Misfire Under Light Load 2000 – 2500 RPM| 1996 Chevrolet Blazer | 4.3

96chevyblazer 300x225 Photograph

1996 Chevy Blazer

This case study is on a 1996 Chevrolet Blazer with a 4.3 V6. The complaint on this Blazer was that it had a misfire that was bad enough to make the tach jerk. I drove this SUV for a few miles with no issues. Fuel trims at idle and at cruise were in a fairly normal operating range of -5 to +9. The truck actually ran really good. Idle was as smooth as I would expect. It didn’t miss a beat even while power braking the engine. A P0300 was stored in history codes. The freeze frame did not show any data that was out of normal range. Misfire history showed misfires scattered on all cylinders with the majority on cylinder 3 and the next large chunk on cylinder 4.

A visual inspection showed that new spark plugs, spark plug wires, ignition cap, and rotor have been installed. Further information from the customer indicated that the truck misfired the worse in drive, not overdrive, at light to normal cruise around 2000 rpm to 2500 rpm. The next test drive indicated that he was right and it was very repeatable. Anything outside that range and the misfire went away.  During the misfiring event cylinder 3 was the worse followed by cylinder 4 with a few misfires on the other cylinders. Firing order on this engine is 1-6-5-4-3-2. You can see that #3 is right after #4. During the misfire event, all the data was fairly normal. Even the fuel trims were on the money. The only data pid that was a little off was the cam retard.

Typically on the 4.3 liter engine I see a cam retard around -11 to -12. This engine was at -14 with an occasional drop to -13. I graphed this pid on the Tech 2 during the misfire even and it stayed rock steady at -14. Now most of the time when I have a worn distributor gear or cam sensor that is causing problems, I will also see the cam retard jumping all over the place.  Seeing the cam retard hold steady and no DTC P1345 Cam/Crank Correlation code, I wanted to move on but none of the other data gave me any reason too.

Tech Tip: If your cam retard is off to far you will set trouble code DTC P1345 Crank/Cam Correlation. The 5.7 and other V8 engines have an adjustable distributor. When you move the distributor you do not effect engine timing on this platform but only how far the spark has to jump from distributor rotor to distributor cap cylinder terminal. The 4.3 engine’s distributor is not adjustable and is locked in place. On the V8 engine, you should try to adjust the cam retard as close to 0 as possible. You can watch the scan tool cam retard pid as you move the distributor. The engine has to be above 1000 rpm for the cam retard pid to update and be accurate.

96blazerwornbushingH4V0 300x202 Photograph

1996 4.3 Worn Distributor Bushing

I decided to research these symptoms before I moved on. My research showed that a cam retard outside -12 commonly caused misfire on cylinders 3 and 4. I removed the cap and check the rotor play. There wasn’t much rotational play but there was a lot of back and forth play. With the distributor out and disassembled you could see the wear on the upper seal, top bushing or bearing, and also on the shaft. The distributor gear only had slight wear. With the new distributor lock in place, I checked the cam retard which was steady at -12.

96blazerdistdisassembeled 300x225 Photograph

1996 Chevy Blazer Worn Distributor Disassembeled

 

 

 

 

 

 

 

 

 

Leave any questions or comments below. Thank you.

73 comments

  • Dustin Palmer says:

    very interesting i keep this one filed in the back.

  • bodyman36 says:

    ok well using a scanner i noted that the timing advace, then under ever so slight tps movement,the advance jumper from 15 to 31 then a neg 61 and back and forth and the misfires were very radid, i have concluded that replacement of the worn dist will be the next step

    • Travis Dewitz says:

      The timing advance number on your scan tool is not accurate until you rev the engine to about 800-1000 rpm. If it is jumping around that much you may also have a bad crank position sensor. The PCM uses the crank position sensor and cam position sensor (in distributor) to get the cam retard reading.

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  • Rev Bonanza says:

    I’ve had this problem on a newer model 4.3. Problem was caused by worn distributor gear. One side of the gear was worn thin and giving an erroneous cam signal.

  • steve bates says:

    Ok my tach was also jumping and miss firing. I replaced the distributer, plugs, wires, cap, rotor and speed sensor today on it. The blazer is still miss firing and the tach is still jumping around. Now what do i need to do??. Its a 2001 blazer. What could be causing this problem?

    • Travis Dewitz says:

      Fuel injectors are a common issue on that model. What is your cylinder compression?

