dougnstein

Ok, so I have a 2003 C5 with a LS2 402ci stroker. I bought this with a few issues (oil going through exhaust like it crazy was primary problem) 2 years ago. The car has always appreciated alot of engine RPM to take off and has always felt like without 3k RPM it would drop off and stall. My assumption was this was a characteristic of the engine/clutch configuration so I always drove around it but I'm starting to think that is very incorrect. I replaced the valve seals with new ones and it ran great and didn't spit oil on my bumper for 1 week or so then back to crap and more oil out the exhaust. I felt that I had some bad valve guides so off went the heads and 3+ months later I get those back and reinstalled them with ALL new valves and freshly decked heads. The machine shop didn't have any real complaints with the valve guides. Just a bent pair of valves from my valve spring compressor binding on the first set and one of two with ground tips on first install whoever that was...

It runs again now, but poorly so I went back to my performance shop to re-adjust some idle trim settings back to where they were before I had them changed to compensate for the leaky valve seals.

I was getting the "surging idle" while coasting so I brought a new MAF sensor with me to replace the suspect for this as well. At the shop, we confirmed the new MAF would help, and it did with the surging but now it is trying to die almost all of the time when letting off the gas, shifting, or coming to a stop. The shop is thinking it is a bad ground or maybe a bad PCM.

NOW: Given a little more detailed history, I'm really thinking I have a bad catalytic converter. I'm taking the exhaust off today to see if I can confirm this. Does anyone have any input to what direction I should go if this is not the problem?

Reasons I am led to a bad cat or both:
1. Performance is wayyy down right now.
2. Gas mileage is between 5-10 MPG average after new MAF. 12 MPG avg before. I imagine should be 18 MPG+ average. (just not 5MPG)
3. Does not overheat but does run hot in the summer when in traffic/idling longer than needed.
4. When I removed the heads several of my new valve seals were completely loose.
5. Oil in intake, I've removed all vacuum lines to intake and cleaned inside. Could back pressure send oil that direction?
6. Readings for o2 sensors look good. I still suspect since they have had soo much oil through exhaust.
7.I have inspected my wiring and removed/capped 90% of my vacuum lines going to the intake so I can pretty much rule this out.
8. No relevant DIC Codes to report.


Any thoughts or suggestions would be great. I'm looking at the exhaust tonight and will report findings but if I don't find anything there then I really don't know where to go next. Looking for another direction in advance! I'm sure I have left out MANY details but I tried to cover the basics. Let me know what you think and I'll give more details if needed. I appreciate everyone here and know you will have good info to follow!

Setup: '03 C5 w/ LS2 Stroker (402ci) w/ maybe 20k miles (incl. 10k since I've had it about 2 years ago), Kooks LT Headers, Random Technologies exhaust with High-Flow Cats and Mufflers, Ram-air intake, OEM replacement MAF, PRC LQ9 Heads, FAST 90MM intake, NEW NGK TR6 plugs.

chaase

Check the catalytic converters and see if they are breaking up or are clogged up.

Bill Curlee

WOW! Where do we begin!?!?!?!
Please answer the following questions:


- Who tuned the PCM and when was it done?


- Use a VACUUM GAGE and get an IDLE vacuum reading.

- Read and post ALL the DTCs that you have. If there are a LOT of DTCs, clear and re-read the DTCs after you take the car for a drive.

Report ALL the DTCs listed and the suffixes( i.e. H and or C )

If you had clogged CATs I would say it would run WORSE at the higher RPMs vise idle. Sounds to me like you either have a VACUUM leak issue OR you tune is HOSED!

If you hand over hand ALL the vacuum lines and they are properly attached and in good condition, use a can of BRAKE PARTS CLEANER and spray around the intake manifold. If the idle get s better all of a sudden when you spray one area,,, , that is a vacuum leak that needs to be CORRECTED! The ducting between the MAF and the TB and the flange at the TB “MUST” be air tight and properly sealed.

Do you have access to tuning software or a scanner with live sensor capability?? If so what are your O2 sensors doing at idle and what are the Long Term Fuel Trims doing at idle?????????????

Bill

dougnstein

I just got in from checking the cats. As expected, they appeared fine to me. I'm not sure of their entire construction but the visible screen material looked somewhat clean and free from obstructions or deformaties.

I have checked for vacuum leaks but not since the heads have been reinstalled. I will recheck and post my findings with a vacuum pressure also.. If there was a vacuum leak, wouldn't that make it run lean?, not rich?

While at the shop, the DIC showed two codes. One was the MAF sensor prior to replacement. I would expect this to be gone. The other was unrelated - something related to the clutch pedal sensor. I wasn't too interested in checking it until the idle was worked out but I prob should check that too.

The o2 sensors along with everything else leveled out after the maf sensor was replaced. the o2 sensors were responsive and seems to be working fine on the live readings.

Bill Curlee

You need someone to datalog the engine. There are several things that make engines run VERY RICH.

Coolant Temp sensor issues
Bad O2 sensors
MAP sensor issues

Are you running stock injectors for an LS2 or LS1 and were what ever your running properly rescaled to your PCM?

BC

tiojames

Don't have any suggestions about the other problems, but don't pull the cats off to see if they are plugged. Any muff shop can test them. They drill a hole in the pipe ahead of the cat and insert a pressure gauge feeler. If they are plugged pressure will be up. If the cats are breaking up you can hear the pieces rattling around in the pipes with the car running.

heavymetals

Strokers build more crankcase pressure.

With the catch can properly plumbed, you should not have any oil in the intake.

Even a bad case of ring "blow by" shouldn't leave oil in the intake.

Sorry, confused OP with the poster with the catch can in his sig.

To the OP: you need a catch can.

dougnstein

...it was the MAF wires picking up interference from the throttle body motor... duh.

Vicarious.

Check to see if you have a hole drilled into your throttle body - throttle plate.

That ended up being the cause of my idle surge, after chasing my tail around for months.

I guess rookie mechanics pull that kind of crap, if they can't get your car to idle right.


Also, are you sure your PCV system is set up properly?

Lothar34

What kind of PCV setup are you using?

VGLNTE1

While i cant explain the milage, other than the car is dumping fuel and cutting it out. Maybe washing the cylinders. I dunno. Could be 100 things. But as far as the surging...my buddys 383 ls6 had the same issue. Tuned it non stop and couldnt fix it. Turned out his pcv was wrong and he needed a fresh line ran from the valve cover to the air tunnel. Good as new after that. But sounds like you need a good tune shop

whitehooptie

Do you have a different intake manifold you could put on it? I've seen some crack between cylinders and whatnot. It sounds like a real bad vacuum leak to me and/or a really crappy tune....