2004 Disco Misfire Problem

[Kacers]

37K miles. Had the SES light come on. Blinked then steady at idle. Blinks if you rev it above 1500 RPM or while driving so keeps throwing the codes. OBD scan shows 0300, 0305, 0308 (misfire). So I figured some issue on #5 or #8. Here is what I've done:

Checked #5 and 8 plugs and wires. Replaced all with new champion plugs and new magnecor 8mm wires and had same issue and throwing the same codes. Put test light on #5 and #8 plugs and saw very weak light so figured bad coil as 5 and 8 are served by the same coil and read that 1/2 the coil pack on one side could go bad.

Ordered new LR coil packs and just installed. Same thing and same codes. Put test light on #8 plug and its firing very weak still with the new coil. Note I replaced both coil packs (not just one) and wiring is correct as car runs just as before.

I am stumped.

Problem started last week about 5-10 miles after I filled up with supposedly 93 octane gas. I thought maybe bad gas but not sure why only those three codes would be thrown. Added gas dryer and also an octane boost to no effect.

I always go to check the last thing you did when diagnosing something that worked before. Buying gas was the only thing that happened. I get alot of water out of exhaust as it idles. It does not smell like gas but small puddle forms under tailpipe.

Could bad gas alone do this? Something with the ECU?

Any other ideas?

AndrewT

 

[Loydster]

did u replace the wires? Might be a bad wire. Might as well check the plugs as well. When my 03 did this i ended up with 2 new coils. all new wiring and new plugs.

 

[Kacers]

Yes - as posted above, I have installed 2 new LR coils, new Champion plugs, and new Magnecor 8mm wires in there now with same issues.

AndrewT

 

[Kacers]

Note I also swapped in another MAF from a friend's truck and still had same issue. I didn't think this had anything to do with the MAF but I wanted to completely rule that one out.

AndrewT

 

[Kacers]

Good vacuum at intake with no variation in reading at idle so this does not indicate valve issue.

Anyone with insight?

AndrewT

 

[cboy903]

I still say it's bad fuel.

 

[Kacers]

I still say it's bad fuel.

Hoping you are right Eric.

Abe is here now and we are draining the tank. Bought some Fuel system cleaner and also octane boost to use if needed. Will report back after this is done.

AndrewT

 

[agbuckle98]

are those codes you mentioned the ONLY ones it's throwing? If so, have you run a compression test yet? I've always hated the "bad gas" diagnosis, but I guess it's not impossible. I would think bad fuel would effect ALL cylinders.

 

[Kacers]

I have not run compression test yet but its on my list for today.

I agree on not understanding why bad gas alone would only cause the misfire codes on those two cylinders.

Yes - those are the only codes. I sometimes get 1300 but that is catalyst damaging event and only happens if I drive car a short distance - its sympathetic to the misfire issue as best as I can tell and unless someone tells me otherwise.

AndrewT

 

[Kacers]

Compression on #8 was at 140+. Checked #2 as a reference and its around 135. Gonna check 5 later with the rest of them but don't see any issues with those numbers.

AndrewT

 

[Kacers]

Drained whatever gas was left in tank and re-filled with 93 octane. Added some Techron fuel system cleaner in there with the new gas as well. Drove it a mile or so and no difference yet. Not sure if I should drive it more right now given the flashing SES and lack of power.

The only other thing that I know is common to cylinder 5 and 8 is the ignition signal going to the coil from the ECU. Since coil, plugs and wires are new, I can only guess ECU issue though that seems like a stretch as I don't see how just that pin (pin8 on Connector 5 (C0638)) could go bad???

AndrewT

 

[Chris-St Louis]

Flashing SES? Thats a bad thing. That means it is misfiring so badly that the cat is actually melting internally. In my experiance, that is a leaking/dripping injector.

 

[agbuckle98]

I know you have replaced a bunch of ignition stuff, but have you actualy checked to see if you have spark at # 5 and 8? Also, have you hooked up a fuel pressure gauge? What does it read? Does it drop down after shutting the engine off?

 

[Kacers]

Chris and others,

Spark test is weak at 5 and 8. Very dim orange glow on the tester. All other cylinders have bright orange healthy spark. Its a very noticeable difference. Thats why I am stumped. Weak spark with new coils (LR), plugs (Champions) and wires (Magnecor 8mm) and only on those two cylinders. I don't see how a leaky fuel injector would cause bad spark to the wire unless someone else can explain.

Very doubtful its a fuel pressure issue but I can check that. Car revs fine and doesn't stutter if you drive like it would with fuel starvation. No bucking or serious hesitation. It just has much less power like two cylinders are not firing well.

I am thinking the wire from the ECU (pin 8 on Connector 5 (C0638)) to the coil may be corrupted as that is the only other thing I can think of common to 5 and 8 besides the coil and the coil in there is brand new.

AndrewT

 

[lordkenyon]

Chris and others,

Spark test is weak at 5 and 8. Very dim orange glow on the tester.

AndrewT

You just using an inline tester? I never liked those much. Here's a thought you might want to try to help narrow it down: Remove the plug and wire to the number #5 for example that you're having an issue with and hook it up to the coil pack at another position of one that's working well, like the #4 on the coil pack. Obviously, don't put the plug in the cylinder to just test, but you could at least determine if the problem is like you think at the coil or further back (like to the ECM). Crank it a few times with it out and see what kind of spark you're getting. Also with a plug out, there won't be any potential injector issues in seeing if that spark was weak. You could also pull the plug (#5 or #8) with the plug out to see the spark and eliminate an injector issue.

I take it you're still pulling all those codes after you cleared them when the new stuff was in?

 

[Kacers]

Just checked pin 8 on Connector 5 (C0638) at the ECU to the red wire on the PS coil plug (pin 1) that fires 5 and 8 cylinders and got good continuity (almost no resistance) so I guess that shoots that theory.

I can only guess ECU at this point. Unless someone else has any ideas.

AndrewT

 

[Kacers]

Yes - in-line tester. And yes, still pulling same codes after I cleared prior codes plus also kept battery fully disconnected (removed all grounds from battery) for 5+ hours while removing intake and doing other things.

I also removed plugs and rested against exhaust manifold to see spark. 5 and 8 very weak while noticeable strong spark from the other plugs I removed to test in same manner.

AndrewT

 

[agbuckle98]

I think you are on the right track checking the wiring from ECM to coil. I'm not all up on that stuff, but I would guess maybe a power supply to the 5/8 section of that coil? IIRC there are 4 wires that go to each side of the coil pack, I'm thinking a power supply and signal to each pair? Just kinda throwing out ideas here, and it seems your signal to 5/8 is fine, maybe it doesn't have enough power to begin with?

 

[Kacers]

There are three wires going to each side of the coil pack. Two are ignition signals that each control 2 cylinders. One is power. So, it must be getting power as the other two cylinders on the PS coil are firing fine. I need to re-test that 5/8 ignition signal wire between the ECU and the coil when someone else is here. I couldn't really move it around and such to see if there may be a loose or intermittent connection. Will do that later if I can get someone over here to help.

AndrewT

 

[roverthen]

I still say it's bad fuel.

We drained the gas into three fuel cans. I poured two into the RRC. Truck is running perfect since last night. So far I ran 35 miles on that gas......