1999 4.3 misfiring

I would look at replacing your coil, moisture will commonly mess them up if they have a crack in them. If the coil is going out, the more you require from it, the more the engine will miss. It would create the code given. That would be my best educated guess from the information you gave.

Just as a possibility I may also suggest trying a MAF cleanner, it should be about $8 a can. If it improve significantly, then I recommend replacing the MAF. Even though the frequency could be operating within specification, if it does not change as quickly as load on the engine changes, the engine may not be able to fire at the right time.

your problem is a bad throttle position sensor, i had the exact sme problem, changed everything out but that sensor and it finally stopped doin that once i replaced the TPS. same exact conditions, whebn it got humid or after a rain it ran like shit lol

I have this same problem with my 99 s10 4.3. the code reader says miss fire cylinder #2... so I replaced wires, plugs, distributer cap and rotor. after 2 days this problem got worse. again I read the codes and it read the same code, cylinder #2. good old GM I guess

I know this an old thread, but seen alot of questions across these forums while searching for an answer to problem that seems to be irritating all of us 4.3 vortec owners. "The dreaded mystery coldstart misfire." I like many others had the shudder between 1500-1800 rpms until the engine came up to operating temperature. I just found the root of all evil hiding under the EGR valve. Removed EGR to clean and realized there was a blow by in the gasket. Removed gasket and noticed it was on the topside and underside of the gasket. Scraped the remainder off, cleaned the surface, and installed new gasket with original EGR valve. Instant success! I can see why people got rid of the issue when replacing spider injector because they probably removed and reinstalled new gasket when replacing EGR. Less than an $2.00 fix compared to the $300.00 cost of new injectors.(I still need to replace injectors as mine has over 300,000 miles on original but thats another project) Also remedied some annoying valve chatter. Worked for me...hope this information helps others!

I have a ? I have a 99 s10 blazer that is doing the same thing. idels great but misses bad when taking of even when your going about 65mph it wil wanna miss and jump. punch it down and runs great. and cheap fixes possible? Id sure appreciate any info

Have you checked your EGR gasket? I would start there.

On a 99 where is its location? I've been told by several that is typical for the 4.3. Egr valve

Its on the front of the intake. When you raise the hood, its a smaller cylindrical part with a plug with two wires I think. Two bolts and it lifts right off. You will need to have replacement gasket, scrape and clean old gasket off, blow carbon out of EGR and replace. Maybe a 10-15 minute job

I sure appreciate it. i hope that solves it.

I hope it fixes your problem. Please let me know. Thanks

Does the gasket on egr valve require rtv?

No rtv. It is a graphite gasket

I cleaned the egr very well and the plunger worked good and did have carbon but not bad. put new gasket and still same problem . idels good and ribs up good. misses taking off and in neutral about half throttle it misses. put the coal to it and just fine until you plain off and it goes to jumping . I'm looking for the nxt remedy?

I should have asked earlier. Is your check engine light on or have you been able to get any codes?

Just finished a 2000 S-Blazer with a 4.3 that had random cylinder misfires. Here are the steps we took. First we replaced the plugs that were the wrong ones. With the computer conected we monitored the live data and were recordeing most of the misfires in cylinder 6, 4, 2 with an occational misfire on 5. We had no codes recorded in the ECM but the fuel trims were way off. We pulled the thermostat and found ijt rusted in an open position with the car cold. We replaced it and retested and the fuel trims were right in line to spec. We changed out the coil that was outputting 1/3 the voltage required. The next change was the distributor which had a worn gear and a cap with corroaded terminals. Again we still had misfires but they were becoming less. We next changed the crankshaft plug. Wires were bare and soaked in oil. We addied octane boost since we had a full tank and the customer really did not know if the gas was 87 octane or F-85. The misfires still occured but only when the RPMs were above 800. The last test was to ohm the ignition wires. They tested out poorly. We changed them and then went for a 2 drive cycle to clear the internal codes since we needed to get the vehicle smogged. The vehicle passed with flying colors. This vehicle has not had any parts replaced for 15 years. The point to the whole senario was that the vehicle needed everything we replaced. It now runs like a new car and the customer left for California and not worried about having problems. Reading on the Internet about the same problem others were having but they were only trying just 1 or 2 things and were giving up. We stuck with it until the vehicle was road worthy nd the problem was eliminated.

awesome write up this will help me in isolating my problem

my 99 s10 extreme 4.3 W missed at the idle driving speed range for years got worse when it was damp outside.stomp down on the gas and fine.it also started and idled well it was just the normal driving range. I went through everything cap and button, plugs and wires, fuel position sensor, cleaned the EGR, new coil pack and still the same problem. today 7-10-15 i simply unplugged my MASS AIR FLOW sensor because of a tip i read about, problem solved my truck has power now that i have never notice since i owned it. hope this helps people out.

