, since the key is functioning properly, and since the car cranks, what difference will bypassing the starter enable relay make?

I wasn't clear enough in a few things...I understand that the ECM is the same from '86-'89...'85 is specific, as is the '90-'92 speed density ECM...only the memcal/prom differs, but apparently the '89 prom is a one year specific model, since '88 and earlier didn't have VATS, and '90-'92 uses speed density rather than MAF.

I have a hard time believing that both the CTS and IAT are bad, although it is possible...they share the same ground line along with the TPS, and all 3 have a good ground signal and 5v reference. The reason why i suspect the prom is due to the lack of datastream and intermittent datastream, along with the lack of a check engine light when it's running rich, no code retrieval, and strange data I AM recieving when the scanner is on the car. The CTS appears to ohm out ok, as does the IAT.

The one problem I can see with using the '86 ECM and PROM is that this combination is setup to use the 9th injector, and the '89 model doesn't have a 9th injector...other than that I don't see any serious differences, at least to determine if the PROM/ECM is the issue....

 

1st up ; understanding how VATS works
https://www.thirdgen.org/vats_passkey_system

A VATS enabled tune in the ECM looks for a fuel enable signal FROM the VATS module ; not TO it

Only relates to doing a full VATS bypass to completely eliminate it
Have you confirmed that nobody has already done the common starter relay bypass if which case you would get crank/ no start regardless

Just means you may have longer cranking on a cold start before engine will fire up


When all said and done ;
I would be correcting the CTS indication problem 1st
and then seeing where you are at