Really bad missfire [Archive] - Forum


View Full Version : Really bad missfire

07-20-2008, 03:13 AM
Hi all,

I have a 2001 GA SE 4 cyl with 152k miles. The crazy thing is missing really badly whenever my foot is on the gas.

Here are the circumstances as best I can describe them:

When the engine is running at normal operating temp (approx 200 on the temp gauge) and I put my foot on the gas the car will start shaking and sputtering really badly. But if the engine is not under any significant load, i.e. if I'm on a flat wide-open freeway, the car runs fine (as long as I'm not accelerating). But as soon as I attempt to accelerate it starts missing again and becomes completely gutless. This especially makes merging on the freeway really difficult.

Here's the weird part. When the engine is cold it runs fine. No missfire whatsoever. I can floor it with no problem. But the second the temp gauge hits 200 it starts missing. Every morning I start the car it runs perfectly until that blasted 200 mark.

In an effort to fix this problem I have replaced the following parts:

4 spark plugs
4 spark plug boots
both coil packs
coil housing
ignition module
4 fuel injectors
both oxygen sensors
knock sensor
fuel filter

All replacement parts were new OEM AC Delco parts

Ocassionally it will store code P0300 - random missfire. Once it stored code P0303 - missfire on cyl 3. A couple days ago the SES light started flashing and code P0304 - missfire on cyl 4 showed up.

It is also now consistenly storing code P0420 which indicates a problem with the cat converter. This thing already burned through the original cat converter and if this keeps up it's going to burn through the second one.

The only other thing I can think of is the EGR valve. Call me an idiot, but for the life of me I can't find the EGR valve on this thing. I have the printed 2-volume shop manual I bought from Helm and I can't find anything in there about EGR. Does this thing have an EGR valve??

Any suggestions would be greatly appreciated. This thing is driving me insane!!


07-20-2008, 07:18 AM
I've had many Q4s come in misfiring due to a burned IDI tower (probably what you're calling a coil housing). New plugs, boots and tower corrects the root problem, but then the car suffers random multiples (your P0300) during the test drive. This is commonly due to a melted down cat, caused by the initial misfiring problem -- it doesn't seem to take much with these. Since you state cat codes are being repeatedly set at this time, check backpressure and/or listen for the rattling that indicates a chunked converter. Furthermore, I don't recall EGR being used much after '98 -- veh may not be so equipped?

07-20-2008, 10:01 AM
The thing I'm calling the coil housing is a white plastic bucket that holds the coils and the ignition module. The spark plug boots connect to it. Is that what you're referring to also?

I have gotten underneath the car and pounded on the cat converter with my fist and there's no rattling. It sounds solid (I know that of course doesn't necessarily mean it's good). Would the backpressure be messed up even if things don't seem to be rattling inside?

The P0420 code only shows up after the car starts missing. If I clear the code, then start driving (and I'm gentle on the throttle) the code will not show up. But if I get on the gas, maybe 5 or 10 minutes after it's started missing, then P0420 will show up. So if the problem was the cat, does it make sense that the code doesn't show up until after it's been missing for some period of time? If the cat was destroyed wouldn't it start showing codes immidiately after the engine was started?

Another really weird thing I should mention (and I'm sure some of you guys are going to laugh at this) is that I have tried splitfire spark plugs. Oddly enough, when the splitfire plugs are in there the missing is reduced significantly from when the OEM AC Delco platinums are installed. It still misses, but it's about a 50% improvement.

Thanks for your help. I guess I just don't know very much about cat converters.

07-20-2008, 04:26 PM
Usually, a cat sufficiently damaged to cause a runability problem will rattle, but not always. The enable criteria for P0420 are structured to help prevent 'false' cat codes (ie, based on a misfire condition -- cat efficiency is tested by varying fuel delivery and monitoring the response time of the upstream vs downstream O2s), but again, nothing is infallible.

If you are ill-equipped to perform an exhaust backpressure test *while the condition is manifesting,* at least verify the basics before proceeding -- recheck ignition, verify proper fuel pressure and pressure rise, maybe even verify compression. A proper scan tool, if available, would be nicel here -- ideally you need to be able to see data while the fault manifests, and you need to be able to recognize out-of-range items and interpret meaning from them. Obviously, myriad problems can cause a transient misfire. Given the scenario described, the suggestion to verify converter integrity (with a proper backpressure test at this point) was made only as it 'felt' like a strong possibility based on experience.

07-21-2008, 01:15 PM
Yeah I hear you. Thanks for the reply. I guess I'm going to take the car to a local shop that has all the GM tools. GM cars are their thing so hopefully they will find something. I think this problem is more than I can diagnose with the tools in my garage.

Thanks very much for your input. I will post back here if/when I have something conclusive.

in the mean time, if anyone else has more input please feel free to make any suggestions you have.