Pontiac bonneville engine miss

my engine codes as a no 3 injector miss. sometimes it misses more severe than others. when accelerating it doesnt seem to be missing. i have changed out injectors, ignition control modules, spark plug wires etc. driving me crazy. sometimes it doesnt miss or code out at all but this is rare.

Here we go again…

What engine, model year, miles? What maintenance has been done? What is the exact code, in the P0XXX format?

Compression check seems to be in fashion lately.

2002 pontiac bonneville se, 3800 engine, 200000 miles, mostly highway. changed all the electronics except the computer. replaced injectors, ignition control module, spark plugs, wires, etc. checked pressure, fuel pump. used the 'nerd" lights to check firing everything okay. the code was PO300. switched injectors, switched wiring still the number 3 shows misfire. someone made the comment that the internal was plastic and may be cracked. didnt seem likely to me since it was only the no#3 coding out and it doesnt misfire all the time. but dont claim to be an expert.

compression seems to be okay

P0300 is random/multiple cylinder misfire. If it was cylinder 3 it would be P0303.

You mention most ignition components except the coil packs. Did you replace those?

When you checked the fuel pressure did you use an actual fuel pressure gauge and is the fuel pressure within specs for this engine? Did you just check the static pressure or did you do some testing under load?

What do you mean by “compression seems to be okay”? You check compression with a gauge and you get actual numbers - like xxx psi

I have a GM of similar vintage. I have learned not to trust the computer - it is particularly dumb. And when I say not to trust it I don’t mean like its gone bad - I just mean not to trust it for picking up errors and picking up the right errors. Check for vacuum leaks and drive around with your EGR valve unplugged for a while and see what happens. Yes - those things should set codes if they’re a problem but like I said - that computer is dumb like a rock. (If you unplug the EGR it will set a code).

sorry it was P0303. i also changed the coil packs. additionally i changed the fuel filter. i did use the guage for fuel pressure but not quite remembering what the numbers were but seem to remember they match the specs. i checked for vacuum leaks but couldnt find any. i will try the egr valve. thanks. could another type of sensor cause the same symptons. also would it be possible the ecm not getting enough power to properly control the sensors.

I mentioned vacuum & EGR assuming a P0300 code. Neither of these is likely to affect only one cylinder.

You need to get a compression gauge on there to check the compression - especially look for an anomaly on the third cylinder.

Define what is meant by the compression seems to be ok.

Checking the compression should always be Step One when working with an engine performance problem no matter if the engine has 25k miles on it or 200k.

The spark plugs are out so kill that bird right then and there, and for what it’s worth compression should be in the 180 or higher range on this engine.

It Could Be An Intermittent Throttle Position Sensor.