lt4 fd

Ive posted in the computer/ecu section about it but thought I could get more responses here... basically it runs very rough when accelerating and under heavier loading... Ive replaced the ENTIRE ignition system, fuel filter, cleaned the tb and replaced all the gaskets, pcv valve, egr solenoid and maf sensor( these were throwing codes), and checked fuel pressure which is high 30's and when I get on it goes to 43ish untill redline... there is also a miss at idle and very very light loads. I had open longtubes with extensions for a year and a half and it ran VERY well then I decided to put the exhaust on and maybe after about 6 months of having it on the car sort of changed tone from a refined smooth scream to a rough grumble, dont know how fast or gradual the transition was I dont remember. A year after that about, I took the exhaust off and added the extensions back on because I thought my car blew out the cats and maybe one got stuck funny in there and now instead of the loud crackly idle it had without exhaust before its got more of a bassy humm with a miss here and there and instead of the loud yet smooth note it had its got a hammered *** 60's truck sound with a very strange tinking noise emmiting from the exhaust here and there at lighter throttle( not an exhaust leak not a missfire but a tinking noise that isnt engine noise because I could only hear it with the exhaust off) and talk about rough acceleration, I get on it and its not a smooth climb up the rpms it a pretty nasty rough one and a lot slower one than it was stock... I dont think its o2 sensorsbecause its also effected at WOT and open loop but I have them ready to replace, its got 92k miles on it and bolt ons. I sprayed it down with carb cleaner and didnt detect any vacuum leaks, I have my oil anylized every change sine I bought it at 67k and there has been nothing unusual although I still have the one for the last 5k miles to send it but the problem was there before that anyways... Im at a loss I dont know what else could make it run this way.... any tips or ideas???? Its been almost a year since my car has been fun to drive

Dave '97 Z28 M6

It could be valvetrain-related.

lt4 fd

Im all ears if you were thinking about anythign specifically, FYI Ive never messed with anything under the valvecovers.

Dave '97 Z28 M6

I had a Camaro exhibiting the same symptoms as you have, and it was caused by a broken valvespring. The car was OBDII at the time, so the SES light would flash, indicating missfires. Mine was idling/running rough, missfiring, and making a "tinking" sound. The key that (could have) given it away in my case is the sudden change in the behaviour of the car... the car was running fine, then suddenly started running poorly.

Off the top of my head, here's some other things that could be the problem:

plug wires
EGR function
injectors

Ya see, I "fixed" everything related to the ignition system, checked plug wires, checked injectors, etc., etc., and it ended up being a broken valvespring.

A compression test should reveal if your valves are operating properly. A trained ear with a stethoscope (or screwdriver/broomhandle) could tell if anything in the valvetrain is making strange noises.

If you could look at your O2 readings, and see how they compare between the 2 banks, you might be closer to finding your problem. I just suggested valvetrain because of the problem that I had. I'm sure there's lots of guys on here that can offer you more advice.

lt4 fd

Thanks for the advice, sounds similar enough to be worth checking out, Ill let you know, but in the mean time if anyone has anymore suggestions they are welcome

93zted

poorly running, excellent possible solutions to your rough running car so far. Valve springs and /or valve lifters a possible cause of rough running. Did you conduct a "leak down" test or a injector leak test or a injector ohm(s) test? Ihave had several Stock lifters go bad on me the last several months causing similar running problems as yours. The new lifters and springs solved the problem! Good Luck! 93ZTED

lt4 fd

No I have not, I am not familiar with them... I have heard of a leak down test so Im sure I could gather some info on it, but what do you need for an injector leak test... does it require a special setup or is it something anyone can do. BTW I cannot hear any strange noises from the engine itself it sounds fairly quiet so I dont know if a bad lifter could be it or if they always make noise when they crap out. Thanks so far!