    • David M says:

      Steve you have had this for awhile, just thought I give you some suggestions.
      GM trucks have an odd security system and with it’s computer it will cause odd symptoms with the pass lock and may or may not through DTC I have seen everything from no-start, stall out and back fires and if there is a problem with the BCM it could cause the problem your are having too. Now earlier models have the BCM tied in to the instrument cluster like your tach. Others are in a different place like above the heater control switch.
      I would check for lose wire connections around the steering column also at the fuse box. If all checks out go for the BCM “Body Control Module” or the ICM “Ignition Control Module” The ICM directly controls the Tachometer and could also cause misfires.

    • Andy Steed says:

      Check your timing gear play. Remove dist. cap. Have somebody bar engine back and forth. Rotor and serpentine belt should move together in both directions. If not, pull timing cover and check gear. Mine was toast! I had replaced everything you did. Still misfired. Code P0300 .

  • corey says:

    i have the same problem but the miss fire is on the #one cylinder when you changed the worn out distributor did it help the problem go away. mine only miss fires when its cold after it hit temp and 15 minutes its fine any other suggestions?

    • corey says:

      and i dont have no check engine light on either no codes.. i did see they have replaced the disturbtior with a used one from the junk yard i think i might try a new one and get back to you

    • Travis Dewitz says:

      Did you check your fuel injector flow for each cylinder? Is the pressure drop equal?

  • steve bates says:

    My miss fire is on #6. I am going to check compression and fuel pressure this week. I have already did everything for a tune up and it still does it. The car runs great at a idle. The tach starters jumping around at about 2000rpm .

  • steve bates says:

    I checked fuel pressure today. It was 60psi and about 55 at a idle. The alternator went out last night so i replaced it today. The blazer tach still jumps around and feels like its miss firing. Probley going to take it to a garage next week to see if they can find the problem. Im think fuel injector.

  • steve bates says:

    All the injetcors read 13ohms.

    • Travis Dewitz says:

      Resistance isn’t an issue on those injectors. The poppets stick which is why you need to confirm with a flow test.

  • steve bates says:

    How do you do a flow test? Thanks

  • steve says:

    Im still having problems with blazer.. i done replace the dist,cap, roto button, plugs, wires, fuel filter, computer, speed sensor, crank sensor, cam sensor. The blazer tach is still jerking. and now it is missing when i come to a stop and when going 55mph it feels like its hitting the fuel shut off.. i took it advance auto and the codes kicked out.
    mas air flow,
    knock sensor.
    cam shaft sensor.
    crank sensor.
    sec air injection.
    control ignition off timer.
    WHAT CAN I DO? I DONE REPLACED. THE CRANK SENSOR AND CAM SENSOR AND ABUNCH OF OTHER STUFF AND IT STILL KICKS THE CODES OUT? AND WHAT IS CONTROL IGNITION OFF TIMER?

    • Travis Dewitz says:

      I think you will find your answer in doing these tests. What is your fuel pressure? Fuel volume? Injector pressure drop test? Exhaust back pressure?

  • David M says:

    After replacing all that and doing some real tests it almost has to be a wiring problem “bad grounds” maybe a bad dash cluster
    But then again may I didn’t see a replacement Spider on his list.
    Like you said before Travis, the fuel injection systems are known on these GMs Just like coil packs are known on Fords.

  • steve says:

    I took the blazer to a buddy of mines house and we hook it up to a snapon computer and when was having trouble geting the crank sensor and cam sensor to relearn with the new computer i replaced. We finally got it to relearn and it runs alot better now. But we are now thinking that the new cam sensor is bad. keeps kicking out 0341. I am going to try replace that next. The service engine light comes on and flashes every once inawhile. The tach still jumps but i think its in the gauage itself. Thanks for all your guys help.

  • dale says:

    have 2000 blazer changed everything including fuel injector and found out it was worn distributor gear and worn shaft gear is 95% worn chevy made the the shaft gear metal to soft and the disrtibutor gear to hard it should have worn out first defective part by gm ,did not do recall just gave notice these 2000 blazers are cheap junk replace all those parts from codes the only way to find its bad is to take off distributor and go in with a camera or lighted scope sell the piece of junk gm is biult to fall aprt at 100,000 miles by foreign honda or toyota i pity the fool who buys late gm

    • Bart Wallers says:

      Hey dale. Ive owned a fix all shop for years. I wouldnt agree with you about some Honda or Toy? These are ok cars but not at all better quality. Then our chevy ford or even dodge.

