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1992 TBI Silverado Idles fine but has only 1/2 throttle after 1/4 mile of travel.

Jesse405

By Jesse405
in Ask A GM Technician

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Jesse405 Enthusiast

Jesse405

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This truck idles fine. ONLY when you put it under load, drive it for 1/2 mile or so, it will not rev, stutters at about 1/2 throttle. If you turn the ign off and right back on - something resets and it is fine - UNTIL you drive it again. It will sit and idle fine and rev fine - as long as you don't move the truck! We have replaced the following:
Fuel Pump
Fuel Filter
Spark Plugs
Spark Plug wires
Throttle Body
Fuel Injectors
Grommet in Valve Cover
Air Positioning Sensor / Throttle Position Sensor
Idle Control Valve
MAP Sensor
(Codes were Cleared)
(Checked for Vacuum leak)
Oxygen Sensor
MALF CODES: OBD1
22
44
33
45
idles beautifully... and revs to 3500rmp mightily UNTIL you move the truck a mile or even less... then 1/2 throttle flat spot... won't go more than 30mph.. then ign on/off and good again
Results of fuel pressure test:
1. Opened mid-run union (near filter) and KOEO bam! 18# but bleeds off pretty swiftly to 9ish,
then gradually down. (Rear section bad?)
2. Re-connect union, KOEO, similar.
3. Open mid-union again and idle - bam! 18# and good flow to bucket.
4. Re-connect union, Idle - 18# (shut off.. it bled down all the way after a minute)

HoosierZ Enthusiast

HoosierZ

Posted
Posted

The TBI engines run at 9-13 psi fuel pressure. The regulator is at the back of the throttle body. It’s a small stamped housing about 1” diameter with 4 small screws hold it on. If you’re seeing 18psi then it’s likely that your regulator is bad or you have a clogged or smashed feed or return line that’s not letting fuel flow freely. The fact that it runs fine for a short distance makes me think it’s a lack of sufficient flow from the tank. Where are you checking pressure? At the throttle body or near the filter?

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Jesse405 Enthusiast

Jesse405

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Thanks for the response! I am testing by the filter where the line splits.  Also left it open and flow was great from the return line and it tested pretty much the same.  My next test was going to blow air from behind filter to tank to confirm that the section behind filter is clear.  If so replace The Fuel Regulator?

HoosierZ Enthusiast

HoosierZ

Posted
Posted

To accurately check pressure, you need to tee into the feed line before the throttle body. It sounds like you’re hooking your gauge to the line coming from the tank without the regulator coming into play. That’s giving you your pump’s max pressure and not the regulated pressure at the injectors which should be 9-13 psi. 
 

I find it odd that you have error codes for rich and lean O2 conditions. 

Jesse405 Enthusiast

Jesse405

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HoozierZ

Thanks! I suspected as much.  

I was thinking I might get a more accurate reading at the line split on the RETURN line?? (as that is where the regulated pressure is? after leaving the TB?)

YES! very odd 44 AND 45? New O2 sensor and double checked. 

I am going to run codes again today or tomorrow. 

I will also try to get a pressure reading right at the Line In on the Throttle Body as you suggest. (Not sure I have the right tap.)

HoosierZ Enthusiast

HoosierZ

Posted
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Jesse, you’re welcome. 
The regulated pressure will be in the line between the pump and the regulator. The injectors are fed fuel just before the regulator. The regulator is simply a diaphragm with a spring behind it. It releases any pressure above it’s set point back to the tank. The fuel in the return line is open to the tank so it isn’t under pressure unless you block it’s return to the tank. Make sense?

 

On my ‘94, I cut the feed line between the firewall and throttle body and added a tee with some rubber fuel hose and clamps and added a gauge to check fuel pressure. I believe at one time someone sold an adapter fitting w/ gauge port that didn’t require the fuel line to be cut. 

HoosierZ Enthusiast

HoosierZ

Posted
Posted

Jesse, you’re welcome. 
The regulated pressure will be in the line between the pump and the regulator. The injectors are fed fuel just before the regulator. The regulator is simply a diaphragm with a spring behind it. It releases any pressure above it’s set point back to the tank. The fuel in the return line is open to the tank so it isn’t under pressure unless you block it’s return to the tank. Make sense?

 

On my ‘94, I cut the feed line between the firewall and throttle body and added a tee with some rubber fuel hose and clamps and added a gauge to check fuel pressure. I believe at one time someone sold an adapter fitting w/ gauge port that didn’t require the fuel line to be cut. 

Jesse405 Enthusiast

Jesse405

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GReat! That's what I was thinking... cut where it's more accessible... I'm going to take a look at what taps came with the kit from O'Reilly's today and maybe go into the back of the TB where the fuel supply line puts in.  Otherwise will go to your method!  If it comes to it, you have to remove the top of the TB to get to that regulator right? Do those diaphragm  springs come in different pressure settings?

HoosierZ Enthusiast

HoosierZ

Posted
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The regulator is on the bottom of the injector housing so you’ll need to pull the top off to get to it. There are no different springs available that I’m aware of. An old trick to raise pressure is to drop a quarter into the regulator to compress the spring a little more. Jegs and Jet both sell adjustable regulators. I believe that the injectors can handle around 20psi before they start to stick open.  
 

Check out Harristuning.com  for troubleshooting and TBI engine mods. 
Lots of good info there. 


 

 

Jesse405 Enthusiast

Jesse405

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7 hours ago, HoosierZ said:

The regulator is on the bottom of the injector housing so you’ll need to pull the top off to get to it. There are no different springs available that I’m aware of. An old trick to raise pressure is to drop a quarter into the regulator to compress the spring a little more. Jegs and Jet both sell adjustable regulators. I believe that the injectors can handle around 20psi before they start to stick open.  
 

Check out Harristuning.com  for troubleshooting and TBI engine mods. 
Lots of good info there. 


 

 

Good info! Thanks... I checked out Harristuning and his TS page is a wealth of good stuff.

 

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