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Discussion Starter · #1 ·
Hey guys! Im new to the forum and can't wait to learn!


So here is what I've got going on. Any input would be awesome!

I own a 1995 GMC Sierra 3/4 ton pickup with the 7.4 liter 454 that is throttle body injected

Recently I took the truck on a trip that is about and hour and a half to two hours up the canyon to camp. When I got to the cabin my truck was running a little weirder then normal, but not crazy, it acted like it wanted to die and lurch a little. Thought nothing of it and stayed the night. In the morning I ran to the gas station to grab some food and it felt like the same thing, but I wasn't too worried. PS I was pulling a four wheeler on a small trailer and had four dirt bikes in the back. on the way home the truck was still acting funny but I thought limp it home and see what I can figure out when I have my tools. Out of no where after I crested a hill the truck died and wouldn't start again. I thought the problem was electrical because I wasn't getting any spark. I replaced the ignition control module and the ignition coil and was able to get the truck started and running but it was backfiring and running really rough. I pulled into town and the truck died again. I replaced the cap and rotor, as well as the spark plug wires and a couple broken vacuum lines but still ran terrible. I was able to get it started and get it home but still ran terrible. At home I replaced my fuel filter, air filter and all of the spark plugs as well as ran injector cleaner through it. But no such luck still. Im stumped. I don't think it is a fuel problem because I can see the injectors spray really well and the truck has all the power it normally has at full throttle it just backfires and sputters a little. I can't get the truck to idle either. I have to hold the brake in and rev it up to keep it running. I took off my muffler and cat as well to see if they were restricting it but still ran the same. When it idles it sounds like the engine is staying a pretty constant speed but it randomly backfires and sputters but my Tachometer jumps around a lot. I haven't been able to locate a crankshaft positioning sensor yet, does it have one? Could this be the problem? Ive also heard of it being a sensor on the TBI. Like an air idle control sensor.

Side note when I put in the new ignition coil you have to grind the rivets off of the old mount to put the new one in so the ignition coil isn't bolted to the block. its just chilling. Could this make a difference? I've seen them like that before so I don't know? sorry for the rookie question, still learning haha.



Sorry guys that was long! Im just so stumped and don't know where to know next. Is there any other information or pictures I can provide to help?
 

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1. Grounding the coil bracket is a good idea.
2.unless I'm mistaken, crank pos sensor is directly behind the balancer on the bottom. Like 1 8mm bolt head and an odd l shaped bracket.
3.IAC can do this, so can fuel pressure, bad crank SIG, bad cam SIG ...let's just say a lot of possibilities... A few are common.

We have to start somewhere, so what tools do you have available - on hand?

Fuel pressure tester, multi meter, test light, vac gauge, spare electronics, scope? Tell me what you have, we can work from there...
 
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Hey bud, I see you have 2 of the same posts up, might be a little hard to keep track of...unfortunately I have no clue how to fix a double post issue...maybe someone knows how to merge them, or delete one?
 

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Discussion Starter · #4 ·
1. Grounding the coil bracket is a good idea.
2.unless I'm mistaken, crank pos sensor is directly behind the balancer on the bottom. Like 1 8mm bolt head and an odd l shaped bracket.
3.IAC can do this, so can fuel pressure, bad crank SIG, bad cam SIG ...let's just say a lot of possibilities... A few are common.

We have to start somewhere, so what tools do you have available - on hand?

Fuel pressure tester, multi meter, test light, vac gauge, spare electronics, scope? Tell me what you have, we can work from there...

Thank you for the input!


I’ve got a test light, multimeter, and those are about all the specialty tools I have. But if I need to go buy something I can.
 

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Discussion Starter · #5 ·
Hey bud, I see you have 2 of the same posts up, might be a little hard to keep track of...unfortunately I have no clue how to fix a double post issue...maybe someone knows how to merge them, or delete one?

Oh shoot I will have to look into that, thank you!
 

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After rereading... You say the engine idles smooth, but backfires occasionally, but tac jumps wildly? Tac is run off coil...may likely be the solid ground that is missing from that assembly... Try that first. Recheck wiring especially at connection for corrosion... Loosness all that...make sure you have batt v at coil and bat volt is 12 or above. Let's try to eliminate that part of the system before we move on...how do your old plugs look...black, white crusty, slightly browned? With an old plug you should be able to see a lot... Also you can use it as a spark checker...ground the threads with a jumper wire and plug it into a plug cable...
 

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Discussion Starter · #7 ·
After rereading... You say the engine idles smooth, but backfires occasionally, but tac jumps wildly? Tac is run off coil...may likely be the solid ground that is missing from that assembly... Try that first. Recheck wiring especially at connection for corrosion... Loosness all that...make sure you have batt v at coil and bat volt is 12 or above. Let's try to eliminate that part of the system before we move on...how do your old plugs look...black, white crusty, slightly browned? With an old plug you should be able to see a lot... Also you can use it as a spark checker...ground the threads with a jumper wire and plug it into a plug cable...

The old plugs were all brown but had some black on them. I’ve checked the spark and I am getting good spark.


As far as idle goes it’s hard to say. It jumps around and isn’t really constant
 

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OK, get fuel pressure checked... Then look up how to do an IAC reset, jumping 2 pins on the dlc...see if IAC pushes all the way in, it will prob him at you also...if it pushes all the way in, unplug it and restart truck. Now we can do leak checks without IAC interfering. Will probably idle very low, or you may have to keep your foot on throttle for a min and slowly release so the ecu figures out how to time engine to run. If it just won't idle we can work from there.
 

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Light brown is not bad, black is too much unburned fuel.
 

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spot on zach. you way better at this than i am. deffinatley smarter to check what is working and responding instead of throwing parts at it. very smart.
 

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I had a 94 with the 5.7. Distributor was original, but I replaced the engine due to my own stupidity. Anyway, the tach bounced from 1200-1800 randomly while driving down the road at about 35 mph. There was a noticeable stutter in the engine at the exact same moment. Ended up replacing the distributor and the problem went away. The reluctor core inside was cracked.

Inspect yours.
 

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Very likely, just don't want to through anything at it...without eliminating a few things first...
 
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