lt4 fd

I have recently done some compression tests and some spark tests on the cylinders I was concerned with to confirm that the engine is in good order and getting a strong spark. I will probably finish doing it to the rest of them for peace of mind is nothing still turns up... From everything so far I gather that the missfire at idle and rough running is fuel or emission device related. Since fuel pressure is good and the long term fuel trim at idle and part throttle is 114ish on both banks is it wrong to think the evap purge is purging far too often? Would that make the engine run very rough under heavier loading like WOT enough to drastically change the sound of the engine and bring down the long term fuel trims like that? I pressed on the egr valve , toward the firewall, and it started to stall the motor like it should have but could it still be bad? Like have a leak or something, and like before would it have an effect on the long term fuel trim and have a serious effect on heavier loading and power? I swapped out the PCM today and it runs exactly the same so thats not the culprit... I also unplugged the maf and that wasnt it, it ran differently but not any better, Ive also checked a few of the vacuum lines and sprayed carb cleaner around the engine bay and came up with no leaks. I also checked out the diff betweeen the 2 banks o2 readings and one seems to be consistantly higher than the other by a tad but they are bouncing all over the place as well. In open loop though they do seem a bit high and one still seems a touch higher than the other but dont take that as fact Ide have to double check and actually look for it this time... still need some help if anyone has any advice...Thanks in advance.

lt4 fd

Can anyone make anything else of what I mentioned above?

Maybe I can post some old video clips of the car when it was running like a raped ape and some new ones if judging by sound would help any?????

ACE1252

This may sound like a corny idea....and it may be just that....but can you unplug 2 of the injectors at a time on the proper cylinders to make it a 6 temporarily just to see if it is localized to a cylinder?

I've always wondered if this was possible, but I was not sure based on how the cylinder order is on the crank.

Dave '97 Z28 M6

Have you pulled out all the plugs, to see what they look like and compare them?

If the engine is dropping cylinders (missfiring) randomly, then it's a different problem - compared to if it's missfiring on the same cylinder(s) consistently. We need to find that out.

lt4 fd

Ill do that tomorrow or later tonight, I only have to check the passenger side as I have already pulled and looked at the drivers side. The drivers side looked all the same, kind of a tad sooty around the rim, the arm thingy is brown and not really sooty at all and the electrode and white ceramic base are clean as could be...

lt4 fd

Took a while to get around to doing it, I pulled them all out finally. The one that makes me wonder is #6 which looked like the arm over the electrode was almost clean except it had the same green marks from I guess oxidized copper on the bottom of it right over the electrode and the normal soot around the rim except for some extra black crust on the ceramic part right below the electrode in the recessed area below the rim. and the fact that all the threads looked clean and shiny like new where as all 7 other plugs had some heat rainbowing or darkening till about half way down the threads??? dont know what that means, the 2 and 4 were a bit frosty, not snow caps or anything just a little frosty and the 8 plug looked about like the 6 plug except it had the heat marking going down the threads. On the opposite side they all looked pretty much exactly the same and pretty good. I thought that once I got it all back together that I would see which cylinder was randomely missfiring by pulling an injector one at a time....well I couldnt tell and later when I drove the car it was barely doing it here and there. So I assume since my sensors SEEM to be functioning properly and giving reasonable info to the pcm from what I saw on the scanner and the pcm swap turned up nothing different then its an actuator of some sort that is probably bad. BTW I tried doing a compression test and started with the #4 and the tester wouldnt not fit between the plug hole and the header primary, so I just pulled the rest of the plugs and figured I would take a look... Does that give anyone any ideas?

lt4 fd

I just got though driving it and I noticed that when I accel hard, full throttle or very near it, at lower rpm's it kind of stumbles... only at or near full throttle. Also at very low rpms like below idle when you let off the clutch and the rpms dip really low and then give it some gas to get moving briskly before it stalls it will kind of kick off and on like a snap of the finger like a small abrupt stumble...???? BTW a long term fuel trim of lower than 128 means it is pulling out fuel correct?

lt4 fd

....hello?