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Random Cylinder Misfire

larryk 97

By larryk 97
in Ask A GM Technician

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larryk 97 Enthusiast

larryk 97

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Veh is a 97 Blazer s10 4x4 w a 4.3l 4 door auto transparked the truck one night and it was running fine. next day it did not want to start, now during this i had a known bad starter ( hold in coil on selanoid was bad). tried spraying ether into the intake, still no start, finally by jumping it, i managed to get it to start, but it is running very bad, FP tested ok, spark present, finally decided spark was weak so tested and replaced coil, and fixed starter.Truck now starts fairly promptly, but still runs rough at idle and at it's worst when cold, (Scan reports random cyl misfire ), plugs 4 & 6 are fouled, new rotor, cap, cables and plugs.engine will smooth out at higher rpm's, but sucks gas, does not appear/smell to be running rich except at initial start when cold.No coolant loss or excessive pressure, and oil levels are fine with no evidence of coolant, plug fouling simply appears as not firing, no build up or evidence of coolant in combustion chamber.Disconnect MAF yeilds no differance in idle, but scan will report MAF error.I have continued driving the truck for the last 2 weeks, conditions have not changed either worse or better.Suggestions... Idea's ?????

minibike132 Senior Enthusiast

minibike132

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Hi Larry welcome to the site. Fuel pressure running is _______. Fuel pressure at initial key up with engine off is _______. After that one turn the key off and monitor fuel pressure for 2 minutes fuel pressure is now_____. Miss fires are hard to diagnose over the computer but we'll give it a shot. Let me know what these are and we can go from there.

larryk 97 Enthusiast

larryk 97

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Hi Larry welcome to the site. Fuel pressure running is _______. Fuel pressure at initial key up with engine off is _______. After that one turn the key off and monitor fuel pressure for 2 minutes fuel pressure is now_____. Miss fires are hard to diagnose over the computer but we'll give it a shot. Let me know what these are and we can go from there.

 

 

I will get you all those numbers tomorrow, BUT i did notice when i first encountered the no start, on key on Pressure would jump to 60psi, then fall to 0psi immediatly after the initial 2 second pump cycle, if i cranked it a bit, the pressure would hold , used this to cause the pump to siphon off some gas through the Pressure gauge so i could check for water contamination. ( fuel was clean BTW), have never encountered a pump acting this way before, and more suspect vacuum simply was insuffecient for the Fp regulator, as i said at the time i was also in the middle of an issue with the starter, but i will have those number tomorrow.And Thanks in advanceBTW truck has 223k on the clock, and pump assy was replaced approx 1 year ago. New strainer and filter installed at that time.Tank was hand wiped out also during the R&R.

minibike132 Senior Enthusiast

minibike132

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60 is normal but it shouldn't drop fast, I think the acceptable drop is around 8 psi over 2 minuites but I just check them and play it by experience. Dropping to zero is bad. You are looking at a fuel issue but I wouldn't go for the pump first. This is a stuck open poppet (injector in this engine) or regulator usually. As for too little vacuum on the system for a regulator to be the issue this regulator is not like others because it sits inside the manifold so it can just pour out of it. Also most people don't think so but regulators can fail two ways. One is the one everyone thinks of where there is a rupture and fuel comes out the vacuum port. The other is one you don't see where the spring gets weak or broken and lets fuel flow back to the tank without the help of vacuum. Recheck those numbers just to make sure and I'll have you do one more test you can do at home to rule out the pump if the numbers support that.

larryk 97 Enthusiast

larryk 97

Posted
  • Author
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Hi Larry welcome to the site. Fuel pressure running is _______. Fuel pressure at initial key up with engine off is _______. After that one turn the key off and monitor fuel pressure for 2 minutes fuel pressure is now_____. Miss fires are hard to diagnose over the computer but we'll give it a shot. Let me know what these are and we can go from there.

 

 

,FP on Key on starts to climb to 30psi very slowly but fails before pump cycles off after 2 sec. But if i crank the engine over ( genarate some vacuum) pressure will immediatly climb to 40+ psi and hold. Engine idle Pressure is 54 psi, and will climb 2 - 4 psi on throttle up Pressure drops to 46 psi on key off after idle, and holds there for 4 min.+

larryk 97 Enthusiast

larryk 97

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was reading some posts from other users on rnd misfire, noticed one involving a 5.7L about a possible vacuum leak, never thought about this one, i get a very definate vacuum build up on the pass side tube that feeds from valve cover to the plastic housing on the TB. disconnected at TB housing and plug it with my thumb, vacuum definatly builds up. This might also explain why i get occasional wiffs of coolant into the cabin?

minibike132 Senior Enthusiast

minibike132

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I'm having a little trouble following you.