  • Jeremy says:

    I have a 1998 4.3 Blazer with a 106828 miles on it, that has a major misfiring problem. I got it cheep from a family member thinking it would be an easy fix, yeah right Ive replaced the cap, rotor button and plug wires so far. It mainly misses from idle to around 3000 rpm, after hitting 3000 it smooths out and only misses occasionally. The engine has plenty of power even with the misfiring. I’m going to change the plugs today but I have a feeling its not going to help. Any advice would be appreciated.

    • Travis Dewitz says:

      Testing is really the only right way to figure out the exact problem. Internal engine issue like valves or fuel injectors come to mind. Bad replacement caps are also very common.

  • Henry Lewis says:

    99 4.3 express with misfiring and bucking problem. Changed distributor, thought problem was gone, It returned. Took connector off distributor expecting it to stop, to my suprise it kept running and missing and surging stopped. Under hood fuse block checked and is corroded on fuse to cam sensor circuit. Problem is non-existent with 3 pin connector removed. Why?

    • Travis Dewitz says:

      May have a cam to crank correlation issue that is causing a misfire. By eliminating the cam signal, it helps from confusing the PCM possibly.

  • STEVE CRUSH says:

    I HAVE 2000 GMC SIERRA MISFIRE BELOW 2000 RPM AND CHECKED TIMING CHAIN BY TEARING IT APART IT WAS GOOD CAN`T FIGURE OUT WHATS WRONG

    • Travis Dewitz says:

      Would need more testing data. Hard to diagnose without more data and some test results. Compression, running compression, fuel injector drop test?

  • David M says:

    Steve, Like Travis said we need more informationn to help you like what motor to have and what dtc are you getting plus what tests have you done other than timing chain check etc…

  • Cory says:

    Hey. I got a 2000 s10 with a 4.3. At first it misfired very seldomly. Then one day my light came on and started flashing with a very bad misfire. Got the code checked and it was a random cylinder misfire. Can’t tell which one. Mainly misfires between 1000 – 3000 rpm. Put new plugs in about 300kms ago

    • Travis Dewitz says:

      A lot more than spark plugs can cause a misfire. If you don’t have the tools to test you may need to bring it to someone who does. Codes only point a direction in which my hands on testing starts.

    • tony says:

      I have a 2000 Silverado with a 4.3L and started missing. Was light at first, but got really bad to the point of almost no acceleration. I changed the cap, rotor, plugs and wires, still had flashing light. I dropped the exhaust system BEFORE the cats, and the truck ran new! I suggest checking your cat…or cats.

  • bob says:

    I have fought a p0300 code for the last 2 yrs. After replacing everything and taking my 2001 silverado 4.3 to my local mech. and the local chev. dealer with no answers and mucho dinero later i came across a service bulletin from GM stating that from 96-2002 there has been an ongoing problem with the valves sticking on 4.3, 5.0, and 5.7 vortec engines while towing or going up a grade. There recommendation was to pull the heads and hone the valves .002 of an inch. I ended up bending a push rod. Frustrating as hell when your local chev. dealer is’nt aware of an on going problem. I have seen several other people that were having the same problem with no answers.

    • Travis Dewitz says:

      The valve issue has had a TSB for a long time. It is a hard problem to verify and is a last resort once you have correctly verified everything else.

  • fredbently says:

    i just changed a distributor 2001 blazer …driving down the road and it just quit! come to find out the rotor inside the distributor came off…i think it was because the worn bushing on the distributor let the insides wobble enough in side that the rotor hit the cap enough to rattle loose…you’ld a thought it would have given some sort of clue what was going on my advice would be just to get a new one of those first btw IT COMES WITH a camshaft SENSOR i got a remanufactured for 100 bucks. ps if you do just replace a cam sensor DON’T absolutely DON’T break a screw off

    • Dave says:

      Yeah, all those screws in the 96 and up gm 4.3 distributor’s are tough to work with. The fine thread along with the thread lock make it hard, not to mention the steel aluminum thing going on…

  • alan eskew says:

    I have a 2001 blazer 4.3 vortec. My distributor was wore out so I replaced it. I now have a p1345 code and it only misfires sometimes. Around 55 and 75. I’ve read a lot on this and after all I’m still not sure where to start

  • alan eskew says:

    And I had none of these problems before replacing distributor so I’m nor really leaning toward the cam or crank sensor

  • alan eskew says:

    Also oil pressuee is fine when first crunk up but then after driving it starts dropping

  • Dave says:

    Alan, make sure your distributor cap, rotor, coil and coil wire are good, look for fire marks if ok then look at fuel pressure.
    These 4.3′s can be tricky sometimes the TPS can act up too…
    Remember its all computer controlled and counts on sensor input to set timing and fuel.

