OK, here's what's going on: during normal driving, the car runs OK, but during heavier excelleration, it seems to be misfiring at around 4-5k rpms to the point that I never hit the redline. It'll rev fine, but under power I'm getting a chugging like one of the cylinders isn't putting out power. This has been going on for two weeks. My gas milage is still around 26 mpg btw.

I had Autozone pull the codes: both were evap air leaks (I don't think that would cause the misfire). My plugs have 30k on them. I only use premium gas. Oil level is good. So, I went home, did the ECU reset proceedure, pedle position reset, and idle reset. Started the car and I had a knocking at idle. The knock seems to be coming from the rear and low in the engine (I crawled all over the thing trying to figure out where it came from). So, I pulled the front plugs and found a white chalky residue indicating oil in the combustion chamber. As some of you know, I'm burning oil badly. Knock went away after driving it around the block and coming back home. Idle, however, was at 850 per the tach.

So here's what I'm thinking. Any suggestions would be appreciated:

1) Oil burning may have foulded a rear plug
2) Possible problem with a lifter or bent valve stem?
3) bad coil?
4) clogged injector? (I ran some Seafoam last weekend to see if it made a difference and no noticable change from it)
5) bad pre-cat? (main cat was changed 10k ago)
If it is a bad pre-cat, is it possible that my thermostat needs to be changed. I've never replaced it so now I know it's stuck in the open position (heat comes out of the vents before the car is up to temp. This means that coolant is circulating when the car is cold). If the car may not get up to full temp, the pre-cats may not get a chance to do their job and may be plugged. Just a thought.

I just need to keep this thing running for another two months before I can do an engine over-haul, so anything to keep her going will be helpful.

 

Bump. Help!

(yes I know its only been up for 5 minutes, but there were 8 posts that pushed it down)

 

First thing to do would be to check all six plugs for oil fouling, as you've done. You say you found a "white chalky" residue; that's more likely to be from the Seafoam than from oil fouling, which usually leaves wet dark black deposits, or carbon fouling, which would leave crusty black/brown scale-looking deposits. Clean or replace the fouled plug(s) with OEM NGK's.

Next, if your thermostat is stuck open, you are correct that the precats might not be lighting off because the CTS will be getting warm coolant way too early and switching the computer to open-loop mode (leaning out the mixture in the process.) I'm not sure how you can test for a clogged precat. If you are really planning to overhaul the engine, just pull the precats and run test or straight pipes. IIRC, thermostat on our VQ's is located to the lower right of the engine, viewed from the belt drive (right front side) under the hood. I found it easier to get to mine by putting it up on a lift and pulling the wheel and inside fender liner off.

Next up: Back off the timing back down to the stock 15 degrees and see if this mitigates the high RPM misfire. It's possible that due to carbon buildup, etc. even high octane gas doesn't prevent misfires.

Bad coils? Swap the coils from two cylinders and see if the problem changes cylinders or RPM range.

If you haven't modded your car with Irish's blow-by catch can or something similar to that, now might be a good time to do so, since you state you have excessive oil consumption, which will ruin your new plugs in a hurry.

If all of this only provides a short-term fix, or no fix at all, then it's time to run a compression test on all six cylinders. However, usually on motors with a bent valve, stuck lifter, or broken valve spring, the misfire is pronounced and active throughout the rev range. Most common cause of a broken valve spring would be an RPM overrun (exceeding redline.)

 

I appreciate it. Also, the white chalky residue could indicate a lean condition as well as oil in the combustion chamber, so from what you've said about the precats and the thermostat, I'll be changing that tomorrow.

if anyone else has any suggestions, please post 'em.

 

bump. 10 char.

 

What codes were pulled up ?
Your idle needs to be reset, perform Idle Volume Learn again or in the worse case, you have an 03, which might still be in warranty ?
If you are having an oil burning issue, you should be at a dealership and have that resolved.
BTW sounds like you need a Throttle Body, and its not a misfiring, more than likely a leak either at throttle body gasket.(coverage is 8/80k )
Vehicle may need air flow meter, and possibly if you can word it correctly.. you have 'lack' of power when accelerating...coverage for MAF and reprogramming (8/80K is for reprogramming + related parts)

 

I have 138k on my '03. It's well out of warranty. However, the throttle body gasket, ect, may be the place to look next. I just cleaned it about three weeks ago so I may have messed up the gasket. I'll give it a look. Thanks for the suggestion.

 

You mentioned you had a friend at Nissan in another thread, have him check which program is on your ecm, and check injectors, esp on rear bank, which I have seen go bad or leaking manifold gasket.

 

You might also want to check your fuel pressure, your fuel pump might be dying a slow life and isn't able to pump out enough fuel under heavy load and high RPMs. Put a gauge on the supply line and watch it as you are accelerating.

Another thing is cap off you EVAP purge line into the manifold, to make sure you are not sucking in air through the EVAP system at higher RPMs- which could lean out the mixture to the point of missfire.

 

the other thing could be that there is too much air (read: leak) and its leaning out and misfiring. This happened to mine, but when I used a larger diam MAF sensor.

 

I think I fixed it.... sort of, that is. I was talking with a guy a work this morning about the problem. When I showed him this thread to cover everything I've gone through, he suggested that it sounded like I was using a low-octane gas. So, just on a hunch, I bought some STP octane boost when I filled up. Half a tank later (I was doing sales calls today), and my problems seem to have gone away. It was gradual, so I don't think it was the octane of the gas. Instead, I think the octane boost clean my injectors (just a theory). Regardless, it seems to be much better. Seafoam didn't do the trick at all btw.