: 93 octane + WOT = rattle?



stngh8r
07-01-08, 07:31 PM
Well, I have noticed that I have some rattle or detonation briefly upon heavy throttle. This seems to happen for just a few seconds, but usually by that time, I have let out a little? I have noticed this since I have owned the car.

FYI:
-New 41-950 AC Delco plugs gapped .50 and MSD wires
-New FPR
-FP ~40psi key on, ~42-45psi idle, ~52psi with a short blip of the throttle.
-Doesn't seem to matter if I have been running mid-grade or 93. Even if its had 93 for multiple tanks.
-Freshly cleaned TB
-Coolant temperature is normal. New water pump, hoses and coolant 11/07.

Concerns:
-Fuel pressure at idle bounces. I suppose this could mean a weak pump? On the other hand, with a quick blip of the throttle, the pressure is where it needs to be?
-bad knock sensor? No codes.
-What reasons could there be that I am able to hear the rattle during heavy or WOT?


Thanks for your help guys

Eric

Ranger
07-01-08, 08:06 PM
Try cleaning the EGR valve.

Submariner409
07-01-08, 08:30 PM
Yep, EGR. Classic case of slow or no EGR "tip in". FP bounce at idle is of no consequence.....injector pulse variation at 650 rpm. Read up on EGR cleaning/replacement in these threads.....piece of cake.

stngh8r
07-01-08, 10:14 PM
Thanks for the speedy responses guys. I will get to it and report back.

cvpiftw
07-02-08, 11:09 AM
Is EGR active at WOT on these motors?

Ranger
07-02-08, 11:22 AM
I THINK so, but would have to check the FSM to be sure. Sub may know for sure.

Submariner409
07-02-08, 01:18 PM
There will always be some EGR activity in any installation, but intake manifold vacuum is at or very close to 0" at WOT, so EGR would be close enough to nothing to make any difference in fuel mixture and timing offset. EGR is highest at part-throttle cruise in order to lower combustion temperatures when leaner mixtures create higher concentrations of nitrous oxides. A sluggish valve generally shows up first as a "rattle" when slight power increases are needed, as in going up a part-throttle hill.

WOT actually is a PCM recognized control position which defeats several engine emissions control systems and modifies the torque control settings as well as shuts down the a/c compressor temporarily.

tateos
07-02-08, 01:27 PM
The fact that you have no codes and octane has no effect, plus the fact that detonation is not common on these engines makes me think it is something other than pre-ignition. I had a rattle on my '97 that the dealer tired everything to fix, ultimately including replacing the motor at 34K. It ended up being a catalytic converter with a cracked matrix that was doing the rattling. Try getting under the car and hitting the cat hard with your hand and/or a rubber mallet - that may be the source of the rattle.

stngh8r
07-02-08, 07:20 PM
It really sounds like detonation.

Ranger
07-02-08, 08:51 PM
I had a rattle on my '97 that the dealer tired everything to fix, ultimately including replacing the motor at 34K. It ended up being a catalytic converter
They replaced a motor for a bad CAT? :histeric: Bet GM was thrilled with that one (if they ever found out).

Submariner409
07-02-08, 09:12 PM
stngh8tr and cvpiftw, After a couple of EGR oriented replies you said that it really sounds like a rattle and that you "would get to it" 24 hours ago. This evening it sounds like "detonation". Did you ever "get to it"??

As tateos opined, these engines are not known for preignition, but if you had a sticky/sluggish EGR then you're definitely in the preignition ballpark.

EGR problems are almost always presented as a part-throttle, off-idle, or downshift "spark knock", "detonation", or "rattle".

Just for grins, in a properly running engine installation, disconnect the vacuum or electrical connection(s) to the EGR valve(s). (Plug the disconnected vacuum line). Drive. What happens ???

cvpiftw
07-03-08, 01:54 PM
Sub, my engine is in good running condition i believe. I have a motor from a 10k 99 STS within my car, the headgaskets took a dump.


Either way, Nothing happens. The engine only detonates when it wants to. I believe it was a case of faulty fuel. I am partially afraid to open the EGR in fear of snapping the bolts.

Ranger
07-03-08, 09:04 PM
What do you mean by "faulty fuel"? It should run fine and NOT detonate even on 87 octane.

eldorado1
07-03-08, 09:20 PM
Do you regularly open it up?

You could have carbon buildup on the heads causing the piston to rock in the bore at TDC. Some TEC would clean that up. If it doesn't fix it, at least you'll know one thing that it isn't!

stngh8r
07-04-08, 01:12 AM
stngh8tr and cvpiftw, After a couple of EGR oriented replies you said that it really sounds like a rattle and that you "would get to it" 24 hours ago. This evening it sounds like "detonation". Did you ever "get to it"??

As tateos opined, these engines are not known for preignition, but if you had a sticky/sluggish EGR then you're definitely in the preignition ballpark.

EGR problems are almost always presented as a part-throttle, off-idle, or downshift "spark knock", "detonation", or "rattle".

Hey Sub, thanks for the EGR tip and also backing it up again in this post.:thumbsup: I can't wait to clean it and see if that helps. The reason I havent "got to it" is due to the car being about 700+miles away at the moment. My wife took the car out of town about the same time I started this thread. It will be the 10th before I see her or the car again.

stngh8r
07-04-08, 01:16 AM
Do you regularly open it up?

:yup:

digitalcaddie
07-25-08, 04:27 PM
Getting the EGR out is really not that bad. Check the Tech Tips section for a step by step post including pictures of how to do this properly. Reading your post reminded me that my car used to rattle a little bit at 4k+ rpm. I cleaned the EGR trying to solve a different problem and there's no rattle any more. I'm not completely sure that the EGR was the issue since I've done some other things, but it could have been. I wasn't thinking about this problem since I only rev it up over 4,000 when I'm doing the ole WOTs, but it is smooth all the way up to 90mph in 2nd gear now. Have fun.

stngh8r
08-25-08, 08:44 PM
Finally got around to cleaning the EGR. NO MORE RATTLE!! :thumbsup:

I can't believe how easy it was to get to as well. I tried to take the EGR off my truck and it was impossible.

Anyways, pulled the two bolts off, sprayed the bottom down with carb cleaner, then poured some Berryman and gasoline into a coffee can just deep enough to cover the bottom of the EGR and let it soak a few hours. After soaking, I sprayed it again with some carb cleaner, rubbed it a little and sprayed again. Let it dry and reinstalled.

Thanks for pointing me in the right direction guys :thumbsup:

tateos
08-26-08, 09:51 PM
Thanks for the update