Key on starts to climb to 30psi very slowly but fails before pump cycles off after 2 sec.
Does that mean you turn the key on and it builds to 30 but once the pump shuts off it goes down alot? I should have also been more clear about the key on test. You need to key it on a couple of times to take up any air let in by connecting the gage. In any case your fuel pressure is low on all tests that you did. You can chase the vacuum thing if you like but I think you will find your problem on the fuel side. I would find the rubber part of the lines (usually next to the trans about in line with the drivers door) and get a helper to turn the key on. Pinch off the return line thats the thinner one, if you can't tell follow the line from the fuel filter and obviously that is the feed side. Have a helper turn the key on while watching the gage and see what the pressure is. If it builds to 80psi let the hose loose over 90psi is dangerous to the system. It should build to somewhere around 70psi.
larryk 97 Enthusiast

larryk 97

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I'm having a little trouble following you.
Key on starts to climb to 30psi very slowly but fails before pump cycles off after 2 sec.
Does that mean you turn the key on and it builds to 30 but once the pump shuts off it goes down alot? I should have also been more clear about the key on test. You need to key it on a couple of times to take up any air let in by connecting the gage. In any case your fuel pressure is low on all tests that you did. You can chase the vacuum thing if you like but I think you will find your problem on the fuel side. I would find the rubber part of the lines (usually next to the trans about in line with the drivers door) and get a helper to turn the key on. Pinch off the return line thats the thinner one, if you can't tell follow the line from the fuel filter and obviously that is the feed side. Have a helper turn the key on while watching the gage and see what the pressure is. If it builds to 80psi let the hose loose over 90psi is dangerous to the system. It should build to somewhere around 70psi.

 

 

 

yes, repeatedly cycled the ignition key on , pressure starts to build but when it fails to reach 30 and the initial cycle on/off completes, pressure drops to 0 psi. only if i let it breifly crank will it build pressure above 30 psi and hold it. also while running i did hit the bleed off on my gauge, indicate pressure held at 18psi steady, engine running. i will try pinching off the return and see what that yeilds

minibike132 Senior Enthusiast

minibike132

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Ok If it doesn't build you have an issue there. Do that return flow test that is going to test for an internal leak on the regulator. They fail in two ways we are basically checking the spring force. If the spring gets weak then you get fuel returning to the tank when it shouldn't be. The other way it can fail is a diaphram rupture which is what most people are familliar with, in that case there is a tear in the diaphram and fuel flows through to the vacuum side of the valve. In your truck the regulator is inside the manifold so there is no hose connecting to the vacuum side it just uses the vacuum that is in the manifold for control. To check for a rupture you can follow the big vacuum hose from the brake booster to the point where it goes into the manifold. It should connect to a plastic nipple that you can give a quarter turn to and take out. Take a flashlight and look inside the manifold while someone turns the key on and off a couple of times to charge up the system. You should not see any fuel and there should not be an overwhelming smell of fuel present. If the return flow test passes and you dont have any sign of fuel in the manifold let me know and we will keep going. If you do get a problem there then you need a regulator.

minibike132 Senior Enthusiast

minibike132

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Posted
Not sure on the 4.3 vortec, but shouldn't fuel pressure be somewhere upwards of 60psi? All the numbers I saw were substantially less than this.

 

 

Yes it should be, now I'm trying to help him figure out why it is so low.

larryk 97 Enthusiast

larryk 97

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ok, pressure now is 60psi KOEO

engine idle pressure is now 55-56 psi

leakdown is < 10psi over 1 hour w initial 2 psi leakdown on engine off.

 

Skip is still present, spark is present on indicated cylinders

 

i should also note that scans also indicate no activity on o2 Bank1 sensor 2, ths O2 sensor was swapped for the O2 sensor just ahead of the Cat, now scans report O2 sensor Bank1 sensor 2 no activity, and no slow/no activity on downstream sensor.

i have assumed bank1 sensor 2 to be the R/S in the exhaust pipe

down stream to be the O2 before the Cat ( and no idea what they call the one after the Cat)

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