Weird diagnostic prob on GMC tbi 350 truck. Blms are 0??

[GNVAIR]

I have been trying to diagnose a drivability problem with my 1990 GMC K1500 truck with a tbi 350. It has been sputtering really bad when it is cold. It runs fine when warmed up.
It has always idled high (1000-1100 rpms) so I figured I would fix that as well.
I hooked up my Snap On scanner and the tps at idle is .56 volts
The IAC counts are 3
The blms are 0. Yes I said 0.
When it was in open loop they stayed at 126. When it went into closed loop they dropped to 0 and stayed there.
This truck is a 90 and has an ecm that is similar to the Turbo Regals with the small removable cal pack and the larger E Prom chip.
Anyone heard of something like this?

 

[larry33kc]

I just got done dealing with my 89 that had drove me nuts with a sputtering and popping problem, after changing every sensor, fuel pump, filter, sending unit,tank,by passsing lines, checking grounds everything, even motors ( I took out a 388 stroker motor) I found out the ECM went south, so don't rule that out

I didnt want to belive the ECM was bad so I changed everything because I kept listening to my Bro in law about how much the ECM's cost, till I found out they were about $80 at the parts store, but I got one from my other BIL for free.

If you need one check the Full size chevy board they are about $25 there

HTH

 

[Drac0nic]

if you need a box I've got a 1227747 I'll ship your way for $20+shipping, I kept one around after my Astro fried one, along with the alternator and battery. Beats paying $80-90 for a reman.

 

[GNVAIR]

Hey guys. I appreciate the replys and am already leaning towards the ecm. I hooked up the scanner again and now it wont go into closed loop. The TPS is completely missing from the data stream. The O2 cross counts stay in the single digits even though the mv's are reading properly.
It pisses me off since the truck had the ecm replaced once before for something stupid.
But I am going to try locating an ecm this week

 

[steve hughes]

May be the throttle body gasket. Very common problem on tbi chevys.

 

[bruce]

Originally posted by GNVAIR
I have been trying to diagnose a drivability problem with my 1990 GMC K1500 truck with a tbi 350. It has been sputtering really bad when it is cold. It runs fine when warmed up.
It has always idled high (1000-1100 rpms) so I figured I would fix that as well.
I hooked up my Snap On scanner and the tps at idle is .56 volts
The IAC counts are 3
The blms are 0. Yes I said 0.
When it was in open loop they stayed at 126. When it went into closed loop they dropped to 0 and stayed there.
This truck is a 90 and has an ecm that is similar to the Turbo Regals with the small removable cal pack and the larger E Prom chip.
Anyone heard of something like this?

A 0 in the data stream for one item is odd. Try removing the chips and reseating them a couple times, and the connectors at the ecm, before swapping it out.

 

[GNVAIR]

Thanks for the advice Bruce. Your advice is always appreciated.

 

[Little6pack]

My 87 has same problem

Wow my 87 TBI truck does this, but all the time.

 

[TurboJim]

Silly thought, but I have seen it before. make sure it has the right ECM in it and you ID it right on the Snappy Scanner. If not, the data stream will not coincide with what the Snappy is deciphering, and yo will get erroneous data. Kinda like when it says Communications mismatch, but it doesnt KNOW its wrong.

 

[GNVAIR]

I had to replace the ecm 2 years ago and it was replaced with the correct # according to what was in the truck when I got it.
The Snap On scanner did not ask for ecm id# though.

 

[TurboJim]

No, it doesnt ask for the ECM number, but when you ID the car to it, you know, 10th vin, 8th vin, 3rd vin and sometimes 4th vin, then the scanner knows data is supposed to be received in a certain order. I have seen over the years only maybe a handful of times, cars come in with a driveability concern for example, and found it to have the wrong ECM in it, by the data getting spit out of the ALDL.

If you know yours is right, then thats cool and the ECM is probably bad. The Snappy is usually very good with GM's. I have only had problems with mine on Chryslers, whick a firmware update from Balco took care of

 

[bruce]

Re: My 87 has same problem

Originally posted by littlesixsteve
Wow my 87 TBI truck does this, but all the time.

You might pull your ecm, and read the 4 letter code on the prom. Then call the local dealer, and see if there is an update for what you have. I think at one time they had a problem with some false O2 codes in the 747s. But, the original thread was about the numbers changing to invalid entries. While looking at the SES light is the same end result, might be very different causes.