Misfire under Boost please help!

Hello everyone,
I have a real head scratcher here. I have a misfire only under when I demand boost. I have replaced plugs wires and coils but it still misfires like a bastard, I have lost alot of fuel mileage and lost some boost as well. Usually pumps about 14-15 psi of boost but has dropped to about 11psi. If I floor it it will misfire and then go and my Check engine light will blink then go off. I do have a code right now but its an evap code for a vaccum leak not sure if that's the cause or not. Not sure if it's low compression in a cylinder or a fuel delivery problem, If anyone has Ideas on where to start that would be great.

Blaine

The evap code will not come up under accel as you describe, get the code and post back.

Max, you know what engine, model and year this is?

I assume it is a supercharged 3.8? Let's also assume the OP will check the thread and post it. !

Yes is a 3.8 supercharged, the code is PO174

My car is modded as well in serveral areas not sure if that will have an impact or not too?

Yes, it will help to know the mods, and how long after the mods, did the misfiring start.....

This has to be a late model 3.8, to have a O2 sensor on each bank.....however, P0171 is not setting, which means the misfire is on Bank #2(2-4-6).....

Since the misfire is bad, it is causing the CE to flash.....even though you have changed coils, wires and plugs have you checked spark output, with a spark tester?

Since it seems only one bank is affected, I doubt it's a MAF problem(this can be checked though, just by disconnecting the MAF and see if the problem is changed)......also, I doubt it's a fuel pressure problem because both banks would be affected....

I have seen problems with the gasket between the upper plenum and the lower intake....it could develop a crack in the gasket, and during an accel, could be leaking air in either cylinder 2, 4, or six, causing a lean condition in that bank.......sometimes it can be detected by spraying carb cleaner around the base of the upper plenum......raise rpm's until the misfire starts and then spray...if misfire goes away you have found your problem.....sometimes, the leak may be more subtle, and you will have to observe the O2 sensors while spraying.....

Other possibilities are an exhaust leak on the bank 2 side....

Bad O2 sensor, or harness problem....

Could also be an injector problem on bank #2 side.....

Rare would be an PCM problem......also check for a rodent problem eating harnesses.....

Thanks for putting some insite on the problem, the mods have been on the car for over a year or 2 maybe even more now most of them were applied before I bought the car. The year of my car is 1999 so in the middle years of that series.

Now I'm not sure about the spark I had a garage install them because it was way to cold to do it myself right now. We also did the ICM because of the high rpm misfire So I kind of ruled out the ignition system but stills not sure.

Also, If i put it in park and i slowly rev or build the rpms right at 4000 rpms right before the supercharger kicks in it starts to studder and misfire then if i push the pedal down even more it punches threw it.

Before i did all the ignition work it had a misfire code on cylinders 5 and 6 so it made sense because those are the later cylinders.

I will also say that its not the exhaust system because i had a custom one put in about 10 months ago from the headers back. There's no CAT but has a chip for that :biggrin: 3' downpipe to the y pipe into flowmasters. Sounds nice and helps because you can hear every little misfire it may have.

And my modds are CAI, intense PCM, 3.4 pulley, headers, exhaust system, cooling system for headers, ram air intake, performance flex plate with 3.29 gear ratio tranny and boost oil pressure and fuel pressure gauges. I have a bunch of cosmetics too but that's my mechnical upgrades that I know of.

Hope that helps maybe a little more but I do appreciate the insite already a ton. Thank you

Having the car in idle and revving to 4K, tells you nothing, because at that point I think the PCM has a built in rev limiter, and it will cause the engine to feel like a misfire.....

I think you need to drive the vehicle with a scan tool attached, and watch the misfire(s), so you can isolate cylinder(s)....then you can concentrate on a particular cylinder.....as I said before, P0174 is a bank #2 code, so the problem is in 2, 4, or 6 cylinder.....you said you had a misfire on 5 and 6.....they are not paired together........so it's helpful to know what the misfire code is after all the ignition work(does 5 still misfire?)........

So let's say #6 is misfiring under a load, with the scan tool attached.....then we check for spark output of the 3/6 coil, with a spark tester......if ok, then we check compression......if ok, then you have to check the injector....this involved sophisticated equipment to check actual flow through the injector compared to other injectors in the car.....another alternative to this, is to pull the fuel rail, and swap it to the other bank(1, 3, or 5).....then run the vehicle and see if the misfire follows the injector.....if all these tests turn up nothing, then I would be leaning towards the gasket between the upper plenum and lower intake.....did you try checking it out with carb cleaner? Like I said you may need to observe both O2 sensors as you spray.....

By the way, you say it's a 1999, but what vehicle is it? I assume a Bonneville?

opps sorry its a pontiac grand prix GTP supercharged., So when I scan the car should i try to do it while the SES light is blinking? I was told with wouldn;t be able to read it. Cause as it sits right not im just getting that po174 code

Blaine

Do you have a scan tool that reads data?

If so, you want to watch and see which cylinder is misfiring when the vehicle acts up......also look at at the O2 sensor data....when the bank #2 misfires, your O2 sensor should go lean, because the oxygen is not being burned up during a misfire....can also watch the O2 sensor to see if it goes rich while you spray carb cleaner at the base of the upper plenum area.....

Unfortunately I don't, I'm going off what the Garage has told me and a scan from autozone. When it acts up the check engine light won't flash unless I'm on the highway and push it for more then like 5 secs I want to say. And the garage said that they wouldn't be able to scan it as it blinks is that true?

Of course that is not true.......either they don't have a scan tool or they don't know how to use it....

So I finally had time to bring it down to the shop and get it scanned so what pooped up for a code doesn't really narrow down the problems really. The code that came up was a p0300 random misfire thats all so I am back at square one thinking of starting with the crank sensor and then going from there...any ideas?

Blaine

did you replace plugs, wires and coil at the same time?

They were like a week apart

i have run into carbon tracking on wires, replaced them and the carbon tracking was still on plugs and ended up back on wires, misfire didn't go away. pull the wires off plugs and look for a straight line, it will be faint, down the inside of the boot.

Generally, a crank sensor doesn't cause a misfire under a load....

The usual problem is secondary breakdown.....plugs, wires, or coils.....

Swingline is correct....carbon tracking on the plug can cause a misfire....if that is the case, BOTH plugs and wires have to be replaced...replacing one or the other, fixes it for awhile but it will return.....

Have you even removed the wires one at a time from the coil towers to check for corrosion/carbon buildup? See that a lot....if found, most times the coil tower can just be sanded clean, but the wires have to be replaced(I would do plugs at the same time, using only A-C Delco's)....

i second the ac-delco plugs for gm products

So after a nice weekend of dicking around with the motor for a while, I do have brand new plugs wires and icm with wires to the coils. Now the coils I got were not brand new there were only 2 i got I have put a new order in for new ones to see if that is the problem. I did clean off my air filter and also put in 2 doses of fuel injector cleaner and the misfireing has gone down but will appear under load here and there with different levels of misfire too. There is a p0300 random misfire code coming up so hoping to narrow it down with the new coils or then it could be fuel delivery...?
Putting it out there to see what you guys think
Blaine