1987 TBI 350 Fuel Pressure

[Tim K]

The truck is throwing a Code 44.
The problem is not the O2 sensor or the pig tail.
The truck feels very lean, the new Cat looks like it was very hot.
It starts and idles great. Throttle tip in and it dies.
Both tanks are showing 11 psi, at idle and road test.

My GM service manual is kinda contradictory. Under "Fuel Control 4-3" it states 18 psi to the throttle body. Computer Command Control 3-29, Chart A-6, specifies 9 psi or above.

Any ideas?

 

[91burb]

Had the same code a few months back, replaced and it went away. Even put in a 3 wire.

 

[Tim K]

So you replaced the fuel pump? Did you measure the line pressure before hand?

Also, the fuel filter was replaced less than 20k miles ago along with one of the fuel pumps and i don't buy the absolute cheapest crap gas, only branded.

 

[MrMarty51]

I think He was talking about the o2 sensor except, He installed a 3 wire heated type.

 

[Tim K]

Anyone have a link on this 3 wire? How is it incorporated into the wiring to the ECM?

 

[Tim K]

With it showing 11 psi after the new fuel filter I decided to replace one of the fuel pumps just to confirm the pressure being not the problem. With the tank on the ground and with a new sending unit and pump it produced 17 psi. Close enough to 18 for me.

I also lengthened to sending unit pigtail and the fuel lines just to make it easier next time I need to drop the tank.

Get it all back together and the truck runs like a top for about 20 miles. Now I would think with the new pump with the greater pressure would show a difference when I switched from one side tank to the other. It made no difference in the running.

It then reverted right back to the same lousy running. You can start it up and it will run great. Start it again and it runs like crap and sometimes it will be running great and then go bad as its rolling down the road. The two things left that I can try is run a wire direct from ECM to the (new)O2 sensor and if that don't work, try a new ECM. Any ideas?

 

[Jims86]

With it showing 11 psi after the new fuel filter I decided to replace one of the fuel pumps just to confirm the pressure being not the problem. With the tank on the ground and with a new sending unit and pump it produced 17 psi. Close enough to 18 for me.

I also lengthened to sending unit pigtail and the fuel lines just to make it easier next time I need to drop the tank.

Get it all back together and the truck runs like a top for about 20 miles. Now I would think with the new pump with the greater pressure would show a difference when I switched from one side tank to the other. It made no difference in the running.

It then reverted right back to the same lousy running. You can start it up and it will run great. Start it again and it runs like crap and sometimes it will be running great and then go bad as its rolling down the road. The two things left that I can try is run a wire direct from ECM to the (new)O2 sensor and if that don't work, try a new ECM. Any ideas?

That 18psi should be directly from the pump. No more than 13 regulated at the throttle body. TBI regulated pressure never went over 13psi until the 94-95 TBI 454 went to 30.
Sounds like its running great in open loop, then when the ECM goes to open loop, it runs like ****. An o2 can show lean from an exhaust leak, or un commanded air from a faulty diverter valve from the smog pump(if you have it) being stuck and sending air to the drivers side manifold.
Is the engine modded at all?
Have you ever rebuilt the TBI and istalled new injector filters?
The 3 wire O2 has 2 white wires, one +, the other ground, and the black wire is the signal. power is from a keyed source, usually from one of the pink/black stripe wires.
They were used in the 93 up models when the sensor was moved to the Y pipe.
It will help the system get into closed loop quicker.

 

[Tim K]

I had replaced the OE throttle body with an Edlebrock (sp) TBI. It didn't make any difference other than make worse milage. My mechanic suggested putting the OE back on it. The engine is bone stock.

Ran it again this am. Tried first with the o2 sensor disconnected. I figured it would stay in closed loop. No voltage signal made it worse, didn't run very well at all.

Let it cool, hooked the o2 sensor back up and ran it in open loop, still runs like crap.

It starts and idles great.

Barely feather the throttle and it dies. It feels just like it ran out of fuel. Drop the throttle wide open and it will take off but surges, does not want to rev and with not much power.

Any more ideas? Time for a new ECM?

I should state that about two years ago the original cat plugged and the truck got quite hot, enough that fuel was boiling out of the evap can. After cutting the exhaust open I ran it a few hundred miles home. Ran great the whole way home. About a year ago it started acting up and I have been chasing this thing ever since. Started with replacing both head gaskets, valve springs and valve seals (blown gasket and broken valve spring) and now every sensor on it. It has good compression.

 

[Jims86]

I had replaced the OE throttle body with an Edlebrock (sp) TBI. It didn't make any difference other than make worse milage. My mechanic suggested putting the OE back on it. The engine is bone stock.

Ran it again this am. Tried first with the o2 sensor disconnected. I figured it would stay in closed loop. No voltage signal made it worse, didn't run very well at all.

Let it cool, hooked the o2 sensor back up and ran it in open loop, still runs like crap.

It starts and idles great.

Barely feather the throttle and it dies. It feels just like it ran out of fuel. Drop the throttle wide open and it will take off but surges, does not want to rev and with not much power.

Any more ideas? Time for a new ECM?

I should state that about two years ago the original cat plugged and the truck got quite hot, enough that fuel was boiling out of the evap can. After cutting the exhaust open I ran it a few hundred miles home. Ran great the whole way home. About a year ago it started acting up and I have been chasing this thing ever since. Started with replacing both head gaskets, valve springs and valve seals (blown gasket and broken valve spring) and now every sensor on it. It has good compression.

