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I have a 1994 Chevy 1500 truck with a 350 in it. Just recently it has developed a really weird problem. It will start just fine and it will idle just fine but when you start to raise the RPM’s anything past 1000 (it idles just a bit below it) it will start to miss then it will start to Backfire. You can drive it and it will start off just fine but a little bit after you start to drive(less than a mile) it will start to miss and then do the whole backfiring thing. Once you let your foot off the gas the RPM’s drop and will will idle with no problems.The only other thing that I have noticed is that the problem goes away a little if I manually shift threw all the gears(its a auto) I am pretty stumped with this because I have checked quite a few things and have not been able to figure it out. I check all of the plugs and wires they look fine. They are almost a year old but if it was a spark problem wouldn’t be all the time not just some? ( They are not that old and were replaced at the same time) I put a new air filter and a new fuel filter on. The problem is still there. The truck does have a carb on it and I don’t know a lot about them. This is the first thing I have ever worked on that had a carb. I tried to spray some carb cleaner in there while the engine was running but that hasn’t really done much besides the carb now looks cleaner. The fuel filter is new and the fuel pump is less than a year old. Anyone have any ideas?
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Its going to be hard to figure out whats been done to that if its been Frankensteined like that. Carbs went the way of the dodo on all Chevy trucks after 1986 due to emissions regulations carbs couldn’t meet. If you’re not somehow misidentifying a throttle body injection system with a carb, someone has swapped in an older style motor and thats going to make if a real pain to diagnose since a hatchet job would have been done to the factory systems to make that happen and the 4L60E transmission is computer controlled and needs engine data to shift properly. You won’t be able to used a scanner since the wiring will be no longer going to the sensors. If you could take off the air cleaner assembly and post up a pic of your engine that would probably be a good starting point to find out what this thing even has.
ok after doing some research it does not have a carb it has Throttle Body Injection? it just looked like a carb or what I thought was a carb. I know even less about that. This is the first that I have ever heard about this. Does this make anything more clear?
Carb cleaner didn’t hurt it, just probably didn’t help. The TBI system is actually a pretty good system and easy to work on. I have the same thing in my 95 Suburban. Can you check the fuel pressure? You would need an adapter that tees into the pressure line going into the base of the throttle body. Pressure should be between 9-13 psi. You can also remove the air cleaner assembly and look down the bores while its running with a flashlight. Look at the spray pattern of the two fuel injectors at idle. Are they fine mist pulses or is one of them dripping or throwing globs of fuel? Has anyone screwed with the distributor position? Are there any codes? You can pull blink codes off this with a paper clip and your check engine light, no scan tool needed to do that. Can you verify what the timing is at idle with a timing light?
No one has messed with the cap at all. I really thought it was a clogged fuel filter for the longest time. I do know how to pull the codes in OBD1 but its not throwing any codes. But I was thinking since its backfiring and all shouldn’t it be throwing a code? I guess it could be burned out? If that’s the case can you pull the codes with out getting a scanner? I have one for OBD2 but not one.
Would some fuel injector cleaner help? I have always been a huge fan on the Lucas injector cleaner! Always been good in all of my other cars.Here is a link on how to read the codes:
http://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php
As previously stated. It is either going to be fuel related and injector cleaner that you put in your tank is a wast of money. Or, the mechanical timing is off for some reason. If you don’t find a problem with either of the former take the distributor cap off then clean and inspect the inside for cracks. If it needs changing move only one wire at a time.
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