1991 R2500 454 suburban drive-ability issue

[Klink K5]

Hello,

Brief history. Stock 1991 2wd TBI 454 sub with 4L80 trans.

I've been fighting a cold start up miss & running driving down the road miss in my 454. The engine once vehicle is cold / sat for awhile & when its cold, it likes to pop / slight back fire thru the exhaust. The engine idles slightly rough but does function all as it should. I've done a cold compression test on it and all cylinders are within 15% of each other as well

Now when fully warmed up and driving 60-65 mph with you foot barley into the throttle 5-10% it misses / jerks going down the road as well. You put your foot into it more 10-100% the truck runs great or when I'm hauling a trailer loaded, runs fine with more fuel thrown at it.

Today I tried to unhook my computer grounds and same symptoms occur with the grounding wire off the back of the cylinder head. I did a direct ground from ECM to battery - and no change as well. Im assuming my ECM should be dead with grounds unhooked as well.............

I've been fighting this issue since the ECM blew 3 years ago upon a start up.

I've changed everything..................... 2 delco computers, plug wires / complete distributor, cap rotor, injectors, coolant sensor, IAC, spark plugs, ect ect ect.

Im reaching out to see if anyone has had the same problems with their TBI 454

 

[YOUNGBLOOD13]

You need to check fuel pressure before throwing any more parts at it. Have you put a fuel filter in it? If not, start there, and when you get the old filter out, tilt it up so inlet side drains onto paper towel. You are looking for dirt/rust.
Hard starting and exhaust pop indicate lean mixture probability. An exhaust leak could also give bad info to oxygen sensor, but not during cranking.
You may want to have someone with a scope look at primary and secondary ignitio . Waveforms, TPS, O2, and fuel pump current. You will have to pay them, but it won't be guessing.
One other thing, have you checked for vacuum leaks? Keep at it, you'll get there.

 

[Klink K5]

You need to check fuel pressure before throwing any more parts at it. Have you put a fuel filter in it? If not, start there, and when you get the old filter out, tilt it up so inlet side drains onto paper towel. You are looking for dirt/rust.
Hard starting and exhaust pop indicate lean mixture probability. An exhaust leak could also give bad info to oxygen sensor, but not during cranking.
You may want to have someone with a scope look at primary and secondary ignitio . Waveforms, TPS, O2, and fuel pump current. You will have to pay them, but it won't be guessing.
One other thing, have you checked for vacuum leaks? Keep at it, you'll get there.

Thanks for the reply. I forgot to add I have put in a new Delco fuel pump, PSI is over 10 pounds , new fuel filter, new fuel pressure regulator on TBI & new injectors,,,,,,,,,,,,,,,,,,all delco parts.

no vacuum leaks know, I,ve used carb clean & either all around the engine, and sprayed in the valve cover with PCV valve out with no change as well. Vaccum at idle is 15 hg

Again, This problem all started when the PCM / ECM blew over 3 years ago.................... chasing my tail. I pulled EMC Ground off today and no change with idle symptoms.......... did something melt together somewhere in the ECM wiring near computer or did ground contact burn off or ?

It seems like the computer is trying to ground itself somewhere else, not thru the correct wiring circuit for the truck??? Grasping for straws here...........

 

[H20 Toie]

I thought the 454 was suppose to have around 30psi of fuel pressure did you put in a fuel pump for a 350 ( they want 13psi)

 

[diesel4me]

"off idle" cruise is when the EGR valve usually opens...that can make it run weird..
If it were starved for fuel,I'd think the engine would bog down or stall under heavy acceleration or loads..

 

[Klink K5]

I thought the 454 was suppose to have around 30psi of fuel pressure did you put in a fuel pump for a 350 ( they want 13psi)

The 1991 suburban with 454 TBI is supposed to be 9-13 PSI on the fuel pressure. In 93-95 454 trucks they went to a smaller size injector and increased fuel pressure to the 25 PSI range. I did the research on it.

I made sure to cover my bases before the regulator, fuel pump, new delco TBI injector replacements................. Ive thought of all the same things..........

 

[Klink K5]

"off idle" cruise is when the EGR valve usually opens...that can make it run weird..
If it were starved for fuel,I'd think the engine would bog down or stall under heavy acceleration or loads..

