: 00 N* still rough idle after changing plenum, cleaning TB



SmoothTS
04-26-08, 05:33 PM
What we all thought was a cracked plenum on my 115,000 mile, 2000 STS turned out to some loose rubber on the side of the plenum. After removing the plenum and intake manifold, I could not see any cracks anywhere. I spent all day cleaning the TB from both sides, installed new intake gaskets, new fuel injector o-rings, and installed my new plenum anyway. I also tried a blash of MAF cleaner and installed a new air filter. My plugs were replaced with new OEM plugs about 500 miles ago. After all of this, I still have my rough idle.... the engine sounds fine, but sitting in the car or looking at the engine with the hood open, you can see the engine "shudder" and "wiggle" about every second or so. This is extra annoying at stoplights, as it can be felt in every seat.This "shudder" at idle has been getting worse since about 80,000 miles. Fuel economy dropped from about 27 highway to about 25 highway around 100,000 miles. Still no codes... and Fuel Pressure REgulator works just fine.

Any ideas? Should I check the EGR valve even though I'm not getting any codes?

Also, the PCV sounds fine (it makes the clunking sound) but should it fit so loose? It easily slides out of its rubber socket/slot/holder on the rear valve cover. Seems like it should be tighter, but I don't know for sure.

Any other thoughts on what I should check? The only other thing I can think of is to have my fuel injectors cleaned and maybe change the fuel filter.

91parkave
04-26-08, 06:31 PM
sounds like a vacum leak, Did you check the rear of the manifold IIRC theres a vacum port there make sure you have that hooked up as well.

SmoothTS
04-26-08, 07:09 PM
I have both PCV hoses connected, FPR hose connected, brake booster hose connected. I'm really not sure what more I can do vacuum-wise. However I am still concerned about how easily the PCV valve itself can slip out of its rubber slot in the rear valve cover. It's a little more tight once everything is warmed up, but when the engine is cold the PCV valve and its attached hose slips right out of there with a very light tug.

Ranger
04-26-08, 07:21 PM
Try replacing the PCV grommet. Clean the EGR valve. Test the FPR again, but this time add vacuum to the FPR with a hand held vacuum pump or by sucking on a vacuum hose connected to it. I remember McCowden was having a problem and the FPR seemed to test fine until I suggested adding vacuum. That in addition to the fuel pressure from the other side showed it faulty.

SmoothTS
05-01-08, 01:49 AM
Well, a buddy and I looked over the PCV and PCV grommet and it seems to be tight enough. Brakleen sprayed in the area doesn't make a difference, so there doesn't seem to be a vacuum leak around there. We also tested the FPR and it too seems to be fine. I have not yet tackled the EGR. Still no codes but I still have the rough idle (a sort of shudder or vibration of the entire engine, visible and can be felt in all seats... every second or so). MPG has improved by about 15% since the TB cleaning and new intake gaskets. Could the vibration be due to fuel filter? Broken motor mount? Bad O2 sensors? Fuel injectors? I'm thinking about sending the injectors to be tested/cleaned. I'm still surprised I don't have any codes. The only codes I have seen thus far are voltage related from when I had the battery disconnected. And one code from when I put the trans into neutral while I had cruise on. Any other ideas in case the EGR cleaning doesn't help?

CadiJeff
05-01-08, 02:03 AM
list of probabilities(my experience) in no particular order
plug
coil pack
fpr
carbon buildup in cyl
fuel injector(s)
loss of fuel pressure
fuel pump
loss of cyl compression
exhaust restriction

Ranger
05-01-08, 05:26 PM
I'd suspect maybe plugs first. Especially if the mileage is high.

SmoothTS
05-01-08, 08:14 PM
I replaced the plugs about 500 miles ago. New plugs and a jug of Techron didn't improve the rough idle, but they did improve my acceleration and 0-60 time! Replacing the plenum and intake manifold gaskets improved my fuel economy by about 15%, but didn't improve the rough idle.

Once my EGR gasket is here, I'll clean it and see if that helps. Hopefully that's the ticket and hopefully I can simply clean it, new EGR isn't cheap. FPR was tested with a vacuum pump and seems to work fine. If it's cheap enough I might replace it and its gasket (a new FPR gasket came with my fuel injector gasket kit). I will also send my injectors in to be tested and cleaned.

So do you think I should replace my fuel filter? Is there any way to test the coil packs (this is a 2000 with coil-on-plug)? The coil packs are pretty expensive to replace on only a hunch. What are the symptoms of a clogged cat or muffler? My engine and exhaust sounds normal and it still has tons of performance and acceleration. It's just the rough idle that bothers me. Is there a chance that O2, IAT, or MAF sensors could be causing this problem? I'm not getting any codes, but maybe the sensors are giving slightly off readings? Sensors are pretty expensive to replace on a hunch, though.

AJxtcman
05-01-08, 08:31 PM
Any other thoughts on what I should check? The only other thing I can think of is to have my fuel injectors cleaned and maybe change the fuel filter.

:histeric:

I will be right back :hmm:

AJxtcman
05-01-08, 08:34 PM
Read this thread. It is a short one.
http://www.cadillacforums.com/forums/northstar-performance-technical-discussion/122654-northstar-cam-specs.html

SmoothTS
05-02-08, 12:10 AM
Read this thread. It is a short one.
http://www.cadillacforums.com/forums/northstar-performance-technical-discussion/122654-northstar-cam-specs.html

Well heck... I wish the previous owner would have known about this back when the car was still under warranty! That would have been a nice fix! Grrrrr :helpless:

derone
05-02-08, 08:41 AM
Hey does yours do it all of the time at idle? Mine does it intermittently, some days its smooth as silk and other days it rocks a little every second or so, though no change in the exhust note. I tried the fuel filter and new plugs and it didn't make a difference. I'm leaning towards the EGR valve and will check it this weekend. I doubt the cams would make the problem intermittent on mine. If it isn't the EGR valve, I may have to buck up for a coil pack.

