Rough Idle, Engine hesitation..?
 

Car now has 94k miles. Done tons of mods and maintainance on her this past couple months.

Engine is idling rough, it shakes at stop lights and coming to a stop. When taking off from a stop, engine hesitates to go. It doesnt happen all the time and but when it does, the roughness varies from crazy to moderate. No codes stored.

Things i have done so far:
NGK Iridium IX plugs
MAF
CPS
Air Filter
New Valve Cover Gaskets
Techron Concentrate in tank
Oil/ Oil Filter
Reset ECU/TCU and Erased all stored codes

-I checked fuel pressure in the system, its good, engine is getting enough fuel.
-Injectors, plugs, wires for each cylinder is all working at optimum level.
-Air filter, MAF is getting enough induction

Whats up with this?! Ive literally tried everything and yet the engine refuses to act normal. Already searched around but couldnt get a definate answer to the problem around the forums. Ive been in n out of garages and mechanics, money is flying out like water the past few weeks, what should i do? :nix:

Vacuum leak??
Engine Mounts?? (but it doesnt happen all the time, remember)
... those are my only thoughts for now.
How do i check to see if theyre bad?

Performace has remained the similar, MPG went down due to the NGK plugs, but i notice a lil loss in low end torque but in the higher end, it pulls hard.

HELP, thank you! :bow:

Ever change your fuel filter?

^ I was going to, but fuel pressure was indicating the engine was getting enough fuel. There are no start up problems either.

Right. That's what I thought, at first, too. My car started fine, etc, but I had a misfire that just would not go away after just about all other things had been out-ruled. Fuel filter change was what did the trick, though. My $0.02 would be to change the fuel filter and go from there.. :nix:

....hm, now that got me thinking. Thanks though, laneshift.

Anyone else care to share some other inside knowledge about my problem?

No problem. Do remember, though, every 65k is the interval that is recommended to change the fuel filter. You may be a bit due...

Change that fuel filter!!!! - You are pressure testing the system with no fuel demand I imagine? This can be misleading. I see you have done the CPS. The other thing that can cause engine stumble is O2 sensors cycling too slowly & reaching the end of their useful life. This could answer why it is storing no misfire codes. It could also be a vacuum leak - this should throw a fuel trim code though.

Yea, im beginning to think its the O2 sensors and the fuel filter is causing the rough idle and hesitation.

I just want to make sure still though cause the parts alone for this is already gonna be expensive. Ive already spent more on repairs/maintainance on my car than whats its worth already. :bs: :X

Yeah! - the early cars could be a nightmare. You should have a really good car now!

Do the filter first & if you do O2 sensors only do the upstream ones. The downstream sensors only tell you if the Cats are good & I would not do them until you have a a persistent CEL.

I should have a really good car by now. :rolleyes:
Ive changed nearly everything on her, everything from regular/preventative engine maintainance to suspsension components, to all the fluids/filters, sensors... the list goes on...

BUT, i still love it.

Drove the car again today and its definately getting worse and worse. Im getting it diagnosed tmrw, lets see if the fuel filter could be the curlpit. Thanks Glyn

...oh another thing i noticed, it idles fine in drive or in neutral. But idles rough when in gear. :nix:

I really wish the CEL was on so i dont have to keep guessing what the problem is.

Plays up the minute there is some load on the engine!!

Gotcha

Oh, thanks for your update john. I really hope the fuel filter is our rough idle/engine hesitation issue. Will be getting it done in the morning.

I hope that is our issue too, as that is a cheap easy fix. Probably won't get to mine until this weekend. I look forward to your results.

Found out i wont get the parts till prolly next week, when i actually have the time, john.

Yea, soo after much diagnosing and such... rough idle and hesitation is still present. Damn.

Got CEL now, codes read: misfire in cylinder 2,3,5.
My next guess would be wires? coils? :nix:

Why would there be multiple misfires all at once?
I lost half my cylinders when driving, and my exhaust made a deep muscle car grumble. It went away obviously after restart.

She shut down on one bank - are misfire codes the only codes? Yes - wires & coils are next check & maybe an O2 sensor. Sounds like lane's car all over again.

wouldn't a clog cat cause one bank to misfire?

Yes it's possible

Codes were ONLY misfire codes, nothing else.

How can i tell if i have a clogged cat? Would i know i need to replace o2 sensors via a code? Like i said, there were only misfire codes, meaning i might just have to replace wires/coils next.

Any last calls? Thanks

If all of your cylinders started to misfire on one bank all of a sudden, I would suggest a clogged cat. It's rare three coil packs/sets of plug wires go out all together like so. If it's random, then I would start with wires and coil packs. Just my $0.02.

If you have a cat problem the Cat should either rattle or the car should wheeze as though one exhaust is blocked on one bank.

O2 sensor should throw a code but we seem to have many problems that should throw or store codes & don't - who know's why.

Rough idle can be caused by a dirty or damaged throttle body. This does not cause misfires though.

Wires & coils are the logical next step but might not cure the problem. Swap around & see if code changes cylinder. This is still the best DIY.

EDIT - the damn misfire codes are not all on one bank!

Yea, thats exactly what i was thinking and all my other mb techs said that too.

-Dont think its cat, as its quiet, and misfires arent all at one bank.
-Cant be dirty throttle body
-What are symptoms of a bad o2 sensor???
-Could it be a vaccum leak somewhere? how can i tell?
-Coils/wires are my next, probably my last resort, as they are VERY pricey to do.