  • alan eskew says:

    Ok well I still had a problem with distributor. Wrong one actually lol. But I still have a question. I have no skip or anything wrong. What is this scan tool and cam retard people talk about. Is it like you might have to do it or you may not have to? And why

    • Travis Dewitz says:

      That is the only way to time the trucks correctly but most 4.3s the distributor is locked and can’t precisely timed but still needs to be checked to make sure it is within 12.

  • alan eskew says:

    With it being a non adjustable timing motor how does it time it. Is it basically cheating it? Andif timing is off even a hair the car wouldn’t crank would it?

    • Travis Dewitz says:

      Timing is computer controlled but you need to have the gear in the correct place. It can start and run with the gear off a tooth many times.

  • Dave says:

    There are alot of factors that determine timing both ignition and fuel in the 4.3. 96 and later. The computer will set the timing based on all sensor input and starts with the crankshaft positing sensor.
    If the timing is off the engine will still crank, act like it wants to start but won’t.
    A good scanner will show the ignition timing while the engine is cranking and running.
    Keep in mind the with the new computer systems you don’t need a distributor anymore, some engines have coil packs instead.
    You can buy a good scanner at the auto parts store for about $200 or online like ebay for a better price, and you almost have to have one if you work on these late model cars and trucks.
    I had alot of trouble with my astro 4.3 and turned out to be a passlock antitheft problem, they are very tricky, even the dealer has trouble with then.

  • Ekelley9548 says:

    Having trouble with 2003 4.3. Misfires at idle but goes away at 1,000rpm. Replaced ink spider, cap , rotor , wires ,plugs and camshaft sensor. Comp is equal on all cylinders at 155 psi. Fuel pressure is running at 55psi with no bleed down. Scanner shows misfire is on no.1 and no.2 cylinders only. Rechecked all ignition components several times.

  • alan eskew says:

    As far as I know everything is fine now. No more skip ir random misfire in diagnostic but I still get code p1345 and my oil pressure keeps dropping. Could my distributor still not bw right?

  • alan eskew says:

    Is it possible that distributor isn’t sitting quite right and no engaging oil pump

  • Dave says:

    Alen, as far as the oil pressure, you need a separate gauge to check that, but if your distributor wasn’t into the oil pump you wouldn’t get oil pressure at all and the engine wouldn’t start because the fuel pump circuit checks oil pressure.
    I think you know what the P1345 code is; so here is what you need to check for that: The plug connectors and wires on both the Crank and Cam Sensors, the timing position & correlation between the distributor rotor and crankshaft timing plate, should be at TDC or 0 deg., As Trav outlined before the engine will still run one tooth off on the distributor gear.
    Here is something else you need to know is the engine will still run with the cam sensor unplugged too, so the sensor could still be bad throwing the code.

    • Travis Dewitz says:

      The oil pressure to fuel pump circuit is actually the other way around. If your fuel pump relay fails the oil pressure switch can also feed power to your fuel pump.

  • Dave says:

    Kelley, I don’t know what issues you had before you replaced that stuff but I have seen issues related to the misfire on #1 and #2 low speed or idle. Most of the time it’s related to the coil or coil wire and ignition control module or wires at these locations…
    I’d like to see the condition of the spark plugs after running this for about 20 to 50 miles for condition, since you changed them, but check them all not just 1&2 and if all are close to the same “clean no carbon”; then I would rule out a fuel issue.

  • alan eskew says:

    Any correlation with cam and crank sensors and oil pressure acting up?

  • Dave says:

    Alan, from what I remember, the oil pressure sending unit is right by the distributor and may have been damaged when the distributor was R&R “removed and replaced”. You more then likely have a bad oil pressure gauge or sending unit to the gauge, you need to put a different oil pressure gauge in line to check the oil pressure; remember to check for bad wires or plug connections too, very common on older cars and small trucks.

  • alan eskew says:

    I now have 05 chevy 1500. Oil pressure is always like 80. Was told that’s fine?