I've double checked the EGR Valve, the only thing I haven't done yet is remove it and put a plate over it. I've thought of trying this test as well if the exhaust is having enough back pressure (restrictive) to blow open the egr valve with little throttle input. I cant see it open while holding the throttle @ 2300 rpm in my shop.

I've unplugged it and still runs the same..................

 

[Chevman88]

Edit: Make sure the ground pin on your connector is actually ground. Only thing I can think of is maybe it's grounding through the body

Checked coolant temp sensor (one with two wires, not the single wire one - that's for the gauge) and made sure it is reading correctly? Coolant temp sensor is what the engine runs off when cold, when the engine warms up it goes into closed loop and switches to the o2 sensor.

Do you have stock exhaust? If not, is the o2 sensor up close to the heads. The stock single wire o2 sensor needs to be close to heat to work - it's pretty easy to upgrade to a 3 wire heated sensor if you have aftermarket exhaust and your sensor is far away. How old is it?

Then there's the normal checks: grounds, tps set correctly, IAC set correctly, vacuum - does it flutter around at all, etc.

One other thing to check is your wiring, make sure everything is getting sufficient voltage - clean contacts, check for corrosion, etc.

 

[diesel4me]

Wont hurt to take a fuel sample and ensure no water is in it..
Often water in the gas will make for poor cold start/running conditions but subside once it warms up..

 

[Klink K5]

There have been several tanks of gas run thru the engine, I have been filling it with 91 octane premium as well. The tank was taken down to install the new fuel pump 4 years ago, there should be now water in the fuel. Maybe the rubber hose split on the pump? I've had that happen before as well in the past on customers vehicles................

Still all my problems don't explain the ground issue when all ground wires are off chassis (that I know of the ECM is still working)

Yes the ECM coolant sensor has been replaced twice With a delco unit, along with the pig tail going to it. This has always been the #1 TBI problem to check first. I have a Snap On scanner and I can not see any glitches in any the sensors as well. But this TBI ECM system is slow as a apple computer back in the day so I understand this............. might be had to pick up "glitches" with sensors on the old scanner

I'm going to dig into wiring today a little more

100% stock exhaust with new A/C delco 1 wire O2 sensor has been installed as well.

 

[black dawg]

Do you have the correct prom in the computer?

 

[rampage]

Why are you pulling the ground wire off, are you purposefully trying to F your ECM up? Most all electronic devices have a ground plane which is attached to the case it’s enclosed in. So the screws that hold it in place also act as a ground.

 

[black dawg]

Do you have the correct prom in the computer?

another thing I have run into with these, too much base timing can actually cause a light throttle misfire due to rotor vs distributor cap terminal position. The ecm adds lots of light throttle timing.

 

[Klink K5]

Why are you pulling the ground wire off, are you purposefully trying to F your ECM up? Most all electronic devices have a ground plane which is attached to the case it’s enclosed in. So the screws that hold it in place also act as a ground.

Not Trying to F@&* up my ECM, just troubleshooting. I added a additional ground wire from the grounding points on the back of the cylinder head, attached 12 gauge wire with a soldered eye and ran it directly to my radiator support grounds............ No Change in problems

 

[Klink K5]

another thing I have run into with these, too much base timing can actually cause a light throttle misfire due to rotor vs distributor cap terminal position. The ecm adds lots of light throttle timing.

My base timing is factory set @ 4 degrees with spout connector / timing connector pulled. I agree with you, when at cruising speed, my scanner says the ECM puts this truck into 50 degrees advance with little throttle input............ should I try to trim off 10 degrees from the total advance by setting my base timing at -6 degrees?

Thanks

 

[Klink K5]

Do you have the correct prom in the computer?

Prom is the original one I pulled out of the ECM. I can not verify it is the correct one for the truck.............Ive tried to research it but no luck doing so ..............I've even tried to reach out to have the TBI Chips .com guy help me out but never can catch him.............. Harris Performance

 

[black dawg]

data on scan tool should show prom ID# I used to have a book that would tell application and prom id, but lost it.

Try the timing at 0 and see how it does.

 

[cuervo]

How old is the coil?