Misfire

[mkerr2112]

96 Discovery 128K miles

I bought this truck about six months ago and have always had a check engine light on. I replaced the plugs and wires before I started monitoring fault codes. First code I read was P1193 - Oxygen Sensor Heater Circuit Open Downstream of Cat Conv. I changed both downstream O2 sensors and that went away. I was also getting P0308 - Misfire Cyl 8 and P1314 Misfire Catalyst Damage.

After the replacing the O2 sensors, I got the following:
P0300 Misfire multiple cylinder
P0308 Misfire Cylinder 8
p0305 Misfire Cylinder 5
P1316 Misfire Excessive Emissions
P1314 Misfire Catalyst Damage Fault - Bank B

My guess was Cylinder 5 and Cylinder 8 are on the same coil pack, so it must be that pack and replaced that coil pack. Now I am down to two codes.

P0308 Misfire Cylinder 8
P1316 Misfire Excessive Emissions

But the truck is running as rough as ever. I have been monitoring a few things. I notice that it runs much better in "open loop" condition - basically when you floor it. It tends to hesitate in lower rpms.

I read a bunch of posts here and the possibilities seem endless. Any recommendations for a starting point would be appreciated. Upstream O2 sensors probably make the most sense first? I have also seen a lot posts about using 8mm wires and higher end plugs. I understand going for higher performance upgrades, but would new stock parts cause faults or misfires?

Thanks!

[Spike555]

Cheap plug wires wont cut it, cheap plugs are fine.
Replace the front O2's just to rule those out.
Check the crank sensor wires harness for chaffing, the crank sensor controls spark.

[mkerr2112]

Thanks so much!

Working through this list and will let you know.

Are denso O2 sensors acceptable?

What is your opinion on wires, Magnacore? Again, lots of opinions on here - many conflicting. My mechanic actually bought the cheap wires for me - but he is a series guy with a 95 Discovery. Is it the switch to GEMS and multipack coils that made the 8mm wires necessary.

[Spike555]

8mm wires are not necessary.
I have Magnecores, STI are fine too as well as Kingsbourne.
7mm came on it from the factory.
I bought my O2's off of Amazon, NTK, $50 each, yes Denso's are fine, whatever brand you use just make sure they are not universal's.

Here is the thing about mechanics and their cars, they do the very least possible and go the cheapest route possible, unless its their hobby car, then they use only the very best.
Never buy a car from a mechanic.

[mkerr2112]

Thanks for the info!

Got the Kingsbourne wires on the way.

Working on the upstream O2 sensors.

With that fault code (P1314) catalyst damage and the mileage (128K) should I go ahead and replace the cats while I am at it?

My friend has a 95 Y pipe and cats. Is there a way to do away with the downstream o2s or modify the pipe? I noticed you mentioned in another thread that they have no effect on engine performance.

[Spike555]

I would not replace the cats unless they are bad and you have inspections where you live.
If no inspections you can cut them out.
You can use spark plug non foulers in the rear O2 bungs and then put the rear O2's inside those to keep the MIL from coming on.
Or you can find a shop with a T4 computer and have the rear O2's turned off.
If you have inspections you will need to keep the rear O2's no matter what.
We dont have them here.

[mkerr2112]

I think I have been smelling a little rotten egg, I think the media is a little loose in the drivers side, and I did have the fault code. I have a stripped O2 so I decided to take the cats off. Is there anyway to test them off the vehicle? I saw a post here that said cats are usually OK if they are not glowing red after a normal run, but that seems to be a really clogged convertor. I never saw them glowing red for sure. On the vehicle a vacuum test may tell me?

We have emissions inspection here in MD. I believe that they only do an OBD Test and visually inspect the emissions equipment. No tailpipe sniffing for 1996 on.

With your method, I could cut the innards out of the cats and fool the rear O2 sensors with the non-foulers? My friend with a 95 Disco was successful with this method - but he has gotten around inspection by having a receipt for a diesel engine and doesn't have to worry about the downstream sensors.

[Spike555]

If they inspect the the exhaust and they see the non foulers they will fail you.
If they hook upto your OBD and scan they will find the "EURO" (no rear O2's) program and you will probably fail.
The reason they do these inspections is to get money out of you and prevent people from doing what you want to do.
All I can really say is to move to a non inspection state.

[mkerr2112]

Thanks!

Do you agree with my diagnosis that the cats do need to be replaced or would you test them further before replacing them? I am looking at the Davico replacement pipe, not too bad at $413 including shipping.

[Spike555]

I disagree with replacing the cats just because you had a catylist damage misfire code.
Would it hurt?
No.
Would I save my money and "wait and see?"
Yes.
That is a good price.

[mkerr2112]

11/24:
I replaced the upstream O2 sensors and kept the old cats.
Hi idle seems to be more under control but still getting P0304 Misfire Cylinder 4

11/28:
Replaced spark plug wires with Kingsbourne
Triple checked wire assignments (no cross wiring)
Now getting P0308 and P0300
But only as pending codes, check engine light not illuminated yet. Driven multiple cycles

The misfire seems to happen under low load, gentle acceleration after deceleration at speed. The truck starts very easily and has power when not misfiring. Seems to run better cold. It seems to be better now at getting over the misfire with the new plug wires. Increasing throttle tends to stop the misfire.

Misfire happened in open loop at 1900 rpm / Load=36 / 170deg water temp

Today:
Checked wiring harness to crankshaft sensor, the corrugated wire loom is intact from where it splits (under the coil packs?) to the sensor/shield at the flywheel adaptor plate. Should I remove the sensor/shield and wire loom and inspect the wires? Worried that I will disrupt the spacer.

[mkerr2112]

After reading some other posts, I went ahead and pulled the crankshaft sensor. It was a little loose but the wires seemed OK. Taped around the ends of the wires, just to make certain good separation/insulation. Reinstalled the sensor but I am still having hesitation. The misfire does not seem as pronounced with the new wires, but still very sluggish. No CEL or pending codes yet, but very few miles driven since last clearing of the codes.

Any other ideas would be greatly appreciated!

[Spike555]

Try this just for shits and giggles, unplug the VSS (vehicle speed sensor), its by the trans-t-case, its what sends the road speed signal to the ECU, it is also the speed limiter.
When it fails it sends a false overspeed signal to the ECU, the ECU then cuts spark to slow you down.
If you unplug it you will not have a speedo but we just want to see if the problem goes away or not.
If not plug it back in, if it goes away replace it.
You are not having the typical VSS symptoms but its free to check so lets rule it out.

[mkerr2112]

I am getting 0 mph on the Scan Gauge. Thought that was just a compatibility error or set up issue. I will try the VSS tomorrow. Thanks!

[Spike555]

I have a Ultra-Gauge, its shows MPH.
It very well may be a VSS issue then, unplugging it will find out for sure.