1998 4.3L Vortec backfires while starting HELP! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-25-2007, 08:35 AM
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1998 4.3L Vortec backfires while starting HELP!

Ok, here's the rundown. I have a 1998 4.3L Engine I plan to use for racing. It is currently in a Blazer I purchased from my friend. It has not run for about 1 1/2 years but run perfect before it was parked. I personally have seen it run. It has about 110,000 miles on it. Ok, We cranked on the motor after putting a new battery on and it started but missed very bad like it was only hitting on about 4 cylinders. I changed the plugs, rotor button, and distributor cap, no change. Catalytic Convertor has been cut off and pipe welded in. Someone told me about the fuel pressure regulator. I changed that then it wouldn't start at all. It would backfire and pop like mad! I changed the cam position sensor, map sensor, and ECM. It still acted like it was out of time or something. Checked plug wires. Seem ok, not crossed. Ok, so I figured, it did run until I put the fuel pressure regulator on and now it don't so I decided to try it again. I put another fuel pressure regulator on it....presto!! Must have been a bad regulator. It started, but still was missing very bad. Barely could keep it running. Ok, so I went and bought new plug wires ( and did them one at a time making sure they weren't crossed and have diagram how they go on. Now, since changing the plug wires, it won't start again. Backfiring out the tail pipe. Last night it sounded like a bomb went off!!! Very Loud Backfire out the tailpipe while trying to start it. I've checked the Fuel pressure at the rail, it's 60lbs. It acts like it's loading up or not in time. I've heard you can pour 10 gallons of gas down the intake and it wouldn't backfire unless it's out of time!! This is making my head hurt!! So first, why is it backfiring??!! Second, why is it missing when I do get it to start? Third, What else could it be!! Would faulty injectors or fuel leak inside the intake cause it to backfire or is it something else!!?? PLEASE HELP!!

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Old 09-25-2007, 09:43 AM
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when you changed the fuel regulator I hope you bled it there after, otherwise you get air and no fuel. Here is a list of prabable causes from the book.
Engine backfires:
EGR system not functioning properly.
Ignition timing incorrect.
Thermostatic air cleaner system not operating properly.
Vacuum leak.
Valve clearence incorrect.
Damaged valve springs or sticking valve.
Intake air leak.
I don`t think these systems use the thermostatic air cleaner assembly, but I would still check the filter, try starting it without one. I would check the whole filter assembly to be sure there was nothing in it. Being it sat a while you`ll know that several valve springs were in the open position and this can lead to the spring being so weak it can`t control the valve well enough for it to run.
Here is the list of misses at idle, you maybe able to cross reference between the two lists and find what your looking for.
Spark plugs faulty or not gapped correctly (slightly misleading)
Spark plug wires faulty.
Wet or damaged distributor components
Short circuit in ignition, coil or spark plug wires.
Sticking or faulty emissions system.
Clogged fuel filter or dirty gasoline, remove fuel filter and inspect.
Vacuum leaks at intake manifold or loose connections.
Incorrect idle speed or idle mixture vehicle must be checked by a dealer.
Incorrect ignition timing
Low or uneven cylinder compression.
Clogged or dirty fuel injectors.
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Old 09-25-2007, 02:48 PM
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re: 1998 4.3L Vortec backfires while starting HELP!

("when you changed the fuel regulator I hope you bled it there after, otherwise you get air and no fuel. Here is a list of prabable causes from the book.
Engine backfires:
EGR system not functioning properly.
Ignition timing incorrect.
Thermostatic air cleaner system not operating properly.
Vacuum leak.
Valve clearence incorrect.
Damaged valve springs or sticking valve.
Intake air leak.
I don`t think these systems use the thermostatic air cleaner assembly, but I would still check the filter, try starting it without one. I would check the whole filter assembly to be sure there was nothing in it. Being it sat a while you`ll know that several valve springs were in the open position and this can lead to the spring being so weak it can`t control the valve well enough for it to run.
Here is the list of misses at idle, you maybe able to cross reference between the two lists and find what your looking for.
Spark plugs faulty or not gapped correctly (slightly misleading)
Spark plug wires faulty.
Wet or damaged distributor components
Short circuit in ignition, coil or spark plug wires.
Sticking or faulty emissions system.
Clogged fuel filter or dirty gasoline, remove fuel filter and inspect.
Vacuum leaks at intake manifold or loose connections.
Incorrect idle speed or idle mixture vehicle must be checked by a dealer.
Incorrect ignition timing
Low or uneven cylinder compression.
Clogged or dirty fuel injectors.)"


