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Discussion Starter #1
160k miles
P0300 Random Misfire
Sometimes P0303 #3 Cylinder misfire

When I use an OBDII scanner to look at misfire counts, it seems most concentrated on the #3 and #4 cylinders, but all cylinders will have a few misfires.

Testing fuel pressure with an Autozone fuel pressure tester, I was getting 52psi with the motor running (spec is 60-68psi). After replacing the fuel filter, I am getting around 54-60psi with the motor running, even under acceleration.

Even when actively misfiring and service engine light soon light is flashing, the motor seems to be running smooth. Not jerky at all.

I've replaced:
  • Spark plugs (NGK Platinum, gapped to .060")
  • Spark plug wires (NGK)
  • Ignition rotor (Delphi)
  • Distributor cap (ACDelco OEM)
  • Fuel filter (GM Genuine)

Was starting to look into testing the fuel injectors, but for the life of me, I cannot see them on top of the engine. I guess I have the spider fuel injection? Does the 2002 have the SCFI design or the newer MFI design?
 

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Discussion Starter #2
A few updates.
1. Checking the RPO codes, I have the L35, which means it came with the older SCFI design from the factory.
2. I removed the distributor shaft and removed the screens per TSB 05-06-04-007. There is no rotational play or side to side play, but there is some up/down play. This link says up/down play must be within 0.6-1.2mm. I measured just shy of 1.2mm.
3. Passed compression test. Got between 140 and 170 on all cylinders. Cyl #3 was lowest at 140psi, Cyl 2 was highest at 175psi. The rest were 160psi. Spec's 100psi and all cylinders must be withing 25% of the highest cylinder.
4. I removed the intake plenum, it was super nasty in there. Gas pedal was sticky and always had some resistance when stepping on it. Back of the throttle plate was gummed up real bad. I figured if everything was so badly gummed up, it wouldn't be a stretch for the poppets to be gummed up too. I went ahead and ordered the updated GM MPFI injector retrofit. Throwing parts at it... Fingers crossed.
 

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2013 GMC YUKON XL
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Had a 2002 S-10 Quad cab with the L35 for 10 years(see avatar) Same problem. Went thru all the systems like you're doing. Biggest changes were getting rid of the Dexcool in the 4.3 (it destroys the intake manifold gaskets) Felpro intake gaskets, New Delphi spider (MFI), Autolite Spark Plugs(See below) and 8mm H.D. wires. When I pulled the distributor I found the gear was highly chewed up on the back of the teeth. Like a rear end jackpot out of alignment. New stock cam, lifters, and an ALL Aluminum distributor (United Motor 9363 @ rockauto), and it pretty much eliminated the P0300 and P0304 codes. Went from 3 codes a week to 1 code about every 6 months (same codes.) Reset and kept on going. Never could eliminate it totally. Don't use a Distributor cap with steel contacts. Brass will last longer and work better. Also the Rotor needs to have 2 attachment points on the rotor arm. United Motor Cap and Rotor have these and work really well in the 4.3, AND with the aluminum body you won't break off the cap attachments on the side of the distributor. Buy it once and it's 1/2 the cost of a "rebuilt" from Autozone or Advance where all they do for the attachment tabs is JB weld on new tabs. These will break too. Autolite AP605 Plugs. Mine hated the AC DELCO Plugs, but ran better, less CEL's and got better mileage with the AP605 or XP605's.

Big problem with the cooling system on the 4.3L. Lack of Cathodic protection (Dex-Cool, Mix of Aluminum and steel, lack of grounding on the CORE!) Get some flat ground strapping and attach it to the radiator core and then to the core support(make sure of clean metal at contact points). As the Dexcool (or any antifreeze) reacts to the metals in the engine you get galvanic deterioration from voltage differential(DexCool is notoriously bad for the 4.3L). Take your volt meter and set to DCV (20V setting) Engine off, and COLD, place meter Red probe in the anti-freeze in the tank, Black probe to a CLEAN Chassis point. Should be less than 50mv or so. Mine was a whopping 2.6V with the Dexcool in it (1 yr old). Dumped the Dex-cool (changed all the other parts at the time.) flushed the engine, put it back together, and ran H.D. cooling system cleaner for a week with distilled water. Got 95% of the crap (sludge) out. 55% Prestone green/45% Distilled water and a can of water wetter. 195*F Thermostat. 2 ground straps on the core (each end). Voltage in the cooling system ended up in the low 20mv range. After 2yrs (normal flush time) Voltage was up to mid high 20mv range.

Make sure the Fan clutch is working, replace or switch over to electric fans (you may already have them).

Larry
 

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Discussion Starter #4
Jeeze, sounds like you've had a lot of problems with your truck

My distributor gear doesn't look worn. I plan on checking for rotational play after I reinstall it.

I've been planning on dumping out the dexcool and throwing in G-05.

How does a bad fan clutch contribute to misfires? My fan clutch doesn't slip when the motor is cold, so it sounds like a bus for the first minutes before it warms up and the fan slows down
 

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2013 GMC YUKON XL
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It doesn't contribute to misfire, but it can contribute to higher than normal temps up to overheating if it doesn't lock up. Remember the 4L60E isalso cooled by the radiator thru the cooler connections. Higher temps in the passenger side tank causes higher temps in the trans.

I traded it in 2 years ago on a 13' Yukon XL. I still keep in touch with the current owner. No increase in occurances of P0300 or 0303/0304.
 

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Discussion Starter #6
Replaced the fuel injectors tonight. While reinstalling the plenum, I noticed the PCV hose elbow is cracked badly, will need to replace that tomorrow.

I did notice the ignition rotor does have some rotational play when the distributor is installed and tightened down. I'm not sure if that's normal or not, so I will have to research that.

I messed up reinstalling the distributor, so the van didn't start upon reassembly. Got it re-timed successfully, and the van started up smoothly after that.

Also, the gas pedal was super smooth, so that's a small win.
 

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Discussion Starter #7
And the misfire is still present. I can get it to happen 100% of the time by flooring it and getting on a highway on-ramp. SES light starts flashing and misfire counts start going up on cylinders 3 and 4 after the first shift.

Here's a video that shows roughly how much rotational play I have with my distributor:

If I pulled the distributor and hold the gear, there is zero play in the shaft itself. So that play must be from where the gears on the distributor shaft connects with the camshaft.

I attached pictures of the gear, I couldn't spot anything really wrong there, but I don't have much experience with these motors
 

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