In response to maxhazard. I had a simular problem with it missing and bucking at normal cruise speeds. Give it gas or slow down and it was fine. I spent a bunch on multiport fuel injectors and wires,plugs,cap rotor,etc. What finally fixed it was replacing the anti-knock sensor in the back of the engine. The ECM advances the timing until it hears a knock then backs down a little for fuel econ. If the knock sensor is bad you will get detonation.

I'm having the same problem I got a 96 s10 4.3 vortec and I've put spider, plugs, wires, distributer, maf,coolant sensor, checked fuel pressure, idk what to do

replaced the MAF sensor and the Ignition control module that was the root of my evils. and cured the problem. I have over 150k miles on mine so both needed the change anyways with a proper tune up.

I had bad misfire between 1000 to 2000 rpms. With 260000 miles and the gear for the distributer was ground down. And that fixed mine

My morning misfire was caused by a bad intake manifold gasket (a very common problem with most gm models around 2000 model year. Caused by crappy dexcool coolant ). After sitting overnight a small amount of coolant would get into right rear cylinder. It would miss for a bit and then run fine. Doesn't have to be leaking a lot either.

I'm having the same problem but mine misses all the time I would recommend replacing the whole distributor and cleaning the egr valve

I have a 2000 s10 with the 4.3l in it. It starts and runs fine, but when you let it sit and idle you can see and feel the rpms drop and come back up. When you Rev it and let it drop back down it acts like it's going to die. Any ideas anybody?

i have an 03 silverado 4x4 4.3 and my truck just runs weak. like i cannot gain speed going up a hill. the only was to keep a contant speed is to the floor. i changed plugs and wires and distrubter cap and rotor cap, plugs and wires. changed the oil and filter, tranny fluid is good. has a miss when i'm barely giving it gas. the cat is cut off the exhaust and comes out right behind the cab. it has like 133k miles. i really like the truck, i just cant figure out why its running so weak

@chris, I've read that the stock 4.3 vortecs requires the catalytic converter and/or OEM muffler to have proper backpressure on the engine or it will burn up the valves and rings prematurely. That being said, I would think the catalytic converter would be most important because there should be an oxygen sensor right after it measuring the converters efficiency.

Ideas and suggestions are great..Would be nice to include a rough guess of the cost involve in the repairs as well. Thanks

96 S-10 v6 4.3 vortec. Misfiring at cruising speed or at intermittent times. And I get a low voltage tps code.. I've replaced; new plugs and wires, dist. cap and rotor, Ignition coil, Throttle Position Sensor, TPS adapter (soldered the wires), MAF sensor, PCV valve, EGR valve/gasket (replaced with a used one). Running out of ideas, and have pretty much ran thru everything in this thread. Any thoughts?

Misfiring at cruising speed or at intermittent times. could be a fuel regulator

I have a 98 s10 4.3 and I'm having this same issue, the issue the thread was originally over. Every morning the truck has a horrible bucking at regular driving idle speeds,meaning when you reach your local speed limit it jerks sometimes violently, I have replaced the Cap,Rotor,plug wires,CTSensor, unplugged the MAF, replaced the MAP, plugs, EGR, everything that has been mentioned here and other places. The problem only occurs early in the morning when it's cool or the engine is cold, and it does it badly under very wet humid rain days. My timing is right. The distributor looks good. I'm needing some help figuring the problem out. The first post on this thread, the one who created it originally has the exact problem I'm having. If you read these threads still please let us know what you did to fix that problem, anyone else that has a suggestion please

You mentioned unplugging you MAF. This could be your problem. You should check to see if it functions properly, clean, or replace. Here is a link that may be helpful http://easyautodiagnostics.com/gm/4.3L-5.0L- 5.7L/how-to-test-the-maf-sensor-1

I had a problem with a 96 GMC Safari van. On rainy days it would run rough and sometimes die completely. A new coil pack helped some, but it still had problems when it rained. I thought that maybe some rain splashed up on the engine, so I made a raincoat for the coil pack. I put a plastic bag over it and rubber bands to hold it in place- no more problems on rainy days!

