Any GM TBI experts? Need Help!

bannerp

bannerp

Well-Known Member
Joined
Dec 4, 2011
Location
Newland NC
1994 Suburban 350 TBI auto trans. A little back story truck was towed in to our shop last year say August found no fuel pressure replaced pump, strainer, fuel filter. Replaced dizzy cap, rotor, plugs and wires ignition coil.Dude basically said hey I'm gonna restore this thing it was my dad's make it right. Got the plugs out 3 out of 8 were covered in severe oil ash and carbon. Got truck running and it oil smoked severely bad. He said he knew it smoked and always had.

Fast forward to October towed back in found distributor pickup was bad no injector pulse. Replaced with new unit truck ran fine.

Fast forward to End of November oil cooler line blew apart he kept driving it till it locked down. Replaced engine with Jasper reman. Noticed he had it to another shop and the TPS, IAC, and EGR valve were all new. Got the engine installed and it runs like crap for about 1 minute. Choking spitting sputtering pouring black smoke like it was running real rich. So I Replaced coolant temp sensor and pigtail no change. Replaced map sensor no change. We let him drive it for a few weeks and still no change. Now I didn't have access to an OBD1 GM scan tool so basically j was just going off what I knew could be the culprits. Coolant temp reads correct. Map voltage is correct. Cleaned all throttle body vacuum ports and passages. Still runs rough when cold. Once you let it go about 1-2 minutes it clears right up runs perfect rest of the day. I've checked injectors they don't leak after truck is shut down. Here's the rundown

New TPS IAC valve EGR valve ignition coil distributor fuel pump ( pressure checked within spec) timing is 2 degrees with spout unplugged. New Echlin Map sensor New Echlin coolant temp sensor. Ran axillary grounds to battery and block to frame. Map is reading 4.18v koeo map has ground no resistance. Coolant temp reads correct. No Codes in ecm.

Any input would be appreciated. I don't like throwing parts on cars without diag but till we got our hands on an Old Snappy Modis I had no choice. These are the things I have checked. Thanks NC 4x4 Fam.
 
Anybody put an O2 sensor in it yet?
 
There is thing (i forgot what called) its on the rear of the TBI.
Its does the fuel pressure, its easily rebuild-able. its li ke 2" square.
[Its like a fuel pump gasket with a spring in there.]
They can get wonkier at times.
.
I think Mac5005 right....
also
check for missing or torn gasket on the bottom of the tbi.
 
I'm not calling myself an expert but I have a 4.3 TBI in my Jeepster and have had to chase issues with it. I tested and replaced everything and read the codes and chased those items before finding out that my ECM was actually bad. It caused every issue in the books. We eventually hooked it to an old ignition reader and saw that it was firing erratically. ECM was $169 and replaced that and it did great. If you are trying to read the codes with the jumper wire, record it with your phone and then you have a video that you can replay to verify the codes that it has thrown.
 
I campaigned an AMC 304 with a built from scratch (parts) GM TBI (custom fuel/timing tune), built another for my brothers 258, and may have answered some questions on @Granny 's old YJ (now Brad's, unless he's sent it down the road?)...

ALL of the above advice is spot on... and I can attest the "Parts Cannon" approach is both wallet draining and futile 🤬
 
Junkyard 7747 ECM.
Harder to find these days on trucks.
So find a van and pull the kick plate beside the drivers side rear passenger seat.

Ba Zing!
 
There are no codes I borrowed a Modis from a friend to check @Shreddinlettuce O2 sensor is new as well @Croatan_Kid thought it was going into closed loop before warm up. Also didn't mention all TBI vaccum ports are clean and block sanded the base plate to make sure no warpage and sucking air via vaccum leaks. TBI base gasket is new.


Just tried a reman PCM from orielly no Change. Gonna rebuild the regulator and go from there.
 
bannerp said:
There are no codes I borrowed a Modis from a friend to check @Shreddinlettuce O2 sensor is new as well @Croatan_Kid thought it was going into closed loop before warm up. Also didn't mention all TBI vaccum ports are clean and block sanded the base plate to make sure no warpage and sucking air via vaccum leaks. TBI base gasket is new.