Try unpluging the vac line from the MAP sensor, and see how it runs. Have you ran any trouble codes?

 

[Tim K]

Try unpluging the vac line from the MAP sensor, and see how it runs. Have you ran any trouble codes?

The only code it is throwing is 44.

I was also thinking that the MAP may be fooling the ECM into throwing a 44 code. Yesterday, I probed the "B" green wire with the power on and the engine off. It was hard to get a good reading through the plug but I briefly saw 5 volts. So, I assumed it was a bad MAP and replaced it with a new one. The new MAP didn't make any difference in the crap running.

After reading your most recent post I plugged the vacuum line to the MAP and left the MAP plugged into the harness. It started rich and immediately threw a 33 code (MAP problem). It quickly leaned out to where it idled pretty good. I took it for a ride and it ran pretty well. On throttle let-off I could tell the IAC was trying to regulate the air mixture. It warmed up quick. When it was up to temp I reattached the vacuum line and it immediately went back to its old ways.

Bad new MAP??

 

[Jims86]

The only code it is throwing is 44.

I was also thinking that the MAP may be fooling the ECM into throwing a 44 code. Yesterday, I probed the "B" green wire with the power on and the engine off. It was hard to get a good reading through the plug but I briefly saw 5 volts. So, I assumed it was a bad MAP and replaced it with a new one. The new MAP didn't make any difference in the crap running.

After reading your most recent post I plugged the vacuum line to the MAP and left the MAP plugged into the harness. It started rich and immediately threw a 33 code (MAP problem). It quickly leaned out to where it idled pretty good. I took it for a ride and it ran pretty well. On throttle let-off I could tell the IAC was trying to regulate the air mixture. It warmed up quick. When it was up to temp I reattached the vacuum line and it immediately went back to its old ways.

Bad new MAP??

Either that, or there is something messing with the map signal, or there is a ground issue with the system.
Low Vac to the map wiil cause a rich condition, and vise versa.
The MAP, TPS, o2, and CTS are all on the same ground circuit, which is grounded to the bolt by the water neck.
I would go ahaead and try to start with a clean slate, and return your original throttle body to service until this is figured out, but go spend $40, and get a rebuild kit from NAPA, and clean out the IAC passage really good. Make sure your timing is at 0*.
If I wasnt 300 miles up the street(I-5), I would come help ya!

 

[Tim K]

Either that, or there is something messing with the map signal, or there is a ground issue with the system.
Low Vac to the map wiil cause a rich condition, and vise versa.
The MAP, TPS, o2, and CTS are all on the same ground circuit, which is grounded to the bolt by the water neck.
I would go ahaead and try to start with a clean slate, and return your original throttle body to service until this is figured out, but go spend $40, and get a rebuild kit from NAPA, and clean out the IAC passage really good. Make sure your timing is at 0*.
If I wasnt 300 miles up the street(I-5), I would come help ya!

I should have been more specific, as of a week ago, the OE TBI is back on the truck. Both of the grounds next to the outlet are clean and tight. New ground strap from the cab to the engine and the battery is tied direct to the alt/engine and the frame. Cranks fine so the engine ground is good. Timing is spot on. Fairly new IAC. You can hear the IAC valve regulating the airflow.
I went and measured the MAP voltage again. 5.2 dcv on the reference and 1.3dcv on the signal at idle. Had my son blip the throttle and the voltage did vary, obviously following the manifold pressure.

I am looking at page 3-53 and it asks; "Does "scan" display a MAP of 2.5 volts or over?" What exactly are they asking? This is an OBD1 system. Is there a way to scan any of these circuits through the ALDL?

I hate guessing and that is all I am doing at this point

 

[Jims86]

I should have been more specific, as of a week ago, the OE TBI is back on the truck. Both of the grounds next to the outlet are clean and tight. New ground strap from the cab to the engine and the battery is tied direct to the alt/engine and the frame. Cranks fine so the engine ground is good. Timing is spot on. Fairly new IAC. You can hear the IAC valve regulating the airflow.
I went and measured the MAP voltage again. 5.2 dcv on the reference and 1.3dcv on the signal at idle. Had my son blip the throttle and the voltage did vary, obviously following the manifold pressure.

I am looking at page 3-53 and it asks; "Does "scan" display a MAP of 2.5 volts or over?" What exactly are they asking? This is an OBD1 system. Is there a way to scan any of these circuits through the ALDL?

I hate guessing and that is all I am doing at this point

Yes there is. However, you need an ALDL cable, and a free version of tunerproRT. This will enable you to see what every sensor is reading, engine rpm, veh speed, iac position, and fuel trims.

 

[Tim K]

Yes there is. However, you need an ALDL cable, and a free version of tunerproRT. This will enable you to see what every sensor is reading, engine rpm, veh speed, iac position, and fuel trims.

SOLD! What make and model do you recommend?

 

[Jims86]

SOLD! What make and model do you recommend?

I personally like this one, http://www.aldlcable.com/sc/details.asp?item=aldlobd1u.
It's USB, and TunerPro RT is an easy program to use. They do ask for a $30 donation, which is worth it in my opinion. It also keeps you from having to wait 10 seconds to use the program.