So, last week I'm driving home and the engine starts to stutter, like it's losing power. It's missing when I get home. Next day, it fires up and runs fine for a while and then starts doing it again. After a couple days of running that way, it starts missing from startup. A friend of mine said he smelled gas in the oil so we changed the intake manifold gasket. Still missing. Changed out the coils, plugs, and wires. Still missing. Got the codes read. P0303 Cylinder 3 misfire, P0203 Injector Circuit Open Cylinder 3, P0420 Catalyst system efficiency below threshold (Bank 1). So, I switched injectors 1 and 3 to see if the misfire followed. It didn't. I reset the codes and P0303 and P0203 came back. The connector to injector number 3 reads 14V just like all the others and from what we can tell there there is no short in the wires. We think its an ECM. My question is, is there anything mechanical that can cause those codes? There is no sign of it burning oil so it should be getting good compression. I just have no clue when it comes to these damned computerized motors. Any input would be very helpful.

Thanks!!

Doc

 

MAF can cause a similar problem. Try wiggling all the sensor connectors from the airbox to the intake to check for a loose connection.

 

Thank you for the reply! I appreciate the suggestion. Unfortunately, I wiggled all the connections and it is still missing. I'm hoping to get a little more sure about my ECM diagnosis before changing it. Any ideas what else might be causing it?

 

PCM issues are rare. These are top of my head ideas (with out researching the codes):
- You listed "P0420 Catalyst system efficiency below threshold (Bank 1)". Bank 1 is the upstream O2 on the rear exhaust manifold. CHANGE IT! This sound like it may be failing. If it's not, it's good to consider changing it every 70,000+ miles. Get an AC Delco replacement (as some others may not work properly).
- When the car is running bad, look under it, if the cat is red, STOP the car. You have a clogged cat and this MUST be replaced.
- To test against the MAF, with the engine off, just unplug the MAF. THe computer will throw a code for now MAF but substitue with values that should permit it to run. If this yields results, get MAF cleaner from the parts store and follow the directions (better then a new MAF if it works).
- Take the ICM off (sits under the coils) and take it to the parts store. Most stores can test it.

 

I think P0303 & P0420 are related to P0203. A malfunctioning injector circuit will cause a misfire, and the unspent fuel from the misfire will make the cat efficiency appear below threshold. Check the harness between the injector and PCM for frayed/pinched wiring. Clean the connection at the injector and PCM. If that doesn't resolve the problem then you'll probably need a Tech II (or equivalent) scanner for further diagnostics.

 

So, new development. On the way into town this morning, it ran perfect for about two minutes and then went back to misfiring. When we changed the intake manifold gasket, it was leaking around the number 3 cylinder. The car runs perfect other than the misfire. I'm losing confidence that it's a computer problem.

 

Ran perfect until operating temperature? That could be the o2 sensor still, since they run open loop until the car is warm.

 

No, it ran bad for about 10 minutes, then good for two, then bad the rest of the way.

 

So from a COLD start (such as sitting over night), the car ran BAD? but then might run good, then back to bad?

If on a cold start it runs fine, I'd still consider a clogged cat or O2 sensor. 03SSLE brought a good idea of it could be a damaged wire.

When the car is running BAD, just check under it and look at the cat, if it's glowing/redish in color, that should be your tell-tale sign that your exhaust is cloggeed. Again, this is a speculative guess.

I know some people hate fuel injection and prefer the old carborated cars (much less complexe to work with and high priced scan tools were not needed to help diagnose problems), but most problems people blame on a PCM end up not being the PCM. This can boil down to a damaged wire, bad sensor or a clogged cat, but not a computer.

 

I checked the cat. It doesn't glow red even when the engine is hot and it's misfiring. Plus, there is good pressure coming out of the exhaust so I don't think its the cat. Today, I'm going to trace the injector wires all the way back to the computer to see if there is a break or fray or something.