Just tried a reman PCM from orielly no Change. Gonna rebuild the regulator and go from there.
Click to expand...
If you still have scanner,

look up blm #s and STFT #s

Also should be able to see o2 voltages and/or back probe o2 sensor plugs with a multimeter.

Also should be able to see loop status from a decent obd1 scanner.

Last time I was chasing issues I datalogged a warm up sequence so I could play back and see what’s going on.



youtube.com

454 o2 sensor values. Video 1

78 k30 454 tbi swap. Monitoring o2 sensor value with timing bypass (tan wire) disconnected. Note smooth idle , but rich. Also not engine does not stall when ...
youtube.com youtube.com

The blm # pegged @ 105 (max) and the o2 voltage ( supposed to be switching from low volt near .1 and high near .9) confirmed it was too rich at idle/and lower rpm.

At idle the o2 voltage would stay near .8/.9 at upper range.

This was to confirm fueling issue and not an ignition issue.

You can pinch off return/supply of fuel lines to test fpr function, but likely in factory app there is no test port or rubber line to do so.

I made up a filter canister with a test port for checking psi.

Some fuel inj psi kits have adapters to add test ports inline between the sections of hard lines and the fittings they hook to.
 
Mac5005 said:
If you still have scanner,

look up blm #s and STFT #s

Also should be able to see o2 voltages and/or back probe o2 sensor plugs with a multimeter.

Also should be able to see loop status from a decent obd1 scanner.

Last time I was chasing issues I datalogged a warm up sequence so I could play back and see what’s going on.



youtube.com

454 o2 sensor values. Video 1

78 k30 454 tbi swap. Monitoring o2 sensor value with timing bypass (tan wire) disconnected. Note smooth idle , but rich. Also not engine does not stall when ...
youtube.com youtube.com

The blm # pegged @ 105 (max) and the o2 voltage ( supposed to be switching from low volt near .1 and high near .9) confirmed it was too rich at idle/and lower rpm.

At idle the o2 voltage would stay near .8/.9 at upper range.

This was to confirm fueling issue and not an ignition issue.

You can pinch off return/supply of fuel lines to test fpr function, but likely in factory app there is no test port or rubber line to do so.

I made up a filter canister with a test port for checking psi.

Some fuel inj psi kits have adapters to add test ports inline between the sections of hard lines and the fittings they hook to.
Click to expand...
 
@Mac5005 just found the rough idle stall flowchart on Mitchell. Unplugged the pip and started and ran normal (minus the timing idle low) gonna make advance warranty the Dizzy as it would seem per the test the pickup coil is bad or intermittent failure when cold. I swear if it's a failed new part I wouldn't be surprised. Nothing is of quality anymore in the auto industry
 
bannerp said:
@Mac5005 just found the rough idle stall flowchart on Mitchell. Unplugged the pip and started and ran normal (minus the timing idle low) gonna make advance warranty the Dizzy as it would seem per the test the pickup coil is bad or intermittent failure when cold. I swear if it's a failed new part I wouldn't be surprised. Nothing is of quality anymore in the auto industry
Click to expand...
Check the dizzy gear they are known to wear on these
 
When the new TPS was installed was the voltage checked? Could check to see if it needs adjustment. It's been a while, but I think there's a process for setting the base idle that would include dicking with the IAC.

I'm not 100% sure it's the same on the 350 as it is the smaller 60* v6's I'm used to, but on mine, timing needs to be set with the ETC unplugged, and also with the motor fully warmed up.
 
Here’s my experience from rebuilding my Tahoe with TBI. I rebuilt the throttle body and there is a diaphragm and spring in it that controls fuel pressure. After rebuilding everything I ran into an issue where the truck would flood out all brand new sensors from the dealership. My issue turned out to be the wire connected to the sensor was degraded and wasn’t sending the proper signal to the computer this flooding the truck out.
 
Didn't read thru the thread entirely, but I do have a 7747 ECM that was on my Jeep 304, same setup as @Caver Dave , harness came off a 90 sub with a 350. Chip is burnt for my RV cam but it ran when I pulled it off. You are welcome to plug it is to see if there is any change.
 
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