Know this is an old thread but I had multiple misfires and change the plugs wires rotor and cap and still misfired then I plugged in a snapon scanner and watched the misses and saw that it was on cylinder 1.3 and 5 so i figured it was an o2 sensor on bank 1 I unplugged it and it runs great Lol

I have been wrestling with my d 2003 ZR2 for awhile it was lazy climbing hills, spark knocking & throwing all the codes you guys are dealing with and getting worst. Yesterday I remove the front O2 sensor and drove it to see if the converter was plugged it did run better up hill. So we removed the catalytic converter and the truck behaves fine now. Just thought I would let you know this has been an on going fight for a couple months and wasted money on other parts. I checked with the part store and they said they sell plenty of converters.

Short explanation. Your distributor cap counts the cylinder as #3,#1 and #5 instead of #1,#3 and #5. longer explanation. I had misfire aswell on my chevy s10. After closer look at the Destributor cap i noticed that the numbers are not in correct order. When we count the cylinders on the V6, standing infront of the engine, from the Right side cylinder closest to you is Cylinder #1, behind that towards the driver is cylinder #3 and last in the row closet to the driver is cylinder #5. oppisite side the cylinders start with #2, #4 and #6. Now if you take a closer look at the distributor cap it counts right side cylinders as #3, #1 and #5, so your misfire will be fixed by correcting the order you plugged the wires to the cylinders.

Way off there bud. They are numbered like that for a reason.

Mine's been missing for 11 yrs since I bought, couldn't figure out. New wires, rotor, cap. Put new coil on today. Now lays rubber. ;-)

My 2003 s10 is missing and bucking real bad at low rpm but smooth once you rev it. I was revving it at the engine and got shocked. Any ideas? Changed plugs, no difference

I rebuilt my 96 S 10 4.3 I kept having a misfire code. I replaced the spark plugs three time in the process realized i was cracking the porcelain part of the spark plug on number 3 because it was so close to the steering shaft you can take one bolt out of the steering shaft push it up move it out of the way put the plug in and the 90 spark plug wire seen people install the straight plug wire which hit the steering shaft slide the shaft back in place adjust steering wheel where you like it and slide shaft back on the knuckle and reinstall the one bolt.

I have a 98 Chevrolet Astra 4.3 and I'm having misfire cylinder#5 and#6, I have replaced the plug wires, plugs, everything that has been mentioned here and other My timing is right. The distributor looks good. I'm needing some help figuring the problem out. If you read these threads still please let us know what you did to fix that problem, anyone else that has a suggestion please. thank you.

check your injectors .mass air.and valves ..look into your timing and see if the chain is stretched

I had a problem with my 99 s10 getting a P300 random misfire and after replacing all kinds of parts and even having chevy look at it they had no idea waste of money their. I finally pulled out the distribitor and found the gear was chewed up replaced it and I have 268,000 miles on it and still running good. 4.3 great engine

I have a 2002 GMC Sonoma with a 4.3 vortec engine. I've been noticing that at low idle the engine would hesitate some. I had to change the intake gaskets (Very common problem) and when I pulled the distributor and inspected it i found that the gear was worn considerably and the shaft was loose in it's housing. Replaced distributor when I reassembled the engine and runs better than it has in years.

I got Chevy 5.7l vortex and it has P0306 and P0122 TPS and a MAF code P0102, runs rough at idle but at the right RPM about 700 and it hesitate like hell goes into limp mode. I replaced TPS still the same unplug the MAF it cuts off some times other time it will conversate for it once unplug. I have replaced distributor and it has CPS as one check timing chain sensor no marks on it, spider injectors went out replaced them fuel pump went out after that replaced it still have these codes for hat come back vehicle like in limp mode. I did a lower in upper intake gasket and can't find wear if there unmetered air coming in with spraying it while she cold and nothing???

I ended up changing map sensor throttle positioning sensor coil wires plugs cap rotor and it still did it. Took the cap off dried it out clean the contacts every time I would do that it ran fine it was moisture getting in the cap .... Maybe this will help somebody else before they change for five different parts for no reason

Pressure sensor on intake mine was doin same thing changed cap rotor plugs wires injectors mass air sensor plugged in new pressure sensor and solved my problem

I'm not a mechanic but I checked my air filter even tho the gage said it was fine it looked dirty so I started it up without the air cleaner and the miss was gone. I replaced the air cleaner and i'm now a happy camper

If parts weren't so cheap, I'd drive this thing over a cliff. 2003 S10, 4.3 and she drops a random cylinder when under load whether it's with a trailer in tow or just going up a hill. I replaced the entire ignition system. Plugs, wires, dist., coil, etc. Still dropped. 175k on truck and the CAT was about shot, so I replaced it as well as all O2 sensors. Still drops. Next step will be EGR cleanup and gasket and hope it's the final straw. Loosing my patience.

