Misfire issue

[INF3RN0666]

Hey guys,

I'm getting desperate about a car issue. The vehicle is a 2000 Impala 3.8L with 350 000 Km. It has a randomy shaking to it and someone on the forum said it was a misfire. The thing is, the wires & plugs had been replaced and no misfire code was registered. The only thing that registered at the time was MAF sensor malfunction. The last scan showed a MAF malfunction and a Random Misfire code. So The MAF was recently replaced. Any idea as to what is causing this shaking because it's still pretty much there? Could it be the coils?

[UncleBob]

misfires can cause odd readings on a MAF. For example, if the misfire is causing backfires through the intake, the MAF will do some really strange things.

Its important to take any code with a grain of salt if you can't verify the component failure.

There are many things that can cause a misfire. If its a hard misfire, it should be easy to diagnose at a shop. Unfortunately, without some useful data, I can't offer any suggestions on what is the most likely culprit

[INF3RN0666]

Haha, I COMPLETELY AGREE WITH YOU ON THE FIRST THING YOU SAID. I never take the computer codes as 100% fact. However, I disagree with you on the second thing. This is a hard misfire, but for some reason, they can't diagnose the problem. The mechanics I deal with (and not just one, but rather 4 or 5) seem to be hard headed. They take the codes as gospel and they refuse to do any further diagnosis without "fixing/take care of" the codes. I've had to replace the EGR valve, MAF sensor, oxygen sensor, and wires/plugs without even knowing if they were faulty. I'm sure that the MAF sensor must have been acting up, kuz the new one didn't set a code, but i'm not so sure about the EGR valve. The next thing on the list is a compression test. And now that the codes have been taken care of, the electrician will take a look at it. Atleast I don't have to go to the Dealers, kuz those guys are next to useless. If you wanna hear a funny story about Chrysler's dealers, then I've got a good one for you.

[curtis73]

Uh... listen to Uncle Bob. He speaks wisely. I had a misfire code on a 96 Impala SS that was caused by a cracked plug. It set off MAF codes, misfire codes, and O2 sensor codes... and trust me, it was a HARD misfire. Like, the first time it happened, I looked down at the fuel gauge because it QUIT.

Uncle Bob knows what he's talking about. One little misfire can cause a cascade of other codes and vice versa.

[curtis73]

Were the wires replaced with factory or aftermarket wires? I had terrible issues on a 3.1 with aftermarket wires. I'm not saying that GM original is the best, but the particular aftermarket wires I had weren't the same resistance and caused issues.

[UncleBob]

if the tech(s) can't figure out a hard misfire, then its time to find a different tech IMO

[INF3RN0666]

I wasn't doubting what uncle_bob was saying about his assumption of what it is, but I'm telling him that it's not as easy to diagnose.

Everybody says that it should be easy to diagnose, but it's not as easy as it seems. Turns out that I'm not the only one with an impala 2000 who's having this problem. BTW, even the dealers can't figure it out, and they charge you money for not finding the problem (go figure).

[UncleBob]

trust me, I know how hard it can be to diagnose runnability problems. Misfires, though, are usually a little easier, simply because they are constant and consistant. Testing things is much easier in these circumstances than, say, an intermittant problem that only happens when its cold, or when accerating with a load over 4K RPM's. Just examples.

There are 2 types of misfires. Global system problems causing random misfires across all the cylinders, or seperate issues effecting some cylinders.

A bad MAF sensor, for example, can only cause a problem with all the cylinders, not isolated cylinders. A bad O2 sensor can cause problems with all the cylinders, or one bank of cylinders, depending on the system (some have more than one O2 sensor).

Were as a bad coil will cause only one or two bad cylinders. Compression issues, one cylinder only (per low compression cylinder, of course)

So if you can tell me what type of misfire we're working with, I can offer some suggestions, but it really comes down to examining the parameters of the computer, and checking inputs and outputs to narrow down the problem. Pulling spark plugs and checking inconsistant colors, and disabling ignition one cylinder at a time to attempt to narrow down which cylinders are the problem, and which type of variables are causing it.

[INF3RN0666]

Here's the events in sequence:
1. random vibration of car (car was overdue for tune up)
2. wires and plugs replaced --> no effect on vibration
3. Mechanic assumes it is a failing engine
4. computer diagnostics output codes for: MAF, EGR valve, RANDOM misfire (usually it would say which cylinders if it was isolated)
5. MAF & EGR valve replaced --> no effect
6. computer diagnostics output codes: bad post cat converter O2 sensor, random misfire

A bunch of mechanics didn't want to speculate untill all the codes were "repaired" by replacing the failing electronic components. Now they're saying they don't know. The dealer took $100 to diagnose and didn't come out with anything as usual.

The misfires (which are hard misfires) tend to increase when the engine gets warmer. I should really start unplugging wires to see if it makes a difference but I doubt it.

[INF3RN0666]

BTW, I appreciate your help and any advice you give me. I'm just getting frustrated with the BS. The one who pissed me off the most is the dealership. If you can't diagnose the problem, then don't charge me for a useless test. The mechanic I deal with atleast has a policy that computer scans are free, and if the code doesn't go away after we replace the part, then he won't charge me for the part.

[UncleBob]

the assumption that the codes are individual problems is rather dangerous. ALL the codes could very possible have one root cause. But I can't guess from what you've offered. Really, I'd have to work on the car to offer any real advice.

[INF3RN0666]

k kool. thanks though.

[534BC]

If you haven't got anywhere yet. Please post the conditions when and how it misfires. So far I am getting that it gets worse when warm and they are hard misfires (define) and it causes a vibration.

Some pertinant info for diagnosing may be

load/tps, rpm , engine temp, anmbiant temp, speed, ect. and then exactly what happens and how often the misfire is.

[bthust]

i had a 99 silverado with misfire issues it was a weak fuel pump

[INF3RN0666]

Hey buddy (bthust)

I agree with you, and I'm about to get the fuel pump changed (against the advice of 2 mechanics). One of my friends ran after me to tell me that his impala's problem was fixed by a fuel pump replacement (same random misfire I had). Funny thing is, some of the mechanics didn't want to speculate about the problem when the car had error codes. Now that it doesn't have any, they say it's not the fuel pump (without even testing the line pressure) but they don't want to speculate on what it might be because they can't figure it out. For some reason, most of th mechanics I've talked to (including mechanics who are my friends) seem to think that a pump is either dead or it's working fine. I realize that such a thing was the case back in the 90's, but there's no reason to keep thinking that way with newer cars.