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Discussion Starter · #1 ·
Good day I'm new here. I have a 1992 gmc sierra with the 350, 4x4. 9 months ago the truck started back firing when accelerating. I checked the plugs and replaced them all and no issues until recently. Now when the truck is in any gear other than neutral, it lopes quite noticeably. I'm thinking its a fuel issue as I take my air filter off and nothing changes. Either that or a vacuum leak somewhere. Any help would be appreciated, thank you.
 

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The 1st thing I'd do is check your fuel pressure. It s/b between 10 and 13 PSI, 10 being almost not enough IMO. Then take a spray bottle and spray water on your ignition coil, not the wires. See if it stumbles. These are free (well almost) checks before you start throwing parts at it. If you can, put it on a scanner, not a code reader, and see what's going on.
 

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Discussion Starter · #3 ·
Ok 10 to 13 psi. Now is that off the easy connector by the TBI? I got the haynes book for reference. Is there a flow test if I can't get a guage? Like x amount of litres or gallons per minute kind of thing for it? I work at a shop and am going to see if they have a pressure tester that may fit my vehicle on Monday. What is the water test for?

Thank you for replying.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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The TBI engine does not have a schrader valve. You have to get a TBI test kit. It puts a test port inline. Put it in down line of the filter.
 
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Discussion Starter · #5 ·
This is the picture in my Haynes manual. I haven't checked physically on the truck but to me its saying there is a port to hook up the tester too. Maybe I'm reading the picture and book wrong though, I'm no mechanic.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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That’s on the Vortec engine ‘96 and up.
 
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Discussion Starter · #7 ·
Ok I see. Yeah in the book it says on TBI systems you have to install a T-fitting between the TBI unit and the inlet fuel line and attach the gage to the t point. I'm assuming the inlet line is the main line from the tank/pump. And then I guess you find a place before the throttle body to install a T fitting.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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The TBI test kit is available on loan from the big name parts stores. The “easiest” one is on the frame rail about right under the driver seat.
 
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The “easiest” one is on the frame rail about right under the driver seat.
^^^ yep. If you want to make the investment, ~$116 you can install a permanent fuel pressure gauge and regulator on your throttle body. These guys sell the "T" adapter, pressure gauge, and adjustable regulator.

It's not too hard to install, you drill a hole in the plate for the thumb screw, and bend the MAP sensor vacuum tube down a little.
IMG_0832.JPG

IMG_0833.JPG


Here's what it looks like on the engine.
IMG_0835.JPG
 

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Discussion Starter · #10 ·
Awesome thanks I will get the equipment from work on Monday. So when I ran the truck today, it ran and sounded fine for about 10 mins. It started loping after that and would struggle when put into gear. Eventually I was going back and forth in my driveway, until I tried to go forward one time and the truck quit on me. Have not been able to get it started since. With the key in run position you can hear the pump in the tank prime for 2 sec and shut off like it should. I measure the resistance of the two injector connectors. Each one was .2 ohms on my meter when you take away the wire resistance of the meter. The book tells me they should be between 1.16 to 1.36 ohms. Now I'm not certain of what is in an injector but I'd guess its a sort of coil that operates it. Now would a lower reading like I got indicate a burnt out injector?

Thanks again.

P.S. I noticed there was a break in a vacuum line from the front lower of the TBI going to the valve cover on the passenger side. I put a new hose on and no change to symptoms.
 

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Each one was .2 ohms on my meter when you take away the wire resistance of the meter. The book tells me they should be between 1.16 to 1.36 ohms.
When you're testing resistance that low, your meter may be off enough to show that little variance (what does it show when you short the leads). Did you try spraying water on the coil yet? It's starting to look like your ICM and/or coil are going out but, do the free checks 1st. A test bench/Ohm meter can tell you if an ICM or coil is bad but it can't tell you if they are good.
 

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Discussion Starter · #12 ·
When I short my leads I get .1 ohm. During test I got .3 to .4 for each injector. Spray water on it? It just occurred to me I haven't changed my fuel filter in 2 years. I know stupid me. I had a 75 monte carlo and it didn't have a fuel filter so by not seeing one in the engine compartment I figured maybe my truck didn't have one either. Turns out it's under the bed along the fuel line. I'll get one tomorrow because it definitely needs to be changed and see if that does anything at all to fix anything.
 

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Spray water on it?
Yes, spray water on your ignition coil near the distributor, not on the wires coming out the top of it, just the body. The insulation inside the coil can short out but the engine will still run. Here's a picture of one that was only ~2000 miles old, note the burn marks on the iron and body. That's where the insulation failed. The coil can/will take out the ICM due to voltage spikes sent through it. Don't buy cheap a$$ components, only good quality stuff.

IMG_0938.JPG
 

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Discussion Starter · #14 ·
Ok whats the water for? I'm just spraying the body or case of the coil right? I will check tomorrow when suns out for scorch marks around it. When I spray it what am I looking for to happen?

Thank you
 

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When I spray it what am I looking for to happen?
The engine will stumble or die if the coil is bad. All you're doing is making an easier path to ground which will short out the coil more. Some people add a little salt to make the water more conductive but, if you're using tap water the minerals in it will probably give you enough conductivity.
 

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I will check tomorrow when suns out for scorch marks around it.
You may or may not see those burn marks. After I sprayed the coil and it stumbled, I pulled it and, after inspection, then found those burn marks.
 

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If the engine runs fine when first started and shortly goes in trouble, it could be open/closed loop causing a mixture problem.

Try disconnecting the front O2 sensors (tie the harnesses up so they don't hit the exhaust). This will keep the computer in open loop and it will use the internal mix tables.
If the idle stays calm, you've got the right circuits, then we need to fine the why.
 

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Try disconnecting the front O2 sensors (tie the harnesses up so they don't hit the exhaust). This will keep the computer in open loop and it will use the internal mix tables.
That's a good point Ray. Just keep in mind the TBI only has a 1 wire O2 sensor in the driver's side exhaust manifold.
 

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Good point Peter, I forgot that.
I know, I'm still living in that past when everyone's moved on to the new fangled technology where they actually care what's going on with both banks of an engine and Cats, LOL

But seriously, that's a good exercise to go through,before you spend any cash on guesses as to what's the real issue.
 
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