1998 WS6 - Spark Knock Won't Go Away

[88MonteSS98WS6]

Hello,
I have a 98 Trans AM WS6 (4L60E) which is essentially bone stock except for a cat-back exhaust system. It has approximately 155K well-maintained miles...full synthetic oil changed every 3K & I only use 93 Octane from either Shell or Mobil.

Issue:
- Engine has spark knock coming from the driver's side bank.
- Knock only seems to happen once the engine has warmed up...which happens very quickly here in the Florida summer.
- It will only knock when under a load, it will not knock when revved in neutral. It is loudest when it shifts from 1st to 2nd and 2nd to 3rd after the engine is warm from sitting in traffic.
- SES Light - P0420 Code (there were other emission related codes, but they have since been cleared after completing the fixes listed below.

Here is what I've checked/changed:
- New AIR Pump (To clear SES code...I know, should've just taken it out but I want to keep the car in as original condition as possible)
- New AIR Pump Check Valves (both sides)
- MAF cleaned with MAF cleaner spray
- New air filter (paper, not K&N)
- Checked all four plugs & wires on driver side, they all seem ok. Plugs are AC Iridium with correct gap and wires are MSD 8.5's, about 20K miles on both. The plug closest to the firewall had a little bit of fresh-looking oil on the threads, but from what I've read that seems to be somewhat normal for an LS1 with this many miles. (?)
- New rear O2 sensor -Bank 1/Sensor 1 (to clear SES code)

What I'm planning to do:
- MCCC upper end cleaning
- New Fuel Filter
- PCV valve replacement

If these fail to resolve the issue what else should be checked next? New MAF? Appreciate any info!

Thanks,
Chris

[Firebirdjones]

I always like to check fuel pressure, especially under load. Might be fine at idle but a weak pump would be more likely to show a problem under a heavy long rpm load. Low pressure in that situation can cause lean conditions and contribute to detonation.

May not be your issue but something I always check. At 155k, if the pump is original I tend to want to replace them anyway when they get much past 100k miles.

[pajeff02]

^^ Agree. What is it that makes you say it is only knocking on the driver side? Is the engine pulling timing in response to the sound?

My other thought was excessive carbon buildup in the cylinders. Does she burn any oil?

[2002ssslp]

^^^ I agree with the carbon buildup situation and the low fuel pressure condition. Lean condition will kill pistons with excessive heat and must be addressed asap. I am puzzled that it is only coming from one side of the engine. Are the knock sensors working correctly, you may want to pull them and have them tested.

[az gt eater]

Just throwing this out there. Prob wrong, as I haven't been wrong enough yet today, but only trying to help out..https://answers.yahoo.com/question/i...6163919AA6AAbt

[88MonteSS98WS6]

Thanks everyone for the feedback. A few follow-ups...
PAJeff02: the knock is only audible on the driver side, it could be knocking on both, I just can't hear it from the passenger side. It does go through about 1/2 to 2/3rds of a quart every oil change, so I'm sure there is plenty of build-up in there after 155K miles.

I agree it wouldn't be a bad idea to change out the fuel pump, was just hoping to rule out other things first since that can get messy if I don't feel like cutting her up to get to the pump. Does the pump flow more when its cold? I ask because I never hear the knock when the engine is cold.

About the MCCC: I read the "how to" where it is sprayed into a hose that is connected where the PCV goes into the intake. I tried pulling the PCV hose off of the intake several times, but that thing isn't budging! I'm afraid of ripping up the PCV system, could I just spray the MCCC directly into the throttle body instead? That is actually what it says to do on the can...but you know how that goes!

Appreciate all the help!
Chris

[bigrondownhiller]

Check the egr valve also for carbon buildup.

[pajeff02]

I will say that the PCV system on our cars are poorly designed and feeds oil to the intake -- a catch can setup is highly recommended. This oil draw will add to the carbon deposit buildup. You have a couple of options - the harsh one being Seafoam, the longer to do one being TC-W3, and the old school method of misting water... yes, I said H2O, into the intake.

[88MonteSS98WS6]

So I replaced the fuel filter and did the MCCC (MOPAR combustion chamber cleaning) and can still hear it knocking. Fuel pump will probably be next.

I also just capped off the PCV inlet on the intake manifold with a vacuum cap. From what I've read that shouldn't cause any issues since it is effectively renders the PCV system useless since it relies on the vacuum from the intake.

A quick question about the MCCC cleaning...I followed the process exactly as outlined on LS1Tech.com (used the connection where the PCV goes into the manifold) and when I started it up and let it idle afterwards till warm there was hardly any smoke. I was expecting quite a lot of smoke as I had seen on some videos, but mine only had a few puffs of smoke once it was warm and that was it. As I said before, I have over 150K on the engine, so I was expecting to see a lot of smoke. Can this be a symptom of some other problem or maybe related to the knocking?

Thanks,
Chris

[pajeff02]

Any chance you have a mechanical knocking -- ie. bent push rod, bad rocker, rod bearing, etc...?

[88MonteSS98WS6]

Pretty sure its not mechanical as it will only do it when its under load and normally when it shifts from 1 to 2 and 2 to 3 after the RPMs drop (ok at higher RPMs before it shifts). I can rev the hell out of the engine in neutral and I don't hear a thing wrong.

Per Bigrondownhiller's suggestion I also checked the EGR today. I actually had a EGR delete kit so I took the EGR out and ran it for a while, but it still made the same noise. I ended up just putting the EGR back in after cleaning everything really well as there was some definite buildup in there. (the stupid code won't clear now though, d@mnit!)

So I think I'm down to the MAF (already cleaned it thoroughly) and fuel pump at this point. After that my only other thought is some type of exhaust/cat converter rattle, which would really surprise me because it definitely sounds like spark knock.

[cammed goat]

I am thinking cat rattle. I used to replace cat converters on the fleet trucks at my old job when we scanned and got a PO420.

[88MonteSS98WS6]

Changed the MAF today, no luck there either, still made the noise. Also had a friend hook up a scanner and he said the fuel pump seems to be good. I am starting to agree that this could be cat rattle, which is a bit irritating since I had a mechanic tell me its definite knock. I'm going to crawl under there this weekend and see if that is indeed the cause.

If it does end up being the cat, does anyone recommend a place to buy new ones? I would want the direct fit replacements, not universals that I would have to cut up. I'm not really interested in gutting them or just going without them.

[Firebirdjones]

I would suggest while your friend is there with the scanner, go for a drive and log/watch the knock. Most any scanner should be able to do that. See if it is "actual" knock that you are hearing, and whether it's "actually" pulling timing out of it. You should see both happening if you have a detonation problem.

That would at least tell you what you're looking at so you're not chasing your tail, or thinking it might be something else like a cat converter. You're starting to get into some pretty expensive parts now so I'd definately do some hard diagnostics with a scan tool and fuel pressure gauge before sinking a bunch of coin.

You say fuel pump "seems to be good" What was the fuel pressure? Did you check it under a full load while driving?

[pajeff02]

^^ Agree. You need to check fuel pressure at the rail. If you do not have a gauge, you can either buy or rent one at most chain parts stores. I may have a driver side GM replacement cat with less than 20k on it. I sold one side last year, but cannot remember for sure which one it was without looking.

[cammed goat]

Tap on the pipe with a hammer to see if there is a rattle. Let us know what you find.

[pajeff02]

I just went and re-read this entire thread. Unless you have installed breathers in the valve covers I do not recommend capping off the PCV system. The engine will build pressure in the crankcase and you could cause other problems.