SmoothTS
05-02-08, 01:05 PM
My idle is always "rough" although some days it doesn't rock/shudder quite as much, might have something to do with temperature or humidity or who knows what. The rough idle started around 80,000 miles. I now have 115,500.

I'm going to clean my EGR valve as soon as the gasket arrives, I'm sure it's due for a cleaning anyway. I'm also thinking about changing the fuel filter, it too might be dirty anyway. Coils, injectors, and other more expensive options have made me a little gunshy, as I don't want to be replacing the more expensive things on just a hunch.

AJxtcman
05-02-08, 01:25 PM
:hmm: I lost some posts. :hmm:

OK my bosses 00 DTS with 110K had a terrible idle. I replace the cams in it and it idles smooth as glass

I first installed the rear cams and the software patch. I needed to run the car out of the shop we got busy. I got back to it about a week later to do the other cams. The car ran about the same as it did when it came in, but the check engine light was flashing (misfires). Sure enough the front 4 cylinder all were misfiring. :yawn: I replaced the front cams and the car ran so smooth you couldn't even tell it was running. I used 2003 VIN Y cams in it.

The car was misfiring since NEW, but all the cylinders misfired evenly. When performing Crank Variance learn Procedure it compensates for that. That is why it did not set a DTC

chevyhighperformance
05-02-08, 06:35 PM
I'm chasing down the same problem on a 2000 STS with 50k miles. I inspected the boots, did a voltage breakover & peak current test on the plugs while they were in place, and replaced the rear coil pack when I saw the ground current change with temperature. I checked all the hoses and inspected the plenum gasket. I checked the fuel pressure when the FPR was under vacuum and under atmospheric pressure. I tried a couple sea foam treatments. I listened to the injectors with a stethoscope and measured all the injectors' resistances. I checked the EGR which was clean.

SmoothTS
05-02-08, 07:37 PM
What do the cams cost and what's the book time for changing them and installing the software update?

chevyhighperformance
05-04-08, 12:03 PM
I read through this.

http://www.caddyinfo.com/howtoroughidle.htm

This was written in 2002. Is this information still current? Is the rough idle just the way it is?

SmoothTS
05-20-08, 03:21 AM
OK my bosses 00 DTS with 110K had a terrible idle. I replace the cams in it and it idles smooth as glass

I first installed the rear cams and the software patch. I needed to run the car out of the shop we got busy. I got back to it about a week later to do the other cams. The car ran about the same as it did when it came in, but the check engine light was flashing (misfires). Sure enough the front 4 cylinder all were misfiring. :yawn: I replaced the front cams and the car ran so smooth you couldn't even tell it was running. I used 2003 VIN Y cams in it.

Well I am ready to get new cams for my 2000 STS. Do you suggest I order the new replacement cams designated for the 2000? Is there any reason why you used 2003 cams for your 2000 DTS?

How much work is it to install the cams? I looked at the service manual and the procedure seems pretty simple, except for that one alignment tool and having to flash the PCM software. Maybe I could do most of the work at home and tow the car to the dealer to have to PCM flashed?

How many hours does it take the dealer's shop to replace the cams? Seems like a very staightforward project.

Submariner409
05-20-08, 12:52 PM
According to Alldatadiy the shop rate to replace all cams, no other work, is 15 (yes, 15) hours. In-house GM warranty rate is a bit less. The entire chaincase (whole front of the engine) has to come off as well as the ignition system, intake, and cam covers. The current list for the 4 cams is around $1,000. Other parts (lash adjusters, followers, chains, etc.) add more. I'll bet you're looking at $2,500, dealer.......

gdmarques
06-09-08, 08:25 PM
I was having a rough idle and a very unusual whistling noise in my '98 Deville with N*. I was sure the noise was a bad bearing on an accessory then I read on the posts here that it might be a leaking intake manifold gasket. So I changed the gaskets and it made no difference in the noise. Finally after finding my auto stethoscope and using just the rubber hose to listen around the engine I found the noise. It WAS coming from the intake manifold however not the gaskets. For some reason at the end of the plenum on the passenger side there is a spring loaded trap door in a round recess molded into the manifold. If you pull this open with your finger (you have to raise the manifold to get at it) there is a gasket molded into the "door". I wiped this clean with a rag on both the gasket and seating surfaces, applied a very thin coating of Permatex high temp orange gasket maker and let the flap close. Reinstalled the manifold and started the engine and both the whistling noise and the rough idle were gone.

I'll post pictures on a separate thread as I have seen many responses around the manifold gaskets themselves, but no mention of this strange valve.

Ranger
06-09-08, 09:05 PM
For some reason at the end of the plenum on the passenger side there is a spring loaded trap door in a round recess molded into the manifold.
It is called an over pressurization relief valve. in the event of an over pressurization (backfire), the valve is forced open to relieve pressure rather than having the manifold explode.

SuperC
06-28-08, 12:27 PM
Well guys, I too replaced the plenum and I still have rough idle. I am no mechanic and replacing the plenum was not an easy job. I think I am gonna have to see a pro for this one.
-C

Murphyg
06-29-08, 03:13 AM
.................................................. .... The only other thing I can think of is to have my fuel injectors cleaned and maybe change the fuel filter.

So how did that work for ya ?

Just had that done on my 01. Made a difference.
Also did it all on the wifes 99 Bonneville......and what a "major" difference.