Well, I replaced my barbecued '97 Mark III with a '98 Cargo/Work Van. Not my first choice for a new ride , but I have to get back on the road.

The '98 came with MIL illuminated, codes are:

P0301 - Cyl 1 Misfire
P0303 - Cyl 3 Misfire
P0305 - Cyl 5 Misfire
P0138 - Oxygen Sensor Circuit, High Voltage, Bank 1 Sensor 2

I knew this before purchasing & used it as a price negotiating point. Figured it would be unlikely to have burnt valves all in a row in the same head and reasoned the MIL codes could be a loose PCM connection or a crappy cap/rotor/coil.

FIRST thing I did was check for a cheapo aluminum cap & rotor (found them sure enough) and replaced with brass/copper Echlin from NAPA.

Cleared MIL but engine still running rough untill reaching operating temp. Going to replace OXS at the rear of the CAT tomorrow but was wondering if anyone has opinions about what might be causing the 1-3-5 misfire if cleaning the PCM harness doesn't get me fixed up.

Engine only has 76K miles on it so I'm thinking carbon build-up is not my problem at this point.

 

Welcome to my White Whale. I've been trying to solve that same 1-3-5 problem on my van since I bought it 4-1/2 years ago.

 

Crikey. Have you changed your PCM?

 

I've changed: PCM, plugs, wires, injectors, intake/plenum gasket, cam sensor/pick-up coil, crank sensor. Not to mention having cleaned everything like the throttle body, intake, fuel rail, IAC valve, etc. real good.

 

One would think that's enough to solve the misfire problem. I'm going to change the downstream O2 sensor since my 1-3-5 was coupled with a P0138.

Maybe there's a glimmer of hope since I don't see OXS in your laundry list. Let you know what happens...

 

Heh, heh, sorry to ruin your day. But like the cap and rotor, I also failed to mention that I changed both the upstream and downstream O2 sensors. Oh and I also did the spark plug wire routing TBS thing.

 

No worries, maybe with both of us working on it we'll stumble onto the cause of the issue. No surprise then -- after changing the downstream OXS the trouble persists. Except of course now the P0138 is gone.

Let me better understand something about your van's issue: Does your MIL just illuminate with no sign of the 1-3-5, or does your engine actually exhibit the symptom along with the report of the misfires?

Second question - what "cold start" engine management function(s) occur between the 15 second to 1 minute mark after the engine is started? (Van left to sit overnight, cold engine.)

In my case I believe engine management driven by the PCM is doing something improperly within this timeframe.

 

My CEL will come on but you'd never know there was an issue, the engine runs like a champ.

Having used two identical PCMs and getting the exact same misfire issues with each, I'm quite confident this isn't a PCM issue. It is my current belief that the problem lies with the cylinder head.

 

How many miles on your engine? Mine just turned 76K and I have a stack of receipts indicating stellar maintenance. Previous owner really did change oil & filter every 3K miles and fixed everything else at the dealership when required. (Except, of course, the current list of stuff which has now become my responsibility.)

 

I haven't paid much attention to the odometer because I always have it on trip. But I think I'm around 110K. I bought with 86K and it had this 1, 3, 5 misfire issue from day one. With my van, with the previous owner, the alternator had froze which caused the belt to snap. I highly suspect that he continued to run the engine without the water pump and overheated it. This is why I haven't ruled out a bad head. The compression is good, about 135 all the way around, so there's no blown head gasket. But there could be cracks in the head.