Hello All

I never notice a check engine light when the misfire happens and it is random once the car has heated up. Sometimes I will get multiple misfires that are about 30 seconds apart and other times I will never encounter one. I never notice it when the engine is cold or above idle speeds.

An example would be that I stop at a stoplight where the car usually idles around 700 RPM, it will misfire and drop to about 500 RPM and then recover, sometimes the car will stall. Usually I can start the car right back up, but I have to give it a little gas. Also, when I try to start the car when it is cold it starts right up, when it's warm I have to give a little gas (about 1/8th the pedal) in order for it to start; otherwise it will start rough and sometimes stall. When it's cold I never have this issue.

The misfire originally started over 2 years ago but was very small and hardly noticeable. About a year ago I replaced a failed fuel pump, and fuel filter. About 3 months ago I replaced the spark plugs, plug boots, coil packs, and coil housing. I have also previously changed the water pump and I have ensured that the engine is correctly timed.

I am not sure what else could be causing this misfire. The engine has 215,000 miles but I feel it still has life left. There are no problems above idle speed and the engine power is there.
I bought an OBD2 reader and have checked the O2 sensor outputs and they look correct (the post-cat is a little weird). When I log the misfires with the OBD2 the most recent log shows them across all 4 cylinders.

I have added two images from my OBD2 reader on my most recent log (1/15/2016). It shows that the misfires are across all 4 cylinders. Additionally, the "Timing Advance" is all over this place. All this data was taken at idle while warmed up.

Thanks in advance,
Barry

Attachments

misfire.png (19k)
timingadvance.png (40k)

I am not savy on the grand am to start with.You can do two things based on my dated idea's.Check the OHM'S reading with a multimeter for the injectors.Make sure they are with in spec.(I do not have a manual to give the standard's on that).I would 2nd idea remove the egr valve and clean it as (these build up carbon) and can do two things.One is the car can lose vacuum in the egr or the carbon build up makes the (pintal) that move's up/down stick and make the idle go all over the board or make the car run like crap or yes cut off.In my opinion you have done the tune up idea's and fuel pump and IF it were the idle air control valve this problem would be present cold or hot and the idle would yo yo.Unless your having a catalytic convertor issue which can cause a Ton of problems like cel,lack of power,choking to death cold or hot (this if really bad will be consistent) every time you drive period.Go simple and try the Easy route 1st.I would add one full can of seafoam or chevron techron injector cleaner for the first trial.I would also check and remove your Egr valve and clean that up.WIth 215k I imagine the carbon has built up inside that!!!! I would check the injector's ohm reading last.If one injector is failing it is like a domino effect if this is the case.Granted I could be dead wrong on any of the idea's I suggested and if so may other's add/correct me if needed.

Just one other thought as it could be the crankshaft position sensor.I know it should kick a code,however if the connection is weak or corroded it could pop the random issue as this specifically controls the timing and worth checking visually and verify the sensor is clean,terminals good and not loose.

That sounds exactly like the problem that I had with one of my 1996 2.4L Cavaliers, which has the same engine as your Grand Am. I'm pretty sure you can easily, cheaply solve the problem by replacing your coolant-temperature sensor (not the thermostat). Delphi part #: TS 10075, AC Delco part#: 213-4396.

The coolant-temperature sensor on one of my Cavaliers was providing inaccurate temperature data to the computer. The sensor was telling the computer that the coolant was colder than it really was, so the computer constantly fed a (too) rich fuel mixture to the engine. (Because the computer thought that the engine was "cold", the computer essentially did the equivalent of turning on the "choke", like we used to do in the old days when a carbureted engine was cold.) That rich fuel mixture was fine when you'd first start/run the car, but not after the engine warmed up. After I drove it for a few minutes it would run a rough (which was especially noticeable when you'd come to a stop/idle). It would hesitate or stalls when I'd come to a stop (or when trying to accelerate from a stop). Replacing the coolant temperature sensor solved the problem.

My mechanic debugged the problem (which took him quite a while) and replaced the sensor but replacing the sensor seems like a pretty easy do-it-yourself job. The sensor screws into the black-plastic water outlet on the right side of the engine (about 4" aft, and 8" below where it says "2.4L Twin Cam"). The sensor seems to be below the coolant level, so removing it will probably cause coolant to leak out by gravity. I guess that you'd either have to drain some of the coolant, or just let it drip onto a pan, and then add fresh coolant to replace what leaked out. If you're replacing it yourself, I'd recommend being pretty gently (especially when replacing it) so that you don't break (or break the seal) on the black-plastic water outlet.

I had almost the same problem, my LD9 would randomly misfire and usually under load... I looked into what everyone said above but my fix came by replacing the Ignition Control Module from one i got from the yard and did the trick. No more misfire.

I wanted to provide an update to let everyone know how I fixed it. Initially I thought the O2 sensor was good but had doubts about the downstream sensor. I was wrong, my upstream O2 sensor needed replaced. When I did this, the reading matched better with the post-cat sensor. Once I fixed this most of the problem was gone, but there was still some slight misfire when I would rev the engine (when the RPMs were coming down they would overshoot the low point and almost stall). I fixed this by replacing the fuel pressure regulator and now the car is running like a champ!

I think the OBDII reader is invaluable for car diagnostics. I could read the correct engine operating temperature so the coolant temp sensor was never a suspect to me.

Thanks for getting back to us! This may save one of us some trouble in the future. (I hope not.)

I'm not sure how the you know if you're reading the correct operating temperature via OBDII because doesn't the computer gets its engine temperature data from the coolant temperature sensor?

But yes, my cheap Bluetooth OBDII reader has paid for itself many times over. I use the $12 Konnwei KW902. Works well, unlike some cheaper ones.

RazzMaTazz wrote:I'm not sure how the you know if you're reading the correct operating temperature via OBDII because doesn't the computer gets its engine temperature data from the coolant temperature sensor?

You're right. I wasn't thinking of the possibility that the temperature the dash/ODBII reads is not actually the correct temp. Is there any way to diagnose this? Could you take a laser reading of the block and see if it is significantly higher than the measured reading?

I have a CP9080A Actron scan tool cost $249.00 (but normally) auto zone or advance will sell ON sale for $199.00.I could run thru what it doe's but do your own research about actron's scan tools so you can see the comparison.I will say with this unit you can see LIVE data showing everything from fuel pressure to yes even engine temps.As for your question in regard to checking the engine temp outside with a tool yes you can.There are tools made to verify engine temp with a scan tool that soley reads temps.There sold at most auto stores or online of course.I will say with my scan tool you download a program to your home pc and you can upload your cars diagnostics to it with the USB port and print off.Actron is one of several that are very low cost but good for under the price I said.The benefit with my scan tool is with the home pc linked it keeps things up to date when the usb is used and shows the code meaning and etc.I have considered going up a notch for the srs benefit on a scan tool But I cannot justify this as this is rarely a issue.I will end my thoughts and say even though scan tools do help sometime's just a good mechanical overview (common sense) can resolve what seems to be a major issue.