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'03 STS, '99 SLS (sold), '02 Yukon, '02 Z28
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Discussion Starter #1
Start the car up and it feels like my '02 Z28. You can barely feel or hear it but I take notice of these things. These things idle like sewing machines so hence my concern. I think there is a drop in midrange performance. Gas mileage is slightly suffering. Was going to check the plugs till I found out it didn't have any. Unless I am wrong? I run 93 octane all the time. Through in Berryman fuel cleaner. No change. Oil looks good but goes as fast as the gas when your liberal with the peddle. Think the valve cover is leaking in the back but not wet and nothing on the floor. Possibly rings. Makes as much motor noise as an LS1 so not bad at all. Have not noticed and codes that would suggest anything wrong. Next step is pressurized fuel service. Any other ideas to what could cause this?
 

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'98 STS (RIP @ 206,xxx miles)
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....Was going to check the plugs till I found out it didn't have any. Unless I am wrong?
You are wrong.
What you don't have is a snake pit of ignition high voltage wiring. Your engine has coil-on-plug ignition like most 21st century gasoline engines. Google is your friend.
If your spark plugs have never been replaced, that would be a logical starting point; eight years times 12,000 miles per year = it's time for new spark plugs.

If/when you replace the spark plugs, you will have the opportunity to inspect the high voltage connections between the coils and the plugs.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Here's your setup.........the rear coil cassette is a PITA. Use only ACDelco Platinum plugs, #41-987. They come pre-gapped to .050". Use a tiny bit of anti-seize on the first 3 threads and torque to 13 ft/lb. As JimD posted, make very sure the rubber plug boots are clean and not carbon tracked.

Your engine is calibrated to run on 87 octane. Unless you pull a trailer or track race every day you're throwing money away. Change the fuel filter. Don't pour snake oils into the gasoline tank - the only recommended fuel system cleaner is Chevron TECHRON (at intervals greater than 3,000 miles) and top tier gasolines (Google). Also do another search and check the FPR - Fuel Pressure Regulator.

Your oil consumption is not rings. Read the entire Cadillac Technical Archive up ^^^ in the black bar, especially the article "Occasional full throttle operation is good for your engine". Do a search for WOT or Wide Open Throttle engine (carbon) cleaning procedures. FWIW my car is run hard, lots of highway time and back road running and it gets just over 4,500 miles to a quart of 5W-30 synthetic. DO NOT keep the oil level at the top of the dipstick hashmark - halfway up or a tad less is the GM-preferred level. There is an extensive GM TSB (Technical Service Bulletin) on this tidbit.............
 

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2003 Cadillac Seville STS 73k Miles, '90 Chevy 1500 Reg Cab
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725 Posts
Start with plugs, gonna cost you about 6 bucks a piece, front 4 are fairly easy, back 4 are...well more of a PITA as expected on a FWD car, and for more emphasis don't skimp on plugs and let the parts man sell you something cheaper that they say crosses across and should be just fine, all that it'll save you is 20 bucks now so you can come back in 5-10k miles and do it all over again. While you're doing that check the FPR, about 40 bucks if it needs to be replaced. With those 2 things done put some miles on it and see what you think. Also pull codes through the DTC, write them down then clear it and come back to it in 2 or 3 weeks and see what you have. If you actually have a miss it should throw a powertrain code.
 

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'03 STS, '99 SLS (sold), '02 Yukon, '02 Z28
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77 Posts
Discussion Starter #5
You are wrong.
What you don't have is a snake pit of ignition high voltage wiring. Your engine has coil-on-plug ignition like most 21st century gasoline engines. Google is your friend.
If your spark plugs have never been replaced, that would be a logical starting point; eight years times 12,000 miles per year = it's time for new spark plugs.

If/when you replace the spark plugs, you will have the opportunity to inspect the high voltage connections between the coils and the plugs.
Plugs were my first thought. Guess this setup just surprised me. I'm use to the coil packs just not the tubes. Guess my milage is doing better though. I'm sitting at 80k and change.

I don't prefer snake oils myself. Usually when I use the Berrymans its on my Yukon. I pour it through the break booster hose while @ 1200rpm then shut off and wait 10 mins. It is to de-carbon the motor. Works like a charm. It's always fun to crop dust the neighborhood every now and then. DONT WORRY. I do NOT do that to the STS. That procedure is probably too rough for a Northstar. Hence putting it in the tank.

I had a '99 SLS and when ever I put 87 octane in it I could hear it detonate pulling out of the driveway. I tried that for 2 tanks. Haven't gone back since. I will give it another go. Hell, who wouldn't want to save some green.

Sub, are u running 87 or 93?