GuruTK173. Have you checked your crank sensor? 4.3s have a tendency to develope oil leaks around the timing cover and have seen the leaks to make them malfunction. If you replace, be aware that the new sensor may need shim if it rubs. Your symptoms sound like a bad crank sensor to me

Don't cheap out on the plug wires!! I bought lifetime wires at AutoZone and O'Reilly and they were junk. I went with the premium MSD's and that made a big difference. Before the MSD's I would have sparks jumping through the wire to the block. The crank sensor is not the problem.

Moisture in the distributor cap?? Get the updated vented caps. It is a known issue that non-vented caps will cause trouble

Same problems with my 97 totally random mifire no one could find anything wrong .i finally grabbed wire harness to distributor and yanked up on it. Foxed mine.

I suffered with the P0300 for nearly 3 months and after months of replacing parts ( a few of which truly needed a tune up ) testing every sensor i coukd think of Koeo and under load... And absolutely nothing i nearly gave up on the truck but as a last ditch effort to save her i pulled the distributor and discovered the once meaty thick gear of this beast was no whittled to mere knife blades i was certain i had found my issues as the new one compared to the old was night and day, after about a 3 hours, a few cuts, busted knuckles and many curse words.... She ran like a scalded dog. Im not saying this is your issue but if youve been chasing the P0300 code i might have just cracked your egg

New to this web site. My daughter has a 1995 Chevy Blazer with the 4.3 Multi port. Its got a bad miss. Especially when you are driving around 2,000 rpm. Sputtering and not smooth like driving normal. I have replace the Map sensor, TPS sensor and the fuel filter. Cap and rotor where replaced not to long ago. It does start and run. A slight miss at idle. No codes.

I replace the cap and rotor today and seems to have made the problem worse. I can give the engine any throttle it just wants to fall on its face. No power.

Make sure you use HIGH QUALITY spark plug wires!! I use MSD now and my 1998 Jimmy runs great! I was using the less expensive wires but that is a BIG mistake! Have to remember that there is 30,000 volts on these wires so the insulation around them must be very high quality. It doesn't take much for the spark to jump through the wires. Spend the $$ and get the high-end MSD's!! I chased the P0300 problem forever as well. If you want to verify your problem, park in a dark area, pop the hood and watch for the sparks out of your wires. You'll see the arcing..

I have the same problem. I see this is not a new problem for 4.3s I have had mine for 2 years. but I have oil on 1 spark plug. I think I'm screwed

Ok I've been looking all over the place for answers for this problem and I found out what mine was. I am at work on break and I had to find out so I unplugged the maf sensor and drove it down the road and it didn't bog or shake not one time so now I know my maf sensor is bad. I hope this helps someone cause I'm glad I figured it out me

Long story short.... Misfire from hell I replaced everything under the sun I finally swapped out my distributor Problem solved

It can be you're camshaft making it misfire havent changed mine but this sensor is literally in the bottom of the distributor cap and rotor it makes sense to me that it would be the cam shaft sensor gone bad giving it misfires to the plugs and distributor cap and rotor . Since our trucks are old it's best to just change all sensors that are important to replace .

Welp are your answers I've tryed, I have a 99 s10 4.3L 4wd 5sp, rebuilt motor .020, ported heads, lower intake, 10.1 compression, comp cams .500 - .510, balanced, tuned with HP TUNERS, motor has 6000 miles still have missing at low rpm, has no egr, knock sensor, front 2, o2 senors all shut off from HP TUNERS, 18° at Idle, 36° total timing, was tuned with rear o2 sensor, I'm out of ideas or options.... any help would be grateful, JR Newman, a Automotive Machinist, in PA, (newmansracing@hotmail.com)...

All too often on those years such as yurs we overlook the fuel press regulator because its under the plenum. This, along with leaking plenum gaskets are both big players in misfiring at idle as well as poor performance. A leaky reg inside a plenum will cause a miss and may even throw a code. The plenum is a sphere which holds the vacuum pressure which controls the regulator at any given speed and by the same token a bad plenum gasket will suck in ambient air and also cause the regulator to perform incorrectly

I have a 94 olds bravada 4.3 6 cyl. Over 45 mph and it would chug and lurch....was cutting out (??) and then it would backfire sooo loud out the tailpipe!!! I replaced quite a few parts looking for the answer to my problem...but I finally found the answer....the Fuel Pump Relay!!! First ill black box on rear firewall on drivers side. At 70 mph it floats like a butterfly!! I did put in a new Fuel Pump and Fuel Pump Regulator as well, but not so sure that I needed to!?!