DOUBLEVISION,
Thanks for the extras. I was out of ideas. I haven't checked the EGR valve. Didn't think that would cause it. Don't see how the engine timing is incorrect since nothing has been altered to effect the timing. Not sure about a vacuum leak even though I didn't put new gaskets on the intake and throttle body after changing the regulator. Can't physically hear any. I'm leaning more towards a valve problem. My initial thinking was a stuck valve, but everyone has told me the 4.3 valves rarely stick. Don't know why that is, but I have heard that from 3 or 4 people. I'm getting a compression tester to test each cylinder. I'll post my findings and hopefully we can figure it out. Thanks alot!
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Old 09-25-2007, 06:30 PM
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1998 4.3L Vortec backfires while starting HELP!

Ok, I checked compression on the cylinders. Number 4 cylinder has NO compression at all! All others had about 165lbs-170lbs. Do you think the rocker arm has fell off? Is the valve spring broke? I'm going to take the valve covers off tomorrow and see what I can find. I have a friend who has a set of 1995 4.3 vortec heads. Will these fit the 1998 model? I know they changed alot of things throughout the life of the 4.3 vortec. If the spring is broke, can that be replaced by me or does a machine shop have to do it? Appreciate the input.
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Old 09-25-2007, 07:08 PM
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IMHO, zero compression would be caused by a valve not sealing against it's seat.
Whether it's burnt, stuck, bent, or broken is still to be determined.

In my way of thinking, a broken rocker arm should result in the valve staying closed ... so I doubt that's it.

Last edited by 66GMC; 09-25-2007 at 07:43 PM.
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Old 09-25-2007, 10:09 PM
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Quote:
In my way of thinking, a broken rocker arm should result in the valve staying closed ... so I doubt that's it.
Unless it's an intake rocker....


One other thing that will cause these to hiccup is a dirty MAF. The '95 heads will fit the block, but the 98' intake manifold will not fit the heads.
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Old 09-26-2007, 08:38 AM
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Quote:
Originally Posted by sbchevfreak
Unless it's an intake rocker....


One other thing that will cause these to hiccup is a dirty MAF. The '95 heads will fit the block, but the 98' intake manifold will not fit the heads.
I am just a partsman by trade, not a mechanic ... I can understand that you would need to have the intake valve open in order to have a full charge of air to compress. I would have thought a closed intake valve might result in LOW compression, not ZERO compression???

I was going to suggest that he get that Blazer checked for codes before throwing any more parts at it ... but I did notice that CRANK position sensor was something missing from the list of possibilities as well.


NAPA (Echlin) # CSS212

I changed my mind about mentioning that because of the zero compression issue.

He has a mechanical problem , IMHO, not an electronic problem.
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Old 09-26-2007, 09:52 AM
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I bet on the Valve Spring

I had one go in a SBC. It had the same backfire that couldn't be cured result. They break on the first turn, after a long sit-up. Also prone to break in an engine that sits a lot even though has been run recently.
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Old 09-26-2007, 12:27 PM
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Ok, I found some 1996 heads. He says the 96-2000 was the same. Is this true? Need to pick these up today if they will work.
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Old 09-27-2007, 12:17 AM
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Hold on a second here, if you haven`t removed the valve cover and inspected what`s going on don`t be so fast to jump on new heads as a fix.
There`s the possibility that the valve is just stuck, and can be easily fixed with the head still on. If the valve spring is shot that`s a easy fix as well with the head still on. get the valve unstuck you can replace the spring and the seal and be back up and riding without the expense and hard work of swapping on new heads.
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Old 09-27-2007, 12:39 PM
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RE: 1998 4.3L Vortec backfires while starting HELP!

Ok, Very good advice. I will try to check this out today. Didn't get to work on it yesterday. The only reason I say change the heads is because I know this guy and he's going to give them to me for $50! And they just came off one that I know was running great. Anyway, I will still check the valve, I just wasn't sure if sitting for 2 years may have weakend more springs. So, the springs CAN be replaced without machine work??
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Old 09-27-2007, 02:01 PM
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You can change every spring on the heads without removing them and replacement springs will be the same thing. there will be no need for machine work either.
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Old 09-27-2007, 02:43 PM
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re: 1998 4.3L Vortec backfires while starting HELP!

Ok, Thanks. I will pull the valve cover this evening and see what I can find. Hopefully it's just a spring and not burned or bent.
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Old 10-01-2007, 09:49 AM
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re: 1998 4.3L Vortec backfires while starting HELP!

Ok, I pulled the valve cover and everything looked fine so we decided to pull the passenger side head and presto! It was a bent valve! I installed the new head and it's good. Had to re-time the motor and I'm waiting on a new o-ring kit to connect the fuel lines back into the "spider". I messed them up when removing the lines. Anyway, I appreciate the help. I should have this thing going today. If I run into another problem I will post it.
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Old 10-01-2007, 01:30 PM
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would doing the oil-in-plug-hole before start trick have prevented the problem? I just think that you can't ever be too careful with engines and stuff, they're expensive to fix.
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