I do get on it every now and then to clear it out but did not know there was a procedure. That would be nice to know. I was just worried about the oil level dropping so fast. I understand these motors use more oil that other cars. I am use to that with my '02 Z28 and LS based Yukon. This one just uses more than either. Don't know exactly how much more. Need to start recording that. Did not know about the GM recommended oil level. I usually top off @ 8qts. I know these are listed @ 7.5qts. I run Valvoline 5-30. Wouldn't mind switching to syn but figured since I drive it daily and change it every 3k I would pocket the savings
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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One reason you're "using oil" is that 8 quarts is way overfilled: the manual calls for 7.5 with filter, and quite a few of us run 7. The GM TSB specifically notes that carrying the oil at FULL (or at the "O" in OPERATING RANGE) is a factor in perceived oil consumption.

Try running it here and see what happens to consumption: <------ ADD XXXXXXXX OPERATING RANGE -------------------

Your 1999 was calibrated for premium fuels. The 2000+ is calibrated for 87 and will be perfectly happy with that fuel. The 2000+ knock sensor/PCM will NOT advance timing to account for higher octane fuels; it will only retard timing to stop knock/detonation. The compression ratio before 2000 was 10.3:1. Your engine is 10.0:1.

www.rockauto.com has ACDelco #41-987 Platinum plugs in packs of 8 for $5.15 apiece.

I run Shell, Texaco, or Chevron 87 and consistently get a shade over 24 mpg long-term highway, 13-16 in town traffic. Google "top tier gasoline".

Your engine will easily go to 7,500 between oil changes, particularly if the car is driven almost daily. Use the OLM and read up on it in the Technical Archive ^^^; it works and it's accurate. (Did you know that your engine has roller cams ?)
 

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2010 DTS
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87,286 Posts
Do not overlook the infamous ripped plenum duct. I'd check that before anything else.
 

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'03 STS, '99 SLS (sold), '02 Yukon, '02 Z28
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Discussion Starter #8
UPDATE!... Replaced plugs with Delcos. Saw a little bit of oil where the plugs were. Have pix. Plugs looked decent. Have pix. Tubes looked good. Just dusty. Cleaned coil cassette terminals. Assuming if one of the cassettes were bad it would show up on the DTC display. Cleared codes. No change. You can still feel it right at start up. Unless the PCM needs to learn.

Noticed that coolant was low by a smidgen from what I remember.
Now things are starting to concern me. I am going fill up with some Chevron and change the fuel filter. Only other thing I can think of to do is have a pressurized fuel system cleaning with out ripping into the unknown. Probably also get the HG check kit. If all of that turns up negative, Will check the plenum duct then on to lifters. Any other suggestions?

Is there a way to check and see if individual injectors are spraying correctly?
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Oil in the plug wells is common and not a problem: the wells are sealed by O-rings in the cam covers and they weep over time. Unless the plug(s) were totally immersed in oil, not to worry (oil is an insulator......).

You don't have "lifters" - the car has roller cam followers adjusted by static hydraulic lash adjusters.

The built-in code system will not display individual coils within the cassettes - it will only give a "miss detected" code. The coil diagnosis is done by a dealer with a Tech-II.

Use a 20 oz. jug of TECHRON in a fillup of 93 octane, otherwise the car is perfectly happy with 87. Listen to each injector with a stethoscope with a length of proper sized vacuum hose substituted for the chest diaphragm. A steady "tick-tick-tick-tick".

While you're doing that, use a shot of brakleen, TB cleaner, or starter fluid to check the bottom of the plenum for air leaks.

Take a hard look at these: (click on each link, and then page back to this post - don't close your browser to change links.......)

View attachment 67158
View attachment 67159
View attachment 67160
View attachment 67161
View attachment 67165
 

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'03 STS, '99 SLS (sold), '02 Yukon, '02 Z28
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Discussion Starter #11 (Edited)
You don't have "lifters" - the car has roller cam followers adjusted by static hydraulic lash adjusters.
I don't know DOHC motors too well. Still learning. I'm use to Camaros and Vettes.

Check the plenum duct FIRST. It's a very common problem.
Don't know why I didn't put that first. Seems to be the easiest to check. Still don't feel that I would be the culprit. I only have 80k on the car. Wouldn't it throw a code from the mixture being too lean/rich. Is it a straight forward fix or call the buddies and supply the beer fix? I have never delt with an intake removal.

A while back I did notice a puff of white smoke @ startup. Might have been from overfilling the oil. Does the EGR sys need cleaning from time to time. I remember that being an issue with the early LS1 Fbodys.
 

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2010 DTS
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It may or may not set some codes. Pretty easy repair. Do a search for plenum or plenum duct. Sub has posted a diagram several times. Might even be a tutorial in the Tech Tips forum.
 

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'03 STS, '99 SLS (sold), '02 Yukon, '02 Z28
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Discussion Starter #13
UPDATE: Saw the pics. Sprayed carb cleaner on and around the duct. No change. Also did the screwdriver to the ear test for the injectors. All seem to be firing the same.
 

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2010 DTS
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87,286 Posts
Did you push on the plenum duct to open the rip (if there is one) while spraying? The rip will be on the bottom and won't be open a lot, so it is important to flex the rubber with your finger or a blunt tool.
 
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