1999 4.3 misfiring


Asked by Jan 05, 2009 at 12:22 PM about the 1999 Chevrolet S-10 2 Dr STD Standard Cab LB

Question type: Maintenance & Repair

I'm running out of ideas on a problem with my 99 S10 4.3
The truck will start and idle fine however as soon as you feed a little gas it starts to miss.
Giving it even more gas is either covering up the misfire or it is clearing and seeems to run better.
I've just replaced cap/rotor wires and plugs.
The problem seems to get worse whenever it gets even slightly wet/humid.
I had Cragen hookup a analyzer and it gave a "random misfire" error code.

Does anyone have an idea of where to look?
Thanks Greg

23 Answers

Michael McGrail

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.

2 out of 2 people think this is helpful.
Danny Wang

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.

3 out of 3 people think this is helpful.
Jeremy Prchlik

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

5 out of 5 people think this is helpful.

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

5 out of 5 people think this is helpful.

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!

7 out of 7 people think this is helpful.

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

4 out of 4 people think this is helpful.

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

3 out of 3 people think this is helpful.

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

3 out of 3 people think this is helpful.

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

4 out of 4 people think this is helpful.

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?

2 out of 2 people think this is helpful.

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

1 out of 1 people think this is helpful.

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.

1 out of 1 people think this is helpful.

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.

2 out of 2 people think this is helpful.
Mark Walther

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

1 out of 1 people think this is helpful.
Ken Goochey

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.

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