: MPG DROP - 22 to 17



krimson_cardnal
12-29-06, 01:08 PM
Driving back from Maine yesterday and had the MIL come on and a message telling me to "Check emmission system". Promptly I run the codes and find PC1520 current. I then notice my MPG dropping so I reset it and it's down to 15MPG. It had been running around 22MPG. After a bit it stabilized at 17-18MPG.

I soon stopped at a service area to let things calm down and relieve myself. I then cycled the ignition to disarm the MIL and took off again. After 5 minutes or so the MIL illuminates, but this time no message accompanies it, so I run the codes and PC1520 is still history and my MPG's are still hanging at 17.

This AM I checked coolant level and it might have been down a bit, but at the angle it's hard to tell so I'm about to take 'er out and warm things up and see how it holds.

The FSM says "PC1520 - Gear Indicator System" should not trip the MIL, and I thought if the MIL trips and a message says check emmission systems their should be a code current???? Two other codes are in history - been their a while - PC00327 and PC00603. They popped up after I replaced the battery.

I didn't notice a change in the RPM to account for the poor mileage so I'm at a loss. Any clues out there???????????????????

I'll be back this afternoon to check in.

Hope ya'll had as nice a Christmas as I did up in Maine. Now all we need is snow in the NE. Colorado got clobbered again, cool. K_C

Submariner409
12-29-06, 02:43 PM
:sneaky: There's another thread running here, somewhere, that talks about FPR (fuel pressure regulator) malfunction. An iffy regulator will raise hob with mpg. Marginal Oxygen sensors will also mess up the air/fuel ratios depending on rpm/load.

STS127
12-29-06, 05:13 PM
^^ I agree. Try a tune up if you haven't already.

STS127
12-29-06, 05:15 PM
Actually a tune up is probably not the soultuion to the problem.

Ranger
12-29-06, 05:58 PM
I believe on OBDII, only emissions DTC trip a MIL. Something does not sound right, but I agree with the sailor. Check the FPR.

dp102288
12-29-06, 10:00 PM
The FSM says "PC1520 - Gear Indicator System" should not trip the MIL, and I thought if the MIL trips and a message says check emmission systems their should be a code current???? Two other codes are in history - been their a while - PC00327 and PC00603.

Why can't I get definitions for those codes (too many digits)? :hmm:

Ranger
12-29-06, 10:09 PM
Yeah, drop one zero.

dp102288
12-30-06, 01:02 PM
^^ Oh! Thanks Ranger!!

P1520 - Transmission Range Switch Circuit

P0327 - Knock Sensor Circuit Low Voltage

P0603 - Control Module Long Term Memory Reset

:hmm:

krimson_cardnal
12-30-06, 05:56 PM
Thanks guys,

Well I haven't had a chance to get under the hood. No garage [I can get into] and the weather has finally turned to winter around these parts, but looking at those codes I can't make sence either. 0603 and 0327 were there, again I think they came up after I had replaced the battery. As far as 1520 and it showing when the MIL came on beats me.

I get no MIL now and no current codes, but mileage still seems a bit off, like 4MPG or so? I do believe I have a vacume leak that I haven't been able to track down. A while back I had a problem with the parking brake not releasing. It isn't consistant and I do not routinely use the parking brake, but my thought is that if there's a vacume line hooking to the FPR and the vacume system is weak or failing this might be effecting things. I'll do the simple test of pulling the vacume line on the FPR and looking for fuel as soon as the sun is shinning again, but that's my thought for now.

I'm also in agreement with submarine on the o2 sensors. Is there any way of evaluating them on board, or must they fail. If they fail that will trip the MIL. That's the part that confuses me. Are they difficult to get to and replace??

The MIL tripped, but no approriate codes were/are showing, either current or history to make sence. Can the MIL come on and NO codes be recorded?????????

My fear was a blown head gasket - my nightmare!!. But looking at the exhaust tips it is obvious the DeVille is running rich.

Ranger
12-30-06, 09:56 PM
Theoretically, any time the MIL is on, there should be a DTC that tripped it.

dp102288
12-30-06, 10:05 PM
Some members have reported their MIL coming on but there being no codes. From a 100% working perspective this doesn't sound possible, but weirder things have happend with the Caddys.

krimson_cardnal
01-09-07, 01:17 PM
Well, after a visual check found nothing I have driven a 700mile round robin and fuel consumption is up to the familiar 23-24mpg highway [70-75] average.

I'm thinking the 02 sensors might be the culprit, only time will tell on that one. Right now, at 113,000m the 96DeVille is running smooth as a top. Wish I could get more than 1000mile/qt oil, but we all know that story...

Tis' a Happy New Year K_C

Ranger
01-09-07, 04:36 PM
Strange as it may seem KC, I kind of miss adding a qt of oil every 1 - 2 K, knowing that was it fortifying the oil additives.

Submariner409
01-09-07, 05:46 PM
I would imagine that replacing the N* O2 sensors would involve a lift and some knuckle-busting. Some N*'s have 3 or 4 of the things in the headers and near the cat. General wisdom says that after 50-70,000 miles, they're on the edge of being able to provide a good oxygen reading to the PCM, thus screwing up the intake air/fuel ratios. I do know that, if you replace one or all, don't touch the tip, or get any oil, grease, or nevr-seize near the tip, or it's gonzo sensor, and don't twist or kink the wiring because that messes up the clean air signal.........Wheeee!!:bouncy:

CadillacSTS42005
01-09-07, 09:46 PM
edit

krimson_cardnal
01-10-07, 01:42 PM
Just this AM went out to the market. Bout 2 miles down the local road after a cold start I stop to pull out onto the main drag. Left hand uphill turn. I pull out and power up to speed, no WOT - just a normal acceleration and BAM - the MIL lights up and the Service - Emission System message" displays. I immediatly run codes - no codes current??? The MIL goes off after three ignition cycles.

When I took the turn onto the main line the engine was not quite up to temp - 160F let's say and I applied a bit of torque to power up normal, no traffic to "blend" into. So loading the engine with it not quite up to operating temp popped something to bring on the MIL but not trip a code. MPG seems to take a temporary hit after this happens.

Today I'm replacing the air filter and will clean out the TB. Then I begin research on the O2 sensors. I believe the 96DeVille has 4 - 1 on each manifold and one each side of the cat. Thanks Sub on the heads up on replacing them. Way things look I might have a shop do it, or at least the ones under the car.

Will probably live with them for a bit as is. Saturday I'm beginning a trip that's ending me up in Tampa and on the long haul everything seems to run okay and I'm into a 13#turkey roast and a daughter home from college and we all know it takes a bit of planning to get the hands greasy. Might do 'em in Atlanta though - anyone know of a good dealer in that town???

I do know what you mean Ranger on the "oil replentishment" - temporary loss of logic on my part....:alchi: K_C

Cad tech
01-10-07, 04:10 PM
DO NOT condem O2 sensors without first watching swing of the 3 precat sensors. Make sure the vehicle is going into closed loop. if you have a slow responding O2 it will set a code. throwing parts at vehicles is to expensive of a method of repairing. Check spark plugs, check for plenum leak by spraying carb clean along base if it changes RPM u need intake gaskets. You are running a rich condition the evidence should be on the plugs, thus if no O2 codes are present there must be a vacuum leak in which the O2s are seeing a lean condition thus dumping more fuel then jneeded into combustion chamber. Most cads I see on daily basis with this complaint without codes is intake leak or coil not hot enough...................

Ranger
01-10-07, 06:03 PM
I am really surprised that the MIL lit up and no DTC was stored.

krimson_cardnal
01-10-07, 09:05 PM
Well it's got me stymied. MIL with no codes... go figure.

Thanks Cadtech for the input. I've suspected a vacume problem for a bit now. Can you expand a bit on the monitoring of the 02 sensors and the closed loop thing? Done thru the OBD I would guess.

Also the spraying of carb cleaner - where exactly, do I need to remove beauty cover? I'm in full agreement in regards to thowing parts at it without understanding wazup.

I am at logger ends on checking out the vacume system as well. Don't really know were to begin and how to go about it. Any suggestions would help big time. Thanksall K_C

Cad tech
01-10-07, 09:21 PM
Ok hopefully you have a tech 2 or another type of scanner that can give you realtime data (they got rid of onboard data on later models SUCK) at any rate the swing will constantly change on the precat o2 sensors from 900-50 Mv if they are swinging at a good rate then that is should always be changing then that is great, now with that hooked up still , remove engine cover spray carb clean around base of manifold (plastic to aluminum head) now have assistant watch any dramatic changes in the O2 swing if there is drastic change or it revs up that is your problem....
Also check relief valve on intake (right by p/s pump) and pcv line to intake they usally rot....check the other vacuum lines....one more thing i just thought of if you have the tech2 also test the egr, command the pintle position to 90% it should die if not we may have a clog there........

clarkz71
01-11-07, 07:48 AM
Ok hopefully you have a tech 2 or another type of scanner that can give you realtime data (they got rid of onboard data on later models SUCK)

I sure do love my OBD I. It is amazing what you can do with it.

Cad tech
01-11-07, 11:00 AM
you can do it on some just access the computer and go to pcm data and I believe it is pd,ps 7 for o2 but data stream is slower....

clarkz71
01-11-07, 01:46 PM
What are you talking about?? Do what on some?

Cad tech
01-11-07, 02:25 PM
access pcm data, inputs and outputs like OBD1

clarkz71
01-11-07, 02:28 PM
As far as I know you can do that on any 93-95 OBD I car. (cadillac)

Cad tech
01-11-07, 02:46 PM
You can access onboard computer on an 80's cads also and gives you the data, OBD1 and some models on OBD2 but on these it is limited.

clarkz71
01-11-07, 02:58 PM
Good to know.:thumbsup:

Cad tech
01-11-07, 03:52 PM
here is an example for 88 deville, can be used for most betwwen 84-92 cads

Cad tech
01-11-07, 03:57 PM
And this for a 96 deville

Ranger
01-11-07, 07:09 PM
Ahh, those look familiar. Wish the '02 & '03's still had that

Cad tech
01-12-07, 10:06 AM
Ranger you know that you can still access the on board comp on those w/o tech2 if not yell ill post how........

Ranger
01-12-07, 12:37 PM
Yeah I know, but I thought we lost the data, inputs, overrides & parameters etc. in latter (OBDII) years. Not so?

krimson_cardnal
01-12-07, 01:30 PM
Cadtech, I'm yelling... post the details!!!

I've been in the FSM going over the PCM DTC's and it's looking like the only errors that trigger the "Service Emission System" message have to do with the MAF sensor. A code should be tripped, and I'm not seeing one, but this afternoon I'm going to pull the air filter and get into the TB and give a good cleaning, after I review the Tech Tip on the TB.

Cad tech
01-12-07, 05:11 PM
anything within the emissions will 02, mat, maf, evap leaks ect no codes though?????alot of the codes have to become a hard set for the on board will pick it up. a steady ses light on ect....it most likely is not triggering it because you are constantly accessing comp....drive the vehicle till you can get a ses light on...I will look and see what I can find on that message coming on....check for those vaccuum leaks they go hand in hand with your gas milage complaint..

krimson_cardnal
01-12-07, 05:15 PM
All I can say is WOW!!

Seems as if I've overlooked the TB in the two years I've driven this DeVille, and apparently the guy before me did as well. Unbelievable. Never saw slimmy black goo like it before. Can some one explain where this comes from. I mean right at the air intake. Phew. How is this possible?

