Posted: Tue Jun 04, 2013 9:28 pm Post subject: 04/ 3.4 rough-miss at idle
Thanks for any suggestions first off!
04 Impala, 3.4, 165ish miles. I got the car a few months ago & it is in good/decent shape (its used as a 2nd car). It did run fine but I got bored & threw a few "tune up" parts at it. I replaced the STOCK 160K plugs with Denso platinum 4511 plugs, new wires (Autolite Premium 7mm), new thermostat, removed & cleaned throttle body, new PCV valve, also relaced a EGR valve that was throwing a code. Since then its slowly developing a bad miss at idle, in gear, when its warmed up. Its not showing any SES codes. The miss is kind of random but I notice it when its fully warmed up & waiting at a stoplight-it feels like a bad misfire. No codes are present. The miss is bad enough to get your attention yet it doesnt throw a code.
I'm thinking it just doesnt like these plugs??? Should I go back to the factory AC plugs? I have done the "poor boys" check of the coils (put a timing light on the wires & look for dead spots) and it passed fine. Gas mileage was only 18ish when I got it, now its 22ish. I'm kind of shooting in the dark now. I CAN hear a vacuum leak at times, usually when its cold & accelerating. I havent found this yet but suspect its just something in the air intake-not really too serious.
I have a new fuel filter on the shelf, just not installed it yet. The one on the car looks factory so that needs to be changed. Stupid question, but where do I check fuel pressure at? I have a gauge, just didnt notice the check valve on the fuel rail anywhere. ??
Change plugs and wires to AC Delco. In all my experiences, I've always noticed Chevy's tend to be more sensitive when using parts outside of AC Delco at least on their engines. The valve to test fuel pressure is on the rail and sticks out right behind the power steering pump. It's accessible but it takes some messing around to get the test hose on the valve after removing the cap.
Hey don't want to be a d-bag here, but your original plugs were most likely ngk's, they were just relabel for ac delco. If you have the original plugs look at the metal part where the spark plug socket goes around. It will even have either an ngk part# or a denso part number. My double platiums were ngk's.
When you replaced the egr valve did you pull the throttle body off to make sure that the egr passage in the intake was free and not carboned over. I have seen a few 3400's with this same mileage throw egr codes and the only problem was there was no flow. Once all the carbon was removed, car ran so smooth. It is an easy thing to overlook and I have done it my self.
No worries 98SS I appreciate the advise.
I didnt look at the stock plugs that close, just noticed the AC part number on them & the .100 gap on them -a little worn! They were a nightmare to get out...
I'm pretty much convinced the Denso plugs I put in it are junk. They were on sale, plus with a discount they were cheap so its not hurting my feelings to toss them out. No big loss!
I did take off the TB & clean it. The EGR port was 100% clogged. I knocked that out & re-installed the old EGR but the light was still on-thats why i slapped on a new EGR. This was before I learned about the EGR relearn process. The old one might have been fine, either way it has a new one now. I have not done that yet (loaned my scanner to a friend, time to go get it back).
On OBD1 vechicles I know you could unhook the end of the EGR going to the exhaust & just put a filter on it (plug the port on the exhaust manifold) & it would not throw a code. This way the EGR would only be pulling in fresh air. Not really looking for any kind of performance gain here, but thought I'd toss that out. It does help keep the throttle body a lot cleaner. No idea if that works on OBD2.
By any chance did you gap the plugs? The denso book calls for 0.50 gap rather than the 0.60 that we are used to seeing? It is pretty easy to knock off or loosen one of the electrodes on that type of plug. However, you would feel that at idle when cold. I would double check to make sure that the plug wires are clicked on, not just on, those back plugs can be a bugger. Sometimes they like to pop on and then pop off. I usually remove the coil pack housing, just a few connectors and 2 nuts and 2 bolts and there is a ton of room to work. Just unplug the battery for an hour if you want the car to relearn stuff like the idle ect.
98SS, I did have to regap all the plugs I put in by quite a bit actually. I think I went .060, so its very possible I knocked off an electrode or something like that. Havent got around to swapping them out yet, the Impala doesnt get driven much since I like driving my big ugly rusty $odge Ram so much. One gas station to the next!
I'll swap out the plugs this weekend & see what happens. I did the valve cover gaskets & plugs last time so the rears (firewall side) were easy, maybe it will be a little exciting this time!
Gas mileage went in the crapper & the car just seemed to idle rough so I tossed the Denso plugs for some NGK's (dont laugh too much, they were freebies!). Just looking to eliminate plugs as the main cause. All the electrodes were still on the Denso plugs i took out. They did show it was clearly running rich (explains the gas mileage).
At the same time I took off the coils, ignition module & mounting bracket. I cleaned & sanded the oxidation from underneath the ignition module-thinking its a grounding issue. Car runs great again, mileage back up to 22+, cant really complain. I'm really thinking it was a ignition module grounding issue. I did some "internet searches" & read that GM had some issues with this (?). This weekend I'm going to run a ground strap from the coil mounting bracket to the block, just in case.
Other suggestions, I did find the fuel pressure test port (thanks SuperImpala). I tinkered with it for a minute with my Matco gauge & said "I need an adapter." With a little more effort I'm sure it can be done, call me lazy!
The vac leak sound I thought I heard is coming from the air intake/filter box. Its not normal, sounds like theres an obstruction but everything is intact & installed correct. I'm calling that an excuse to buy/make a cold air intake so its ok!
The car hasnt had good maintenance & it shows. A little tinkering around & it should be fine, just chasing gremlins right now. Thank you all for the help, we can call this one a success
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum