Help with misfire code

rjaywhit · Post by **rjaywhit** » 14 Nov 2011, 08:51

1995 850NA 130K - A week ago my daughter got of the freeway and the car started running rough. I ran codes and found misc misfire codes for cyl 2 & 4. I replaced the cap and rotor since I knew that they were pitted and the plugs only had 15K on them. Still getting misfire codes. Replaced the plug wires and spark plugs. Now getting codes on cyl # 4 only. Running a little better, but still slow off the start and not idleing as it should.

I have found that the CEL is burned out so who knows how long she was driving with the issue.

How does the ECU know that there is a misfire? Since this is a distributor setup is there any management of the ignition by the ECU? If so, how?

Any ideas on what I should try next? Current codes are 5-4-3 , 4-5-4, 5-4-5, 5-5-4.

Could a damaged cat or fouled O2 sensors be causing a misfire?

Post by **rspi** » 14 Nov 2011, 09:25

I doubt that. The only thing you have left in the path is the coil and the injector. Do you run fuel injector cleaner in the car? I usually use Lucas no less than every 10,000. Make sure you have good gap on the plugs. I also think the MAF can cause rough running. Does your car have the OBD-II port at the coin holder?

Post by **tjts1** » 14 Nov 2011, 09:28

The engine has a knock sensor that can also detect misfires. What kind of spark plugs are you using?

Post by **jblackburn** » 14 Nov 2011, 09:38

Pull the plug out and ground it against the head while the engine's running. IS it firing?

If so, might be time for a compression test to see if it's got a burned valve.

rjaywhit · Post by **rjaywhit** » 14 Nov 2011, 09:46

Autozone didn't have 5 Champion copper plugs in stock so I got Motorcraft copper plugs.

I'm hoping that I don't have a burned valve, I just rebuilt the head 15K ago. That would really suck.

Post by **tjts1** » 14 Nov 2011, 10:51

rjaywhit wrote:Autozone didn't have 5 Champion copper plugs in stock so I got Motorcraft copper plugs.

I'm hoping that I don't have a burned valve, I just rebuilt the head 15K ago. That would really suck.

I would try different plugs. Either Volvo or NGK coppers.

Post by **erikv11** » 14 Nov 2011, 11:04

You can test the injector by swapping it to another cylinder, see if the misfire follows it.

Post by **rspi** » 14 Nov 2011, 11:34

People, please use OEM plugs or something believed to be better. I installed some champ plugs in one of my cars and the MPG dropped the same day about 20%. Only took my anal behind a week to realize it because I was driving about 1,000 miles per week. Took those things out and tossed them in the trash.

If you just rebuilt a head I doubt you have a burnt valve. I have heard in the past that running 87 octane will cause valves to burn a lot faster than 91+. I would swap the plugs and a fuel injector and see what happens.

Post by **jblackburn** » 14 Nov 2011, 11:37

I've never used Volvo plugs because of the ridiculous price, but NGK and Bosch Copper plugs have worked well for me. Just have to change them more often, but at $1.25/plug, who cares?

Post by **boosted5cyl** » 14 Nov 2011, 13:24

You mentioned cap & rotor being replaced, what about the spark leads?

Most commonly misfires are detected by the ECU using the crank position sensor. It can sense very small variations in the motor's rhythm and point to the offending cylinder. If you get a chance to see the wheel for the crank position sensor you will see it has a lt of teeth, its very granular.
I've have read on forums that knock sensors are involved too, but my understanding of their operation does not indicate this. They are basically piezoelectric microphones that are "tuned" to listen for knocking in the motor and they only send a signal if knocking conditions are met. Otherwise they send no signal at all which means they cannot be involved in misfire detection. That's my take on it anyway.

O2 sensor and or clogged cat can cause poor running, but it would be more uniform(ish) across the cylinders. They are unlikely to cause an issue in just one specific cylinder.