Multiple/Random Cylinder Misfire

[98slab88]

The car is a 1998 Olds 88 with the 3800 series II motor, with 90000miles on it. I brought it to advance to check out the code and got the random/multiple cylinder misfire code. this is how it went down... Driving on the thruway (about 75-80mph) went to pass someone, the tach probably hit 4000rpm and then the car kinda cut out, there wasnt any power until the rpm dropped back down. And the rest of the way home if I went to 4000rpm it would do the same thing. the next day it seemed fine but once in a while it would lose power at lower RPMs (~2000 - 3000). If I hold the gas down when its misfiring (losing most of its power) after a few seconds it would regain its power and run fine, then lose it again later on. The poblem happened suddenly, but a friend that works at advance said start with a basic tune up, i doubt that would help because the problem started instantly. I was think a crank or cam position sensor. Any idea?

thanks,
Steve

[maxwedge]

Replace the fuel filter then check the fuel pressure during the power loss, try and use one with a long hose you can tape it to the w/s and drive monitoring the pressure.

[98slab88]

Yeah, I replaced the fuel filter and it seems great! But only one time for about 5 seconds it started to misfire again. The check engine light went off and hasnt come on again... But I will continue to run it to see if it cleans up. I also put in some dry gas to make sure it isnt a moisture problem, I did fill up my gas tank the night before. Thanks for the input!

[olds8894]

Check your spark plug wires. If they are bad, they will cause a misfire.

[98slab88]

Okay.... it started to do it again. I just filled up my gas tank again. The car accelerates great until the pedal reaches about 3/4 of the way down, and then it loses its power and the check engine light comes on. Everytime the pedal is down that far it will do it, it doesnt matter what RPM the engine is at. Any ideas? I just had the plastic plenum replaced by a shop a few weeks ago. Could it be a throttle position sensor?

Thanks for your help so far !!!
-Steve

[maxwedge]

Again, the fuel pressure is what when this happens??

[olds8894]

3/4 the way dow you say? My car was doing that too. I thought it was the EGR Valve or the MAF so I cleaned those and it still did it. Then I thought maybe the Catalytic converter was bad so I took it in and the muffler shop took it off. Now my car accels all through the pedal.

[98slab88]

Sorry... I checked the fuel pressure like you said while driving. During idle its around 60psi, under acceleration its around 65psi, when it starts missing the pressure increases the most, hits around 70psi and higher. Also, it might be valuable to mention this... when the car is missing it doesnt jerk the car or seem rough... it just doesnt have power. Also when it is missing for a few seconds or more the check engine light will flash. I put some fuel injector cleaner in today. When driving it today I found out that the longer I drive it the worse it gets and can start missing before the 3/4 point with the gas pedal is reached. I let the car sit for a few hours and it is much better and will only miss at the 3/4 mark with the pedal.

Thanks again guys

[olds8894]

It sounds exactly like what was wrong with my car. I'm not a mechanic, yet, but I'd try the catalytic converter route.

[98slab88]

I looked under the car yesturday and checked out the exhuast system. I bumped my hand against the cat and could hear something large that was loose in there, I normally would cut it apart and weld in a straight pipe (I always used to do that on my older cars) but there is an O2 sensor located after the cat. So I will pick up a direct fit cat from the autoparts store and drop it in a week or so and let you guys know.

I have also noticed these things: The warmer the engine temp is the more often the car misfires. And the when the car goes from running fine to misfiring, it's like on and off, there is no gradual decrease in power. And if I accelerate very slowly I can go farther past the "misfire point" and pretty much come close to flooring the pedal before it misfires again.

Any other ideas?

Thanks,
Steve

[maxwedge]

Originally Posted by 98slab88

I looked under the car yesturday and checked out the exhuast system. I bumped my hand against the cat and could hear something large that was loose in there, I normally would cut it apart and weld in a straight pipe (I always used to do that on my older cars) but there is an O2 sensor located after the cat. So I will pick up a direct fit cat from the autoparts store and drop it in a week or so and let you guys know.

I have also noticed these things: The warmer the engine temp is the more often the car misfires. And the when the car goes from running fine to misfiring, it's like on and off, there is no gradual decrease in power. And if I accelerate very slowly I can go farther past the "misfire point" and pretty much come close to flooring the pedal before it misfires again.

Any other ideas?

Thanks,
Steve

You can check the cat with an infrared themometer, temp at outlet should be 100 degrees more than inlet after driving, how long are the plugs and wires in the car?

[98slab88]

Well I was going to replace the cat now anyway. As far as I know the plugs and wires are less than 6 months old. But maybe this weekend I will change them out. What plugs do you recommend? I know they use the oridium? plugs, but I have always been a big fan for the bosch platinum 4.

Thanks

[olds8894]

Dont replace the cat. Just put a 'test pipe' in, unless you are an enviromental wacko. I've never had a single problem since having a 'test pipe' put on. The extra power is worth it. Sorry to hear that you have this problem, its not fun and it is very frustrating.

[98slab88]

Haha I'm anything but an enviromental wacko, haha. I normally would weld in a test pipe but the car is a 1998, and has OBDII, and since it has a sensor AFTER the cat I dont want the computer to throw a code. I allready ordered the cat and will install it next weekend. Today was weird though, it did not misfire at all, but it seemed sluggish. Then after backing up a concrete step into my garage, and installed some new light bulbs, I turned the car back on to pull it out it was misfiring. Weird, I'll just replace the cat.

For the temperature thing, I'll have to find someone with an infared thermometer and I will check it out.

Thanks

[90BuickSlab]

yo mayne, dont be a pussy just rip the cat out and slap in a straight pipe, just think of all that horsepower it's robbing.