2004 Discovery NAS (Bosch V8)

Been running great until I started getting a misfire on cylinder 5. The current wires and plugs have about 50k miles on them so I figured I'd change them. The plugs came out looking fine. Put in NGK plugs and wires.

Still getting the misfire on cylinder 5. Ran a can of seafoam through the tank and did the intake manifold induction cleaning with another can of seafoam. Still misfiring. I took a risk and cleaned the MAF. Still misfiring. The MAF and throttle body were clean btw.

I've got 145K miles on the clock so I figured maybe the coils were going bad and replaced them both. Still misfiring.

It misfires at about 1500 - 1750 RPMs when under slight load, like going slow up a slight hill. If I take my foot off the gas for a few seconds or floor it the misfiring goes away. Otherwise the SES light flashes for a couple seconds while it chugs, then it clears itself and is all good. Otherwise the engine runs smooth and strong. No ticking, hardly leaks, never over heated.

I replaced the cats and O2 sensors last year. It's not throwing any other codes and drives like a champ. Engine was pulled 3 years ago and all gaskets, oil pump, water pump, thermostat, and timing chain were replaced.

Any suggestions on what to try next? ECU? MAF? Manually clean the injectors?

 

When mine did this exact same symptom, same cylinder, same rpm, etc, it turned out that a bad spark plug (cracked porcelain) created a bad wire. But I know you changed both. Any chance plug got damaged on install? Try one more new one or a swap?

Other than that I'd guess you're on the right track with injector.

 

Have you swapped plug wire to a different place? See what happens.

Rough on start up, i.e. Coolant in cylinder?

 

Didn't try swapping plugs or wires. I didn't see signs of coolant in any of the cylinders.

I think I'm just gonna bite the bullet and rip the intake manifold off again and clean the injectors.

 

Might be worth clearing codes and quickly swapping the plug and wire. See if the code follows. Just another thing to rule out. Takes 10 mins.

 

I was battling misfires for afew weeks straight. A bitch to diagnose on your own.

Continue with your plans to clean the fuel injectors. Also do as suggested and swap the cylinder 5 spark plug with any other spark plug - see if the misfire returns. If this does not solve the problem swap the cylinder 5 ignition wire with any other correct length wire (keep in mind the ignition wires are different lengths accordingly). If this still does not solve the problem than swap the ignition coils around and see if the misfire follows or continues to misfire on cylinder 5. You also may want to invest in a cheap engine compression kit from local parts store and do a compression test on cylinder 5. Make sure your spark plug gaps are the correct spec.

You recently replaced your cats and I would hate to hear that your new cats melted so verify weather or not the cats are glowing red. Easiest way to do this is wait until its dark out, drive the truck around until it starts misfiring then pull over and look underneath the truck. Your cats might be receiving to much un-burnt fuel which will eventually melt your cats and destroy them. Flashing check engine light is one such indication of this issue so I would be reluctant to drive very far or very often until the issue is resolved.

Good luck mate.

 

If you have a scan tool with live data, I would check out the fuel trims to determine what type of misfire it is. That should give you some good direction.

 

Jobooth, Any progress on this?

 

If you're removing the intake, I'd triple-check that the wires are connected correctly to the coils. The pattern is tricky as hell.

 

Alright. I'm still getting misfires on 5 & 8 immediately upon starting up.

I've ruled out:
-plugs (by replacing and switching them around),
-wires (by replacing and switching aound),
-coils (by replacing)
-injectors (by cleaning, testing with multimeter, and switching around)
-ran seafoam through both the tank and the intake manifold via induction

Next is to get a compression kit and test. I have a hard time believing it's that as I had the engine pulled and all gaskets replaced not 2 years ago (along with oil pump, water pump, timing chain, thermostat).

If the compression is good I guess I replace the ECU?