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Mike S.
Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 7433
Experience:  ASE Certified Master Technician
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What would cause a 4.3 misfire on one cyl.

Customer Question

what would cause a 4.3 misfire on one cyl.
Submitted: 6 years ago.
Category: Chevy
Expert:  Mike S. replied 6 years ago.

Well, that just about eliminates any of the ignition system as the problem and most likely the fuel system as well. That leaves mechanical. I would do a compression test and then if that fails a cylinder leakdown test to determine if perhaps the piston rings are stuck, broken or just plain worn out. That would also check that the valves are seating and the head gasket. On a 2000 the computer should have at least told you what cylinder it is, so if the code is P0301,P0303 or P0305 that would be on bank 1 or the same side as cylinder #1. If P0302, P0304 or P0306 that would be on bank 2, so remove the valve cover and measure the camlobe lift to make sure the camlobes on that cylinder are not wiped or flattened down. It could also be a bent push rod, rocker arm stud pulled out of the head, so let the engine idle and look for anything loose or making noise.


Customer: replied 6 years ago.
pulled boat ot lake no problem. 1/2 way back ran out of gas. put 5 gal in started, returned can & 1/2 of that 1/2 way home started missing.. Oh it's had 3 caps & rotors & 2 fuel filters. After plugs are pulled & cleaned it begines missing on #3 then #5 chimes in ^ then #2 or #6/ comp is 145-150 & spark is blue with red edges & will jump almost 1/2" gap
Expert:  Mike S. replied 6 years ago.
I thought you said it was only missing on 1 cylinder? So what are all the computer codes? When you say clean spark plugs, what are you cleaning off, gas or oil or anything else? If you are cleaning gas off more than 1 spark plug you may want to check the fuel pressure to make sure it isn't too high. It should be 58-64 psi for a 2000 Chevrolet Blazer 2WD 6 Cylinders W 4.3L SFI.
Customer: replied 6 years ago.
OK Forgot new fuel pump as only had 50 psi to start.there are no codes except engine miss fire code. plugs will get black fluffy deposits #3 being the worst.. After cleaning 3 times didn't ttrust there wasn't a carbon trackI couldn't see so put in new plugs with the samr result. Starts missing on 3 & if I continue to run 5 will start to miss then either 6 or 2
Expert:  Mike S. replied 6 years ago.
Ok, but what is the code? I see 2 4.3l engines for this vehicle. What is the 8th digit of your vin, a W or X? Black fluffy is most likely carbon from a rich mixture. Is your air filter dirty and/or pcv valve clogged up? Is the tailpipe blowing out any black smoke? What did you gap the plugs at?
Customer: replied 6 years ago.
l8th digit is W . What has me stumped is it ran fine pulling boat to lake & 1/2 way back before begining to miss or set mis fire code give me a miniute & I'll go retrive code no#. plugs are gapped 60ths as is most everything in gm BRB--------------- P0300B is code set
Expert:  Mike S. replied 6 years ago.

Ok, I was just double-checking cause the vin X 4.3L fuel pump psi was 41-47psi and spark plug gap of 0.045". The vin W is 0.060" so you do have them correct. I am just a bit concerned about the low fuel pressure. Perhaps the ecm and O2 sensors are overcompensating for that and making it run rich. Do the spark plugs smell like gas when you clean them?

Yeah, P0300 is a random misfire code. You might want to consider replacing the fuel regulator.

Customer: replied 6 years ago.

look back it's a new spider and the real misfir is on #3 what time will you be on tomorrow . haven't eaten today & wife is hungry give me a time & I'll meet you back here


Expert:  Mike S. replied 6 years ago.

I'm on by about 8-9 am in the morning. But before you go here is something to think about from the Chilton manual on fuel pressure lower than 55 psi.

"15.If the fuel pressure is less than 379 kPa (55 psi) at idle, it falls into the following 3 areas:

■The pressure is regulated but less than 379 kPa (55 psi).The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. Refer to Engine Cranks but Does Not Run .

■A restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.

■A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved."


"A lean condition may result from the fuel pressure being below 414 kPa (60 psi). A lean condition may cause a DTC to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring."


"Fuel pressure that drops-off during acceleration, cruise or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. You can diagnose a lean condition using a scan tool. If an extremely lean condition occurs, the heated oxygen sensors will stop toggling. The heated oxygen sensor output voltages will drop below 300 mV. The fuel injector pulse width will increase."


Customer: replied 6 years ago.

well it gives me something to do tomorrow. will pull top off for 7th time and put old spider back it & try as don't have vac line to pull off on this one. but it's always #3 that gets black like it not firing. will get hold of you in am have good evening


Expert:  Mike S. replied 6 years ago.

If its only #3 then it may not be the fuel pressure. If the black stuff is oily then perhaps you have a worn valve guide and/or bad valve guide seal and oil is entering the combustion chamber and fouling that spark plug.

Customer: replied 6 years ago.
the part I'm having so much trouble understanding is that it pulled boat TO the lake & 1/2 way back fine. After putting in 5 Gal It ran fine for almost 5 min before starting to miss. If a push rod , rocker or worn guides those things don't just majically happen . guides are a slow process; rocker & or push rod ususlly happen when engine put under a sudden stress & usually make noise . none of this has happened. I'm aware of what oil fouling is & the diff between gas, oil, carbon are. I've been doing this sort of work for 56 yrs . I've got some sort of quirk here that I've never experianced & I'm kinda looking for something other than routine. I'm thinking it has to do with ign but onthoff chance I got hold of a bad (new) spider I'm going to pull it out & put original one that I cleaned back in.
Expert:  Mike S. replied 6 years ago.
Did you ever consider that perhaps there was something wrong with the gas? Although, I don't see how that would effect only one cylinder. But I do see eventually you will have problems on 3, 5, 2 and 6.