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Hello I'm new here and first time posting. I have a 96 gmc 1500 that is running real rough at low rpm and idle and has hesitation. Codes are 0430 and 0300. So far I've done plugs,wires, cap and rotor. Fuel filter, Coolant temp sensor, egr valve, 02 sensor. I have orderered throttle position sensor, fuel pressure regulator, map sensor, idle control sensor or valve or whatever it is also but haven't changed those yet. Nothing yet has helped. Also in the short time it's been running like this I believe the catalactic converters have been ruined and muffler is developing holes, from raw fuel entering exhaust from what I understand. Fuel consumption has gotten real poor. Not super savvy on engines so all the possibilities and no real direction/ lack of knowledge is frustrating. I like the truck, it's in great shape for its age and prior to this ran great. It's just over 200k. Thanks alot in advance
 

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dont replace, test. gotta diagnose a problem first! theres quite a few problems that it could be. you could have a simple vacume leak.and if you cant obviously hear or find one the you need to invistage it further. I'm going to suggest that if your vacume brake booster is original that it could very well be the problem. you could replace everything on the engine and not fix a damn thing if the brake booster is leaking. theres a big hose that runs to the brake booster. plug it with a bolt and pit a gear clamp on it. how does it run? better? perfect?

I personally believe that all the original brake boosters on these trucks are coming to the end of there life. they usually make noise and when the brake peddle is depressed you can hear a hiss inside the cab. but not always. they also degrade over time and we get used to it. been there done that. it sucks.

you can also test this with an obd2 scanner and watching the short term or long term fuel trims rise all the way to 100% if you hold the brakes long enough. the computer compensates for the loss of vacume and when it suddenly comes back when you release the brakes the truck will fall on its face and may even stall.

man if your really stuck with this truck come see us at gmt400.com as these are the only trucks we talk about, for the most part but those guys know everything about them. you may even be able to find a service manual over there for detailed trouble shooting steps.

good luck.

Al
 

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Discussion Starter · #4 ·
Thank you both very much for the responses. I have not replaced the iac. That was on my list of things to try but kinda taking a pause from throwing money at it without learning more about what's going on. Al, yes the brake booster was actually the very first thing I replaced. It might have helped a little bit briefly but not long. It's actually got worse since I made this post. I've been driving to work by necessity and it's literally getting around 6mpg and running horrible. Sounds like real **** now with I think cats clogged. Strong gas smell in exhaust but still no smoke of any kind. And it does run decent going 65 on hwy. Still sounds bad but better than low rpm. I'm not real sure how to go about testing the iac and such components or I would. Really don't want to take it to a shop because the few mechanics I know personally are too busy and I just don't trust any others around here. Not looking for a new motor and a $8000 bill to fix this thing. Plus it's always good to learn and be able to fix I believe. Also is there anything wrong with cutting out the cats and deleting them? I understand it's probably illegal and not environmental friendly but it's ok for the truck right? Thanks again and I'll have to check out that site
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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What’s your fuel pressure?
 

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no I dont think we should cut the cats off. Hell I bought an entirely new y pipe with both converters. I'm all for fixing things the correct way. the ecm fuels the engine based on live data and if that data is not what the ECM needs to see then it will never run right.

after a complete tune up and everything working right my Burb routinely gets above 17 MPG.
 
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