95 Old Ciera Missfire

Okay so I don't think the thread title for my on going story is appopriate anymore, and I can't change it, so I started a new thread.
Here's the old thread. http://www.automotiveforums.com/vbulletin/showthread.php?t=729000

Long story short. I have a missfire. I have changed plugs twice. Replaced the IAC and TPS. Cleaned out the entire TB, and replaced all the vacum hoses and pcv valve.

Miss fire is still there. In fact tonight I put regular Delco plugs in to see if it was my Bosch, and it initialy ran great with only about 1 misfire every 5 sec or so. BUT..... when the engine gets a bit of gas and it idles back down, it is now worse. infact it stalled once, but came back to life. I have checked all the plug wires and they are in the right order.

if i let it run a bit it then seems to greatly reduce, but then it will come back shortly.

Is a coil pack going bad?

I'm at wits end on this...

oops it's a 3.1

I just replaced the wires. Still does it.

On futher inspection, here's the pattern


On start up, runs fine till engine gets gas and idle drops down. If you let it run without giving gas, its fine. After gas, it runs fine, then after it drops back cown to idle it gets rough.

I drive for a bit, and upon initial accel, it is rough, then goes away.

When the engine starts to warm up, it is almost gone. EXCEPT if I let it idle for about a minute or two, then it starts to misfire again.

My next step is a coil pack.

This is getting expensive.

I read about a open/closed loop system for the spark timing... could this be where my issue is coming from? per auto zone it says here...


"
There are 2 different run modes. When the engine is first started and the rpm is above 400, the system goes into Open Loop operation. In open loop operation, the ECM/PCM will ignore the signal from the oxygen sensor (O2S) and calculate the injector on-time based upon inputs from the coolant sensor (ECT), mass air flow (MAF) and mass air temperature (MAT) sensors.
During Open Loop operation, the ECM/PCM analyzes the following items to determine when the system is ready to go to the Closed Loop mode.

The oxygen sensor (O2S) varying voltage output showing that is hot enough to operate properly. This is dependent on temperature.
The coolant sensor (ECT) must be above specified temperature.
A specific amount of time must elapse after starting the engine. These values are stored in the PROM.
The engine speed is above 800 rpm since start up.
When these conditions have been met, the system goes into Closed Loop operation In Closed Loop operation, the ECM/PCM will calculate the air/fuel ratio (injector on time) based upon the signal from the oxygen sensor. The ECM/PCM will decrease the on-time if the air/fuel ratio is too rich, and will increase the on-time if the air/fuel ratio is too lean. "

okay so now i replaced the coil packs.


stil there.

I ran it on the highway for the first time and i noticed that the misfire is very prevelant on the highway as well. It shaks the whole damn car. feels like im driving on a rumble strip.

anyway it seems to be at only very specific rpms, as it goes away with a bit more gas, or a bit less gas on the highway.

What would controll the spark timing at certain rpms?

Have you checked the injectors ?
Have you checked your fuel pressure ??

Seems like you've covered about everything, exceptt for the ignition control module.... (ICM)

to answer your last question- the ECU in conjunction with the crank/cam sensor controls spark timing.

I know the ICM is a bit pricy. Plus I'm not so shure it's the culprit. Would it affect me at only certain RPM'S?

This damn gremlin did not show up untill I went back there to do plugs, air filter, and the belt. We sprayed it down with cleaner, then worked on it.

I take it there is no easy way to get to the injectors huh? I read in an older post about the wires to the injectors geting cracked if they are hosed down with cleaner. How would I verify the bad wire/s?



Also I'm thinking that a plug may have been cross threaded. When I changed them out agian it seemed to come out with very little turning. When I put it back in, I made shure it went down a good ways, but it was tight. I don't see how that would cause a problem at only very specific rpms though.


Thanks for the help, I know I'm tossing alot out here.

Another tought, could I have moved a pully off whack when I replaced the belt?

As far as the cam sensor, I'd imagine that if it were the culprit I'd light up the SES.

If the ICM has a short it can just affect at certain Rpms and sometimes they dont, it can be tested at most part stores.

On hosing it down, if thiers any water/moisture in the plugs, it can cause misfires. But I havent heard of washing it down causing the injectors to crack. but they can crack due to old age and from the excessive heat they are exposed to.

I always warm the car up before spraying any cleaners, even though it says not to. If the engines not warm enough to dry every thing, then this could be contributing to your delima.

The only way to verify wires conditions is to go over every inch of the wires you think is faulty visually, ie: ignition system - to and from the ICM and to the other varous sensors I mentioned above.

Actually the misfire will cause the SES light to come on.
The crank and cam sensors dont usually trip the SES.
Only a good scann would show that.

But they are all testable with a volt/ohm meter.
The Zones free auto repair manuals (http://www.autozone.com/az/cds/en_us/0900823d/80/0f/10/d2/0900823d800f10d2/repairInfoPages.htm)should have specific procedures for those, as I dont know the exact readings off hand.

let me know what you find out.

I didn't realy hose the thing down, just sprayed cleaner on the top.


I tried to do a hard acceleration and it seems to be lacking power.

I'll try the ICM next, but I think you may be right and it could be an injector... however there is no ses. I would think with it being this bad, it should trip it.

Do you know of anywhere that will give a good scan for not that much money?

Again thanks for all your help.

Well finaly got around to getting in there. Took the plenum off (pain in the ass) and took a peek at the injectors. They are all betwen 12.3 and 12.5 ohms. I took off the ICM and had it tested. It passed.

Talking with a mechanic at pep boys, he seemed to think that the injectors could be gumed up. I'm going to bring it in for a feul cleaning to see if that helps. The dump in the tank stuff seems to not be doing much.

If the cleaning does not work, I might be left with not much left. I have not goten to the Cam sensor, as that's a bitch toget to as well. The crankshaft sensor is even more of a pain...