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Old January 18th, 2018, 01:23 AM
Tyler Wirken
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Tyler Wirken
1997 Defender 90 LE
Join Date: May 2007
Location: Kansas City, Mo
Posts: 703
Could use some help with misfire diagnosis

So I am on the road and the truck, 1997 D-90 with a 4.o, is running a bit crappy.

Started acting like a misfire on the highway the other day and I am having trouble nailing it down. Would love some help before I have to head back home on Sunday for the 9 hour drive.

Here is what I have done, checked etc...

Got the codes read when first realized it was running poorly.

Code was a misfire #1 Cylinder. Pulled that plug as the truck burns some oil and suspected a fouled plug. Plug not fouled with oil but didn't look great as it had gunk of sorts packed around the electrode. Decided to change the plug out.

Cleared the coeds and headed down the road. No improvement. Stopped again. Codes this time were P3999 and P1316 . Did some research and could not find 3999 code but found 1316 to be misfire excessive emissions fault. But not cylinder #1 this time. Checked all the plugs after getting to my destination. All look okay but #1 was wet and smelled of fuel. Figured it was not firing. Bought an inline spark tester and verified spark at plug. Then thought perhaps it was fuel related so I disconnected battery in hopes of resetting system etc... Fired it up and it ran like a charm. Engine was cold. Thought I was good to go. Drove it today and back to rough idle like before.

Truck seems to run smooth at highway speeds but not at idle or low speed. Also replaced plugs, wires, and new heads/head gaskets less than two years ago.

Any ideas?

Also, wondering now if it may be bad catalytic converters as mine have been rattling for a while but never got around to fixing it. Could they have gotten worse on the drive causing truck to rough idle when warm? Would that explain the code?

I am a bit lost as to what to do and would love some insight so I can feel comfortable driving it home.

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Old January 18th, 2018, 07:34 AM
1of40's Avatar
Status: Offline
NAS 97SW & 83 One Ten Tdi
Join Date: Dec 2003
Location: Potomac Falls & Wintergreen, Virginia
Posts: 6,250
You might be on to something re bad cats. Perhaps they have compromised the O2’s and sending a false signal. Im assuming no you have a CEL?

The Rangie and Disco guys on DWeb may have insights that are helpful as well.

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