Misfire Under Light Load 2000 – 2500 RPM| 1996 Chevrolet Blazer | 4.3
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.
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.
1996 Chevy Blazer Worn Distributor Disassembeled
Leave any questions or comments below. Thank you.
36 comments
very interesting i keep this one filed in the back.
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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
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.
[...] 1996 Chevy Blazer dewitzdiagnosticsolutions.com [...]
[...] 1996 Chevy Blazer dewitzdiagnosticsolutions.com [...]
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.
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?
Fuel injectors are a common issue on that model. What is your cylinder compression?
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.
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 .
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?
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
Did you check your fuel injector flow for each cylinder? Is the pressure drop equal?
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 .
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.
All the injetcors read 13ohms.
Resistance isn’t an issue on those injectors. The poppets stick which is why you need to confirm with a flow test.
How do you do a flow test? Thanks
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?
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?
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.
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.
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
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.
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.
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?
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.
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
Would need more testing data. Hard to diagnose without more data and some test results. Compression, running compression, fuel injector drop test?
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…
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
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.
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.
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.
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