I swabbed things out w/ tb-cleaner reaching in as far as possible. Seems to have made a difference in MPG - could this be so???

Anyway, I'm off on my journey in the AM, things seem real smooth right now, we'll see what around 3,000 miles brings. Thanks guys, I'll be watching. K_C

Cad tech
01-12-07, 05:54 PM
Hope it works keep us posted;)

Ranger
01-12-07, 06:03 PM
K_C,
It comes from the EGR & PCV. Crankcase gases and exhaust gases are drawn into the manifold and burned, but when the engine is shut down, they settle out or condense on the TB & throttle plate as they cool. I make cleaning the TB an annual spring ritual.

krimson_cardnal
01-12-07, 09:51 PM
Thanks Ranger and Cad tech. Now I'm curious at to what's under the intake manifold. Might even go twice a year, bi-annual. Be back K_C

Ranger
01-12-07, 09:58 PM
Shouldn't be anything under the intake. That oily mess stays inside the intake.

Cad tech
01-12-07, 10:27 PM
better not be an oily mess then your head gaskets are leaking or p/s pump leaked into it..LMAO...starter motor and knock sensor...

krimson_cardnal
01-13-07, 10:21 AM
By "under" I meant inside where I saw all that TB mess. Top of engine looks real nice, bottom tends to be damp - oil... I've resigned to live with that for now. You used the "HG" word Cad tech, hope you did raise it's ugly head on me... LOL. I'm off for Tampa see ya around..... the drive's gonna do me and the DeVille good - I just know it. K_C

Cad tech
01-13-07, 08:09 PM
have a great trip best of luck with that h********* problem LMAO....

krimson_cardnal
01-22-07, 10:48 AM
***UPDATE***
I've come to the conclusion that a clean TB is one of the more, if not most [single thang], valuable requirements to running the NorthStar economically! I know it's been stated before, and I, for some reason, didn't think it applied to my DeVille, but for those who haven't, do clean that sucker routinely.

I'm in Atlanta now, on my way back north to winter, and so far on this trip I've been averaging 24MPG @ 75MPH. As you might remember I was in the 17MPG range before cleaning out the TB. I was thinking O2 sensors and all sorts of horrible things. DeVille is running smooth - 206F, 1500Mile/qt oil, no coolant loss, 24MPG. Life is good, even looked up an old flame!!

Cad tech, I'm not even thinking that way anymore - shhhhh! Don't even think about it. K_C

Ranger
01-22-07, 05:20 PM
:thumbsup:

krimson_cardnal
01-27-07, 02:00 PM
***UPDATE***
I've come to the conclusion that a clean TB is one of the more, if not most [single thang], valuable requirements to running the NorthStar economically! I know it's been stated before, and I, for some reason, didn't think it applied to my DeVille, but for those who haven't, do clean that sucker routinely.

I'm in Atlanta now, on my way back north to winter, and so far on this trip I've been averaging 24MPG @ 75MPH. As you might remember I was in the 17MPG range before cleaning out the TB. I was thinking O2 sensors and all sorts of horrible things. DeVille is running smooth - 206F, 1500Mile/qt oil, no coolant loss, 24MPG. Life is good, even looked up an old flame!!

Cad tech, I'm not even thinking that way anymore - shhhhh! Don't even think about it. K_C

Well got back to NY to sub-zero weather and a bit of snow. After 3,500 miles I figured the first thing to do was sleep, then change the oil. Not being that brave [14F] I took it to my stand-by oil change place. After 12miles into a 20mile drive the MIL illuminates - "Service Emission System" message dislays and the MPG goes to hell. Car performes flawless - no coughs or shudders, it seems to run rich all of a sudden. Seems as if the MAF is on the way south.

Two weeks - Florida and back - and no problem. Back in NY and first time I drive off on a cold start it triggers MIL - NO CODES - Message only.

What-the_ :annoyed: . . . K_C