1992 Suburban 2500 ecm

[Adam Boyd]

I have just picked up a '92 Suburban 2500 with the 7.4.  Got it cheap knowing it need a little work.  Body has around 200k but the motor is a newer crate engine. When you start the truck and will not idle with out adding fuel. Once you get it going it will idle but idle rough.  When it warms up it will run fine for the most part, has a slight miss coming off throttle. I'm assuming a vacuum leak.  When they did the engine swap it looks like they carried over all of the previous engines accessories so I'm going to start replacing things one at a time.  To get a place to start I tried jumping the A and B pins to see what codes were stored. No matter what I tried I could not get the light to flash, it just stays light all the time.  When you first turn the truck on there is no flash, just comes on and stays on.  Tried unhooking the battery for an hour or so to reset it but as soon as I turned the ignition on the light was still there.  First time playing with these older trucks, just looking for some tips before I tear into it.

[riverbanks]

For the hard start,I would disconnect electric connection to c.t.s,default is rich in ecm,any better? inspect wiring at it and all sensors,o2,wiring intact?

[Adam Boyd]

Went ahead and replaced the sensor, checked the wiring, replaced a couple of grounding straps that where broken and replaced the gasket on the tbi. No change. Still takes fuel to keep it idling. It's also running really rich, dumping fuel out the back. 

[riverbanks]

Strange?lean at idle, adding fuel to run,but overloaded, otherwise,give a good look at injectors while it's running,nice cone pattern?

[Adam Boyd]

Yep, injectors had a nice cone pattern. 

[riverbanks]

Darn truck,I am one to not condemn a ecm,on e test to try to access codes,one at a time, disconnect sensors,leaving each one off in a series,I think you have a wiring problem,I think?so with the adding of fuel to start,it's in open loop,then rich is when hitting closed or trying to,so,o2 sensor first,then map sensor,and while there, check vacuum port, clogged? or restricted,and another is to check timing wire,the one you disconnect for base time,firewall or under glovebox,not sure

[Adam Boyd]

That's the plan I think. Going to pull the ECU and look for visible signs of damage. The not being able to pull codes bothers me. I've also now lost the AC and my gauge cluster is being goofy so something electrical is going on. 

[riverbanks]

Start earlier up the supply,fusible link,leaving starter,2 I think,and one in line,leaving alternator,on intake,how about checking voltage at fuse box, battery voltage,both sides of fuses,stuff happens,potholes,just wearing on our wiring

[Adam Boyd]

I have gone through and checked all wiring and fuses.  Checked the battery and alternator.  Went back  and checked for vacuum leaks.  Replaced the tps, iac, map and o2 sensors. Checked the egr valve.  The truck now takes a little bit of throttle on startup for about 30sec when cold then runs and drives fine.  Still seems to be burning fuel though.  Still cannot get any info from the ecm with either the paperclip or a scanner.  I am wanting to replace it just to rule it out but am not sure if it would be the actual ecm or the prom chip.  Any ideas?

[Mikebtte]

Have you verified the timing is correct on the engine? could be getting fuel way to soon and not igniting good. Sounds like a possible issue as you said the engine was replaced. Easy to get off one tooth on the old SBC.

 

ECM's do not fail very often on the TBI engine. Pretty solid set up. Not to say it cannot happen. If it did you would need to replace the ECM, PROM is only what contains perimeters. Would need to be a like for like ECM, not a 350 or 305 ECM. 

92 was a mid year ECM. 91 was old style ECM and 93 was hit or miss on PROM or old chip style. 92 was a test year. If it is a manual transmission you would have better luck as the Auto was switching between 700R and 4L60E at random. 

[Adam Boyd]

I actually have a timing light on order, was next on my list of things to check. The whole ecm is some what confusing. I've found the correct ecm but the prom I can't seem to source. This particular truck has a 4l80e and 4.10 gears. Not sure how much it that makes a difference though. 

[Mikebtte]

I'll look up the old PROM list I have someplace. Upload it if I can find it.

[Adam Boyd]

That would be great. Thank you. 

[Adam Boyd]

Wanted to finish this, got it fixed.  Turns out the ecm was bad.  Replaced it.  Got communication with the ecm back and fixed my transmission's limp mode issue. The timing was also off. Disconnected the wire and it was at 16 degrees. Brought it back down to the recommended 4 degrees and it took care of the rest. Runs smooth now.

[riverbanks]

Thanks for the update,sounds like,party time