  • Dave says:

    That seems to be too high to me,
    Like I said before, you need a separate inline gauge to check for faults.
    The dash board gauges are rarely actuate…
    Much like the temperature gauge reading 212 deg at full running temperature and not boil over; when it should read about 195 deg. is common.

  • Adam says:

    I have read through all comments about the 4.3 misfiring, jerking, stalling etc… I have a 2001 4.3 1500 Silverado and I have had the same problems, I am just curious if anyone has found a solution to this problem? Seems like a lot of guessing and money spent on parts that aren’t the problem! My problem started with my truck not wanting to start, then once started it was missing @ idle, but once truck came up to operating temp problem went away. After changing plugs problem went away but came back with a vengeance, since I have changed plug wires, cap, rotor, mass air flow sensor, both upstream and down stream O2 sensors(4 total), fuel pump, fuel filter, air filter and have ran several bottles of cleaners through fuel system PROBLEM NOT SOLVED! It is still missing between 1800-2500rpms, but idles and starts great now, not sure if this means anything but I have noticed that when descending down hill it seems to get worse in that 2000-2500rpm range but I can give it gas and get over 2500rpm and missing goes away until I get back to cruising speed (70mph) or 2000-2500rpms. CAN ANYONE HELP!!!!!!!!!!!!

    • Travis Dewitz says:

      Many things can cause a misfire. New parts are faulty all the time. Did you put in OE AcDelco plugs? Valve float? You say you have been running cleaners? What kind? I only know of three types that actually work well and that still doesn’t guarantee clean injectors and they also do not clean debris and rust.

  • Dave M says:

    Like Trav said, we need more data; are you getting DTC’s or check engine light?
    But with the information you gave us to me it could be the TPS “Throttle Position Sensor” , Ignition Coil or even poor Ignition Wires. GM did have a problem with the EGR valve control and made a firmware upgrade for the computer too.
    You need to run more tests and post the data.

  • alan eskew says:

    Simple solutions. Spend all your money fixing random stuff. Spend your money at a good shop. Or like I did buy something else lol

  • John says:

    99 blazer with v6, stalling at a stop and firing rite back up, drove it in garage, checked dtc’s, cam sensor code came up, so I pulled out the old, in with the new, also new cap and button, all parts were installed exactly the way they came out, so I go to start it up and the engine binds up, cranks for a second then binds up, strange because all I done was change the sensor and install a new cap and button, sounds like it’s off timing or wired wrong, double and triple checked plug wires , all good, can the distributor go just like that? Help!

    • Travis Dewitz says:

      The distributor can go in at any location as they are not keyed. You may need to bring #1 up and retime mechanical timing or at least verify the distributor is in correctly. Actual timing must be finished via a scan tool.

  • Cuppy says:

    2001 4.3 Blazer….
    Has a very light miss at around 1-2K RPM but no check engine light. Replaced cap & rotor. New plug wires & spark plugs. New fuel filter. New Battery. Replaced leaking EGR Valve gasket and cleaned egr valve, even checked specs on it from fully closed to fully open. Checked dist. for any wear including gear. Looks new. Motor is at TDC and all marks line up. Replaced all vacuum lines I could see. Replaced a rear o2 sensor that went bad when cat failed due to a torn wire. Fuel pressure @59 psi before starting, and holds at 55psi running. After shutting off, an hour later pressure still holds at 55psi. So I’m assuming spider isn’t leaking… Will be getting a new map sensor seal just to make sure.. Even without the cat the engine still has the light miss. Performed the checks for the ICM as stated.. Used this as a reference:

    http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-trobleshoot-a-hard-to-diagnose-misfire-1

    •http://easyautodiagnostics.com/gm_icm_dist/distributor_ign_module_1.php

    And now I don’t know where to turn before actually taking it in to a shop I don’t trust and that’s gonna throw the same new parts I just put in and still have the problem.
    Help please.

    • Travis Dewitz says:

      Do a running compression test. Sounds like possible valve issue. I would also do a fuel injector pressure drop test.

  • Lee says:

    P.S…
    Oh and cylinder compression are all at 195.

  • Cuppy says:

    P.S.
    Oh and cylinder compressions are all at 195psi…

  • Dustin says:

    Has anyone tried oxygen sensors? Stumbles at low end and clears up at higher speed/RPM. Does not sound like ignition or even stopped cats.

  • Dustin says:

    This is a common problem on mustangs.

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